JOURNAL OF
PETROLOGY
Journal of Petrology, 2016, Vol. 56, No. 12, 2341–2372
doi: 10.1093/petrology/egv023
Advance Access Publication Date: 10 July 2015
Article
Field Evidence for the In Situ Crystallization of
the Merensky Reef
Rais Latypov1*, Sofya Chistyakova1, Alan Page2 and Richard Hornsey3
1
School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa, 2SRK Consulting,
Northlands 2116, Johannesburg, South Africa and 3MMG Limited, 32A Jellicoe Avenue, Oxford Corner, Rosebank,
Johannesburg, South Africa
*Corresponding author. E-mail: [email protected]
Received September 30, 2014; Accepted April 16, 2015
ABSTRACT
The Merensky Reef (MR) in the Bushveld Complex has been variously interpreted as resulting
from: magma mixing followed by gravitational settling of chromite and/or sulphide onto the
magma chamber floor; upward percolation of interstitial fluids from the solidifying underlying cumulates; a pressure-induced burst of crystallization of sulphide and chromite; the emplacement of
chromite- and sulphide-rich slurries from a staging chamber; a sill-like injection of magma rich in
sulphur and platinum-group elements (PGE) into pre-existing cumulates; hydrodynamic sorting in
mobilized unconsolidated cumulates containing disseminated ore minerals. We test these models
against field relations in the MR in potholes, roughly circular depressions in which some of the
footwall rocks are lacking. The most telling observations are: local transgressions of the MR by
younger potholes; widespread magmatic erosion of the footwall to the MR; magmatic erosion of
cumulates within the MR itself; evidence of nearly solid rocks a few metres below the footwall prior
to crystallization of the MR; igneous layering in the MR that is concordant with the sloping sides of
potholes; chromitite seams of the MR lining vertical to overhanging walls of potholes; mineralized
dyke- and sill-like apophyses of the MR in the footwall a few metres below its normal position.
None of the suggested models for the origin of the MR are consistent with all of these observations. We propose an alternative hypothesis that involves the following sequence of events: (1) a
batch of new dense magma mixed with the resident melt as it entered the chamber and the resultant hybrids then spread out laterally along the floor as basal flows; (2) as a consequence of mixing,
the hybrid magmas were superheated and caused intense thermochemical erosion of the footwall
cumulates, resulting in igneous unconformities on various scales from dimples a few millimetres
across to kilometre-sized regional potholes as well as dyke- and sill-like apophyses along weak surfaces in the footwall rocks; (3) on cooling of the hybrid magma, chromite crystals and droplets of
sulphide melt formed in situ, draping the irregular erosional surfaces; (4) chromite and the sulphide
melt effectively scavenged PGE from magma that was continuously brought to the base of the
chamber by vigorous thermal or compositional convection. Multiple pulses of magma replenishment led to repetition of this sequence of events, resulting in a complex package of mineralized
rocks. In situ crystallization of basal layers of magma may be a viable interpretation of PGE reefs in
other layered mafic–ultramafic intrusions.
Key words: Merensky Reef; Bushveld Complex; potholes; basal magma layers; platinum-group
elements; chromitite seams; sulphide liquid; in situ crystallization; magmatic erosion; magma mixing; superheated hybrid magma; multiple emplacement; convection; cumulates; layered intrusions;
South Africa
C The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: [email protected]
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INTRODUCTION
The Merensky Reef (MR) is one of the most spectacular
and economically important features of the Bushveld
Complex in South Africa. Owing to its huge lateral extent and enigmatic origin, the reef is a continuous
source of inspiration to igneous petrologists attempting
to shed light on how magma chambers evolve and
eventually result in mafic–ultramafic layered intrusions
(e.g. Vermaak, 1976; Wilson et al., 1999; Arndt et al.,
2005; Prevec et al., 2005; Cawthorn et al., 2006; Wilson
& Chunnett, 2006; Maier et al., 2013; Vukmanovic et al.,
2013). In addition, being one of the world’s largest repositories of platinum-group elements (PGE), the reef presents a great challenge to our understanding of how
igneous processes lead to the extreme concentration of
metals represented by economic deposits (e.g.
Campbell et al., 1983; Cawthorn, 1999a, 1999b, 2011;
Ballhaus & Sylvester, 2000; Boudreau, 2008; Naldrett
et al., 2011). For these and other reasons the MR has
been the subject of numerous studies over many decades. The principal feature of our approach is that, instead of employing sophisticated techniques (e.g.
mineral-scale trace element or isotopic analyses, numerical modelling, etc.), we have returned to basic field
observations that, regrettably, are currently rarely used
to constrain rivalling petrological hypotheses (e.g.
O’Hara & Herzberg, 2002). We will show that this approach provides very strict constraints and unique answers to many of the perplexing aspects of the MR. We
will show that the relationships of the MR with its footwall and hanging wall provide crucial tests of existing
hypotheses for its origin and, more importantly, to
allow us to formulate a plausible alternative interpretation. We suggest that the general aspects of our petrogenetic model may be applicable to other PGE reefs in
layered mafic–ultramafic intrusions.
THE BUSHVELD COMPLEX AND ITS PGE
DEPOSITS
The Bushveld Complex and its many world-class ore deposits have been described in detail in numerous publications (Hall, 1932; Wager & Brown, 1968; Eales &
Cawthorn, 1996; Lee, 1996; Cawthorn et al., 2006;
Mondal & Mathez, 2007; Naldrett et al., 2011, 2012; Maier
et al., 2013; Junge et al., 2014). It is sufficient to say that
the Bushveld Complex is the largest mafic–ultramafic
layered intrusion on Earth and encompasses about a million cubic kilometres of igneous rocks emplaced into the
upper crust 205 billion years ago as part of a major intracontinental magmatic event. The complex is preserved
in several segments, the western, eastern and northern
lobes being the largest (Fig. 1). On the basis of mineralogy, the complex is subdivided stratigraphically into five
major units—the Marginal, Lower, Critical, Main and
Upper Zones—constituting a total stratigraphic thickness
of about 7–9 km. The Bushveld Complex is believed to
be so astonishingly voluminous because it grew
Journal of Petrology, 2016, Vol. 56, No. 12
incrementally by the successive emplacement of numerous separate batches of magma of various volumes and
compositions (e.g. Eales et al., 1990; Kruger, 2005). The
world’s largest deposits of PGE, the MR and UG2 chromitite, are located in the upper, cumulus plagioclasebearing part of the Critical Zone (Fig. 1). Our study
focuses largely on the MR as developed in platinum
mines in the western lobe of the Bushveld Complex
(Fig. 1). The MR is a chromite- and sulphide-bearing
package of broadly pyroxenitic rocks (a few centimetres
to 10–12 m in thickness); these are extremely enriched in
PGE and are sandwiched between cumulate rocks that
are almost totally devoid of chromite, sulphide and PGE
(Cawthorn, 1999a; Mitchell & Scoon, 2007). The MR regionally cuts out several metres of underlying cumulates
(Fig. 2) and thus essentially overlies an igneous unconformity of regional extent (e.g. Viljoen, 1999). The term
‘normal’ MR is used for cases in which a sheet-like reef
covers a relatively planar floor that has experienced rather uniform erosion. For this reason discordant relationships of normal MR with respect to the igneous layering
of the footwall rocks are not usually obvious (Figs 2 and
3a), although locally the transgressive nature of normal
MR becomes visible (Fig. 3b). In addition, the MR covers
local, nearly circular excavations, generally known as
potholes, that cut into footwall cumulates to depths of 1–
100 m (usually 10–16 m). The term ‘potholed’ MR is used
for such areas. Yet another unusual feature, rarely
described in the literature, is mound-like masses of footwall rocks (up to 1–10 m high) that stand above planar
normal MR (e.g. Farquhar, 1986). Morphologically the
mounds are the opposites of potholes and are therefore
referred to here as antipotholes. ‘Antipotholed’ MR commonly drapes over these mounds, and is discordant to
their igneous layering (Figs 2 and 3c). The conventional
explanation for the unconformity is erosion of the temporary floor of the magma chamber during episodes of
replenishment by new pulses of hot magma (e.g.
Vermaak, 1976; Kruger & Marsh, 1985; Cawthorn et al.,
2005; Naldrett et al., 2011). Assuming that antipotholes
are local remnants of footwall rocks that were more resistant to erosion (Fig. 2), the total thickness of the cumulates that was removed on a regional scale is a few
metres, at least. Replenishment of the chamber by new
magma is supported by sharp regressions in the crystallization sequence and marked changes in whole-rock
compositions and isotopic ratios that occur at or slightly
above the MR (e.g. Kruger & Marsh, 1985; Naldrett et al.,
1987; Naldrett, 1989; Kruger, 1992). The origin of the MR
and its anomalous enrichment in PGE should thus be
sought in the specific details of open-system igneous
processes and internal differentiation in the Bushveld
magma chamber.
HYPOTHESES FOR ORIGIN OF THE MERENSKY
REEF
Two competing explanations are traditionally advanced
for the anomalously high concentrations of PGE in thin
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Fig. 1. Location and schematic geological map of the Bushveld Complex (Rustenburg Layered Suite) showing the location of mines
where most observations reported in this study were made [modified after Webb (2009)]. Also shown is a schematic stratigraphic
section of the Bushveld Complex indicating the position of the MR and UG2 chromitite towards the top of the Critical Zone.
reefs in layered intrusions (see a review by Mungall &
Naldrett, 2008; Mungall, 2014; Godel, 2015): settling of
dense sulphide melt from the overlying magma (e.g.
Campbell et al., 1983; Naldrett et al., 1987; Naldrett,
1989; Barnes & Maier, 2002; Naldrett et al., 2009, 2011,
2012) or introduction of PGE by interstitial fluids migrating upward from the underlying cumulates (e.g.
Ballhaus et al., 1988; Mathez, 1995; Mathez et al., 1997)
or fluid (e.g. Boudreau & McCallum, 1992; Boudreau,
1995, 2008; Boudreau & Meurer, 1999). These two
hypotheses are not, however, exclusive, either in general or in particular for the MR of the Bushveld
Complex. During recent decades, several alternative
hypotheses have been advanced for the origin of the
MR. For various reasons most of them are now either
forgotten or mentioned very rarely in the literature. The
hypotheses that are still considered to be geologically
valid are summarized in Fig. 4.
One of the earliest and most widely accepted views
(Fig. 4a) is that the MR is a result of large-scale turbulent
mixing of new and resident magma, followed by settling of chromite crystals and droplets of immiscible
sulphide melt through a several kilometres thick body
of magma to the temporary base of the chamber (e.g.
Vermaak, 1976; Campbell et al., 1983; Naldrett, 1989).
According to this hypothesis, high PGE concentrations
are achieved by equilibration of the sulphide melt with
a large volume of silicate melt (achieving a high
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Journal of Petrology, 2016, Vol. 56, No. 12
Fig. 2. Generalized stratigraphic section illustrating the transgressive relationship of the Merensky Reef (MR) to its footwall. Three
major types of the MR are distinguished: (1) ‘normal’ MR, which overlies relatively planar sections of the temporary floor that have
experienced rather uniform erosion; (2) ‘potholed’ MR, which covers near-circular depressions (potholes) that are produced by
localized erosion of the floor; (3) ‘antipotholed’ MR, which drapes over mounds (antipotholes) representing parts of the floor that resisted erosion. Representative photographs of these types of the MR are shown in Figs 3, 5, 7 and 8. An arbitrary position of the
magma-chamber floor before erosion is indicated by a dotted line. It should be noted that potholes are commonly much larger
than antipotholes. Based on various sources and personal observations.
R-factor ¼ silicate magma to sulphide melt ratio) that
promotes scavenging of a large proportion of the PGE
owing to their high sulphide–silicate melt partition coefficients [up to D ¼ 106 according to Fonseca et al. (2009)
and Mungall & Brenan (2014)]. An essential element of
this model is gravity-induced settling of dense sulphide
melt through a thick magma column to the temporary
floor of the chamber.
Scoon & Teigler (1994) proposed that crystallization
of chromite took place from hybrid melts close to the
cumulate–magma interface (Fig. 4b). Lateral mixing of
hot primitive magma with resident melt over distances
of more than 150 km was suggested as an alternative
mechanism for achieving high R-factor values. PGE
were concentrated in sulphide-poor chromitites as a result of chromite control, a process thought to involve a
combination of direct nucleation of platinum-group
minerals (PGM) and localized S saturation. Naldrett
et al. (2011) have recently reached a similar conclusion,
proposing that the MR was produced by settling of
chromite and sulphide droplets from a basal layer of
mixed magma.
Kinnaird et al. (2002) proposed that a batch of dense,
new magma was emplaced into the Bushveld chamber
as a fountain that interacted with and entrained granophyric roof-rock melts (Fig. 4c). This process contaminated both the new magma and the resident melt with a
silica-rich component that induced crystallization of
copious chromite and concomitant crystallization of
PGM. Small PGM crystals were collected by the more
abundant chromite grains. The magma with chromite
and adherent PGM was then carried to the floor of the
chamber in the collapsing margins of the fountain to
form chromitite layers enriched in PGE.
Cawthorn (2005, 2011) suggested that magma mixing was not required for the formation of the MR.
Instead, the basal injection of new magma caused an
increase in pressure in the overlying resident melt
(Fig. 4d). This triggered the formation of immiscible sulphide melt and/or chromite in the resident melt, which
then settled through the basal layer of a new magma
(which was unsaturated in sulphide/chromite) and accumulated on the floor. While settling, the immiscible sulphide liquid and/or chromite scavenged PGE from the
column of magma. Subsequent rupturing of the roof resulted in a release of pressure and cessation of sulphide/chromite formation in the resident melt.
Willmore et al. (2000), following earlier ideas by
Lauder (1970), Boudreau & McCallum (1992), Mathez
(1995) and Mathez et al. (1997), proposed that volatilerich fluids expelled from the underlying cumulates
migrated upwards and remobilized the minor amounts
of sulphur and PGE present in the footwall rocks. The
fluids escaped from the cumulates and mixed with
fluid-undersaturated magma in the chamber (Fig. 4e).
The mixing of the S-bearing fluid with a fluidundersaturated magma induced local S saturation and
PGE collection in addition to the PGE that may have
been carried by the fluid itself. In this variant, the final
process of PGE concentration took place from the
resident magma, but the footwall cumulates were the
principal source of the PGE.
Mitchell & Scoon (2007) and Kruger (2010) for various reasons came to the conclusion that the MR did not
form sequentially with its surrounding cumulate rocks.
Instead, they proposed its formation from a PGE- and
S-rich primitive magma injected as thin, laterally extensive sills into pre-existing, partially consolidated cumulates (Fig. 4f). Prolonged lateral streaming and mixing
of magmas resulted in chromite crystallization and consequent sulphide and PGM precipitation at the base of
the sill to form the MR. The laterally flowing magma
Journal of Petrology, 2016, Vol. 56, No. 12
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was viewed as a persistent supplier of Ni, Cu, and PGE
to the crystallizing MR.
Hutchinson et al. (2015) have suggested that the
Bushveld Complex was periodically replenished by
pulses of new magma from a staging chamber that carried with it suspended chromite crystals, sulphide droplets and discrete PGM (Fig. 4g). The new magma
entered the chamber as high-energy, basal flows that
caused erosion of the floor cumulates, with subsequent
accumulation of phenocrysts at the base of the chamber
to form the PGE-rich sulphide-bearing chromitite seams
of the MR. The model thus appeals to crystallization in
underlying mid- to upper-crustal staging chambers.
Maier et al. (2013) presented a radical reinterpretation of the origin of PGE deposits of the Bushveld
Complex (Fig. 4h). They argued that these deposits
formed when the complex underwent central subsidence in response to crustal loading, resulting in slumping of unconsolidated cumulates towards the centre of
the complex. The slumping was accompanied by the
hydrodynamic redistribution of minerals in slurries to
form layers enriched in PGE-bearing sulphides, chromite, and magnetite. The model thus relegates the PGE
deposits almost exclusively to the post-depositional
stage in the evolution of the solidifying magma
chamber.
There have been several attempts to evaluate the
validity of the various hypotheses for the PGE reefs (e.g.
Cawthorn, 1999a, 1999b, 1999c, 2005, 2011; Ballhaus &
Sylvester, 2000; Cawthorn et al., 2002, 2005; Maier,
2005; Maier & Barnes, 2008; Maier et al., 2013). The general consensus is that each model has its own particular
strengths and weaknesses, but none of them appears to
successfully explain all of the features of the PGE deposits. We would like to stress here that most of these
models [except that by Maier et al. (2013)] are essentially based on the chemical, mineralogical and isotopic
compositions of the cumulates. The detailed field relations of the MR as revealed in excellent exposures
underground are either mentioned only in passing or
largely ignored. This is unsatisfactory and we attempt
to redress the balance by employing basic field observations on the MR to evaluate the different hypotheses.
We then propose an explanation that is most consistent
with the current state of knowledge.
Fig. 3. (a) Normal MR consisting of pegmatitic orthopyroxenite
with its lower and upper chromitite seams from the
Thembelani Mine. There may be, however, up to 4–6 chromitite seams in the normal MR. It should be noted that the MR is
planar and concordant with layering in the footwall norite and
anorthosite. (b) Normal MR consisting of orthopyroxenite with
only a lower chromitite seam; Eastern Platinum Mine. It should
be noted that the MR is also tabular but discordant with layering in the footwall norite and anorthosite (photo courtesy of
Jan Van der Merwe). (c) Antipotholed MR consisting of orthopyroxenite (it is not clear if chromitite seams are present) from
the Brakspruit Shaft, Rustenburg Platinum Mine. It should be
noted that the MR drapes over the side of a mound and truncates the layering in the underlying norite and anorthosite. The
conceptual positions of these types of the MR are indicated in
Fig. 2.
OBSERVATIONS AND DEDUCTIONS
This study is principally based on underground observations of the MR, made by one of the authors (Alan
Page) over a period of 8 years (1999–2006) during his
employment at the Rustenburg Platinum Mine in the
western Bushveld Complex. These data have been supplemented by surface and underground observations
on the MR by another co-author (Richard Hornsey), who
has spent more than 20 years (1992–present) working in
mines and exploration projects in the western, eastern,
and southern Bushveld Complex and its satellite intrusions. Several underground visits to Dishaba and
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Journal of Petrology, 2016, Vol. 56, No. 12
Fig. 4. Sketches illustrating some of the most popular hypotheses for the origin of the Merensky Reef in the Bushveld Complex.
(See text for discussion.)
Rustenburg Platinum mines on the western Bushveld
Complex by the principal authors (Rais Latypov and
Sofya Chistyakova) in recent years were instrumental.
Most of our observations come from potholes (Fig. 5),
which are near-circular to elliptical depressions with
gently to steeply inclined sidewalls in which some of
the footwall cumulates are absent. Potholes are relatively common at the base of the MR and locally occupy
up to 15–20% of mined areas (Viljoen & Hieber, 1986;
Viljoen et al., 1986a, 1986b; Carr et al., 1994, 1999). The
Journal of Petrology, 2016, Vol. 56, No. 12
size of potholes varies from a few centimetres to hundreds of metres in width and from a few centimetres to
several tens of metres in depth. Several so-called regional potholes are known that are several kilometres in
extent and about 20 m in depth (e.g. Viljoen, 1999;
Smith & Basson, 2006; Roberts et al., 2007). For example, the Brakspruit Pothole in the Rustenburg
Platinum Mine is over 33 km long, up to 22 km wide
and about 21–92 m deep. There appears to be little or
no discernible pattern in the spatial distribution of potholes at the base of the MR (Viljoen, 1999; Hoffmann,
2010), although on a regional basis there are areas with
an increased frequency of potholes (Carr et al., 1994,
1999).
The emplacement of magma into the chamber: a
fire-fountain, a basal flow or an independent sill?
Exactly how was the evolving Bushveld chamber replenished by new batches of magma parental to the
MR? Two possibilities are commonly considered. The
first is when the new magma is less dense than the resident magma in the chamber and either rises to the roof
or stalls at some intermediate level in a compositionally
stratified magma chamber and spreads out laterally
across the entire chamber (e.g. Fig. 4a). In this case the
new magma does not come into direct contact with the
floor cumulates. Alternatively, the new magma entering
the chamber can be denser than the resident magma
and, if emplaced with sufficient momentum, it may
form a turbulent fountain that rises an appreciable distance into the resident magma. Extensive mixing results from entrainment of the resident magma in the
margins of the fountain. Eventually negative buoyancy
forces overcome the upward momentum and the dense
hybrid magma collapses and spreads out across the
floor, producing a basal layer that, depending on
the volume of magma emplaced, may be several
hundreds of metres thick (e.g. Fig. 4b) (Campbell et al.,
1983; Campbell & Turner, 1989). In this case a hybrid
layer of magma comes into a direct contact with the
floor cumulate rocks. A third possibility that has to be
considered is the emplacement of a new magma as an
independent sill directly into pre-existing cumulates
(e.g. Fig. 4f).
Which of these three scenarios was realized in the
case of the MR? Geological observations allow us dismiss the Merensky Unit, including the MR, as a separate
late injection into pre-existing partly consolidated cumulates. This view implies that magma was intruded
along the junction between the Bastard Unit and its
footwall rocks (Fig. 6a). However, there are underground and opencast observations demonstrating that
potholes of the Bastard Unit cut through cumulates belonging to the Merensky Unit (Fig. 6b; unpublished mining reports, e.g. Page, 2006) and even its footwall rocks
(Fig. 6c), precluding the late formation of this unit. This
simple geological observation makes a sill injection
(Fig. 4f) for the origin of the MR untenable.
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To distinguish between the two remaining options, evidence from the MR potholes is crucial. The margins of
potholes beneath the MR are clearly transgressive and
truncate cumulates exhibiting igneous layering and
lamination in footwall rocks that were already indurated
(Fig. 5b and c) attesting to the erosional origin of the
base of the pothole. Erosion is particularly evident from
in situ autoliths of footwall rocks hosted within the MR
(Fig. 7) that are almost certainly connected to each other
and with the adjacent cumulates in three dimensions.
Irrespective of the exact mechanism of erosion (mechanical, chemical or thermal), the removal of pre-existing
cumulates in potholes requires that the magma parental to the MR was in direct contact with the cumulates
forming the temporary floor of the magma chamber.
This could be the case only if the new magma was relatively dense and spread out as a basal layer along the
interface between the resident melt and the chamber
floor. This is an important deduction that must be incorporated into any successful hypothesis for origin of the
MR. The field data are not compatible with the popular
model that involves fire-fountaining and lateral spreading of new magma somewhere above the floor within a
compositionally stratified magma (Fig. 4a), because this
would not result in erosion of the chamber floor. The
emplacement of any basal layer of new magma would
involve some mixing with the resident melt. Mixing is
inevitable during forceful entry of denser magma into a
chamber as the consequent fountain would be highly
turbulent (Campbell & Turner, 1989). An excessive viscosity contrast between the magmas may hinder mixing, but this is not likely for magmas of broadly basaltic
composition (Campbell & Turner, 1986). The field observations are thus consistent with the hypothesis proposed by Scoon & Teigler (1994) that appeals to
fractional crystallization of hybrid magma in a basal
layer (Fig. 4b).
There are three other independent lines of chemical
evidence that support the basal emplacement of
magma. First, there is a sharp whole-rock compositional break between the MR and its footwall (Naldrett,
1989) that can best be explained by its crystallization
from a magma that was not parental to the underlying
cumulates. Second, the osmium isotopic composition
in the MR (initial 187Os/188Os > 017) is distinctly different from its footwall and hanging-wall rocks (initial
187
Os/188Os ¼ 012) (Kruger, 2010) which can best be interpreted as a result of its formation from a separate,
basal layer of magma that had a different isotopic composition than the resident magma. An alternative interpretation of the reef as an independent sill intruded into
the solidified rocks of the Bushveld Complex (Kruger,
2010) is not consistent with the field observations of
potholes in the immediately overlying sequence that
cut through the MR (Fig. 6b and c). Third, an exponential decay of PGE tenor in sulphides from more than
2000 ppm to almost zero through an interval of about
1 m above the upper chromitite of the MR is most compatible with scavenging of PGE from a thin basal layer
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Fig. 5. (a) Part of the Klipfontein opencast on the Bastard and Merensky Reefs showing a section through a large near-circular
pothole; Bokoni Mine. Photo courtesy of Daniel Mudau. (b) The edge of a pothole showing the MR chromitite and pyroxenite crosscutting igneous layering in the footwall anorthosite and norite. Shaft 3 of the Karee mine. (c) A small pothole at the base of the MR
that cuts through igneous layering in the footwall anorthosite. Dishaba mine.
Journal of Petrology, 2016, Vol. 56, No. 12
2349
Fig. 6. Sketch illustrating the inference that the Merensky Unit cannot be produced from a sill-like magma emplacement into preexisting cumulates (a, and see Fig. 4f) because it is transgressed by the overlying Bastard Unit (b) as indicated by both underground
and opencast observations reported in unpublished mining reports (e.g. Page, 2006) and confirmed by our own observations (c)
showing how pegmatitic pyroxenite of the MR sandwiched between two chromitite seams is cut off by pyroxenite of the Bastard
pothole at Thembelani mine. The sketch (b) is based on the work of Bennie Cilliers, who observed Bastard potholes at the Impala
and Waterfall mine opencasts. It should also be noted that the truncation of the pegmatitic pyroxenite close to the temporary floor
of the chamber suggests that pegmatitic textures developed early, probably at a cumulate–magma interface. Following Cawthorn
& Boerst (2006) we argue that the pegmatitic textures result from recrystallization and chemical modification of ordinary pyroxenite
adjacent to superheated magma emplaced as basal flows along the chamber floor.
of magma rather than from the entire magma reservoir
(Naldrett et al., 2011).
Added magma: crystal slurries or superheated
liquid?
Were the new batches of magma that excavated the MR
potholes full of phenocrysts (i.e. crystal slurries) or free
of phenocrysts (i.e. liquidus or superheated melts)?
Crystal slurries are believed by some to be much more
capable of forming potholes (by physical erosion) than
crystal-free magmas. Other types of suspended flows
(e.g. sand particles carried in air), can very effectively
erode solid rocks, given sufficient time. Potholes in rivers, which form through abrasion, are generally circular
in plan, suggesting that MR potholes may also be due
to mechanical erosion. There are, however, several objections to this view. First, we make a strong case below
that footwall rocks a few metres below the temporary
floor of the Bushveld chamber were almost completely
solid at the time of pothole formation. To scour large
potholes in solid rocks is physically extremely
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Fig. 7. Magmatic erosion of footwall rocks beneath the MR in the Brakspruit Pothole, Rustenburg Platinum Mine, gave rise to angular in situ autoliths of footwall norite and anorthosite that appear to have retained their original positions and orientations. The
transgressive nature of potholes with respect to footwall rocks is evidence for the emplacement of hot, primitive magma parental
to the MR along the base of the magma chamber.
demanding. Second, scouring of cumulates by flowing
crystal slurries would be expected to result in linear
channels, not near-circular depressions. Circular potholes do indeed form in river beds by abrasion
involving particles and pebbles that rotate in the flow
and grind their way into the river bedrock. This is not,
however, comparable in any sense with the formation
of the MR potholes. River potholes always occur in the
Journal of Petrology, 2016, Vol. 56, No. 12
beds of channels. All available underground maps
show that Merensky potholes are not aligned along linear trends. They seem to be distributed randomly, making their distribution almost impossible to predict
during mining activities. Third, crystal slurries would
act like high-viscosity gravity currents, and form elongate fingers flowing down slopes like avalanches. This is
a very different dynamic situation to sand blasting,
which is caused by impacts of particles travelling at
high velocities in dilute suspensions. Fourth, the amplitude of potholes can be anywhere from a few millimetres dimples to ten of metres in depth, but the
average thickness of the MR above them changes very
little (on average 1 m; Fig. 2). Scouring by gravity currents is very unlikely to be on a scale much greater than
the thickness of the current itself, which if it was forming the MR would have to be a few tens of centimetres
at most. It is very difficult to envision kilometre-sized circular and regional potholes being formed by such gravity currents. Fifth, a pertinent question to ask is: where
are the fragments that could be derived from potholes?
If one accepts, for example, that footwall rocks from the
Brakspruit Pothole of the Rustenburg Platinum Mine
(33 km long, up to 22 km wide and about 21–92 m
deep) were mechanically removed from this pothole (as
opposed to being completely dissolved in hot or superheated magma), then large numbers of transported
xenoliths of various shapes and sizes would be expected to occur in and around the pothole. The same
must be true for all other potholes of the MR. However,
several decades of extensive open pit and underground
mining of the MR clearly indicate that xenoliths and
even in situ autoliths (e.g. Figs 7 and 8) of footwall rocks
are extremely rare both within and around potholes.
The vast volume of footwall rocks that disappeared
from potholes, apparently without trace, is an insurmountable problem for models involving mechanical
erosion by crystal slurries (e.g. Fig. 4g).
We claim that the field observations provide no support for recent models attributing the origin of the MR
(and associated potholes) to the replenishment of a
magma chamber by crystal slurries from underlying
staging chambers (Fig. 4g). In fact, we believe that crystal slurries would not necessarily cause any appreciable
erosion of pre-existing cumulates because phenocrysts
would tend to settle to the floor and protect the footwall
from erosion (Fig. 9). The field evidence (Figs 7 and 8)
rather indicates that the footwall rocks in potholes were
melted or dissolved by new magma, thereby providing
the simplest explanation for the dearth of footwall rock
xenoliths in and around potholes. Thermal and chemical erosion appears to be the most probable cause of
the circular potholes (Campbell, 1986a). The formation
of large, kilometre-sized potholes may have additionally
been aided by rupturing and downslope slumping of
unconsolidated cumulates in response to the entry of
new magma into the chamber (Carr et al., 1994, 1999).
Some alternative views on the origin of potholes by
non-deposition owing to fluids introduced into the
2351
magma chamber from country rocks (Ballhaus, 1988) or
by excavation associated with the violent discharge of
fluids from underlying cumulates (Boudreau, 1992) are
not supported by definitive field evidence. In particular,
there is no evidence for fluid flow in the form of veins,
alteration zones or bleached aureoles. Pegmatitic rocks
are indeed common but, as discussed below, their origin is most plausibly related to reconstitution of footwall rocks by superheated magma from above rather
than to fluid activity from below.
To excavate potholes, the basal layer of magma
must have been in thermal or chemical disequilibrium
with the floor cumulates (e.g. Woods, 1992; Kerr, 1994;
Robertson et al., 2015). Thermal disequilibrium may
lead to bulk or selective melting, where the rate-limiting
step is the diffusive movement of heat into the rock;
that is, the thermal conductivity of the cumulates.
Chemical disequilibrium can cause dissolution, where
the rate-limiting step is the diffusive movement of
mass. The crystals and the magma may have identical
temperatures, but if they are not in chemical equilibrium then the crystals can dissolve, and mass is
removed and dispersed in the contaminated magma.
Let us envisage a new hot olivine-saturated magma that
enters the evolving chamber of the Bushveld Complex
as a basal flow that comes in contact with plagioclaserich floor cumulates. The new magma may be hot
enough to exceed the melting temperature of the
plagioclase cumulates and therefore it may melt them
to varying degrees to a depth of several metres below
the crystal–liquid interface (Fig. 10a). Alternatively, if
the new magma has a temperature below the melting
temperature of plagioclase cumulates then, instead of
melting, which is now impossible, magma will dissolve
the cumulates (Fig. 10b). Dissolution can take place only
along the crystal–liquid interface, and crystals beneath
the surface will remain totally unaffected until the front
of dissolution advances downwards to reach them.
Interestingly, phase equilibria indicate that assimilation
of hot, plagioclase-rich footwall cumulates by an olivine-saturated magma (by both melting and dissolution)
can result in a superheated hybrid melt, hence delaying
magma crystallization and prolonging corrosion. In
general, dissolution is always slower than melting because the rate of mass transfer is typically several
orders of magnitude less than the rate of heat transfer.
On the other hand, it is much easier to maintain the
chemical gradients required for dissolution and this will
prolong dissolution of the floor. These two scenarios
are not mutually exclusive. One can imagine a situation
in which erosion starts with melting during an initial
stage and then gradually gives way to dissolution as
the magma cools. Melting followed by dissolution may
explain why, despite an erosion of several metres to
tens of metres of footwall rocks in potholes (up to 92 m
in the Brakspruit Pothole), the rocks immediately below
the MR often show no textural or compositional evidence of reconstitution by new magma. All that can be
attributed to this effect is anorthosite rims a few
2352
Journal of Petrology, 2016, Vol. 56, No. 12
Fig. 8. Erosion of footwall rocks beneath the MR in the Brakspruit Pothole, Rustenburg Platinum Mine, resulted in discordant contacts (a) and apparently isolated relics of the pre-existing cumulates (b). It should be noted that further erosion could have led to
the complete separation of parts of the footwall rocks from the underlying cumulates to form in situ autoliths (see Fig. 7). The
observations indicate that the magma in contact with the footwall must have been superheated to be able to melt or dissolve the
footwall rocks in this way.
centimetres thick that commonly underlie the MR (e.g.
Fig. 11c). If these rims are due to partial melting of
footwall norite, then it is not clear why the norite immediately below them looks unchanged. Thermally
induced melting of footwall rocks by new hot magma
would be expected to have taken place several metres
below the MR. The puzzle becomes understandable if
one accepts that formation of potholes was initiated by
melting and completed by dissolution, a process that
has no immediate effect on rocks below the crystal–
liquid interface (Fig. 10b).
Magma superheat may significantly enhance the
melting/dissolution of footwall cumulates. It has been
long established that superheated magma does not
Journal of Petrology, 2016, Vol. 56, No. 12
Fig. 9. Schematic illustration of the inference that emplacement
of crystal slurries will not result in thermal/chemical erosion of
footwall rocks because settled phenocrysts blanket and protect
the temporary floor of the chamber.
generally result from fractional crystallization or mantle
melting and natural magmas are rarely superheated.
They commonly arrive in crustal chambers close to
their liquidus temperature or carrying intratelluric
phenocrysts (e.g. McBirney, 1979). Magma superheating requires special circumstances. One possibility is
assimilation of footwall rocks by magma lying on the
opposite side of a cotectic, as discussed above (Fig. 10a
and b). An alternative is mixing of two compositionally
different melts lying on opposite sides of a thermal minimum (e.g. an eutectic or cotectic) (Fig. 10c) (e.g. Irvine
et al., 1983; Campbell, 1986a). It seems reasonable to
assume that mixing occurred within the Bushveld
chamber as resident magma was entrained into a turbulent fountain. Following Irvine et al. (1983), the resident
magma in the Bushveld chamber may have been saturated in plagioclase or both plagioclase and orthopyroxene, whereas the new, more primitive magma was
saturated in olivine or olivine and orthopyroxene. The
mixing of the disparate magmas may have resulted in a
hybrid that could be as much as 100 C above its
liquidus (Irvine et al., 1983) and would therefore be able
first to melt and then to dissolve the footwall without
crystallization intervening to hinder it. Of equal importance is the fact that, upon cooling, such hybrid magmas
may have been able to crystallize spinel (chromite in
natural systems) as the first liquidus mineral (Fig. 10c).
it is necessary to emphasize, however, that simple
phase diagrams and mixing lines, although being informative, should be used with caution; they illustrate
only general principles. In particular, the notion that
mixing of liquidus melts across a cotectic can generate
superheated liquids is a valuable one, but one should
bear in mind that these phase diagrams have only a
passing resemblance to the multicomponent reality.
2353
More detailed petrological studies involving numerical
modelling in multicomponent systems are required to
understand the relative significance of the various factors (thermal versus chemical disequilibrium, footwall
rock composition, and magma superheating) in the
thermal/chemical erosion of footwall rocks in potholes.
There is a potential objection to an origin of potholes
by thermal/chemical erosion that warrants special mention. Carr et al. (1994, 1999) undertook a test of the thermal/chemical hypothesis using initial Sr isotope ratios,
which are known to be much more radiogenic in the
MR (R0 > 07066) compared with its footwall rocks
(R0 < 07066). They found that the isotopic signature of
potholed MR (R0 ¼ 07069–07078) at the Western
Platinum Mine is comparable with normal MR (where
erosion was presumably minimal or lacking) and took
this as evidence against assimilation of, or reaction
with, footwall rocks in potholes. This argument fails,
however, to take into account the following salient
points. (1) Normal (planar) MR does not mean that erosion was lacking at these locations. Rather it may be
that the cumulates have experienced rather uniform
erosion. Antipotholes indicate that erosion was, at least,
a few metres (Fig. 2). In contrast, in potholes the erosion
presumably was concentrated at initial points of weakness in the surface of the cumulates. A coincidence in
initial Sr isotope ratios between normal and potholed
MR may simply suggest that the degree of magma contamination by footwall rocks in these two domains was
similar. (2) Comparison of the rate of mineral dissolution/precipitation (05–10 cm a1; Morse, 1986) with
that of thermal/compositional convection in a magma
chamber (km a1 to km day1; Morse, 1986) suggests
that any contaminant from the footwall rocks in potholes can be evenly distributed into the entire basal
layer of magma before the onset of crystallization, leaving no record of higher contamination of magma in potholes. (3) It is conceivable that the magma that
crystallized the MR may not be identical to the one that
eroded its footwall; the initial magma that was intensively contaminated by footwall rocks may have been
flushed away by new batches of magma entering the
chamber. For these reasons, isotopic arguments about
the degree of magma contamination in normal versus
potholed MR are unconvincing and cannot be used to
negate thermal/chemical erosion, particularly when this
process is so overwhelmingly supported by field observations (Figs 3, 5b, c, 7 and 8).
The floor cumulates: poorly consolidated mush
or almost solid rocks?
The rheological state of cumulates in layered intrusions
is disputed. Some researchers argue that cumulates are
incompletely crystallized for several hundreds of metres
below the temporary floor of a magma chamber and
are characterized by pore interconnectivity and a high
degree of permeability for interstitial melt. As a result,
cumulates can be affected by large-scale slumping,
2354
Journal of Petrology, 2016, Vol. 56, No. 12
Fig. 10. Schematic illustrations and simple binary phase diagrams illustrating the inferences that (a) new magma that is hotter than
the melting temperature of footwall plagioclase-rich rocks would melt them because of thermal disequilibrium, whereas (b) new
magma with a temperature lower than the melting temperature of the footwall rocks (for simplicity, they are taken to be of the
same temperature) would be able to dissolve them because of chemical disequilibrium. In both cases contamination of the new
magma by plagioclase-rich rocks tends to move the magma into the melt field. (c) Mixing of magmas evolving along different liquid
lines of descent may produce a superheated hybrid magma capable of both melting and dissolving footwall rocks. It should be
noted that, on cooling, the superheated hybrid magma may crystallize spinel (chromite) as the first liquidus phase. The Fo–An
pseudobinary phase diagram is redrawn from Osborn & Tait (1952). In (a) and (b) a field of spinel is ignored for simplicity. Fo, forsterite; An, anorthite; Sp, spinel.
Journal of Petrology, 2016, Vol. 56, No. 12
compaction and upward migration of interstitial melt or
fluids (Boudreau & Meurer, 1999; McKenzie, 2011;
Maier et al., 2013). In contrast, others believe that cumulates are nearly solid and that porosity decreases very
quickly to a few volume per cent within a few metres of
the magma–mush interface. Consequently, interstitial
melt is trapped and is unable to migrate. As a result, cumulates are not prone to any large-scale, post-cumulus
modification (Campbell, 1986b; Morse, 1988; Cawthorn,
1999c; Tegner et al., 2009; Holness et al., 2011).
The angular outlines of in situ autoliths of footwall
rocks (Fig. 7) and the abrupt truncation of fine-scale igneous layering (Fig. 11a and b) around potholes indicate that the footwall to the MR several metres below
the floor was already layered and rigid. Even more telling is the fact that pyroxene oikocrysts in the footwall
mottled anorthosite are truncated by pothole margins
(Fig. 11c; see also Hutchinson et al., 2015). The oikocrysts fill interstitial space and are therefore the latest
material to crystallize in the mottled anorthosite,
strongly suggesting that the footwall rocks, only a very
few metres below the MR, were almost solid. This inference is also supported by the observation that
high-density and low-viscosity sulphide melt was able to
percolate downward into the footwall of the MR for distances of only 1–2 m, indicating the very low permeability of the underlying cumulates (Cawthorn, 1999c). There
is a general consensus that, as a rule, there is a relatively
thin mush zone (i.e. only a few metres thick) at the top of
cumulate sequences (Morse, 1988; Tegner et al., 2009;
Holness et al., 2011). In particular, meticulous mapping
of parts of the Skaergaard intrusion by Irvine et al. (1998)
showed that angular anorthositic blocks that fell from
the roof deformed modally graded layers in the floor cumulates. This was taken as an indication that the temporary floor of the magma chamber was sharp and that
modal layering was already fully developed beneath the
magma–cumulate interface (Irvine et al., 1998). Similar
relationships between autoliths and layering in cumulates are present in many other layered intrusions
(e.g. Kiglapait intrusion; personal observations).
The observations reported above have several important implications. They present serious obstacles for
hypotheses for the MR involving intrusion-scale hydrodynamic sorting of mobilized cumulates containing disseminated ore minerals (Fig. 4g). It is hardly
conceivable that almost consolidated cumulates could
be mobilized to trigger post-depositional sorting of minerals within it. For the same reason, large-scale transfer
of interstitial melts or fluids from the underlying cumulates is unlikely if they were nearly solid just a few
metres below the floor of the magma chamber (Fig. 4e).
In addition, Mungall (2015) has recently shown that
even in partially molten cumulates the upward motion
of vapor bubbles is highly unlikely to be significant and
will therefore be unable to provide sufficient metal
transport. Even more critical for the latter model is the
fact that the UG2 and MR packages of cumulates in places directly overlie volcano-sedimentary rocks of the
2355
Transvaal Supergroup; for example, in the southeastern
sector of the eastern Bushveld Complex (Van der
Merwe, 2007). The absence of cumulates below these
packages persists downdip for distances of several kilometres and there is therefore no source from which
PGE could be scavenged to form the UG2 and MR. We
thus infer that post-cumulus processes cannot be regarded as principal agents in the formation of the PGE
deposits. They may, at best, have played a minor role in
the post-cumulus modification of the primary magmatic
features of these deposits.
The mechanism of differentiation: gravity
settling or in situ crystallization?
There is a long-standing debate in igneous petrology
about whether layered bottom cumulates form by settling of crystals or by in situ crystallization (Wager &
Brown, 1968; Campbell, 1978, 1996; McBirney & Noyes,
1979; Morse, 1988; Frenkel’ et al., 1989; Jaupart & Tait,
1995; Naslund & McBirney, 1996; Latypov & Egorova,
2012). This has implications for the MR, as even a
superheated basal layer of magma would eventually
cool to its liquidus temperature and start crystallizing.
The question is whether phases would nucleate and
grow within the basal layer itself or against pre-existing
minerals along the floor of the chamber. In the former
case, nucleation and crystallization could be followed
by settling of dense crystals towards the floor of the
chamber, whereas in the latter case the crystals would
grow in situ; that is, attached to the cumulate–magma
interface. Several lines of field evidence indicate that
the latter was the case.
The settling hypothesis predicts that potholes that
are filled by settling crystals will show igneous layering
that is subhorizontal, planar and discordant with respect
to the inclined sidewalls of the pothole (Fig. 12a). A remarkable example of such gravity-induced layering has
been documented at the erosional bases (potholes) of
several cyclic units in the Vesturhorn Intrusion, Iceland
(Fig. 13a). In contrast, the in situ crystallization model
predicts that potholes that are filled by in situ growth of
crystals should have igneous layering that follows the
outline of the pothole margins (Fig. 12b). In situ crystallization may possibly be accompanied by redeposition
of crystals that originally grew in situ but were torn
loose and transported along the floor of the chamber by
flowing magma. The most likely places for the sedimentation of transported crystals will be in structural traps
and depressions in the floor of the chamber, particularly
potholes. In this case the layers will tend to become
thicker in the deepest parts of potholes (Fig. 12c). It is
the two latter types of concordant layering that are
commonly observed in potholes at the base of the MR
(Fig. 13b), suggesting its predominant formation by in
situ crystallization, with or without sedimentation.
The settling hypothesis also predicts that the MR
should be absent along subvertical to overhanging
edges of potholes, simply because any chromite and
2356
Journal of Petrology, 2016, Vol. 56, No. 12
Fig. 11. Magmatic erosion of footwall norite (a, b) with well-developed igneous layering and footwall mottled anorthosite with pyroxene oikocrysts (c) below the MR from the Frank Shaft, Rustenburg Platinum Mine. Truncated pyroxene oikocrysts are indicated
by yellow arrows. The observations suggest that the cumulates were almost solid just a few metres below the temporary floor of
the magma chamber. Photos courtesy of Ray Brown.
sulphide droplets that settled from the overlying
magma cannot have accumulated there owing to the
pothole’s shape (Fig. 14a). They could accumulate only
on the floor of potholes. There are no such obstacles for
in situ crystallization; chromite crystals and droplets of
sulphide melt can form attached to interfaces with any
orientation (Fig. 14b). As shown above, chromitite
seams of the MR are present along steeply inclined, vertical and even overhanging sections of the walls of potholes, commonly without any systematic changes in
Journal of Petrology, 2016, Vol. 56, No. 12
Fig. 12. Predictions of the geometry of igneous layering within
potholes owing to crystal settling (a) and in situ crystallization
(b). Settling of crystals from flowing/convecting melt will form
layered rocks that are discordant with respect to pothole margins. In contrast, in situ crystallization should result in layering
that follows the outlines of pothole margins. The latter may be
accompanied by the transport and subsequent redeposition of
crystals that originally grew in situ by magma flowing along
the floor (c). In this scenario, layers will be thicker in the deepest parts of potholes. Crystals are not shown to scale.
thickness (Fig. 15). This is persuasive evidence of in situ
crystallization rather than crystal settling—a process
that is commonly invoked for layers of chromitite in layered intrusions. It should be noted that there are similar
2357
chromitite seams in other layered intrusions. For instance, in the Rum Complex, Scotland, laterally extensive millimetre-thick chromitite seams are developed
along the margins of culminations and depressions at
the bases of cyclic units, even where these are vertical
or overhanging. These relationships have also been interpreted as a result of the in situ growth of chromite on
crystal–liquid interfaces (Young, 1984; O’Driscoll et al.,
2010; Latypov et al., 2013).
Mungall (2014) has presented an interesting means
of generating chromite in situ by partial melting of
orthopyroxene-rich cumulates rather than by direct
crystallization from chromite-saturated magma. The
central thesis is that partial melting of such rocks will
generate liquids containing considerably less chromium than the original orthopyroxene, and hence the
residue must contain chromite together with olivine.
Narrow chromitite selvages mantling zones of pegmatitic norite in the MR have been attributed to this kind of
fluid-induced partial melting process (Nicholson &
Mathez, 1991). From the phase equilibrium point of
view, this appears to be a realistic process and some
chromitite seams in the MR may have indeed formed in
this way. It is unlikely, however, to represent the principal origin of chromitite seams in the MR. First, an origin
of most chromitite seams as restites is not consistent
with the fact the footwall of the MR is mainly anorthosite and leuconorite (e.g. Figs 3, 5b, c, 7 and 8) rather
than pyroxene-rich cumulates. In addition, the chromitite seams of the MR cannot be viewed as simple reaction rims because in many instances they overlie
different lithologies along their strike (e.g. orthopyroxenite and anorthosite) with no change in thickness.
These observations support in situ crystallization of
chromite directly from chromite-saturated magma on
footwall rocks regardless of their composition. It should
be noted also that in situ growth may not always produce layers with uniform thicknesses [as argued by
Hutchinson et al. (2015)] because of the possible transport and sedimentation of detached in situ grown crystals in depressions from magma flowing along the floor
(Fig. 14c). It is not surprising then that the thicknesses
of some chromitite seams of the MR are sometimes
controlled by undulations in the basal contact; that is,
there are noticeable local thickenings in small depressions in the floor of the chamber.
An unusual and enigmatic feature of potholes that is
of paramount importance in this discussion is so-called
undercutting MR. Leeb-du Toit (1986) and Ballhaus
(1988) were probably the first to describe this feature in
detail, but subsequently it has largely been neglected
by researchers. An astonishing fact is that around MR
potholes, there may be up to six extra undercutting sections of the MR that extend laterally from pothole margins into the footwall (Fig. 16). These sections of reef
are sill-like apophyses of medium- to coarse-grained
harzburgitic and pyroxenitic meso- to orthocumulate
that generally contain disseminated sulphides (Fig. 17).
They vary in thickness from a few centimetres to
2358
Journal of Petrology, 2016, Vol. 56, No. 12
Fig. 13. (a) Map showing igneous layering above erosional bases (potholes) of cyclic units in the Vesturhorn Intrusion, Iceland.
Potholes are filled by layered rocks that are subhorizontal, planar and discordant with respect to the inclined walls of potholes. This
suggests the formation of the layered rocks by gravity-induced crystal settling. Modified from Roobol (1972). (b) Generalized crosssection of a typical MR pothole from the Western Platinum mine that is filled with layered rocks that are concordant with the
stepped sides of the pothole. This suggests the formation of the layered rocks by in situ crystallization (see Fig. 12b and c).
Modified from Carr et al. (1999). FW, footwall rocks; NW, hanging-wall rocks.
around 50 cm and can be traced away from pothole
margins for distances of more than 20 m in some cases
(Ballhaus, 1988). Most undercutting sections of MR tend
to be concordant with the layering in the footwall, but
crosscutting relationships are not uncommon (Fig. 17a).
In places, undercutting MR splits into thinner offshoots,
with some of them projecting up-section (Fig. 17d).
There appears to be a lack of deformation of igneous
layering and rotated xenoliths that would suggest forceful magma injection to form the undercutting MR. It is
therefore likely that most of the apophyses are formed
in much the same manner as the potholes themselves
(i.e. by melting/dissolution of footwall cumulates along
particularly amenable horizons within the footwall) followed by in situ crystallization within the resulting cavities. Some undercutting reefs exhibit thin chromitite
seams along both margins (Figs 18 and 19). One
undercutting MR extending outwards from a regional
pothole in the Union Section Mine about 20 m below
the normal position of the MR is a sill-like body of sulphide-bearing harzburgite to olivine-rich orthopyroxenite. The apophysis is about 300 m long and 30 cm thick
and has chromitite seams along both margins. Part of
this MR with its upper chromitite seam is shown in
Fig. 20. Because of its high PGE content (up to 10–
20 ppm), this undercutting MR has been mined for c.
120 m along its length before it became subeconomic.
Not all undercutting reefs are rich in PGE, nor are they
so extensive.
The implications of undercutting MR are manifold.
First, there is no likelihood of crystal settling in the development of the MR within the apophyses. Single crystals of chromite or droplets of sulphide melt cannot
simply penetrate many metres of the floor of the
Journal of Petrology, 2016, Vol. 56, No. 12
Fig. 14. Predictions of crystal settling (a) and in situ crystallization (b) for the development of a chromitite seam of the MR
within a pothole with overhanging walls. Settling of chromite
from flowing/convecting melt cannot form a chromitite seam
along the overhanging walls of a pothole and will produce only
a thick seam at its basal part that acts as a trap for settled chromite grains. In contrast, in situ crystallization can result in a
chromitite seam that follows the outline of the pothole margins, including its overhanging portions. This process may be
accompanied by the transport and subsequent redeposition of
crystals that originally grew in situ by magma flowing along
the floor (c).
2359
Fig. 15. The lower chromitite seam of the MR is found even on
vertical to overhanging sections of the walls (indicated by yellow arrows) of potholes. Such relationships are indicative of
the formation of the MR by in situ crystallization. Photo (a) is
from the Frank Shaft of the Rustenburg Platinum Mine and
photos (b, c) are from Shaft 3 of the Karee mine. Photos (a), (b)
and (c) are courtesy of Chris Lee, Grant Cawthorn and Colleen
Meissner, respectively.
chamber by gravity settling (Fig. 21a). It has been suggested that undercutting MR could represent olivine–
pyroxene–sulphide–chromite slurries that were injected
into the footwall, as there are simply no melts of comparable composition. We reject this scenario because
crystal slurries are not compatible with the architecture
of MR potholes, as discussed above. In addition, the
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Journal of Petrology, 2016, Vol. 56, No. 12
Fig. 16. Generalized cross-section of a typical Brakspruit-type pothole from the Rustenburg Platinum Mine R.P.M.). It should be
noted that the normal MR (black) around the edge of the pothole is accompanied by six undercutting MR that are sill-like mineralized protrusions extending laterally from pothole margins into footwall rocks. Various marker horizons (Footwall, Brakspruit,
Pioneer, Boulder Bed) in the footwall rock sequence are denoted by dotted lines. Simplified after Ballhaus (1988).
geometry of the undercutting MR with respect to the
sidewalls of potholes (Figs 16 and 21) excludes lateral
injections unless the potholes were entirely filled with
crystal slurries. This was certainly not the case, as the
MR pyroxenite constitutes only a small portion of potholes (e.g. Figs 13b and 16). In addition, it appears to be
physically impossible that small chromite grains could
separate from viscous orthopyroxene/olivine slurries to
form the chromitite seams present along the margins of
the apophyses (e.g. Fig. 19). However, the latter is no
problem during in situ crystallization provided that convection continuously supplied fresh magma from the
main magma reservoir (Fig. 21b and c). We stress that,
in contrast to common thinking, the formation of mesoto orthocumulates in such sill-like bodies is not an issue
because the rate of mass transfer by convection
(km a1 to km day1) is typically several orders of magnitude higher than the rate of crystallization (05–
10 cm a1). This means that magma in the apophyses
could exchange with the main magma in the chamber
rapidly compared with the rate of crystallization in the
bodies. Second, the apophyses in the footwall require
the magmas parental to the MR to have been emplaced
as basal flows between the temporary floor of the
magma chamber and the resident melt. Third, the fact
that some of the apophyses are mineralized shows that
high PGE concentrations in stratiform horizons of layered intrusions can be achieved during crystallization
on crystal–liquid interfaces.
To summarize, layers following pothole margins,
chromitite seams developed along vertical to overhanging walls around potholes, and mineralized apophyses
in the footwall are indicative of in situ crystallization of
the MR from a basal layer of magma. Crystal settling is
not compatible with the field relations and therefore all
models involving this process (Fig. 4a–d, f and g) are
questionable. As mentioned above, this is not to say
that crystals and droplets of sulphide melt that were originally attached to pre-existing surfaces were not later
entrained and deposited in potholes as a result of convection in the basal layer of magma or subsequent
magma influxes (Figs 12c and 14c). It is this mechanism
that is probably responsible for the increased thickness
of cumulates in the deepest part of some large potholes
(e.g. Farquhar, 1986; Viljoen & Hieber, 1986) as well as
for the local thickening of chromitite seams and elevated amounts of sulphides (up to 15–20%) in some
small potholes (Fig. 5c).
The Merensky Reef: a result of a single or
multiple replenishment events
At issue is whether the MR is a product of the emplacement of a single batch or several batches of magma.
Most researchers are now in favour of the latter, although the exact number of magma pulses claimed
varies from three (Cawthorn, 2011) to five (Page, 2006)
or even more (Mitchell & Scoon, 2007; Hutchinson
et al., 2015). Multiple emplacement events are supported by the occurrence of several chromitite seams
(e.g. Cawthorn, 2011), differences in their textures (e.g.
Vukmanovic et al., 2013) and variations in the tenors
and ratios of PGE in the MR (e.g. Barnes & Maier, 2002).
However, separate replenishment events are most
clearly recorded by internal discordances that are
commonly demarcated by chromitite seams. One
Journal of Petrology, 2016, Vol. 56, No. 12
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Fig. 17. Examples (a–d) of undercutting MR with no visible chromitite seams along their margins. It should be noted that the
undercutting MR in (a) cuts across the layering in the footwall rocks and in (d) it splits into two branches, one of which intrudes
up-section. The existence of undercutting MR is persuasive evidence for the basal emplacement of parental magma. Brakspruit pothole, Rustenburg Platinum Mine. Scale bars in (b) and (d) is 25 cm.
informative example of the MR with two chromitite
seams that envelop pegmatitic pyroxenite is shown in
Fig. 22a. Some textural features of the pegmatitic pyroxenites (e.g. large orthopyroxene grains containing
cuspate inclusions of plagioclase) were advanced by
Cawthorn & Boerst (2006) as evidence for formation by
recrystallization of ‘ordinary’ pyroxenite at the cumulate–magma interface owing to reaction with overlying
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Journal of Petrology, 2016, Vol. 56, No. 12
Fig. 18. Photograph (a) and sketch (b) of undercutting MR with chromitite seams developed along both margins. Such chromitite
seams, especially underneath a screen of the footwall rocks (b), indicates that the MR is produced by in situ crystallization against
the sides of the apophysis. Brakspruit pothole, Rustenburg Platinum Mine.
superheated magma. Based on this idea, geological relationships in this outcrop can be explained as portrayed in Fig. 22. At first, the footwall norite/mottled
anorthosite was dissolved by superheated magma (Fig.
22b) and the resulting unconformity was covered by in
situ grown chromitite 1 and pyroxenite 1 (not preserved
in this outcrop) (Fig. 22c). Later, pyroxenite 1 was partly
removed and partly reconstituted into pegmatitic pyroxenite in contact with a subsequent pulse of superheated
magma (Fig. 22d). Finally, all the cumulates were
eroded again (Fig. 22e) and the resulting erosional
surface was covered by chromitite 2 and pyroxenite 2
(Fig. 22f). There are underground outcrops with up to
five or even six chromitite seams, each developed on
successive erosion surfaces. Such outcrops of the MR
are most common in the deeper parts of potholes, but
may also occur at its normal elevation along the edges
of potholes where the MR is more tabular. In these
cases the basal part of the MR is commonly composed
of olivine-rich rocks (harzburgite and olivine pyroxenite)
Journal of Petrology, 2016, Vol. 56, No. 12
2363
Fig. 19. Photograph (a) and sketch (b) of undercutting MR in cross-section with chromitite seams visible along both margins. Such
chromitite seams indicate that the MR is produced by in situ crystallization against the sidewalls of a sill-like body. This outcrop is
located about 15 m below the normal position of the MR. Brakspruit pothole, Rustenburg Platinum Mine.
grading upwards, via several chromitite seams, into
pegmatitic pyroxenite and then pyroxenite.
There are several textural and mineralogical features
of footwall rocks that provide additional support for the
basal emplacement of hybrid magmas. One is the pegmatitic texture of the pyroxenites. Cawthorn & Boerst
(2006), following Viljoen (1999), came to the conclusion
that such textures are not a result of late-stage, fluidinduced recrystallization of primary cumulates within
the cumulate pile, as commonly supposed (e.g. Mathez
& Mey, 2005). Rather, the pegmatitic textures are a
result of the modification of ordinary pyroxenite near a
crystal–liquid interface owing to superheated magma
flowing into the chamber (Cawthorn & Boerst, 2006).
There are field observations that support this inference.
In some areas, pegmatitic pyroxenite is cut by the gently to steeply inclined margins of post-Merensky potholes (Fig. 6c). The truncation of pegmatitic pyroxenite
close to the temporary floor of the chamber indicates
that pegmatitic textures developed essentially at
cumulate–magma interfaces. This, in turn, means that
hybrid magmas must have been emplaced as basal
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Journal of Petrology, 2016, Vol. 56, No. 12
Fig. 20. A sample of undercutting MR (a) showing the upper chromitite seam (b) and abundant sulphides in pegmatitic harzburgite
(c) from the Union Section mine. The sample is part of an apophysis about 20 m below the normal position of the MR. The apophysis is about 300 m long and 30 cm thick and has chromitite seams along both margins. It has been mined along its entire length because it is rich in PGE.
flows that were able to reconstitute the uppermost pyroxenites of the cumulate pile. Similarly, anorthosite
selvages that are so common at the base of the MR
(Fig. 11c) and the less common troctolite selvages
(Roberts et al., 2007) are probably a result of heating
and reconstitution of the footwall norites beneath basal
flows of hot hybrid magma.
The observations presented here confirm that the
MR is probably a result of the emplacement of several
successive pulses of magma that caused heating, melting and dissolution as well as associated textural and
mineralogical modification of earlier cumulates. It is
conceivable that the cause of many of the characteristic
and enigmatic features of the MR, such as its enrichment in PGE, lies in the complex emplacement history
and reprocessing of older ore-bearing cumulates.
A HYPOTHESIS FOR ORIGIN OF THE
MERENSKY REEF
Field relations in and around potholes indicate that the
genesis of the MR was not related to or controlled by
such processes as settling of chromite and sulphide
melt from the overlying magma (Fig. 4a–d), upward percolation of interstitial melt or fluids from the underlying
solidifying cumulates (Fig. 4e), sill-like injection of
magma into pre-existing cumulates (Fig. 4f), the emplacement of chromite-rich slurries from deeper chambers (Fig. 4g), hydrodynamic sorting in mobilized
unconsolidated cumulates containing disseminated ore
minerals (Fig. 4h), or crystallization induced by a pressure increase in the chamber (Fig. 4d). These mechanisms appear not to be essential for the formation of PGE
deposits in the Bushveld Complex. Thus, there must be
some other process responsible for these deposits in
layered intrusions. Latypov et al. (2013) have recently
proposed an alternative explanation based on a study
of PGE-bearing chromitite seams at the bases of cyclic
units in the Rum Complex. The field observations presented here support our hypothesis and below we extend it to the MR.
We envisage the following sequence of events. The
evolving magma chamber was replenished by a batch
of hot, dense, primitive magma. Although some researchers argue that even ordinary basaltic magma
would suffice to generate the MR, given the preference
of PGE for sulphide melt (e.g. Mungall & Brenan, 2014),
we prefer to believe that this primitive magma had an
anomalous composition. This better explains why
unconformities associated with replenishment of the
Bushveld chamber are common both below and above
the MR, but only this (and UG2) event resulted in the
formation of a world-class PGE deposit. Therefore, following Naldrett et al. (2011), we suggest that this is because the new magma was unusually enriched in PGE
(and possibly also S) as well as in incompatible trace
elements (Arndt et al., 2005). It is possible that no
chilled margins or mafic–ultramafic sills in the footwall
of the Bushveld Complex preserve the composition of
this remarkable magma. The magma was both hotter
Journal of Petrology, 2016, Vol. 56, No. 12
Fig. 21. Predictions of crystal settling (a) and in situ crystallization (b) for the development of an MR chromitite seam within a
sill-like cavity in the footwall rocks. Settling of chromite from
flowing/convecting melt will not able to form a chromitite
seam within the cavity. In contrast, in situ crystallization may
result in a chromitite seam that follows the outline of the cavity. Sedimentation of crystals from flowing magma along the
inclined sidewalls of a pothole will have little effect on the development of a chromitite seam in the cavity (c).
and denser than the resident magma and entered the
chamber with sufficient momentum to form a fountain.
Turbulent mixing with the resident melt in the fountain
resulted in a hybrid magma that flowed rapidly across
2365
the floor of the chamber (Fig. 23a). As a consequence of
mixing, the hybrid magma was superheated and was
able to melt or dissolve footwall plagioclase-rich cumulates, both locally and more extensively, forming a new
temporary floor of the magma chamber with small to
large potholes and antipotholes as well as dyke- and
sill-like apophyses into the footwall (Fig. 23b and c).
In this process, some cumulates were recrystallized and
reconstituted, forming anorthosite and troctolite selvages. On cooling to the liquidus temperature of the hybrid magma, this was followed by in situ crystallization
of mineral phases at the magma–cumulate interface
(Fig. 23d–f) because heterogeneous nucleation on preexisting crystals is a kinetically favourable process (e.g.
Campbell, 1996). The first liquidus phase in the hybrid
magma was chromite, subsequently accompanied by
the separation of an immiscible sulphide melt and crystallization of olivine and/or orthopyroxene. The dykeand sill-like apophyses behaved as open systems in
which convection continuously supplied fresh magma
from the main magma reservoir. The magma occupying
these cavities crystallized from the margins inwards
producing mineralized cumulate rocks.
During this process both chromite crystals and droplets of sulphide melt concentrated PGE. As they grew in
situ they scavenged PGE from magma that was continuously brought to the crystallization front (Fig. 23e and f)
by convection in the basal magma layer. This allowed
them to equilibrate with a large volume of magma and
become highly enriched in PGE. Two factors were crucial in producing the high concentrations of PGE: the
high sulphide/chromite–silicate melt distribution coefficients (up to 1 106 for sulphides; Fonseca et al., 2009;
Mungall & Brenan, 2014) and vigorous convection in
the magma (km a1 to km day1; Morse, 1986). The rate
of convection is generally 105–107 times higher than
that of solidification, which is only in the range of
05–10 cm a1 in slowly cooled magma chambers
(Morse, 1986). As a result, droplets of sulphide liquid
could theoretically come into equilibrium with magma
105–107 times their own volume (i.e. a high R-factor).
This essentially means that any droplet of sulphide liquid or chromite grain growing at the crystal–liquid
interface had an almost unlimited access to fresh, undepleted magma necessary for the effective scavenging of
PGE. Numerical modelling of this process has confirmed that high concentrations of PGE (several ppm)
can be achieved in chromite/sulphide-bearing cumulates crystallizing in situ at the crystal–liquid interface
(Latypov et al., 2013). Locally, where footwall rocks
were not completely consolidated, PGE-rich sulphide
melt percolated downwards into porous footwall cumulates to form footwall mineralization (Fig. 23e).
The initial stage of magma replenishment was followed by several other events that also brought dense,
primitive magmas into the chamber. Each subsequent
basal flow of superheated hybrid magma caused melting and dissolution of pyroxenite and even chromitite
cumulates that crystallized from previously emplaced
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Journal of Petrology, 2016, Vol. 56, No. 12
Fig. 22. (a) Multiple magma events recorded in pegmatitic pyroxenite of the MR sandwiched between two chromitite seams, Karee
mine (photo courtesy of Hulisani Manenzhe). The following sequence of events is envisaged: (b) magmatic erosion of footwall anorthosite by superheated magma; (c) in situ crystallization of chromitite 1 and pyroxenite 1 on the erosion surface; (d) reconstitution
of pyroxenite 1 into pegmatitic pyroxenite during the emplacement of subsequent pulses of superheated magma (not shown); (e) a
second phase of magmatic erosion of all footwall lithologies by superheated magma; (f) in situ crystallization of chromitite 2 and
pyroxenite 2 against the erosional surface.
Journal of Petrology, 2016, Vol. 56, No. 12
2367
Fig. 23. A general hypothesis for the origin of the MR. (a) PGE-rich, dense, primitive magma enters the chamber, mixes with the
resident melt in a fountain to produce superheated hybrid magma, which spreads across the floor of the chamber as a basal layer.
(b, c) The superheated magma causes thermochemical erosion of cumulates forming the temporary floor of the chamber with potholes and antipotholes as well as dyke- and sill-like projections into footwall cumulates. (d) This is followed, upon magma cooling,
by in situ crystallization of phases (e.g. chromite/sulphide) directly on the irregular erosional surface, both normal (e) and
overturned (f). Chromite and droplets of sulphide melt scavenge PGE from magma that is continuously brought towards the
crystal–liquid interface by flowing/convecting magma. This allows the chromite and sulphide melts to equilibrate with a large
amount of basaltic magma. Locally, PGE-rich sulphide liquid percolates downwards into porous footwall rocks (e).
magmas. In addition, some pyroxenite cumulates were
subjected to recrystallization and reconstitution with the
formation of pegmatitic pyroxenite (e.g. Fig. 22). On
cooling, each new batch of hybrid magma generally
crystallized chromite and separated sulphide melt that
draped irregular erosional surfaces. Sulphide liquid was
not restricted to chromite crystallization and continued
separating as silicate
minerals (olivine
and
orthopyroxene) crystallized above chromitite seams.
Finally, the basal layer mixed with the overlying layer of
resident melt to form a hybrid magma that crystallized
into barren norite cumulate overlying the MR.
Alternatively, the hanging-wall rocks may have crystallized from a new magma added to the chamber that
was not saturated in chromite/sulphide melt. This can
explain why sulphides and chromite are restricted to
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Journal of Petrology, 2016, Vol. 56, No. 12
Fig. 24. (a) Photograph of the normal (planar) MR consisting of pegmatitic pyroxenite with four chromitite (Chr) seams, each of
which may demarcate the arrival of a new pulse of PGE-rich primitive magma that resulted in erosion, redeposition and reconstitution of pre-existing cumulate rocks. The photograph indicates the origin of the MR by multiple replenishment events. Siphumelele
mine. (b) Schematic section of the MR in (a) showing that PGE-rich chromite and sulphides are developed almost exclusively within
the MR. This phenomenon emphasizes the uniqueness of the Merensky mineralizing event in the history of the Bushveld Complex.
The simplest explanation is that the resident magma in the chamber was not saturated in chromite/sulphide and crystallized barren
footwall rocks. New magma that entered the chamber was saturated (either originally or upon mixing) and crystallized chromite
and immiscible sulphide melt, forming the Merensky Reef package of PGE-rich pyroxenitic rocks. Chromite and sulphide are absent
in the hanging-wall rocks to the Merensky Reef, either because of eventual complete mixing of a basal layer with the overlying
magma that was not saturated in chromite/sulphide or because of the introduction of a new magma into the chamber that was not
saturated in these phases.
the MR (Cawthorn, 1999a; Mitchell & Scoon, 2007) and
absent from the footwall and hanging-wall rocks that
formed from pre- and post-Merensky resident magmas
not saturated in sulphide melt and chromite. This scenario is different from the case of the Skaergaard intrusion, in which sulphide saturation occurred as a result
of internal, closed-system fractional crystallization
(Andersen, 2006). This long sequence of events finally
resulted in an MR with up to 4–6 chromitite seams
occurring within normal to pegmatitic pyroxenite that
are overlain by barren norite (Fig. 24).
CONCLUDING REMARKS
We claim that basic field observations remain an important and effective means of testing petrogenetic
hypotheses. Field observations must always buttress
any successful petrological hypothesis. In many cases,
field observations are the simplest way to resolve longstanding petrological controversies. In particular, we
show here that field relations indicate that the MR of the
Bushveld Complex formed from basally emplaced
layers of magma, within which crystallization and collection of PGE took place essentially in situ; that is, directly on magma–cumulate interfaces. The undeniable
field evidence supporting this inference is the development of chromitite seams on the subvertical to overhanging walls of potholes as well as along both
margins of sill-like apophyses extending from potholes
into the footwall. We believe that this model can successfully be extended to PGE reefs in other layered intrusions. In general, the inferred in situ crystallization of
the MR seems reasonable in the light of the current consensus among most (but not all!) igneous petrologists
on layered intrusions as magma chambers that gradually lose heat and crystallize inwards from their margins (Jackson, 1961; Campbell, 1978, 1996; McBirney
& Noyes, 1979; Cawthorn & McCarthy, 1981; Wilson &
Larsen, 1985; Naslund & McBirney, 1996; Tait &
Jaupart, 1996; Latypov et al., 2011). The idea finds
support in recent experimental, textural and numerical
studies (e.g. Pupier et al., 2008; Hammer et al., 2010;
Špillar & Dolejš, 2015) showing that nucleation on preexisting crystals (rather than homogeneous nucleation
Journal of Petrology, 2016, Vol. 56, No. 12
in the magma itself) is the predominant mechanism of
crystal growth in natural magmas. If the rocks hosting
ore deposits in layered intrusions were formed by in
situ crystallization, then it seems logical to suggest that
the deposits themselves should also be produced by
this process.
ACKNOWLEDGEMENTS
Any opinion, finding and conclusion or recommendation expressed in this material is that of the authors and
the NRF does not accept any liability in this regard. We
thank Grant Cawthorn, Morris Viljoen, Chris Lee,
Wimpie Britz, Dennis Hoffmann, Johan Marais, Ray
Brown, Jan Van der Merwe, Hulisani Manenzhe,
Colleen Meissner and many other mining geologists for
fruitful discussions on many aspects of the MR, their
help in organizing underground and open pit visits, and
much assistance and advice during the field work for
this study. They bear, however, no responsibility for the
conclusions we have reached. We especially thank
Steve Barnes and Jesse Robertson for bringing the attention of the first author to the differences between
thermally controlled melting and chemically induced
dissolution of cumulate rocks as well as for pointing out
several relationships that are not compatible with
mechanical erosion of the footwall to the MR. Ian
Campbell and Steve Barnes are also thanked for useful
discussions of some aspects of mixing processes in
magma chambers. Peer reviews of the paper by Grant
Cawthorn, Brian O’Driscoll, Roger Scoon, Lew Ashwall,
Grant Bybee and Paul Nex as well as official Journal reviews by Chris Lee, Wolfgang Maier and Jim Mungall
and editorial handling by Marjorie Wilson are gratefully
acknowledged. We are also very grateful to Brian
Robins for his meticulous editing and comments that
have significantly improved the paper.
FUNDING
This work is based on research supported by the
National Research Foundation (NRF) of South Africa in
the form of Research Grants 87677, 90834 and 91812 to
R.L. and an Innovation Postdoctoral Fellowship to S.C.
REFERENCES
Andersen, J. C. Ø. (2006). Postmagmatic sulphur loss in the
Skaergaard Intrusion: implications for the formation of the
Platinova Reef. Lithos 92, 198–221.
Arndt, N., Jenner, G., Ohnenstetter, M., Deloule, E. & Wilson, A.
H. (2005). Trace elements in the Merensky Reef and adjacent
norites, Bushveld Complex, South Africa. Mineralium
Deposita 40, 550–575.
Ballhaus, C. (1988). Potholes of the Merensky reef at Brakspruit
shaft, R.P.M.—primary disturbances in the magmatic stratigraphy. Economic Geology 83, 1140–1158.
Ballhaus, C. & Sylvester, P. (2000). Noble metal enrichment
processes in the Merensky reef, Bushveld Complex. Journal
of Petrology 44, 545–561.
2369
Ballhaus, C. G., Cornelius, M. & Stumpfl, E. F. (1988). The Upper
Critical Zone of the Bushveld Complex and the origin of
Merensky-type ores—a discussion. Economic Geology 83,
1082–1091.
Barnes, S. J. & Maier, W. D. (2002). Platinum-group element
and microstructures of normal Merensky Reef from Impala
Platinum Mines, Bushveld Complex. Journal of Petrology
43, 103–128.
Boudreau, A. E. (1992). Volatile fluid overpressure in layered intrusions and the formation of potholes. Australian Journal
of Earth Science 39, 277–287.
Boudreau, A. E. (1995). Some geochemical considerations of
platinum-group element exploration in layered intrusions.
Exploration and Mining Geology 4, 215–225.
Boudreau, A. E. (2008). Modeling the Merensky reef, Bushveld
complex, Republic of South Africa. Contributions to
Mineralogy and Petrology 156, 431–437.
Boudreau, A. E. & McCallum, I. S. (1992). Infiltration metasomatism in layered intrusions—an example from the Stillwater
Complex, Montana. Journal of Volcanology and
Geothermal Research 52, 171–183.
Boudreau, A. E. & Meurer, W. P. (1999). Chromatographic separation of the platinum-group elements, gold, base metals
and sulphur during degassing of compacted and solidifying
igneous crystal pile. Contributions to Mineralogy and
Petrology 134, 174–185.
Campbell, I. H. (1978). Some problems with the cumulus theory.
Lithos 11, 311–323.
Campbell, I. H. (1986a). A fluid-dynamic model for the potholes
of the Merensky Reef. Economic Geology 81, 1118–1125.
Campbell, I. H. (1986b). Distribution of orthocumulate textures
in the Jimberlana intrusion. Journal of Geology 95, 35–54.
Campbell, I. H. (1996). Fluid dynamic processes in basaltic magma
chambers. In: Cawthorn, R. G. (ed.) Layered Intrusions.
Developments in Petrology 15. Elsevier, pp. 45–76.
Campbell, I. H. & Turner, J. S. (1986). The influence of viscosity
on fountains in magma chambers. Journal of Petrology 27,
1–30.
Campbell, I. H. & Turner, J. S. (1989). Fountains in magma
chambers. Journal of Petrology 30, 885–923.
Campbell, I. H., Naldrett, A. J. & Barnes, S. J. (1983). A model
for the origin of platinum-rich sulphide horizons in the
Bushveld and Stillwater complexes. Journal of Petrology 24,
133–165.
Carr, H. W., Groves, D. I. & Cawthorn, R. G. (1994). A GIS based
spatial analysis of controls on the distribution of Merensky
reef potholes at the Western Platinum Mine, Bushveld
Complex, South Africa. South African Journal of Geology
97, 431–441.
Carr, H. W., Kruger, F. J., Groves, D. I. & Cawthorn, R. G. (1999).
The petrogenesis of Merensky Reef potholes at the Western
Platinum Mine, Bushveld Complex: Sr-isotopic evidence
for synmagmatic deformation. Mineralium Deposita 34,
335–347.
Cawthorn, R. G. (1999a). Platinum-group element mineralization in the Bushveld Complex—a critical reassessment of
geochemical models. South African Journal of Geology
102(3), 268–281.
Cawthorn, R. G. (1999b). Geological models for platinum-group
metal mineralization in the Bushveld Complex. South
African Journal of Science 95, 490–498.
Cawthorn, R. G. (1999c). Permeability of the footwall cumulates
to the Merensky Reef, Bushveld Complex. South African
Journal of Science 102(3), 293–310.
Cawthorn, R. G. (2005). Pressure fluctuations and the formation
of the PGE-rich Merensky and chromitite reefs, Bushveld
Complex. Mineralium Deposita 40, 231–235.
2370
Cawthorn, R. G. (2011). Geological interpretations from the PGE
distribution in the Bushveld Merensky and UG2 chromitite
reefs. Journal of the South African Institute of Mining and
Metallurgy 111, 67–79.
Cawthorn, R. G. & Boerst, K. (2006). Origin of the pegmatitic
pyroxenite in the Merensky Unit, Bushveld Complex, South
Africa. Journal of Petrology 47, 1509–1530.
Cawthorn, R. G. & McCarthy, T. S. (1981). Bottom crystallization
and diffusion control in layered complexes: evidence from
Cr distribution in magnetite from the Bushveld Complex.
Transactions of the Geological Society of South Africa 84,
41–50.
Cawthorn, R. G., Merkle, R. K. W. & Viljoen, M. J. (2002).
Platinum-group element deposits in the Bushveld Complex,
South Africa. In: Cabri, L. J. (ed.) The Geology,
Geochemistry, Mineralogy and Mineral Beneficiation of
Platinum-group Elements. Canadian Institute of Mining,
Metallurgy and Petroleum, Special Volume 54, 507–546.
Cawthorn, R. G., Barnes, S. J., Ballhaus, C. & Malitch, K. N.
(2005). Platinum-group element, chromium, and vanadium
deposits in mafic and ultramafic rocks. Economic Geology,
100th Anniversary Volume, 215–249.
Cawthorn, R. G., Eales, H. V., Walraven, F., Uken, R. & Watkeys,
M. R. (2006). The Bushveld Complex. In: Johnson, M. R.,
Anhaeusser, C. R. & Thomas, R. J. (eds) The Geology of
South Africa. Geological Society of South Africa, Council for
Geoscience, pp. 261–282.
Eales, H. V. & Cawthorn, R. G. (1996). The Bushveld Complex.
In: Cawthorn, R. G. (ed.) Layered Intrusions. Elsevier, pp.
181–229.
Eales, H. V., De Klerk, W. J. & Teigler, B. (1990). Evidence for
magma mixing processes within the critical and lower zones
of the northwestern Bushveld Complex, South Africa.
Chemical Geology 88, 261–278.
Farquhar, J. (1986). The Western Platinum Mine. In:
Anhaeusser, C. R. & Maske, S. (eds) Mineral Deposits of
Southern Africa. Geological Society of South Africa, pp.
1143–1154.
Fonseca, R. O. C., Campbell, I. H., O’Neill, H. St. C. & Allen, C. M.
(2009). Solubility of Pt in sulphide mattes: Implicatios for the
genesis of PGE-rich horizons in layered intrusions.
Geochimica et Cosmochimica Acta 73, 5764–5777.
Frenkel’, M. Ya., Yaroshevsky, A. A., Ariskin, A. A., Barmina, G.
S., Koptev-Dvornikov, E. V. & Kireev, B. S. (1989).
Convective–cumulative model simulating the formation process of stratified intrusions. In: Bonin, N., Didier, J., Le Fort,
P., Propach, G., Puga, E. & Vistelius, A. B. (eds) Magma–
Crust Interactions and Evolution. Theophrastus, pp. 3–88.
Godel, B. (2015). Platinum-group element deposits in layered
intrusions: recent advances in the understanding of the ore
forming processes. In: Charlier, B., Namur, O., Latypov, R. &
Tegner C (eds) Layered Intrusions. Springer Geology 379–
434.
Hall, A. L. (1932). The Bushveld Igneous Complex. Geological
Survey of South Africa Memoir 28, 560 pp.
Hammer, J. E., Sharp, T. G. & Wessel, P. (2010). Heterogeneous
nucleation and epitaxial crystal growth of magmatic minerals. Geology 38, 367–370.
Hoffmann, D. (2010). Statistical size analysis of potholes: an
attempt to estimate geological losses ahead of mining
at Lonmin’s Marikana mining district. The 4th
International Platinum Conference, Platinum in Transition
‘Boom or Bust’. Southern African Institute of Mining and
Metallurgy, Johannesburg, South Africa, pp. 1–8.
Holness, M. B., Stripp, G., Humphreys, C. S., Veksler, I. V.,
Nielsen, T. F. D. & Tegner, C. (2011). Silicate liquid immiscibility within the crystal mush: late stage magmatic
Journal of Petrology, 2016, Vol. 56, No. 12
microstructures in the Skaergaard Intrusion, East
Greenland. Journal of Petrology 52, 175–222.
Hutchinson, D., Foster, J., Prichard, H. & Gilbert, S. (2015).
Concentration of particulate platinum-group minerals during magma emplacement; a case study from the Merensky
Reef, Bushveld Complex. Journal of Petrology 56, 113–159.
Irvine, T. N., Keith, D. W. & Todd, S. G. (1983). The J-M
platinum–palladium reef of the Stillwater Complex,
Montana: II. Origin by double-diffusive convective magma
mixing and implications for the Bushveld Complex.
Economic Geology 78, 1287–1334.
Irvine, T. N., Andersen, J. C. O. & Brooks, C. K. (1998). Included
blocks (and blocks within blocks) in the Skaergaard intrusion: geological relations and the origins of rhythmic modally graded layers. Geological Society of America Bulletin
110, 1398–1447.
Jackson, E. D. (1961). Primary textures and mineral
associations in the ultramafic zone of the Stillwater
complex, Montana. US Geological Survey Professional
Paper 358, 106 pp.
Jaupart, C. & Tait, S. (1995). Dynamics of differentiation in
magma reservoirs. Journal of Geophysical Research 100,
17617–17636.
Junge, M., Oberthür, T. & Melcher, F. (2014). Cryptic
variation of chromite chemistry, platinum group element
and platinum group mineral distribution in the UG-2
chromitite: an example from the Karee Mine, western
Bushveld Complex, South Africa. Economic Geology 109,
795–810.
Kerr, R. C. (1994). Melting driven by vigorous compositional
convection. Journal of Fluid Mechanics 280, 255–285.
Kinnaird, J. A, Kruger, F. J., Nex, P. A. M. & Cawthorn, R. G.
(2002). Chromite formation—a key to understanding processes of platinum enrichment. Transactions of the
Institution of Mining and Metallurgy 111, B23–B35.
Kruger, F. J. (1992). The origin of the Merensky cyclic unit: Sr
isotopic and mineralogical evidence for an alternative orthomagmatic model. Australian Journal of Earth Sciences 39,
255–261.
Kruger, F. (2005). Filling the Bushveld Complex magma chamber: Lateral expansion, roof and floor interaction, magmatic
unconformities, and the formation of giant chromitite, PGE
and Ti–V-magnetitite deposits. Mineralium Deposita 40,
451472.
Kruger, F. J. (2010). The Merensky and Bastard cyclic units and
the Platreef of the Bushveld Complex: consequences of
Main Zone magma influxes and dynamics. The 4th
International Platinum Conference, Platinum in Transition
‘Boom or Bust’. Southern African Institute of Mining and
Metallurgy, Johannesburg, South Africa, pp. 43–46.
Kruger, F. J. & Marsh, J. S. (1985). The mineralogy, petrology
and origin of the Merensky cyclic unit in the western
Bushveld Complex. Economic Geology 80, 958–974.
Latypov, R. M. & Egorova, V. (2012). Plagioclase compositions
give evidence for in situ crystallization under horizontal flow
conditions in mafic sills. Geology 40, 883–161.
Latypov, R. M., Hanski, E., Lavrenchuk, A., Huhma, H. & Havela,
T. (2011). A ‘three-increase model’ for origin of marginal reversal in the Koitelainen layered intrusion, Finland. Journal
of Petrology 52(4), 733–764.
Latypov, R. M., O’Driscoll, B. & Lavrenchuk, A. (2013). Towards
a model for in situ origin of PGE reefs in layered intrusions:
insights from chromitite seams of the Rum Eastern
Intrusion, Scotland. Contributions to Mineralogy and
Petrology 166, 309–327.
Lauder, W. R. (1970). Origin of the Merensky Reef. Nature 227,
365–366.
Journal of Petrology, 2016, Vol. 56, No. 12
Lee, C. A. (1996). A review of mineralization in the Bushveld
Complex and some other layered intrusions. In: Cawthorn,
R. G. (ed.) Layered Intrusions. Elsevier, pp. 103–145.
Leeb-du Toit, A. (1986). The Impala Platinum Mines. In:
Anhaeusser, C. R. & Maske, S. (eds) Mineral Deposits
of Southern Africa. Geological Society of South Africa, pp.
1091–1106.
Maier, W. D. (2005). Platinum-group element (PGE) deposits
and occurrences: mineralization styles, genetic concepts,
and exploration criteria. Journal of African Earth Sciences
41(3), 165–191.
Maier, W. D. & Barnes, S. J. (2008). Platinum-group elements in
the UG1 and UG2 chromitites, and the Bastard Reef, at
Impala platinum mine, western Bushveld Complex, South
Africa: evidence for late magmatic cumulate instability and
reef constitution. South African Journal of Geology 111,
159–176.
Maier, W. D., Barnes, S. J. & Groves, D. I. (2013). The Bushveld
Complex, South Africa: formation of platinum–palladium,
chrome- and vanadium-rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large, relatively
slowly cooling, subsiding magma chamber. Mineralium
Deposita 48, 1–56.
Mathez, E. A. (1995). Magmatic metasomatism and formation
of the Merensky Reef, Bushveld Complex. Contributions to
Mineralogy and Petrology 119, 277–286.
Mathez, E, A. & Mey, J. L. (2005). Character of the UG2 chromitite and host rocks and petrogenesis of its pegmatoidal footwall, northeastern Bushveld Complex. Economic Geology
100, 1617–1630.
Mathez, E. A., Hunter, R. H. & Kinzler, R. (1997). Petrologic evolution of partially molten cumulate: the Atok section of the
Bushveld Complex. Contributions to Mineralogy and
Petrology 129, 20–34.
McBirney, A. R. (1979). Effect of assimilation. In: Yoder, H. S.
(ed.) The Evolution of the Igneous Rocks: Fiftieth Anniversary
Perspective. Princeton University Press, pp. 307–338.
McBirney, A. R. & Noyes, R. M. (1979). Crystallization and layering of the Skaergaard intrusion. Journal of Petrology 20,
487–554.
McKenzie, D. (2011). Compaction and crystallization in magma
chambers: towards a model of the Skaergaard intrusion.
Journal of Petrology 52, 905–930.
Mitchell, A. A. & Scoon, R. N. (2007). The Merensky Reef at
Winnaarshoek, Eastern Bushveld Complex: a primary magmatic hypothesis based on a wide reef facies. Economic
Geology 102, 971–1009.
Mondal, S. K. & Mathez, E. A. (2007). Origin of the UG2
chromitite layer, Bushveld Complex. Journal of Petrology
48, 495–510.
Morse, S. A. (1986). Convection in aid of adcumulus growth.
Journal of Petrology 27, 1183–1214.
Morse, S. A. (1988). Motion of crystals, solute, and heat in layered intrusions. Canadian Mineralogist 26, 209–244.
Mungall, J. E. (2014). Geochemistry of magmatic ore deposits.
In: Holland, H. D. & Turekian, K. K. (eds) Treatise on
Geochemistry, Volume 13. Pergamon, pp. 195–218.
Mungall, J. E. (2015). Physical controls of nucleation,
growth and migration of vapor bubbles in partially molten cumulates. In: Charlier, B., Namur, O., Latypov, R. &
Tegner, C. (eds) Layered Intrusions. Springer Geology
331–378.
Mungall, J. E. & Brenan, J. M. (2014). Partitioning of platinumgroup elements and Au between sulfide liquid and basalt
and the origins of mantle–crust fractionation of the chalcophile elements. Geochimica et Cosmochimica Acta 125,
265–289.
2371
Mungall, J. E. & Naldrett, A. J. (2008). Ore deposits of the platinum-group elements. Elements 4, 253–258.
Naldrett, A. J. (1989). Stratiform PGE deposits in layered intrusions. In: Whitney, J. A. & Naldrett, A. J. (eds) Ore
Deposition Associated with Magmas. Reviews in Economic
Geology 4, 135–166.
Naldrett, A. J., Cameron, G., von Gruenewaldt, G. & Sharp, M.
R. (1987). The formation of stratiform PGE deposits in layered intrusions. In: Parsons, I. (ed.) Origin of Igneous
Layering. D. Reidel, pp. 313–397.
Naldrett, A., Wilson, A., Kinnaird, J. & Chunnett, G. (2009). PGE
tenor and metal ratios within and below the Merensky Reef,
Bushveld Complex: implications for its genesis. Journal of
Petrology 50, 625–659.
Naldrett, A., Kinnaird, J., Wilson, A., Yudovskaya, M. &
Chunnett, G. (2011). Genesis of the PGE-enriched Merensky
Reef and chromitite seams of the Bushveld Complex. In: Li,
C. & Ripley, E. M. (eds) Magmatic Ni–Cu and PGE Deposits:
Geology, Geochemistry and Genesis. Reviews in Economic
Geology 17, 235–296.
Naldrett, A., Kinnaird, J., Wilson, A., Yudovskaya, M. &
Chunnett, G. (2012). The origin of chromitites and related
PGE mineralization in the Bushveld Complex: new mineralogical and petrological constraints. Mineralium Deposita 47,
209–232.
Naslund, H. R. & McBirney, A. R. (1996). Mechanisms of
formation of igneous layering. In: Cawthorn, R. G. (ed.)
Layered Intrusions. Developments in Petrology 15. Elsevier,
pp. 1–43.
Nicholson, D. M. & Mathez, E. A. (1991). Petrogenesis of the
Merensky Reef in the Rustenburg section of the Bushveld
Complex. Contributions to Mineralogy and Petrology 107,
293–309.
O’Driscoll, B., Emeleus, C. H., Donaldson, C. H. & Daly, J. S.
(2010). Cr-spinel seam petrogenesis in the Rum Layered
Suite, NW Scotland: cumulate assimilation and in situ crystallization in a deforming crystal mush. Journal of Petrology
51(6), 1171–1201.
O’Hara, M. J. & Herzberg, C. (2002). Interpretation of trace element and isotope features of basalts: Relevance of field relations, petrology, major element data, phase equilibria, and
magma chamber modeling in basalt petrogenesis.
Geochimica et Cosmochimica Acta 66, 2167–2191.
Osborn, E. F. & Tait, D. B. (1952). The system diopside–forsterite–anorthite. American Journal of Science, Bowen Volume,
413–433.
Page, A. S. (2006). The Brakspruit Pothole, East Business Area,
Rustenburg
Section.
Anglo
American
Platinum
Corporation, Mine Managers Association Presentation
Abstracts, Johannesburg, pp. 1–32.
Prevec, S. A., Ashwal, L. D. & Mkaza, M. S. (2005). Mineral
disequilibrium
in
the
Merensky
Reef,
western
Bushveld Complex, South Africa: new Sm–Nd isotope
evidence. Contributions to Mineralogy and Petrology 149,
306–315.
Pupier, E., Duchesne, J. C. & Toplis, M. J. (2008). Experimental
quantification of plagioclase crystal size distribution during
cooling of a basaltic liquid. Contributions to Mineralogy and
Petrology 155, 555–570.
Roberts, M. D., Reid, D. L., Miller, J. A., Basson, I. J., Roberts, M.
& Smith, D. (2007). The Merensky Cyclic Unit and its impact
on footwall cumulates below Normal and Regional Pothole
reef types in the Western Bushveld Complex. Mineralium
Deposita 42, 271–292.
Robertson, J., Ripley, E. M., Barnes, S. J. & Li, C. (2015). Sulfur
liberation from country rocks and incorporation in mafic
magmas. Economic Geology 110, 111–1123.
2372
Roobol, M. J. (1972). Size-graded, igneous layering in an
Icelandic intrusion. Geological Magazine 109, 393–404.
Scoon, R. N. & Teigler, B. (1994). Platinum-group element mineralization in the Critical Zone of the Western Bushveld
Complex: I. Sulphide poor-chromitites below the UG-2.
Economic Geology 89, 1094–1121.
Smith, D. S. & Basson, I. J. (2006). Shape and distribution
analysis of Merensky Reef potholing, Northam
Platinum Mine, Western Bushveld Complex: implications
for pothole formation and growth. Mineralium Deposita 41,
281–295.
Špillar, V. & Dolejš, D. (2015). Heterogeneous nucleation as the
predominant mode of crystallization in natural magmas:
numerical model and implications for crystal–melt interaction. Contributions to Mineralogy and Petrology 169, 4,
doi:10.1007/s00410-014-1103-6.
Tait, S. & Jaupart, C. (1996). The producing of chemically stratified and adcumulate plutonic igneous rocks. Mineralogical
Magazine 60, 99–114.
Tegner, C., Thy, P., Holness, M. B., Jakobsen, J. K. & Lesher, C.
E. (2009). Differentiation and compaction in the Skaergaard
intrusion. Journal of Petrology 50, 813–840.
Van der Merwe, M. J. (2007). The occurrence of the Critical
Zone along the exposed southeastern sector of the eastern
Bushveld Complex. South African Journal of Geology 110,
617–630.
Vermaak, C. F. (1976). The Merensky Reef—thoughts on its environment and genesis. Economic Geology 71, 1270–1298.
Viljoen, M. J. (1999). The nature and origin of the Merensky
Reef of the western Bushveld Complex based on geological
facies and geophysical data. South African Journal of
Science 95, 221–239.
Viljoen, M. J. & Hieber, R. (1986). The Rustenburg section of
Rustenburg Platinum Mines Limited, with reference to the
Merensky Reef. In: Anhaeusser, C. R. & Maske, S. (eds)
Mineral Deposits of Southern Africa. Geological Society of
South Africa, pp. 1117–1134.
Viljoen, M. J., de Klerk, W. J., Coetzer, P. M., Hatch, N. P.,
Kinloch, E. & Peyerl, W. (1986a). The Union section of
Rustenburg Platinum Mines Ltd with reference to the
Journal of Petrology, 2016, Vol. 56, No. 12
Merensky Reef. In: Anhaeusser, C. R. & Maske, S. (eds)
Mineral Deposits of Southern Africa. Geological Society of
South Africa, pp. 1061–1090.
Viljoen, M. J., Theron, J., Underwood, B., Walters, B. M.,
Weaver, J. & Peyerl, W. (1986b). The Amandelbult section of
Rustenburg Platinum Mines Limited, with reference to the
Merensky Reef. In: Anhaeusser, C. R. & Maske, S. (eds)
Mineral Deposits of Southern Africa. Geological Society of
South Africa, pp. 1041–1060.
Vukmanovic, Z., Barnes, S. J., Reddy, S., Godel, B. & Fiorentini,
M. L. (2013). Microstructure in chromite crystals of the
Merensky Reef (Bushveld Complex, South Africa).
Contributions to Mineralogy and Petrology 165, 1031–1050.
Wager, L. R. & Brown, G. M. (1968). Layered Igneous Rocks.
Oliver & Boyd, 588 pp.
Webb, S. J. (2009). The use of potential field and seismological
data to analyze the structure of the lithosphere beneath
southern Africa. PhD thesis, University of Witwatersrand,
Johannesburg, 337 pp.
Willmore, C. C., Boudreau, A. E. & Kruger, F. J (2000). The halogen geochemistry of the Bushveld Complex, Republic of
South Africa: Implications for chalcophile element distribution in the lower and critical zones. Journal of Petrology 4,
1517–1539.
Wilson, A. & Chunnett, G. (2006). Trace element and platinum
group element distributions and the genesis of the
Merensky Reef, Western Bushveld Complex, South Africa.
Journal of Petrology 47, 2369–2403.
Wilson, A. H., Lee, C. A. & Brown, R. T. (1999). Geochemistry of
the Merensky reef, Rustenburg Section, Bushveld Complex:
controls on the silicate framework and distribution of trace
elements. Mineralium Deposita 34, 657–672.
Wilson, J. R. & Larsen, S. B. (1985). Two-dimensional study of a
layered intrusion; the Hyllingen Series, Norway. Geological
Magazine 122, 97–124.
Woods, A. W. (1992). Melting and dissolving. Journal of Fluid
Mechanics 239, 429–448.
Young, I. M. (1984). Mixing of supernatant and interstitial fluids
in the Rhum layered intrusion. Mineralogical Magazine 48,
345–350.