JOURNAL OF PETROLOGY
VOLUME 42
NUMBER 10
PAGES 1845–1867
2001
Compositional and Lithological Controls
on the PGE-Bearing Sulphide Zones in
the Selukwe Subchamber, Great Dyke:
a Combined Equilibrium–Rayleigh
Fractionation Model
A. H. WILSON∗
SCHOOL OF GEOLOGICAL AND COMPUTER SCIENCES, UNIVERSITY OF NATAL, DURBAN, 4041, SOUTH AFRICA
RECEIVED NOVEMBER 30, 1999; REVISED TYPESCRIPT ACCEPTED MARCH 14, 2001
The platinum group element (PGE)-bearing Main and Lower
Sulphide Zones of the Selukwe Subchamber of the Great Dyke are
made up of a series of subzones within which the ratios Pd:Pt are
effectively constant, whereas these ratios vary significantly between
the subzones. Fractionation of Pd with respect to Pt varies by a
factor of 10 and cannot be modelled using a sulphide collector
phase and constant partition coefficients. The link with sulphide is
indisputable and the control is likely to have been the degree of
oversaturation of PGE micro-nuggets in the magma. The apparent
partition coefficients for Pt and Pd between silicate and sulphide
liquid are dependent on the degree of oversaturation and thereby
exhibit spurious correlation with the PGE content of the sulphide.
Modelling replicates the Pt and Pd distribution and ratios only by
dramatically changing the effective partition coefficients. Pyroxene
compositions (including TiO2) are shown to be strongly dependent
on the incompatible element content of the whole rock, and specific
linear arrays relating these variables can be related to the PGE
subzones. The overall control is Rayleigh fractionation, but constancy
of the ratio Pd:Pt and the initial pyroxene composition (before reequilibration with trapped liquid) within the subzones is indicative
of equilibrium crystallization. This layered structure may have been
derived from liquid layers in the magma chamber.
INTRODUCTION
compositions
An increasing number of mafic-layered intrusions are
reported to have distributions of platinum group elements
(PGE), Au and base metals characterized by vertically
offset profiles. Offset profiles involve the vertical separation of metal peaks and were first reported in the
Great Dyke (Prendergast & Keays, 1989; Wilson et al.,
1989; Naldrett & Wilson, 1990; Wilson & Tredoux,
1990). The phenomenon has since been recognized in
the Munni Munni intrusion of Western Australia (Barnes
et al., 1990), the Skaergaard intrusion (Bird et al., 1991)
and the Rincón del Tigre Complex of eastern Bolivia
(Prendergast, 2000). PGE are also postulated to be transported in fluids emanating from expulsion of intercumulus
liquid from the crystal pile, and there is strong evidence
to support such processes in several gabbroic sections
of layered intrusions such as the Skaergaard intrusion
(Andersen et al., 1998) and in the Stillwater Complex
(Boudreau & McCullum, 1986, 1992). In contrast, the
mineralized zones in the Great Dyke and Munni Munni
intrusion are located in pyroxenite, and primary magmatic processes have been invoked to explain the mineralization in these intrusions (Prendergast & Wilson,
1989; Barnes et al., 1990; Naldrett et al., 1990). A fundamental question is whether processes postulated for a
particular magmatic environment could be applied to
other dissimilar environments. A thorough understanding
of the mechanisms involved is hampered, in many cases,
∗Telephone: (31) 260 2803. Fax: (31) 260 2280.
E-mail: [email protected]
 Oxford University Press 2001
KEY WORDS:
platinum group elements; sulphide; Great Dyke; pyroxene
JOURNAL OF PETROLOGY
VOLUME 42
by a lack of detailed information on the silicate framework
in the mineralized zones. This paper attempts to link
compositional variations in silicates and whole rocks to
distinct patterns of PGE mineralization in the Main and
Lower Sulphide Zones in the Selukwe Subchamber of
the Great Dyke, and thereby to establish if the same
processes operated in these two mineralized zones of the
intrusion.
The nature of the offset profiles in the Great Dyke
was initially represented as contiguous variations of metal
values giving systematic variations through the sequence
(Prendergast, 1988; Prendergast & Keays, 1989; Wilson
& Tredoux, 1990). Proposed models were largely based
on fractional segregation of sulphide with strong partitioning of the PGE into sulphide (Naldrett & Wilson,
1990). Precise replication of the trends by numerical
modelling using established partition coefficients for PGE
between sulphide and silicate melts proved elusive
(Barnes, 1993), indicating that other mechanisms were
wholly or partly responsible for the observed profiles
(Peach & Mathez, 1996). Transport of PGE by fluid
migration, largely considered to result from compaction
of underlying cumulates, has been proposed in the form
of a chromatographic model (Boudreau & Meurer, 1999).
The association of sulphides with PGE enrichment has
been recognized in most mineralized environments in
layered intrusions, and therefore the role of late-stage
fluids is of major significance because of their potential
interaction with sulphide. Late-stage fluids would also
affect silicate minerals either by their infiltration and
expulsion, or by reaction when contained.
There are two main mineralized zones in the Great
Dyke—the Main Sulphide Zone and Lower Sulphide
Zone—and although an internal structure has been previously recognized (Prendergast & Keays, 1989; Wilson
& Tredoux, 1990) no systematic dependence has been
demonstrated relating these to the silicate host rocks. In
addition, these two mineralized layers were seen as separate entities with no common features relating to their
structure. This paper establishes common aspects between the two mineralized zones and shows a strong
relationship to the silicate framework.
GENERAL ASPECTS OF THE GREAT
DYKE
The Great Dyke is a linear intrusion of mafic and
ultramafic rocks that cuts across the Zimbabwe Craton
(Fig. 1). The Craton comprises dominantly Archaean
rocks and is bounded by the Zambezi metamorphic belt
in the north, the Mozambique belt to the east and the
Limpopo belt to the south. The Great Dyke, aligned
approximately NNE, is 550 km in length and 4–11 km
wide (Worst, 1960). Gabbro and quartz gabbro satellite
NUMBER 10
OCTOBER 2001
dykes are located parallel to the intrusion. These satellite
dykes are closely associated with a major fracture pattern
that is postulated to be the result of a craton-wide tectonic
control that gave rise to the form and alignment of the
major Great Dyke intrusion (Wilson, 1996).
Previous age determinations of the Great Dyke using
the Rb–Sr method yielded ages of the order of 2·4 Ga
(Allsopp, 1965; Davies et al., 1970) with the most precise
being 2455 ± 16 Ma (Hamilton, 1977). Recent studies
(Mukasa et al., 1998; Armstrong & Wilson, 2000) indicate
a significantly older age of emplacement of 2579 ±
7 Ma.
The Great Dyke is longitudinally subdivided into a
series of narrow contiguous magma chambers and subchambers (Fig. 1). Two main magma chambers (the
North and South Chambers) have been recognized, with
further subdivision into a series of subchambers on the
basis of structure, style of layering and continuity of
layers (Podmore & Wilson, 1987; Prendergast, 1987;
Wilson & Prendergast, 1989). The North Chamber is
subdivided into the Musengezi, Darwendale and Sebakwe
Subchambers. The South Chamber comprises the northern Selukwe Subchamber, the subject of the current
study, and the southern Wedza Subchamber. Similar to
the other subchambers, the Selukwe Subchamber has
extensive development of PGE resources in the Main
Sulphide Zone (MSZ) and Lower Sulphide Zone (LSZ).
THE SELUKWE SUBCHAMBER
Structure and location of the studied
section
The Selukwe Subchamber comprises an upper Mafic
Sequence (280 m thick) overlying the Ultramafic
Sequence, which has an exposed thickness of 1600 m. In
transverse section, the Subchamber is synclinal in shape
with essentially the same lithological sequence being
exposed on both sides of the longitudinal axis. Asymmetry
in the layering pattern close to the walls is attributed to
the physical shape of the chamber and the contrasting
nature of the wall rocks, which are greenstones on the
west flank and granite on the east side (Wilson et al.,
2000). The layering has a shallow plunge from north to
south in the northern segment and vice versa for the
southern segment, giving an overall boat-like shape to
the layering configuration.
Repetition of rock units of the Mafic Sequence on
the longitudinal axis arises from a number of major
transverse, steeply dipping faults that interrupt the overall
continuity of the layering. This has important implications
for the exposed strike length of the economic MSZ (Fig.
2), which is located entirely within pyroxenite beneath
the boundary of the Ultramafic–Mafic Sequences, and
1846
WILSON
PGE ZONES IN THE GREAT DYKE
Fig. 1. Location of the Great Dyke and its chambers and subchambers within the Zimbabwe Craton. The inset marks the location of drill
cores MR87 and MR92 in the Selukwe Subchamber. Adapted from Wilson et al. (2000).
consists of a zone of base metal sulphides, the lower part
of which is enriched in PGE.
The current study focuses on two drill core intersections
(MR87 and MR92) of the MSZ in an area east of the
axis of the Subchamber in close proximity to the Unki
prospect shaft (Fig. 1). Both drill cores intersected the
MSZ, but only MR92 intersected the LSZ (Fig. 2).
This is the first intersection of the LSZ in the Selukwe
Subchamber and so permits the most detailed investigation to date on the PGE distribution in the LSZ
of the Great Dyke.
Sequence of Cyclic Unit 1 (Fig. 2) is the P1 Pyroxenite
layer, comprising websterite in the upper 4–8 m, which in
turn overlies an orthopyroxenite unit 220 m in thickness.
Cyclic units below Cyclic Unit 1 are dominated by dunite
and granular harzburgite grading into olivine pyroxenite.
The pyroxenites have 1–15% interstitial plagioclase, together with other late-stage minerals, and range from
adcumulates to orthocumulates (Irvine, 1987), as compared with dominantly adcumulate pyroxenites in the
North Chamber. Except for a narrow zone that separates
the Websterite Layer from the gabbroic rocks of the Mafic
Sequence, pegmatoids are almost completely absent from
the succession.
Stratigraphy and rock types
The Mafic Sequence is exposed in the central region of
the Selukwe Subchamber and is underlain by a series of
cyclic units of the Ultramafic Sequence (Wilson et al.,
2000). The uppermost lithology of the Ultramafic
Textural relations in the P1 Pyroxenite and
their interpretation
The principal development of sulphide in the Selukwe
Subchamber is the MSZ located mainly within
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JOURNAL OF PETROLOGY
VOLUME 42
Fig. 2. Lithostratigraphic sections for the Selukwe Subchamber. The
complete succession is based on the distribution of rock types in the
axial region and is derived from both field and drill core data. Details
of the stratigraphic section of the P1 Pyroxenite are from drill core
MR92. The solid bars represent the vertical extents of the Main
Sulphide Zone (MSZ) and Lower Sulphide Zone (LSZ) and the localities
of two samples at 9·86 m and 89·84 m on which detailed petrographic
assessment was carried out. Adapted from Wilson et al. (2000).
orthopyroxenite, close to the websterite–orthopyroxenite
boundary (Fig. 2). The LSZ is located in the same
orthopyroxenite layer, some 35 m below the MSZ and
with no obvious petrological break between the two
sulphide-bearing zones. The pyroxenite consists of
medium-grained, prismatic to tabular cumulus orthopyroxene crystals enclosed within a matrix of postcumulus
minerals comprising plagioclase and clinopyroxene, together with pockets of late-stage minerals such as magnetite, phlogopite, K-feldspar, quartz, apatite, rare rutile
and zircon. The clinopyroxene and plagioclase occur
mainly as large oikocrysts, the former up to 20 mm in
length, and the plagioclase as zoned crystals up to 10 cm
in diameter. Preferential weathering on the margins of
the plagioclase oikocrysts gives rise to the characteristic
nodular texture commonly associated with exposures of
the MSZ in the field.
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OCTOBER 2001
Sulphide is interstitial to the cumulus pyroxene, and
its distribution is closely associated with the late-stage
minerals, giving rise to local development of a net texture
where it is most abundant. The sulphide also has a
tendency to concentrate around the margins of the plagioclase oikocrysts, further accentuating the weathering feature of the nodules (Wilson, 1992). These textures indicate
the close association of trapped silicate liquid with earlyformed sulphide liquid.
A detailed assessment of the textures of the pyroxenite
is important in understanding the distribution of trace
elements and is illustrated by a study on two samples
located within the MSZ and LSZ (Fig. 2). The textural
forms are largely controlled by the fabric of the orthopyroxene and crystal associations, summarized as follows:
(1) Grains with preserved crystal faces. These make up a high
proportion of the primocryst orthopyroxene assemblage,
and are most commonly (but not totally) in contact with
interstitial plagioclase. In some cases (in the sections
observed), all faces of individual crystals are preserved
and in other cases several faces are preserved and others
are obscured. In general, prism faces [110] show the
strongest retention of facial development, whereas domal
faces [101] have a greater tendency to be obscured by
mutual interaction. Identification of the status of crystal
boundaries as facial, or planar but non-facial, is readily
determined by universal stage measurements. In this
study approximately 400 such crystal boundaries were
measured and related to the textural associations discussed below.
(2) Grains of orthopyroxene with mutual planar boundaries.
Although few crystal faces are preserved it is not unusual
for two crystals to be joined along faces. Commonly six
to 20 grains are in close contact with little or no interstitial
feldspar. Generally, linear boundaries of the crystals meet
at triple points and there is a relatively constant grain
size within individual clusters.
(3) Grains of orthopyroxene enclosed within clinopyroxene oikocrysts. These show resorption textures with marked rounding and grain size reduction. Similar textures have been
reported for the Stillwater Complex ( Jackson, 1961), and
are the result of down-temperature stability relations
between ortho- and clinopyroxene.
Textural types (1)–(3) are consistent with variable degrees of densification occurring on a local scale by which
porosity has been reduced by a combination of processes,
including re-equilibration, overgrowth, enlargement of
crystals, enclosure by late-stage minerals and compaction
(Hunter, 1996). The preservation of crystal faces of
orthopyroxene grains bounded by plagioclase indicates
that cementation of cumulus orthopyroxene took place
at an early stage in domains within the cumulate pile.
In adjacent domains, annealing of grains caused mutual
interaction of orthopyroxene, with recrystallization overcoming face boundary locations. Triple junctions at 120°
1848
WILSON
PGE ZONES IN THE GREAT DYKE
indicate that textural equilibrium has been achieved in
these zones (Hunter, 1987, 1996). There are no instances
of such mutually interacting clusters of orthopyroxene
crystals being enclosed by a single grain of optically
continuous interstitial plagioclase. This is consistent with
the description of oikocrysts for this rock unit (Wilson,
1992).
The range of textural types and domains within individual samples is illustrated in Fig. 3a and b. Annealed
orthopyroxenes have few crystal faces, and are dominated
by mutually interacting crystal boundaries. Framework
orthopyroxene crystals are defined here as largely isolated
crystals commonly having several preserved crystal faces
and tending to be variable in grain size. They also usually
touch other crystals at point contact or along one or two
crystal faces. The stratigraphically lower sample (Fig.
3a), located within the LSZ, has a higher degree of
annealed zones (and less interstitial plagioclase) compared
with that from the stratigraphically higher position within
the MSZ (Fig. 3b) (insets to these figures show textural
type-zones). On a thin-section scale, both samples have
locally developed strongly annealed domains, which are
not specifically located along layering planes. Therefore,
although grain boundary readjustment has taken place
in the annealed domains, it is not uniformly developed.
The framework domains are considered to represent the
earliest stage by which the crystal network was frozen in
place by the oikocrystic and interstitial plagioclase, and
which would have played an important or even a critical
role in the entrapment of interstitial liquid. The P1
Pyroxenite exhibits a range in textures, which reflect
various stages in the sequence of events that led to
complete solidification of the cumulate. Original packing
of crystals in the immediate subliquidus stage could not
have been greater than 30–50% (Wager & Brown, 1968),
although flow or slumping of the crystal mush could have
produced a higher packing efficiency (Higgins, 1991).
Magmatic shearing (Benn & Allard, 1989) may have
caused rotation and interaction of grains, and flow partitioning could have reduced the melt fraction to <20%
(Nicolas, 1992).
Fig. 3. Tracings of textural forms of cumulus pyroxene in core MR92.
Crystals exhibiting several faces are most commonly in contact with
plagioclase and constitute a framework structure. This is in contrast to
the development of locally annealed and recrystallized zones. The
interstitial zones are mainly plagioclase, or oikocrysts of clinopyroxene,
which include reacted orthopyroxene. The small-scale map of each
sample emphasizes the zones constituting framework and annealed
crystals. (a) Sample located in the LSZ at 89·84 m below the gabbro
contact. (b) Sample located in the PGE Subzone (PGEsz) of the MSZ
at 9·86 m below the gabbro contact. Sample locations are shown in
Fig. 2. (Note in both samples the tendency of sulphide to concentrate
at the boundary of the annealed zones.)
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JOURNAL OF PETROLOGY
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Individual domains within the two examples have areas
where pyroxene crystals are annealed to attain textural
equilibrium and although planar boundaries are observed, few of these represent crystal faces. This is in
contrast to adjacent areas where groups of crystals possess
several faces unhindered by growth. Other sides of the
crystals are rounded by dissolution or by interaction with
other crystals. Sulphide tends to be mainly concentrated
on pyroxene–pyroxene boundaries and to a lesser extent
on pyroxene–plagioclase boundaries but is seldom enclosed entirely within plagioclase. Sulphide was confined
to melt-rich zones by the early crystallization of the
plagioclase cement, which rapidly formed zoned oikocrysts (Wilson, 1992). Local alteration of pyroxene is
developed in close association with small pockets of
late-stage minerals (quartz, K-feldspar, phlogopite and
primary amphibole), and this also tends to be most
prevalent in zones bounded by framework-textured
orthopyroxene crystals.
These textures indicate that parts of the rock formed a
rigid framework with cementation arising from the development of plagioclase oikocrysts. Hunter (1996) pointed
out that such poikilitic cementation would restrict growth
of cumulus grains (hence preserving their original crystal
form) and would also restrict compaction. At the same
time, mutually bounding pyroxene crystals may undergo
grain boundary readjustment and annealing. The proportions of cumulus orthopyroxene to oikocrystic and latestage phases indicates that densification must have occurred with expulsion of liquid at an early stage. Melt
(together with liquid sulphide if present) is therefore likely
to have been concentrated in localized zones where orthopyroxene formed a rigid framework and ultimately this
would have formed a closed environment. Sulphide tends
to concentrate at the boundary of the annealed and framework zones in both samples (Fig. 3a and b). These observations imply that consolidation of the crystal mush
took place before the cementation stage, dependent on the
temperature interval between the liquidus and solidification temperatures, defined as Tcem (Hunter, 1996).
In the Great Dyke P1 Pyroxenite, Tcem is indicated to be
relatively small and therefore solidification took place soon
after the compaction stage. The combined, and generally
contrasting effects of early removal of melt by compaction
and annealing of primocrysts, and its retention within the
crystal framework, would profoundly affect the major and
trace element geochemistry of the rock.
VERTICAL DISTRIBUTION OF PGE
AND BASE METALS IN THE MAIN
SULPHIDE ZONE
General characteristics of the MSZ
Vertical metal distributions in the MSZ are reputed to be
remarkably consistent throughout the Great Dyke,
NUMBER 10
OCTOBER 2001
although variations are observed in the magnitude and
vertical intervals over which they occur (Prendergast &
Wilson, 1989; Wilson et al., 1989). The variations are
known to exist on both regional and small scales (Wilson
& Tredoux, 1990; Wilson, 1996). The regional variation
is one of mainly systematic changes of peak values for the
metals and the widths of the mineralized zones in relation
to the position between the axis and margin of the Great
Dyke. No link has previously been established between
the metal distributions and silicate mineral compositions
except for an increase in iron content in the pyroxenes in
the region of the mineralized zone (Prendergast & Keays,
1989; Evans & Buchanan, 1991; Wilson, 1992).
The general pattern of metal and sulphide distribution
in the MSZ gives rise to well-established subdivisions
(Fig. 4). The MSZ encompasses an upper sulphide-rich
Base Metal Subzone (BMsz) and a lower Platinum Group
Element Subzone (PGEsz) generally 1·5–2·5 m thick. The
base of the PGEsz is coincident with the visual appearance
of sulphide in small amounts (0·3–0·5% by volume). The
unit stratigraphically overlying the PGEsz is called the
Hanging Wall section. The Footwall section is the interval
between the PGEsz and the top of the LSZ. The vertical
patterns for Pt and Pd do not show identical form; Pd
is concentrated at the base of the PGEsz and Pt towards
the top. Concentrations of Pt and Pd in the PGEsz range
from 500 to 5000 ppb.
Distribution of base metals, S, Pt and Pd
Distribution of base metals and PGE is represented by
two drill sections (MR87 and MR92), which were studied
in detail using continuous 15 cm samples (representative
analyses are given in Table 1 and the complete dataset
is available at Website http://www.duck.cs.und.ac.za/
geology/greatdyke). As Cu, Ni and S contents start to
rise, so the concentrations of PGE rise to appreciable
amounts (ppm levels) (shown for MR87 in Fig. 5). (Analyses for Pt and Pd were carried out by fire assay using
Ni sulphide collection and inductively coupled plasma
mass spectrometry using an Elan6000 instrument. Blanks
were pure quartz, and the international standard SARM7 returned values of 3640 ppb Pd and 1500 ppb Pt. Five
samples from the Footwall Zone at sub-ppb level were
analysed by radiochemical neutron activation techniques
at the Department of Earth Sciences, University of Melbourne.) In many sections, the BMsz commences one
sample position (15–20 cm) below the Pt peak position.
The peak positions of S, Cu (Fig. 5a) and Ni occur one
or two sample positions (15–30 cm) above the Pt peak,
where the sulphide content is 4·5–6%. The amount of
sulphide decreases upwards in the section by way of
several minor peaks, but at overall elevated levels compared with that in the PGEsz.
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WILSON
PGE ZONES IN THE GREAT DYKE
Fig. 4. Generalized distribution of Cu, Pt and Pd in the MSZ of the
Selukwe Subchamber based on the average of 85 drill core intersections.
The zonal subdivision is discussed in the text. Of particular importance
is the location of the Hanging Wall section to the PGEsz and the
Footwall sequence. The Base Metal Subzone (BMsz) and the PGEsz
together constitute the MSZ. A generalized pattern of variation of Pd/
(Pd + Pt) is also illustrated.
The PGEsz is 1·6–2·2 m in width in this region where
Pt and Pd (as well as the other PGE and Au) rise to
markedly high values. The patterns of Pt and Pd (Fig.
5b) are different, with Pt increasing in concentration
upwards in the profile to its peak position near the
top of the PGEsz whereas Pd tends to decrease in
concentration upwards in the succession through a series
of minor peaks. The position of the highest Pd peak is
60–90 cm below the highest Pt peak, with a clear crossover point of the metal profiles. The general form of the
peaks is a gradual rise on the lower limbs with a sharp
fall-off on the upper limbs to give a pattern of asymmetric
peaks. The displacement of the Pd peaks to slightly below
those of Pt, as well as the PGE to the base metal
peaks in general gives the so-called offset profiles that
characterize the MSZ.
Ratios of Pd to Pt and PGE to base metals
in the PGE Subzone
The ratio Pd/(Pd + Pt) emphasizes the contrasting
profile patterns of Pt and Pd in the PGEsz and the
Footwall section (Fig. 5c). Values for this ratio are relatively low (0·5) in the Footwall to the PGEsz, below the
Pd peak. Upwards in the section in the PGEsz, values
for the ratio rise to a maximum close to the Pd peak and
then decrease, with lowest values coinciding with the Pt
peak. Ratios of Pt/Cu and Pd/Cu (Fig. 5c) have relatively
constant values below the Pd peak position, rise sharply
immediately above the Pd peak and then decrease again
at a position below the Pt peak but to a much lower
level than in the Footwall. A clear crossover point exists
for the Pd/Cu and Pt/Cu curves.
The variation of the ratio Pd/(Pd + Pt) indicates
an apparently progressive change through the section.
However, the combined data for MR87 and MR92 show
clear groups for this ratio in the PGEsz (Fig. 6a). The
pattern is the same in both intersections, with the stratigraphically lowest zone in each group (CPSZ zone) having
the highest values for Pd/(Pd + Pt) and the stratigraphically highest zone (APSZ zone) the lowest values.
The intermediate BPSZ zone is a narrow zone >35 cm
wide, which corresponds to the crossover region of the
Pd and Pt profiles and is therefore intermediate to the
wider APSZ and CPSZ zones.
Therefore, superimposed on the apparently continuous
trends of Pd and Pt variation in the vertical profiles,
there exists a certain consistency of the ratios within these
zones. Data from 16 drill hole intersections (Fig. 6b) in
the vicinity of the Unki shaft exhibit a high degree of
correlation for the APSZ zone (Pt-rich) and CPSZ zone (Pdrich). Using the larger dataset, the narrow intermediate
BPSZ zone is shown to comprise two distinct subsets, which
relate to one sample position above and generally two
sample positions below the crossover point for the Pt and
Pd profiles, and is effectively a transitional zone between
the dominant zones. The percentage population for the
various ratio groups is shown in Fig. 6b.
The results of this analysis show that: (1) irrespective
of the variable absolute values for Pt and Pd in different
drill core sections, the Pd/Pt ratio is remarkably constant
over certain intervals; (2) for any one section, Pd/(Pd +
Pt) values lie between well-defined limits for each zone,
with boundary samples ‘linking’ the dominant zones to
give the apparent continuous variation in metal values
observed in the vertical profiles.
Footwall and Hanging Wall sections to the
PGE Subzone
The 30–35 m thick section of poorly mineralized pyroxenite that lies between the PGEsz and the LSZ is called
the Footwall to the PGEsz (Fig. 4). It is a homogeneous
orthopyroxenite containing very finely disseminated
sulphide of essentially constant composition (100 ppm
Cu and 0·3% sulphide). Apart from the basal 10 m where
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JOURNAL OF PETROLOGY
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Table 1: Representative compositions of orthopyroxene and whole rocks of the Main and Lower Sulphide
Zones in drill holes MR87 and MR92
Drill hole:
MR87
MR87
MR87
MR87
MR87
MR87
MR87
MR87
MR92
MR92
MR92
Height (m):
6·77
8·10
8·71
9·61
9·96
10·33
10·51
11·93
13·07
36·65
60·79
63·17
Zone:
HW
HW
PGEsz
PGEsz
PGEsz
PGEsz
PGEsz
FW
FW
FW
LSZ-U
LSZ-U
A
A
B
C
C
A
A
B
B
B
Subzone:
MR92
Orthopyroxene
% Oxides
SiO2
54·60
54·76
54·69
54·94
54·94
54·75
54·76
54·63
54·46
54·84
54·69
54·74
Al2O3
1·31
1·37
1·41
1·43
1·40
1·36
1·39
1·41
1·32
1·30
1·37
1·41
Fe2O3
1·46
1·40
1·35
1·32
1·34
1·36
1·36
1·41
1·41
1·33
1·31
1·30
11·81
11·33
10·91
10·67
10·81
11·04
11·04
11·44
11·44
10·80
10·59
10·49
FeO
MnO
0·26
0·25
0·25
0·24
0·24
0·25
0·25
0·25
0·25
0·25
0·24
0·24
MgO
27·86
28·22
28·45
28·62
28·61
28·65
28·50
28·30
28·08
28·84
28·76
28·97
CaO
2·04
1·78
1·84
1·84
1·75
1·85
1·80
1·78
1·80
1·71
1·86
1·88
Na2O
0·08
0·10
0·08
0·08
0·07
0·06
0·07
0·07
0·03
0·02
0·08
0·07
TiO2
0·175
0·168
0·144
0·138
0·143
0·161
0·158
0·190
0·184
0·163
0·162
0·148
Cr2O3
0·395
0·439
0·435
0·442
0·437
0·425
0·420
0·427
0·427
0·427
0·450
0·456
NiO
0·076
0·076
0·080
0·077
0·077
0·081
0·079
0·061
0·060
0·062
0·074
Total
mg-no.
100·07
0·8130
99·89
0·8161
99·64
0·8229
99·80
0·8270
99·82
99·99
0·8251
0·8222
99·83
0·8218
99·97
0·8151
99·46
0·8129
99·74
0·8220
99·59
0·8288
0·077
99·78
0·8320
Whole rock
S (%)
1·56
1·10
1·48
0·46
0·48
1·35
0·72
0·06
0·07
0·09
0·15
0·22
Cu (ppm)
1943
1825
2224
723
699
681
1337
89
66
81
229
361
Ni (ppm)
2288
1863
2674
905
1195
1075
1406
495
497
473
710
861
Zr (ppm)
14·3
P (ppm)
79
163
81
57
71
85
80
68
250
134
162
92
Pt (ppb)
<4
<4
46
2851
2450
1450
1975
88
72
14
16
66
Pd (ppb)
<4
<4
26
1065
1850
1950
2875
94
74
26
Pd/(Pd + Pt)
17·0
10·1
0·36
8·5
0·27
7·6
8·6
0·43
0·57
9·4
0·59
7·9
0·51
20·7
0·51
16·4
0·65
16·1
10·4
2
8
0·11
0·11
Pt and Pd fall close to or below detection limits (<4 ppb)
these elements are present in the Footwall to the PGEsz
in the concentration range 5–150 ppb.
Two zones of essentially constant Pd/(Pd + Pt) are
identified in the Footwall to the PGEsz (Fig. 7). Higher
values for the ratio Pd/(Pd + Pt) occur in the stratigraphically lowest zone (BFWZ) and lower values in the
overlying AFWZ zone. The Hanging Wall section to the
PGEsz (Fig. 7) also shows a remarkably constant ratio
of Pd/(Pd + Pt), with a value slightly higher than that
of the underlying APSZ zone.
content does not exceed 0·9 wt % and on average is
0·3 wt %. The PGE are associated with sulphide but the
distribution is highly variable and falls to detection limits
at the base of the Footwall to the PGEsz. The PGE
distribution is cyclical on a scale of 10–15 m, giving a
series of rhythmic units. As will be seen in the following
sections, the LSZ can be broadly subdivided into a Lower
Section (83–142 m below the mafic contact) and Upper
Section (47–83 m) on the basis of a sharp, geochemically
defined break. In this sense there is a direct parallel to
the overlying and more strongly mineralized PGEsz and
the Footwall to the PGEsz.
THE LOWER SULPHIDE ZONE
PGE and base metal distributions
The LSZ (Fig. 8) is a sequence of continuous mineralization which in this area is >85 m thick. The sulphide
A series of well-defined units is superimposed on
general trends of metal variations in the LSZ. Sharp
1852
WILSON
PGE ZONES IN THE GREAT DYKE
Drill hole:
MR92
MR92
MR92
MR92
MR92
MR92
MR92
MR92
MR92
MR92
MR92
Height (m):
65·57
70·83
75·24
77·75
78·95
89·84
91·83
95·12
98·15
102·63
116·73
MR92
130·14
Zone:
LSZ-U
LSZ-U
LSZ-U
LSZ-U
LSZ-U
LSZ-L
LSZ-L
LSZ-L
LSZ-L
LSZ-L
LSZ-L
LSZ-L
Subzone:
B
B
C
C
D
A
A
C
C
C
D
F
Orthopyroxene
% Oxides
SiO2
54·95
54·89
54·92
54·94
54·77
54·90
54·89
54·98
54·90
55·01
55·18
55·31
Al2O3
1·41
1·42
1·39
1·36
1·39
1·33
1·39
1·41
1·34
1·42
1·32
1·32
Fe2O3
1·27
1·27
1·29
1·29
1·29
1·26
1·25
1·24
1·22
1·21
1·21
1·18
FeO
10·28
10·29
10·42
10·47
10·41
10·23
10·15
10·01
9·91
9·83
9·82
9·59
MnO
0·24
0·24
0·24
0·24
0·24
0·24
0·23
0·23
0·23
0·23
0·22
0·22
MgO
29·12
29·01
28·94
28·83
28·93
29·23
29·15
29·15
29·13
29·36
29·58
29·77
CaO
1·88
1·86
1·87
1·87
1·88
1·86
1·88
1·88
1·87
1·89
1·87
1·87
Na2O
0·07
0·07
0·08
0·06
0·05
0·05
0·02
0·08
0·06
0·05
0·07
0·08
TiO2
0·150
0·144
0·146
0·152
0·145
0·149
0·142
0·142
0·136
0·13
0·139
0·131
Cr2O3
0·464
0·467
0·457
0·456
0·457
0·468
0·479
0·484
0·498
0·482
0·434
0·455
NiO
0·078
0·076
0·078
0·076
0·077
0·076
0·086
0·083
0·079
0·084
0·080
Total
mg-no.
99·91
0·8320
99·74
0·8340
99·83
0·8319
99·74
0·8307
99·64
99·79
0·8320
0·8352
99·67
0·8365
99·69
0·8384
99·37
0·8397
99·70
0·8418
99·92
0·8430
0·086
100·01
0·8469
Whole rock
S (%)
0·28
0·2
0·14
0·13
0·04
0·18
0·16
0·03
0·02
0·02
0·02
0·01
Cu (ppm)
413
281
267
293
80
368
232
30
10
13
11
12
Ni (ppm)
911
758
729
737
664
805
901
679
622
663
617
632
Zr (ppm)
10·2
8·1
9·4
17·8
8·5
11·0
7·8
8·4
83
170
90
120
72
Pt (ppb)
150
180
260
205
106
370
195
375
130
94
330
38
Pd (ppb)
14
28
80
56
42
18
14
170
108
70
360
150
0·24
0·21
0·28
0·05
0·06
0·31
0·45
0·43
58
7·9
71
0·13
55
7·0
104
0·08
100
6·4
P (ppm)
Pd/(Pd + Pt)
71
8·4
0·52
70
0·80
HW, Hanging wall to PGEsz; PGEsz, PGE Subzone; FW, Footwall to PGEsz; LSZ-U, Lower Sulphide Zone—Upper Section;
LSZ-L, Lower Sulphide Zone—Lower Section.
discontinuities are observed within the Lower Section.
Although the patterns are broadly similar for Pt and Pd
(Fig. 8a and b), in detail they are different. Pt has a
tendency to increase upwards in the Lower Section
whereas Pd decreases by way of well-defined steps. In
the Upper Section, the fall-off of Pd is at a greater rate
than for Pt. These characteristics are similar to those
observed in the MSZ, but on a much larger scale, in
that a series of peaks is present with gradually rising
lower limbs and sharp fall-off on the upper limbs giving
an asymmetric saw-tooth pattern. The Upper Section to
the LSZ differs from the Lower Section by a smoother
variation, with values for Pt and Pd falling to detection
limits at the base of the Footwall of the PGEsz.
Variation of Cu (Fig. 8c) shows very low values (<10
ppm) in the Lower Section of the LSZ, rising sharply at
the top of the zone before decreasing at the base of the
Upper Section, where it remains at levels of 200–350
ppm through the zone. The concentration of Cu falls to
low values in the Footwall to the PGEsz but remains
essentially constant over 30 m in this section, rising
sharply at the base of the PGEsz.
Variations in metal ratios
Ratios of Pt to Pd through the LSZ (Fig. 9) suggest
apparently variable values, but in reality most of the
samples in discrete vertical intervals exhibit ratios for
these elements within clear limits. Samples located at the
boundaries to these intervals have intermediate values.
In both the Upper and Lower Sections of the LSZ the
stratigraphically lowest zones have the highest Pd:Pt
values, which then decrease systematically upwards in
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JOURNAL OF PETROLOGY
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OCTOBER 2001
Fig. 6. Interdependence of Pt and Pd in the PGEsz. Relative positions
of the zonal subdivision are indicated in Fig. 4. (a) Combined data for
the PGEsz from drill cores MR87 and MR92. (b) Combined data for
the PGEsz from 16 drill core intersections in the vicinity of the Unki
shaft area. A total of 260 analyses make up this dataset. Average values
for the ratio Pd/(Pd + Pt) within each of the zones, together with
standard deviations, are indicated in the inset table, the last column
being the population percentage for each of the groups.
Fig. 5. Distribution of Cu, S, Pt and Pd in the MSZ for drill core
section MR87. (a) Variation of Cu and S. (b) Variation of Pt and Pd.
(c) Variation for the ratios Pd/Cu, Pt/Cu and Pd/(Pd + Pt). Dashed
lines mark the positions of the peak maxima for Cu, Pt and Pd.
each section. The relative values of the ratios are extreme,
with Pd/(Pd + Pt) values ranging from 0·06 to 0·25 in
the Upper Section and from 0·07 to 0·77 in the Lower
Section but the same pattern of variation occurring in
both sections. This pattern shows a striking similarity
with the PGEsz and Footwall to the PGEsz. The sequence
of steps is particularly well defined in the Lower Section.
Therefore, although the overall stratigraphic distribution
of the elements indicates a continuous variation (Fig. 8),
in detail the dominant characteristic is one of ratios lying
within clearly defined limits on a step-wise basis. Four
zones are recognized in the Upper Section (labelled
ALZU–DLZU in Fig. 10a and b) and six zones in the Lower
Section (labelled ALZL–FLZL in Fig. 10a and b). The BLZL
zone should possibly be regarded as the intermediate
zone between ALZL and CLZL. The pattern also shows a
strong dependence on the variation of sulphide, with a
pronounced break between the Upper and Lower Sections of the LSZ (Fig. 10a).
Fig. 7. Interdependence of Pt and Pd in the Footwall and Hanging
Wall to the PGEsz. Combined data from drill cores MR87 and MR92.
Average values and standard deviations for Pd/(Pd + Pt) are indicated
for each of the zones.
The pattern of relative variation of Pd and Pt through
the LSZ is therefore similar to that of the MSZ in
that Pd:Pt is highest in the lowest units and decreases
systematically by way of a series of steps upwards in the
1854
WILSON
PGE ZONES IN THE GREAT DYKE
Fig. 9. Delineation of zones each having ratios for Pd:Pt lying within
clearly defined limits in (a) the Upper Section and (b) the Lower Section
of the LSZ in MR92. Each of the zones is labelled from top to bottom
as A–D for the Upper Section, and A–F for the Lower Section. The
triangular tag symbols identify those samples at the boundaries of the
zones and that have intermediate compositions. The BLZL zone in the
Lower Section may be regarded as the boundary zone between ALZL
and CLZL. The lines mark approximate limits for the arrays, and average
values and standard deviations for each of the groups are indicated.
Fig. 8. Profiles of metal variation in drill core MR92 for (a) Pd, (b) Pt
and (c) Cu. From top to bottom in the section, the zones delineated
are the Hanging Wall (HW) and Base Metal Subzone (BMsz) to the
PGE Subzone (PGEsz), the Footwall to the PGEsz, the Upper Section
of the Lower Sulphide Zone (LSZ), and the Lower Section to the LSZ.
sequence. The apparently continuous variation over the
profile is the result of intermediate samples linking groups
of samples with similar Pd:Pt ratios. This may indicate
that (1) some migration of metals has taken place across
the boundaries of the well-defined zones and must have
occurred after the general pattern was well established,
or (2) the primary processes that produced the change
may not have been instantaneous, but gradual, and the
intermediate samples reflect the rate of change.
the Upper Section. In the Upper Section S rises gradually
and then remains effectively constant. The ratio of Pd
to Cu (expressed as 1000 × Pd/Cu) in Fig. 10b highlights
significant differences in both overall distribution pattern
and trends for the Upper and Lower Section of the
LSZ. A major discontinuity is also observed between the
Footwall to the PGEsz and the Upper Section of the
LSZ.
The concentration of Cu is strongly dependent on S
(Fig. 11a) over the range 10–7000 ppm Cu. Ni variation
with S (Fig. 11b) highlights the dominant control of Ni
by sulphide where it is abundant, but orthopyroxene
dominates the distribution where sulphide is in low
abundance, containing >650 ppm Ni. Discontinuities in
the Ni sulphide distribution are observed particularly in
the Footwall to the PGEsz and the Upper Section of the
LSZ.
Interdependence of S and base metals
The variation of S in the LSZ (Fig. 10a) indicates distinct
patterns with very low values (<0·02% S) in the Lower
Section rising to 0·2% immediately below the base of
Significance of the PGE variations
Previous studies of the Great Dyke have interpreted the
PGE trends as resulting from selective partitioning of the
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OCTOBER 2001
Fig. 10. Variations of metal ratios in drill core MR92. (a) Average values for Pd/(Pd + Pt) decrease upwards in the succession with a stepwise
structure as portrayed in Fig. 9. S content is also represented. (b) Sample-by-sample variation of Pd/(Pd + Pt) together with the ratio of Pd to
Cu. Boundary zones to the steps are marked by the stippled bars. The progressive strong depletion of Pd relative to Cu further emphasizes the
metal fractionation but with a build-up in the Footwall to the PGEsz and at the boundary between the Upper and Lower Sections of the LSZ.
PGE into primary sulphides by a continuous process.
The offset trends reflect the effective efficiency in uptake
of the PGE by sulphide (Prendergast & Keays, 1989;
Naldrett & Wilson, 1990; Wilson & Tredoux, 1990;
Barnes, 1993). However, the apparent systematic and
smooth variations mask a more pronounced relationship
by which zones are characterized by ratios that lie
within clearly defined limits. These zones form a stepped
distribution separated by transitional zones. To maintain
these ratios, a continuous sulphide fractionation model
cannot be applicable to either the strongly mineralized
or the poorly mineralized sections.
PYROXENE COMPOSITIONS
Compositional variations in the vertical
profile of the P1 Pyroxenite
Previous models of PGE mineralization in the Great
Dyke have not considered the silicate framework in
detail. Such information is important in assessing the
interdependence of the mineralization and primary controls of the silicate compositions. Compositions of orthopyroxene have been determined on the basis of close
sampling of drill core in the same sections as analysed
for PGE.
Precise compositional determinations were carried out
on orthopyroxene using mineral separates and analysed
by X-ray fluorescence spectrometry (XRFS). Electron
microprobe studies on the same samples indicate that,
within error of the determinations, no zoning was detected
in the pyroxenes. Advantages of using XRFS are higher
analytical precision for large numbers of samples, particularly for the minor components TiO2, Cr2O3 and
NiO, and that it overcomes small-scale heterogeneities
arising from exsolution. Separates were determined on
carefully sized fractions using the magnetic barrier separator, heavy liquids and final hand picking. Each separation was carried out in duplicate and then each
analysed in duplicate. Analytical errors are indicated in
the diagrams on the basis of standard deviations of
replicate analyses.
1856
WILSON
PGE ZONES IN THE GREAT DYKE
Fig. 11. S and base metal variation in MR92. (a) Variation of Cu and
S. (b) Variation of Ni and S.
Main Sulphide Zone and Footwall to the
PGEsz
The variation of mg-number [as Mg/(Mg + Fe2+)] in
orthopyroxene in section MR87 (Fig. 12a), commencing
with the Footwall to the PGEsz, is a series of troughs
and peaks. There is a tendency for the troughs in mgnumber to be located in the mid-points of these zones.
The variation of Cr2O3 in orthopyroxene (Fig. 12b) is
similar to that of mg-number, except that Cr2O3 attains
a maximum in the Hanging Wall to the PGEsz or in the
upper part of the PGEsz in both drill core sections.
The cyclicity is even more pronounced for TiO2 in
orthopyroxene in MR87 (Fig. 12c), with sharp changes
observed at the boundaries of the zones APSZ, BPSZ and
CPSZ. The variation for TiO2 is strongly antipathetic to
that of mg-number.
The distribution of incompatible elements is not dependent on the abundance of the liquidus silicate and
sulphide phases and therefore has the potential to monitor
changes in the amount or composition of the liquid. The
distribution of Zr (Fig. 12d) in the whole rock indicates
variations that are essentially identical to that of TiO2 in
orthopyroxene. This observation suggests a strong link
between minor element compositions in orthopyroxene
and a fundamental parameter of the whole rock that
would have been imposed by the primary crystallization
process, namely the incompatible element content.
Fig. 12. Variation of pyroxene composition and Zr in whole rock
in the MSZ for drill core MR87. (a) Variation of mg-number for
orthopyroxene. (b) Cr2O3 in orthopyroxene. (c) TiO2 in orthopyroxene.
(d) Zr in the whole rock. Vertical bars mark the peak positions for Cu,
Pt and Pd. Zones APSZ, BPSZ, CPSZ and AFWZ are delineated according
to the PGE zonal structure as in Figs 6 and 7.
Lower Sulphide Zone
Variation in mg-number in orthopyroxene in the LSZ
(Fig. 13a) illustrates an overall systematic decrease with
a very well-defined small-scale structure of humps and
troughs on a scale of >10–15 m. This variation highlights
the same zones as defined on the basis of Pd/(Pd + Pt)
(ALSU, BLSU, CLSU, etc. for the Upper Section and ALSL,
BLSL, CLSL, etc. for the Lower Section). Cr2O3 in orthopyroxene (Fig. 13b) exhibits small-scale fluctuations
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superimposed on major oscillations, which correspond
to the observed significant changes in the distribution of
PGE in the Lower Section (see Fig. 8). In contrast, Cr2O3
in the Upper Section follows mg-number closely in both
the overall and smaller-scale variations. Small amounts
of interstitial magnetite are unlikely to have influenced
the distribution of Cr2O3 and chromite is not present in
this section. The TiO2 content of orthopyroxene (Fig.
13c) increases systematically through the entire LSZ but
the small-scale oscillations are particularly well developed.
In many cases, the boundaries of the zones, defined on
the basis of Pd/(Pd + Pt) (see Fig. 10), correspond closely
to sharp changes in TiO2 concentrations. Also in a similar
pattern to that observed for the MSZ, the boundaries of
these delineated zones in many cases tend to correspond
to low TiO2 concentrations in orthopyroxene, with gradually rising and falling peaks forming well-defined rhythmic units. The boundaries are less well defined in the
Lower Section, but the systematic variations are still
present. A break in the overall trend occurs in the
Footwall to the PGEsz.
Zr in the whole rock (Fig. 13d) has an almost identical
small-scale variation to that of TiO2 in orthopyroxene in
the Upper and Lower Sections of the LSZ. The overall
increase in Zr content upwards through the LSZ is
rather more subdued compared with that of TiO2 in
the orthopyroxene. Again, these two elements provide
evidence of a substantial link between a compositional
parameter for the cumulus orthopyroxene and an incompatible element that would mainly reflect the trapped
liquid in the cumulate.
SILICATE INTER-ELEMENT
VARIATIONS
P and Zr in the whole rock
P and Zr are incompatible elements in mafic cumulates
and therefore they should display similar variation in
chemical profiles for the whole rock. Such corresponding
variation is demonstrated in Fig. 14, with three distinct
groupings of data conforming to: (1) the LSZ and Footwall
to the PGEsz; (2) the PGE Subzone itself; (3) the Hanging
Wall to the PGEsz, which includes the Websterite Layer.
Zr is not completely incompatible in orthopyroxene, and
a small amount of Zr will have been partitioned into the
mineral as is indicated for the Zr intercept of 3–5 ppm
(Fig. 14) (confirmed by ion probe studies in progress).
The slightly greater intercept for the pyroxenes higher
in the succession reflects both the evolving composition
of the liquid and the increasing amount of oikocrystic
clinopyroxene. The displaced trends correspond to the
more abundant development (up to 5% of the whole
rock) of clinopyroxene oikocrysts, particularly in the
region of the PGEsz, and to the development of up to
Fig. 13. Variation of pyroxene composition and Zr in the whole rock
in the MSZ and LSZ in drill core MR92. (a) Variation of mgnumber for orthopyroxene. (b) Cr2O3 in orthopyroxene. (c) TiO2 in
orthopyroxene. (d) Zr in the whole rock. Zones (ALZU–DLZU and
ALZL–FLZL ) are delineated according to the PGE zonal structure as in
Figs 9 and 10. Symbols are the same as in Fig. 10.
50% cumulus clinopyroxene in the Websterite Layer.
Clinopyroxene has a higher partition coefficient for Zr,
which therefore results in the progressive displacement
of the trends. This relationship also indicates that the
clinopyroxene did not form from the interstitial liquid
but as an early primary crystallization phase.
Interdependence of orthopyroxene
compositional components and whole-rock
incompatible elements
The comprehensive and close sampling of this study
shows the existence of a strong inverse relationship for
mg-number of orthopyroxene with Zr content of the
1858
WILSON
PGE ZONES IN THE GREAT DYKE
Fig. 14. Variation of Zr and P in whole rocks for all samples studied
in drill core sections MR87 and MR92. Different groupings, each with
strong linear associations, have different intercepts on the Zr axis,
indicating the incorporation of this element into pyroxene. Three
parallel trends are present and related to lithologies and positions within
the stratigraphic sequence.
whole rock (Fig. 15) for all sections of the P1 Pyroxenite.
The section that includes the Websterite Layer, the
Hanging Wall to the PGEsz, PGEsz itself and Footwall
to the PGEsz (Fig. 15a) has a range of mg-number of
0·81–0·83 in a series of well-defined arrays, or groupings
of data points, which correspond exactly to the zonal
distribution of PGE (see Figs 6 and 7). The same relationship is observed for the Upper Section of the LSZ
(Fig. 15b and c) (with a range of 0·828–0·848). The
arrays for zones BLSU and DLSU overlap and lie within
one field indicating a compositional reversal within the
sequence. The Lower Section of the LSZ (Fig. 15c) shows
very well-defined separation of zones in the Zr vs mgnumber diagram. These zones exactly correspond to the
zonal distribution of PGE in the LSZ (see Figs 9 and
10). P could have been used in place of Zr and gives the
same result. Points that lie on the boundaries between
the various arrays, and are therefore intermediate to the
arrays, are also identified. In a similar fashion to the
PGE variation, zone BLSL can be regarded as intermediate
to ALSL and CLSL.
There is a general shift of mg-number of orthopyroxene
to progressively lower values for each of the arrays or
groupings of data points upwards through the succession.
On the basis of their average slope, these arrays become
steeper progressing upwards in the succession (Fig. 16a).
This may be explained by a slight overall enrichment of
Zr in the magma, which then is also reflected in the
higher Zr content of the trapped liquid. The compositionally defined small-scale rhythmic units are characteristic features of the P1 Pyroxenite and indicate a
primary control on the crystallization process. Variation
of TiO2 in orthopyroxene, plotted against mg-number
(Fig. 16b), also exhibits well-defined linear arrays with
an indication that the arrays become steeper higher in
Fig. 15. Variation of mg-number in orthopyroxene with Zr content of
the whole rock for the stratigraphic intervals in MR92. (a) The Footwall
section to the PGEsz and zones within the MSZ. (b) Zones within the
Upper Section of the LSZ. (c) Zones within the Lower Section of the
LSZ. The points tagged with the triangular symbol represent those
that lie at the boundary of the zones and show intermediate compositions
between adjacent arrays. BLSL is regarded as an intermediate zone
between ALSL and CLSL. The lines indicate the approximate limits of
each of the arrays.
the succession. These arrays highlight the separation of
the Upper and Lower Section of the LSZ and the
separation of the LSZ, as a whole, from the overlying
rock units.
Concomitantly, Zr in whole rock exhibits strong interdependence with TiO2 in orthopyroxene (Fig. 17a). All
data for the Hanging Wall to the PGEsz, the PGEsz and
the LSZ sections fall on a well-defined trend. The Footwall
section to the PGEsz displays a well-constrained linear
trend but is displaced from all other data. This is consistent with this zone having an apparent break in the
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Fig. 16. Variations of mg-number in orthopyroxene as related to overall
concentrations of Zr in the whole rock and TiO2 in orthopyroxene. (a)
Block diagrams indicate the fields of Zr in the whole rock for the
Footwall to the PGEsz and the MSZ, the Upper and Lower Sections
of the LSZ. The average slopes of the arrays in each of these sections
are indicated by the straight lines. The progressive increase of Zr
upwards in the section is apparent. (b) Variation of TiO2 and mgnumber in orthopyroxene for the various stratigraphic groups. Symbols
are the same as in Fig. 10. The linear arrays of data points pertaining
to specific stratigraphic intervals are apparent.
overall evolutionary pattern for the P1 Pyroxenite. This
relationship suggests that compositional equilibrium was
attained between the cumulus pyroxene and the interstitial liquid.
Breaks in the trends of mg-number with TiO2 and
Cr2O3 contents in orthopyroxene (Figs 16b and 17b)
occur at stratigraphic levels that immediately precede
zones where PGE undergoes a sharp increase. A dramatic
break occurs within the lower section of the LSZ. Zones
of enrichment of Ni in orthopyroxene (Fig. 17c) follow
those of Cr2O3. Orthopyroxene in the PGEsz is markedly
enriched in Ni and this may partly result from reequilibration between the cumulus orthopyroxene and
Ni-rich sulphide liquid in the base metal portion of the
MSZ (Barnes & Naldrett, 1985; Li & Naldrett, 1999).
DISCUSSION
The precise determinations of orthopyroxene compositions over narrow sampling intervals reveal variations
on a scale of 2–10 m, which cannot, in detail, be explained
by simple fractionation or by repeated influx of magma,
although the overall progressive change in composition
Fig. 17. Variations of compositional components in orthopyroxene.
Specific stratigraphic intervals are delineated by the shaded boxes. (a)
The relationship of Zr in whole rock to TiO2 content in orthopyroxene.
There is strong linearity for all samples but the group relating to the
Footwall to the PGEsz lies on a separate trend. (b) Variation of Cr2O3
and mg-number for orthopyroxene. Sharp breaks correspond to sudden
decreases in Pd and Pt concentrations in the LSZ, and Footwall to the
PGEsz. Symbols as for Fig. 10. (c) Variation of NiO and mg-number
in orthopyroxene have breaks that correspond to those of Cr2O3 and
therefore also to changes in PGE concentration in the LSZ, and the
Footwall to the PGEsz. Symbols as for Fig. 10.
through the section is most easily attributed to bulk
fractionation. Injection of small amounts of primary
liquid, combined with continued crystallization, is likely
to have occurred at several points within the section.
However, the small-scale patterns of variation, some of
which have a symmetrical disposition within the layers,
are not consistent with magma influx. The role of trapped,
and possibly migrating liquids could also have been
important in both the final textures observed and the
mineral and whole-rock compositions. In addition, there
1860
WILSON
PGE ZONES IN THE GREAT DYKE
exists a striking correlation between PGE characteristics
within specific rock units and compositional variation of
orthopyroxene, which has not previously been demonstrated in the Great Dyke or in other layered intrusions.
Characteristics of the more highly mineralized MSZ
are remarkably similar to those of the LSZ, indicating
that similar processes took place. In addition, the processes that controlled the PGE enrichment are indicated
to have been strongly linked to those that controlled
the compositions of the silicate minerals. The observed
cyclicity in the pattern of mineralization is inconsistent
with previous models of progressive fractional segregation
of sulphide. It is also not consistent with models that rely
on fluid transport of PGE as the dominant control on
mineralization. The interdependence of several geochemical factors requires examination of accepted processes for magma chambers, particularly where these
relate to the concentration of PGE.
Liquidus and post-liquidus controls on
pyroxene composition
Textural and compositional equilibrium is dependent on
many processes subsequent to the first stage of crystallization. These include reaction with trapped liquid
(Barnes, 1986), migration of interstitial liquid through the
crystal pile (Irvine, 1980), solid-state crystal readjustment,
and annealing processes (Hunter, 1996). Observed textures cannot be attributed to any single control and it is
likely that all processes contributed by variable degrees
to the final textural form. It is shown that in the P1
Pyroxenite, zones of strongly annealed pyroxenes are
interspersed within a framework structure of cumulus
crystals that have not all undergone annealing to the
same extent. The degree to which annealing has taken
place is variable and is related to the stratigraphic position
in the sequence, with rocks lower in the sequence showing
greater degrees of annealing. The extent to which annealing took place is dependent on the development
of cementing phases, which then also controlled the
redistribution and retention of trapped liquid. A combination of processes will have contributed to the final
mineral composition and it is unlikely that the preserved
composition would be that of the initial liquidus composition of the early stage mineral phases. The degree to
which liquidus cumulus compositions have been modified
would depend very largely on the relative influence of
the trapped liquid, the passage of evolved liquids expelled
by compaction and the stage at which the system became
sealed, preventing expulsion or migration of liquid.
The linear arrays linking pyroxene compositions to
incompatible element content in the whole rocks indicate
two important controls in the crystallization and
solidification process: (1) equilibrium crystallization gave
rise to the essentially constant pyroxene compositions
that existed initially within each of the layers; (2) the
trapped liquid played an important role in establishing
the final compositions, thereby giving rise to the observed
range of compositions within each of the layers. For each
of the small-scale rhythmic units, the initial (liquidus)
composition of the orthopyroxene must have been constant, or very nearly constant, and reaction with trapped
liquid would have driven the pyroxenes to more evolved
compositions. The degree by which the pyroxenes
changed composition would have been dependent on the
local amount (for a given sample) of trapped liquid.
It may therefore be deduced that there are essentially
no rock types in this succession that contain pyroxenes
of liquidus compositions. An estimate of the liquidus
pyroxene composition for each of the units is given by
the intercept of the array on the mg-number axis where
the concentration of the incompatible element in the
interstitial liquid is zero. For the whole-rock composition,
this equates to the amount of Zr contained within the
pyroxene (2–5 ppm from Fig. 14). Using the approach
of Barnes (1986), the amount of trapped liquid required
to cause the displacement of the compositions is calculated
to range between 2 and 12%. This small amount of
trapped liquid would not have caused the amount of Zr
incorporated within the pyroxene to vary greatly, and
the Zr content of the whole rock remains strongly dependent on that incorporated within late-stage mineral
phases, mainly zircon and baddeleyite. These results also
indicate that the initial porosity of the cumulate was
reduced rapidly at an early stage such that the final
trapped liquid strongly influenced the re-equilibrated
pyroxene compositions.
The role of fluids and re-equilibration
processes
If crystallization accompanies liquid migration then an
assumption of a closed system binary mixture of cumulus
crystals and crystallized liquid is not valid. In the Banded
Sequence of the Stillwater Complex, there is little correlation between the estimates of trapped liquid using
the mg-number shift for the whole rock and the trace
element content (Meurer & Boudreau, 1998). Therefore
using these approaches would not give reliable estimates
of the amount of trapped liquid that crystallized. Meurer
& Boudreau (1998) concluded that in the Banded
Sequence the observed signatures result from the effects
of trapped interstitial liquid combined with a continuous
flux of fractionated liquid driven by compaction.
There are several fundamental differences between the
rocks studied in the Stillwater Complex and the P1
pyroxenite of the Great Dyke: (1) in the Banded Sequence
evidence points to extensive re-equilibration and recrystallization, whereas in the P1 Pyroxenite the degree
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VOLUME 42
of annealing is highly variable between and within
samples, and also indicates a progressive change through
the sequence; (2) an interstitial cementing medium is
present in the P1 Pyroxenite, but not in the Banded
Sequence; (3) there is clear evidence of mutual dependence between the mineral mg-number shift and the
trace element content in the P1 Pyroxenite but not in
the Banded Sequence; (4) there is strong systematic
variation in mineral compositions in the P1 Pyroxenite
whereas none is observed in the Stillwater Banded
Sequence, even over hundreds of metres. This indicates
either that the overall conditions of melt removal or
retention were different in these two intrusions, or that
processes that operated in the pyroxenitic cumulates of
the Great Dyke were different from those for the gabbros
and anorthosites of the Stillwater Complex. An assessment of these processes could provide important information on the manner in which different rock types
may have formed in layered intrusions and also may
caution against a single model for all situations.
In the P1 Pyroxenite layer, there is evidence of transitional conditions at the boundaries of the small-scale
rhythmic units, both for the mineral chemical relations
and for the PGE distributions, which may have resulted
from transfer of liquid between the zones. That densification is an essential part of the process is indicated
by the calculated initial porosity not exceeding 12%, but
the removal of interstitial liquid must have taken place
rapidly and at an early stage in the consolidation process.
The process of densification may have been driven more
by grain boundary readjustment and annealing than by
extensive post-liquidus compaction, which would have
destroyed the compositional integrity of the layers.
Controls on PGE enrichment processes
Models for the processes by which PGE are enriched in
mafic and ultramafic cumulates are extensive and wide
ranging, and a brief review is necessary. Campbell et al.
(1983), using the Merensky Reef as an example, proposed
that the enrichment was largely the result of a magma
mixing process by which PGE were scavenged by sulphide
by virtue of the very high partition coefficients for these
metals between silicate and sulphide liquid. Evidence was
based on major lithological and chemical changes. In
the case of the P1 pyroxenite, chemical changes are
much more subtle and there are no major lithological
changes in the MSZ, or in the LSZ. Central to this
model is the R-factor (Campbell et al., 1983), which gives
the degree of interaction of sulphide with the silicate
magma.
Current models for the MSZ (Naldrett & Wilson, 1989;
Prendergast & Keays, 1989) are based on continuous
fractional segregation of sulphide and the high partition
NUMBER 10
OCTOBER 2001
coefficients of PGE between sulphide and silicate liquids.
Support for this model is the apparent smooth variation
for the PGE with a slow rise in concentration followed
by rapid depletion associated with abundant base metal
sulphides.
A chromatographic mechanism (Boudreau & Meurer,
1999) by which PGE are concentrated in sulphide during
their upward transport as a result of degassing and
compaction of the crystal pile may provide a theoretical
basis for a resorption and migrating front. Direct evidence
for such a process is lacking. Furthermore, it is difficult it
see how this mechanism could produce zones of essentially
constant Pd:Pt ratios superimposed on an overall evolutionary trend. It also gives no basis for a link between
silicate and whole-rock compositions and the PGE zones,
as is observed in this study. The model fails to explain
why in other occurrences, such as the Merensky Reef,
no apparent separation of Pt and Pd is observed (Wilson
et al., 1999).
Fractionation of PGE
Modelling of fractionation of PGE in sulphide-rich environments is problematic (Barnes, 1993) and application
of conventional concepts on elemental partitioning is not
strictly applicable (Tredoux et al., 1995). It is postulated
that there exists a strong tendency for PGE to form
clusters in magmatic and other geological environments
(Capobianca & Drake, 1990; Ballhaus et al., 1994; Tredoux et al., 1995). Ballhaus & Sylvester (2000) examined
the role of sulphide melt in enriching PGE in magmatic
systems and suggested that, for the Merensky Reef, it
effectively immobilized a pre-existing in situ PGE anomaly
in the stratified liquid. Fleet et al. (1999) pointed out that
at high concentrations of PGE in sulphide (a likely
situation where sulphide first appears as an immiscible
liquid), non-Henrian partitioning may take place. Oversaturation with respect to some or all of the PGEs may
lead to the formation of metal complexes or PGE micronuggets, which will be easily transferable to sulphide
liquid if present. The extremely high concentrations of
PGE per unit sulphide in parts of the LSZ and at the
base of the MSZ (illustrated by Pd × 1000/Cu in Fig.
10b) indicates the potential for PGE oversaturation at
these stages.
The extent of fractionation of Pd relative to Pt in the
Great Dyke is greatly in excess of that indicated by
compositions of liquidus pyroxenes, which are subject to
fairly constant partitioning controls. This may reflect the
nugget formation combined with the natural partitioning
into high-temperature sulphide liquid. A summary of the
change of Pd/(Pd + Pt) ratios with concomitant changes
in the estimated liquidus compositions of orthopyroxene
for the various zones is illustrated in Fig. 18. Also indicated
1862
WILSON
PGE ZONES IN THE GREAT DYKE
Fig. 18. Summary of fractionation trends of Pd and Pt with modelled
changes in liquidus composition of orthopyroxene for the Upper and
Lower Sections of the LSZ and the PGEsz. Average values and error
bars (horizontal and vertical lines through each of the points) are taken
from Figs 6, 7, 9 and 15. Open arrows show where sulphide increases
markedly, and closed arrows indicate where Cr in orthopyroxene
increases markedly. In each case, the increase in sulphide is preceded
by an increase in Cr.
are the points at which S (as sulphide in whole rock) and
Cr (in orthopyroxene) undergo marked changes. In the
LSZ, increases in the Cr content are accompanied by
an inflection or a small reversal in mg-number. A change
in Cr content is noted to precede a change in the S
content and may have resulted from minor emplacement
of magma, which also caused the observed major breaks
in the Pd:Pt ratio.
Modelling on the basis of sulphide extraction has been
carried out in an attempt to (1) reconcile the change in
Pd/(Pd + Pt) with observed fractionation of orthopyroxene and (2) see if absolute concentrations of Pt and
Pd may broadly agree with those observed in the rocks.
Estimates of the liquidus compositions of pyroxenes,
together with composition of the magma at this stage in
the chamber (Wilson, 1982), allow the amount of silicate
fractionation in this section of P1 Pyroxenite to be
determined (Fig. 18).
Modelling of the PGE behaviour requires an estimate
of the composition of the magma crystallizing at the P1
Pyroxenite level. The only direct estimate of the PGE
composition of the primary liquid is that of Prendergast
& Keays (1989) for a satellite dyke chill giving 6·4 ppb
Pd and 0·32 ppb Pt. However, this sample contains small
sulphide particles indicating that it may have entrained
or fractionated some sulphide, and this renders it unlikely
to yield a reliable estimate of the initial liquid composition.
The P1 Pyroxenite contains all known enriched zones of
PGE in the Great Dyke, with the narrow chromitites in
the lower Ultramafic Sequence containing inconsequential amounts of PGE. The lower Ultramafic
Sequence also represents that part of the chamber that
was repeatedly replenished and therefore the magma is
likely to have retained its original PGE signature until
the appearance of the first major sulphide zone. The ratio
Pd:Pt can be estimated from mass balance calculations of
the various sulphide zones in the P1 Pyroxenite. As both
Pt and Pd fall to below detection limits at the top of the
Upper Section of the LSZ, in spite of the relatively
abundant sulphide content, this indicates that the part
of the magma body supplying the PGE had at this stage
been completely depleted in these elements. Assuming
re-establishment of the PGE reservoir by further magma
influx (or mixing from within the chamber) combined
with fractionation, to give the Footwall to the PGEsz,
and the PGEsz itself, a similar calculation can also be
carried out for this part of the succession. The ratio Pd:
Pt in the initial magma is calculated to be 0·76 for the
LSZ sequence and agrees well with 0·81 calculated for
the MSZ.
An estimate of absolute concentrations is more difficult
because of the combined effects of fractionation and
magma recharge in an expanding chamber, but the
magma chamber is assumed to have undergone no further
major influxes above this level (Wilson & Chaumba,
1997; Wilson et al., 1998) and therefore the height of the
chamber can be estimated from the compositional change
of pyroxene through the Mafic Sequence. For the Selukwe
Subchamber, this is calculated to be 720 m at the base
of the Lower Section of the LSZ and 640 m at the base
of the Footwall to the PGEsz. Initial concentrations in
the magma at the base of the LSZ are calculated to be
17 ppb Pt and 13 ppb Pd. A similar calculation for the
Footwall to the MSZ gives 8 ppb Pt and 6·5 ppb Pd.
Using the calculated ratio for Pd:Pt, it is possible to
examine the process required to fractionate these elements. Using constant values for Dsulph/sil for Pd and Pt,
modelling fails to produce any trends or ratios that
approximate observed values. The answer to this problem
may be found in the experimental partitioning data of
Fleet et al. (1996) and Crocket et al. (1997) as evaluated
by Fleet et al. (1999) and by Ballhaus & Sylvester (2000).
A spurious correlation has been found to exist between
measured values for Dsulph/sil and the concentration of PGE
in the sulphide. Ballhaus & Sylvester (2000) explained this
as the result of PGE nuclei in the sulphide–silicate binary
melt system being incorporated into the sulphide phase.
Nuclei or micro-nuggets of PGE form in the silicate
magma as the result of it achieving oversaturation in
PGE, and these would be readily incorporated into an
immiscible sulphide melt. Controls on the status of the
oversaturation are complex and poorly understood, but
are variably dependent on enrichment of PGE in the
magma as a result of silicate fractionation, bulk composition of the magma [especially FeO content (Borisov,
1998)], complexing trace elements such as Te and Bi,
and falling temperature (Borisov & Palme, 1997).
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VOLUME 42
NUMBER 10
OCTOBER 2001
Fig. 19. Modelling of apparent partition coefficients (D) for Pt and Pd between sulphide and silicate liquid to replicate the observed changes in
Pd/(Pd + Pt) for (a) the Lower Section of the LSZ and (b) PGEsz. Variation of sulphide content used in the modelling is also indicated.
Modelling is based on the premise that initially, and
where sulphide concentration is very low, apparent
Dsulph/sil for Pt and Pd will be at the upper end of
the measured experimental ranges and should also be
consistent with the calculated concentration of PGE
within the sulphide. Input into the model of the variable
sulphide contents to agree with measured values, degree
of fractionation (as constrained by pyroxene compositions), as well as selection of apparent values for
Dsulph/sil should allow replication of the change of Pd:Pt
ratio through the sulphide zones. The only modelled
scenario that yields results comparable with observations
is one in which the values for Dsulph/sil decrease systematically upwards in the sequence coinciding also with
increasing amounts of sulphide. The results of the modelling are shown in Fig. 19. In the Lower Section of the
LSZ (Fig. 19a), values for Pd/(Pd + Pt) decrease from
0·78 to 0·1 concomitant with the sulphide content increasing from 0·01 to 0·9%. This requires the apparent
Dsulph/sil for Pd to decrease from close to 105 at the base
of the sequence to >104 at the top, whereas that for Pt
decreases from 2 × 104 to 6 × 103. These values are
remarkably consistent with expected apparent Dsulph/sil
values shown by Ballhaus & Sylvester (2000), and the
modelled results also match closely the observed concentrations. The change in Pd/(Pd + Pt) values with
variation of Dsulph/sil for the LSZ is illustrated in Fig. 19a.
The higher sulphide content at the base of the PGEsz
results in lower apparent values for Dsulph/sil (initially 5 ×
104 decreasing to 2·5 × 104 for Pd, and 2·5 × 104
decreasing to >104 for Pt) to duplicate the observed
variation of Pd/(Pd + Pt) (Fig. 19b) and the observed
concentrations. Modelled peak values of 3 ppm for Pt
and 2·6 ppm for Pd agree well with observed values, and
even more importantly, indicate spatial displacement
replicating the characteristic offset profiles.
ORIGIN OF THE MINERALIZED
LAYERS IN THE P1 PYROXENITE
The observed strong rhythmic development of mineral
compositions and incompatible trace elements provides
important evidence for the types of processes that took
place in the development of the P1 Pyroxenite layer of
the Great Dyke, and possibly similar successions in other
layered intrusions. The fact that PGE distribution was
strongly affected provides a further insight into the processes involved. The similarity in structure and compositional characteristics between the MSZ and the LSZ
indicates that the controlling processes were similar and
that only the scales on which they operated were different.
There is strong evidence that liquidus compositions of
orthopyroxene were modified by entrapment of evolved
liquid and that all pyroxenes within a single layer (represented by a compositional array) were essentially also
of the same composition before they were modified. The
well-defined compositional arrays argue against a steadystate process by which one layer interacts with those
below and above, but evidence also exists of a transitional
situation at the boundaries. Although migration of liquid
by compaction is recognized as an important process in
many instances (such as the Banded Sequence of the
Stillwater Complex), the observations for this part of the
Great Dyke would argue against this being the dominant
process. The textural form of many of the orthopyroxene
crystals indicates that following early compaction, a rigid
network was established early in the history of the rock.
Compaction followed by recrystallization and annealing
of orthopyroxene resulted in densification to varying
degrees in the stratigraphy and within local zones.
The close relationship of PGE distribution and pyroxene compositions indicates that the overall process was
fractionation by which pyroxenes were driven to more
1864
WILSON
PGE ZONES IN THE GREAT DYKE
evolved compositions, while at the same time Pd was
strongly fractionated relative to Pt. The silicate magma
became oversaturated in PGE as a result of extensive
crystallization of olivine and pyroxene in the Ultramafic
Sequence. The PGE micro-nuggets, which formed in the
magma as a result of the oversaturation, combined readily
with the first sulphide to form in the LSZ. The degree
of oversaturation in the magma (which was progressively
reduced with continued sulphide formation), imposed a
greater effect on the apparent partition coefficient for Pd
relative to Pt, resulting in strong fractionation of these
metals. Although the process represents the overall evolution of the magma body, in detail it was taking place
over narrow zones within which strong equilibrium was
established with only minor transmission between adjacent layers. Variability within each layer was imposed
by late-stage reaction and re-equilibration of orthopyroxene combined with local redistribution of liquid.
This would have been brought about in a two-stage
process: (1) equilibrium crystallization of silicates and
sulphide, with subsequent densification taking place
within narrow vertical intervals, resulting in overall fractionation of the magma body, followed by (2) re-equilibration and homogenization within each layer with only
minor transmission across layer boundaries. The first
control would have been imposed at the liquidus stage
whereas the second would have been at the subliquidus
stage. The trapped liquid was completely devoid of PGE
and therefore did not allow significant local redistribution
within the layers.
Previous models of PGE enrichment in the Great Dyke
based on continuous fractional segregation of sulphide
are an oversimplification. However, the overall decrease
of Pd relative to Pt in the rhythmic units in the Lower
and Upper Sections of the LSZ, in the Footwall to the
PGEsz, the PGEsz itself, and possibly even in the Hanging
Wall successions is strongly indicative of fractionation of
Pd relative to Pt. The sharp breaks between these units
and the stepwise manner of the final pattern indicate the
superimposition of other processes.
The P1 Pyroxenite layer formed as a result of fractionation of the Great Dyke magma. However, it occurred
at a stage where the magma chamber was no longer
subject to major influxes of magma, which gave rise to
the well-developed cyclic units characterizing the early
stages (Wilson, 1982; Wilson & Chaumba, 1997). However, this does not mean that smaller and more episodic
incursions of magma were not taking place, as indicated
by changes in the Cr content of orthopyroxene. Change
in pyroxene compositions combined with a sudden decrease in PGE concentration, followed by a gradual
increase, are indicative of a small influx of magma at the
boundary of the Lower and Upper Sections of the LSZ
and a further, and possibly larger, influx gave rise to the
Footwall succession of the PGEsz. Between the zones
where magma influx had occurred, a control of remarkable periodicity established distinct variations of
pyroxene compositions and mineralization, which ultimately gave rise to the stepwise pattern now observed
in the rocks.
Fluid dynamic considerations of magma chambers
have indicated that double diffusion layers are set up in
magma bodies (Irvine et al., 1983; McBirney, 1985) as a
result of the combined effects of heat loss upwards in
the system and density controls resulting from both
crystallization and the heat gradient (Campbell, 1996).
The heat and compositional gradients may act in opposition to stabilize or destabilize the system. Invariably,
these layers migrate upwards in the liquid column and
become incorporated into overlying liquid layers as the
solidification process proceeds. On theoretical grounds,
double diffusion convection may exist in magma chambers but direct evidence for it in the rocks is lacking
(Naslund & McBirney, 1996). The observations in this
work may be that evidence. The sudden jumps in compositions may have resulted from the sudden breakdown
of the lowest convecting layer by its incorporation into
that immediately overlying it. Temperature must fall
upwards hence the magma layers overlying the basal
must be cooler, with lower mg-number. Mixing of the
lowest layers may have taken place on attaining density
equivalence as a result of latent heat released from the
lower boundary layer together with the liquid becoming
more evolved. The result of the mixing is postulated to
be a sudden jump to lower mg-number in the boundary
layer, giving rise to pyroxenes of generally lower mgnumber.
Pyroxene and PGE compositions point to effective
equilibrium crystallization within the rhythmic layers
combined with late-stage re-equilibration. The strong
link with sulphide and the breaks in the pattern of
sulphide distribution, which also coincides with dramatic
changes in Pd:Pt ratio, indicates that the PGE enrichment
was controlled by the incorporation of the metals into
sulphide, possibly by way of stabilized metal clusters in the
magma resulting from oversaturation in PGE. Sulphide
formation may have had a dramatic effect on the degree
of oversaturation of PGE in the magma.
ACKNOWLEDGEMENTS
The following are thanked for constructive comments on
earlier versions of the manuscript: R. Grant Cawthorn,
Martin Prendergast and Richard Arculus. Discussion
with Chris Ballhaus added value to this paper. Mr C. Z.
Murahwi of Anglo American Corporation (Zimbabwe) is
thanked for facilitating the project and for his continuing
support of our Great Dyke studies. The University of
Natal Research Fund (URF) and the National Research
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JOURNAL OF PETROLOGY
VOLUME 42
Foundation (South Africa) are acknowledged for financial
support.
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