429
S.Afr.J . Geol. ,1988,91(4),429-438
BIF -hosted gold mineralization at the Fumani Mine, Sutherland greenstone belt,
South Africa
A .1. Pretorius, D.O. van Reenen and J.M. Barton Jr.
Department of Geology, Rand Afrikaans University, P.O. Box 524, Johannesburg 2000, Republic of South Africa
Accepted 15 July 1988
The Fumani gold mine is situated in the Archaean mafic-ultramafic unit of the Sutherland greenstone belt in
South Africa. The ore zone occurs in a metamorphosed and sheared banded iron-formation consisting of layers
of garnet + biotite + amphibole alternating with quartz-rich layers. Gold is associated with As-rich arsenopyrite
formed at 400 - 250°C. Mineralization is probably epigenetic, post-peak metamorphism and might have been
introduced along shear zones during late retrograde metamorphism or hydrothermal activity. The mineralizing
solutions were possibly derived from the dehydration of greenstone successions due to thrusting of high- over
low-grade metamorphic rocks during the Limpopo orogeny at approximately 2700 - 2670 Ma ago.
Die Fumani-goudmyn word aangetref in die Argei'ese mafies-ultramafiese eenheid van die Sutherlandgroensteengordel in Suid-Afrika. Die ertsafsetting kom voor in 'n gemetamorfiseerde en geskuifskeurde,
gebande ysterformasie wat bestaan uit granaat, biotiet en amfiboolryke lae wat met kwartsryke lae afgewissel
word. Goud is geassosieer met As-ryke arsenopiriet wat in die interval 400 - 250°C gevorm het. Mineralisasie
is waarskynlik epigeneties, post-piek metamorfose en is ingeplaas langs skuifskeursones tydens laat retrograad
metamorfose of hidrotermale aktiwiteit. Die mineraliserende oplossings is moontlik gekoppel aan die
dehidrasie van groensteenopeenvolgings as gevolg van die oorskuiwing van hoe- oor laegraadse metamorfe
gesteentes tydens die Limpopo orogenese ongeveer 2700 - 2670 Ma gelede.
Introduction
Debate has continued over the years about the
syngenetic or epigenetic origin of banded iron-formation
(BIF) hosted gold deposits. The syngenetic origin has
been supported by workers such as Fripp (1976) who
emphasized that, at the Vubachikwe Mine in Zimbabwe
the ore bodies consist of stratiform stratabound
sulphide- and mixed sulphide-carbonate facies BIF and
that gold-bearing sulphide formation predated the
metamorphism and deformation of the BIF. Fripp
(1976) proposed that the sulphides, carbonates, chert,
and gold were precipitated on the sea floor from
geothermal brines. Members of the epigenetic school,
e.g. Phillips et al. (1984), argued that the Vubachikwe
deposit may be of epigenetic origin because there is a
strong correlation between gold and arsenic and the base
metal/gold ratios are characteristic of Archaean
epigenetic gold deposits. There is also a correlation
between transgressive quartz veins, arsenopyrite, and
gold distribution on the mine scale.
Following Fripp's (1976) model, Potgieter & De
Villiers (1986) proposed that gold at the Fumani gold
mine in the Sutherland greenstone belt is syngenetic and
was deposited together with sulphide minerals and BIF
in a sea-floor exhalative environment. They based their
suggestion on the stratabound nature of the ore bodies,
general lack of crosscutting features, and the fact that
sulphides and gold (seemingly) participated in all the
metamorphic and deformational events. They proposed
that gold moved into more favourable structural traps
such as along fold hinges during emplacement of late
tectonic granitic plutons. Potgieter & De Villiers (1986)
believed that a later high-grade metamorphic event,
related to the Limpopo orogeny, affected the early gold
mineralization. This was because they described cores of
early auriferous arsenopyrite, surrounded by later,
higher temperature arsenopyrite, implying that Fumani
represents a metamorphosed gold deposit. In this paper
the authors present the results of an integrated
structural, metamorphic, and petrological study of the
Fumani ore body. We propose that the mineralization at
Fumani is probably epigenetic and directly related to the
tectonometamorphic evolution of this area during the
Limpopo orogeny at about 2700 Ma ago.
Geological setting of the Sutherland greenstone
belt
The Sutherland greenstone belt (Figure 1) is of
Archaean age and the lithological succession has been
designated the Giyani Group of the Swaziland
Supergroup (SACS, 1980). No isotopic age has yet been
determined directly for these rocks but they are intruded
by approximately 2650-Ma old granitic rocks. The
greenstones are presumed to be similar in age (± 3500
Ma) to the Barberton and Pietersburg greenstone
successions, and occur as a northeast-trending belt, 60
km long and 17 km wide, which bifurcates into the
Khavagari and Lwaji belts in the southwest (Figure 1).
The absence of persistent marker horizons, generally
poor outcrop plus ubiquitous shearing and alteration,
preclude the construction of a reliable stratigraphy for
the belt. Ultramafic-mafic-dominated and mafic-felsicdominated sequences can be recognized with the
ultramafic-dominated sequences essentially restricted to
the northern margins (Figure 1).
The ultramafic-mafic sequences consist of tremoliteactinolite schists, amphibolites, and intercalated BIF
(Prinsloo, 1977). Most of the known gold deposits occur
along the northern margins of the belt, coinciding with
the distribution of the ultramafic-mafic-dominated
S.-Afr. Tydskr.Geol.1988,91 (4)
430
0
o
30 00
1
~30~3~0~_ _ _- - - - - r - - - - - - - - - - - - - - - - - - - - - - - ' 2 3 ° 0 0 '
I
pNew Union
(j
@LOUiS
Moore
Klein Leta ba
oC:==4_ _ _8
km
Mafic assemblages
Ultramafic assemblages
2650 Ma granite plutons
2800 Ma gneiss
Klein Letaba gneiss
•
Gold occurrence
~~~~~~~~~~~~~~~~~~~~~~_________________~23
o
30
I
Figure 1 Setting and general geology of the Sutherland greenstone belt.
sequences. The mafic-felsic sequences are dominated by
amphibolites with intercalated BIF, minor tremoliteactinolite schists, and quartz-sericite schists.
The contacts between the greenstone belt and the
surrounding Klein Letaba Gneiss are usually sheared,
and Fripp et al. (1980) suggested that at the Gemsbok
locality (Figure 1) the greenstones were thrusted
northwards on to the gneisses. At the Fumani Mine, also
near the northern contact, thrusting towards the south
along steep northward-dipping shear zones is indicated
by rotated garnets and S-C structural relationships (S.
McCourt; pers. comm., 1986). These conflicting
transport directions, determined at localities as little as 8
km apart, illustrate the complex structure of the belt.
1. an inferred early sub-horizontal N-S trending folding
event (F1);
2. the main east-northeast trending folding event (F2);
3. a later event marked by non-penetrative, northwestsoutheast trending upright isoclinal crossfolds (F3)
(Prinsloo, 1977; Fripp et al., 1980); and
4. an east-northeast trending shearing event (F4).
The F2 event accompanied prograde regional
metamorphism and was responsible for the development
of the penetrative northeast-trending schistosity
(Prinsloo, 1977). Emplacement of the unfoliated 2650
Ma granitic plutons probably postdate the F2, and
possibly also the F3, event.
Metamorphism
Deformation
Four regional deformation events have been identified in
the Sutherland greenstone belt
The regional metamorphic grade in the Sutherland belt
is heterogeneous and ranges from very low grade (almost
completely unmetamorphosed rocks), through lower
431
S.Afr.l . Geol. ,1988,91 (4)
v.
______2c5====~50______
75====~100m
v v v v Spotted ultramafic rock
. Micaceous Quartzite
Banded i ron format ion (GQAS)
x
x
x
x
o 6 /)
o()"6,,
t,...
'-'
Dolerite
Pegmatite (2632 Ma)
A
\
s;::;.r
Quartz vein
Shear zone
Gold ore body
by
Dip and strike of foliation
.o~
Mineral elongation lineation
~
Mine shaft
'
. .:''.'; :.'.
.
v
v
xxxxxxxxxxxx
x
x
x
x
x
xx
x
x
x
x
x
x
x
x
x
x
x
x
Figure 2 Geological map of the Fumani gold mine showing lithologies, shear zones and gold distribution.
greenschist facies to upper amphibolite facies (Prinsloo,
1977; Louwrens, 1983). The distribution of metamorphic
facies appears to be controlled by shear zones as is
suggested
by
the
juxtaposItion
of
virtually
unmetamorphosed conglomerates and basic lava against
penetratively deformed and metamorphosed rocks. In
the Khavagari belt (Figure 1) and in the central portion
of the belt, retrograde metamorphism is restricted to
shear zones (Van Reenen et al., 1988).
Geology of the mine area
The Fumani gold mine is situated at the northeastern
end of the Sutherland greenstone belt (Figure 1). The
mine, which has produced about one ton of gold, started
production in 1934 as the Giant Reefs gold mine. After
closing down in 1963, the mine was taken over by Mining
Corporation in 1972 and renamed Fumani. Mining
operations commenced again in 1976 after completion of
a drilling programme. These boreholes proved ore
reserves to a depth of 600 m.
The main rock types found in the Fumani Mine area
are:
1. spotted ultramafic rock;
2. micaceous quartzite; and
3. BIF (Figure 2).
Typical chemical analyses of these rocks are shown in
Table 1. The ultramafic rock may be a metamorphosed
komatiite based on its high Mg content, high Cal Al and
Table 1 Bulk rock chemistry of the main rock types at
Fumani
Ultramafic rock
Si0 2
Ti0 2
AI 20 3
44,72
0,08
3,14
Fe203
FeO
MnO
MgO
CaO
CO 2
3,55
4,50
0,10
31,05
4,55
0,18
0,09
0,02
0,50
7,17
0,56
Total
100,21
Na20
K20
P20S
Cr203
H 2O
45,66
0,07
6,20
1,19
7,36
0,11
20,75
8,36
0,03
0,16
Micaceous quartzite GQAS
68,05
0,26
15,82
1,14
2,30
68,38
0,28
15,94
1,02
2,61
0,27
1,19
3,08
4,34
2,28
0,07
0,01
0,53
6,53
2,86
0,73
0,46
0,01
1,85
1,26
3,34
3,64
0,13
0,02
1,94
0,42
99,81
100,00
100,84
GQAS
49,32
0,21
7,87
3,22
29,10
0,58
3,83
2,40
0,03
2,40
0,09
39,71
0,30
7,93
26,73
13,37
0,91
3,60
2,00
0,09
2,29
0,12
0,01
0,75
3,00
0,35
99,81
100,40
Mg/Fe ratios, and low alkali content. The spotted nature
of the rock is due to randomly orientated porphyroblasts
of olivine, partially replaced by antigorite and
magnetite, in a fine-grained matrix of tremolite and
actinolite.
A light-grey massive micaceous quartzite forms the
S.-Afr.Tydskr.Geol.1988,91(4)
432
Table 2 Rb-Sr elemental and isotopic data for samples from the Fumani gold mine
and other gold occurrences in the Sutherland greenstone belt (determined using
standard analytical procedures - Barton et at., 1979). The uncertainties in the
ages are expressed as two standard deviations. The mica ages are calculated
assuming an initial 87Sr/86Sr ratio of 0,71. The uncertainties in the 87Sr/86Sr ratios
are 0,01 % (1 sigma) while the uncertainties in the 87Rb/86Sr ratios are 0,7 % (1
sigma). The uncertainties in the elemental concentrations are 0,5% (1 sigma)
A.
Location/sample
Pegmatite, Fumani gold mine
B-86-1 m
Pegmatite, Louis Moore gold mine
B-85-32 b
Ore zone biotite, Klein Letaba
gold mine
B-85-31 b
m
Rb(ppm)
Sr(ppm)
87Rb/86Sr
87Sr/86Sr
Age (Ma)
934
12,0
1398
53,93
2632 ± 53
258
35,7
22,59
1,5284
2506 ± 50
218
64,3
10,09
1,0192
2126 ± 43
Rb(ppm)
Sr(ppm)
87Rb/86Sr
87Sr/86Sr2650
0,04
0,04
179
161
0,0007
0,0007
0,7270
0,7287
= muscovite; b = biotite
B.
Location/sample
Calcite veins, Fumani gold mine
B-86-8C
B-86-32
hangingwall and footwall of the ore-bearing BIF and
consists of quartz (60-75%), andesine (5-10%), and
micas (up to 30%, mainly biotite and muscovite). The
precursor to this rock is uncertain and the high alkali
content (Table 1) may be indicative of either a
granodiorite or a greywacke. No intrusive relationships
or sedimentary textures could be observed due to
deformation within the mine area. Alteration is often
observed adjacent to the ore bodies and ranges from a
slight discoloration of the rock to a green bleached rock.
The alteration is expressed by muscovite replacing
plagioclase.
A thin, undeformed pegmatite dyke intrudes the BIF
and micaceous quartzite near the footwall contact
(Figure 2). Analysis of primary muscovite from this
pegmatite yielded a Rb-Sr age of about 2630 Ma (Table
2A) which is interpreted to reflect the time of pegmatite
intrusion. Pegmatites in the Sutherland belt are probably
a late phase of the approximately 2650 Ma granitic
plutons. A late (undeformed) dolerite dyke, 15-30 m
thick, intrudes the footwall micaceous quartzite (Figure
2).
The BIF horizon, with an apparent 70 m thickness, is a
garnetiferous
iron-rich
quartz-amphibole
schist
(GQAS). This rock consists of layers of garnet + biotite
+ amphibole alternating with quartz-rich layers.
Sulphides and accessory minerals tend to concentrate in
the garnet + biotite + amphibole layers. Grunerite is the
main amphibole present and ranges from fine acicular
aggregates and more tabular coarsely twinned to
untwinned crystals which often occur parallel to
Table 3 Microprobe data for the different generations
of garnet
AP7#
N31
Type P core
AP7#
N32
Type Grim
AP 64#
N31
Type S
Si0 2
Ti0 2
AI 20 3
FeO a
MgO
MnO
CaO
K20
P20 S
35,72
0,03
19,90
35,19
0,10
2,47
4,44
36,93
20,47
34,59
0,13
2,52
4,90
35,83
0,08
19,16
27,85
0,72
8,51
5,72
0,03
0,04
Na20
NiO
Cr203
0,02
0,02
0,13
0,04
0,16
0,06
0,02
Total
98,62
100,34
98,62
Sample
analysis
a
All Fe as FeO
foliation. Magnetite and pyrrhotite were observed in the
pressure shadows of rotated grunerite augens in shear
bands.
Three varieties of garnet were identified:
1. anhedral poikilitic garnet with numerous quartz
inclusions (Type P);
2. anhedral to subhedral (relatively) inclusion-free
garnets (Type G);and
433
S.Afr.J . Geol. ,1988,91 (4)
3. subhedral sieve-textured garnet containing numerous
elongated hornblende inclusions (Type S).
P garnet cores surrounded by G garnet is common. In
some cases a further growth of S garnet were observed
around G garnet. The zoning is characterized by an
increase in MnO and CaO content and decreasing Fe
(Table 3). P garnets usually contain magnetite inclusions
and rarely any sulphide. Pyrrhotite inclusions are
common in G garnet and also occur in type S garnet.
Arsenopyrite is rarely found as inclusions in garnet or in
the pressure shadows around it. Magnetite and
pyrrhotite do, however, occur in the pressure shadows.
The zoning in garnet suggests that P garnet represent the
oldest generation and S garnet the youngest. This
suggestion is supported by the grain sizes of garnet in
shear bands where S garnet is larger than G garnet which
is, in turn, larger than P garnet.
The foliation is defined by chloritized green and
brown biotite. Biotite is also replaced by euhedral
tourmaline which grew during or after deformation.
Hornblende occurs as subhedral grains and is extensively
replaced by calcite and chlorite.
Tourmaline, calcite, and chlorite constitute the late
phases in the GQAS. Tourmaline is commonly zoned
and crystallized during and after shearing. Calcite is
associated with pyrrhotite and fills openings created by
deformation. Chlorite replaces most of the other
minerals and chlorite veins were observed to cut across
mylonitized quartz and being displaced by later
deformation.
Figure 3 Photomicrograph of the ore showing textures related
to shearing. Note the rotated garnet and sulphide in the
pressure shadow. Scale bar = 0,5 mm.
A.
POLES TO SHEAR ZONE
PLANES
"/60 " NW
A ..... ,...
",.
/ ,/-1'3 24 ·'/60 °N~ ____ -- -B. POLES TO FOLIATION
Structure
The orientation of the foliation at the Fumani Mine
corresponds with the regional strike and dip (060° and
60 NW) of the greenstone belt. BIF horizons in the
Sutherland belt are generally between 5 m and 15 m
thick compared to the 70 mat Fumani. The thickening at
Fumani is attributed to isoclinal folding. Shearing
developed parallel to the fold limbs during later stages of
deformation and several prominent shear zones, ranging
from 1 - 50 cm in thickness, are developed parallel to
lithological contacts. Mylonitized quartz can be observed
in most thin sections of the GQAS (sequence) (Figure
3). Although several discrete shear zones can be visually
recognized, the whole GQAS sequence itself constitutes
a large ductile shear zone.
Foliation/bedding planes in the GQAS define an axial
plane (056/60° NW) and a fold axis plunging to 324° at
60° (Figure 4B), parallel to the mineral elongation
lineations (Figure 4C). The orientation of the shear
zones is parallel to the axial plane _of the folds (Figure
4A). Shear deformation is also proven by the recognition
of heterogeneous strain in different rocks of the mine
area. The micaceous quartzite and ultramafic rock are
generally relatively unfoliated and undeformed in
contrast to the GQAS which was subjected to intense
deformation.
The presence of a long-lived, or repetitive, shearing
N=260
0
01 ~2
.
2%
- 4%
4 - 6%
18]6 - 8%
Q 8 - 10%
10010 - 15%
iiiI-
C
15%
LINEATIONS
Figure 4 Stereoplots of structural data from the Fumani Mine.
The coincidence of foliation planes and shear planes are taken
as evidence that the GQAS sequence represents a large shear
zone.
event is demonstrated by the fact that all the mineral
phases, even late-stage calcite and chlorite veins, are
deformed. Early prograde garnet and grunerite are
rotated, elongated, and show pyrrhotite concentrated in
the pressure shadows, indicating that shearing started
after peak metamorphic conditions were reached.
Calcite veins developed during a late stage in the history
and veins grew in existing openings (resulting from
S.-Afr.Tydskr.Geo1.1988,91(4)
434
earlier deformation). These veins were rotated into and
broken up by later reactivation of existing shear zones.
Gold mineralization and sulphide minerals are
concentrated parallel to the mineral elongation lineation
within the shear zones rather than in crosscutting veins,
suggesting that the gold distribution is related to ductile
shearing rather than hydraulic fracturing (S. McCourt;
pers. comm., 1986). The competency difference between
the GQAS and the micaceous quartzite resulted in more
intense deformation at the contacts between these rock
types. This is recognized underground by a better
developed foliation in the GQAS as well as by the
presence of quartz lenses and boudins along or near the
contacts with the micaceous quartzite. The shearing
along the contact zones could have provided
channelways for the mineralizing fluids and the best gold
values usually occur less than 8 m from these contacts in
the 70-m thick ore zone. Fluid movement along this
contact is suggested by the apparent correlation of Au ,
As, S and CO 2 (Figure 5) along a short, traverse across
the hangingwall ore body (Figure 2), starting at the
micaceous quartzite .c ontact and ending in unmineralized
GQAS. The gold peaks -closely correspond to high
values of As, S and CO 2 . The absence of a strong As
peak at the third gold peak is not well understood, but a
strong Ca peak (not shown) were noticed at this
position. No correlation between Fe (not shown) and As
or S were observed, suggesting that Fe was not
introduced into the rock but that the As and S in the
fluid combined with existing Fe to form the arsenopyrite
and pyrrhotite.
Metamorphism
The prograde and retrograde metamorphic events
described for the belt as a whole can also be recognized
in the GQAS at Fumani. Prograde metamorphic mineral
assemblages are characterized by the rare presence of
ferro-hypersthene, salite (cpx) , and early poikilitic
garnet ± magnetite. Opx-cpx geothermometry, using
the technique of Wood & Banno (1973), indicates
temperatures in the region of 620° ± 70°C for this
prograde event.
Retrograde metamorphism, accompanied by shear
deformation, is characterized by the assemblage garnet
+ biotite + grunerite + hornblende. Retrograde
textures where salite breaks down to grunerite + calcite
(Figure 6), and where grunerite is replaced by
hornblende, have been observed. The retrograde event
also resulted in the growth of Ca-rich inclusion-free
garnet rims (type G) on the early poikilitic (type P)
3
2
o
10
S%
o
6
Figure 6 Photomicrograph showing retrograde replacement of
salite (S) by grunerite (G). Scale bar = 0,25 mm.
As%
o
10
Au ppm
o
26 m
GQAS Ore zone
Figure 5 Geochemistry of the GQAS in close proximity to the
hangingwall micaceous quartzite.
Figure 7 Photomicrograph showing retrograde zoning of
garnet. Scale bar = 0,5 mm.
435
S.Afr.J.Geol., I988,9I( 4)
garnet (Figure 7). The preliminary results of (inclusionfree) garnet-biotite geothermometry using the technique
of Ferry & Spear (1978) yield temperatures for this event
ranging from 460
380°C. Reappearance of
stilpnomelane and greenalite replacing grunerite
indicates extensive retrogression in the presence of
abundant fluid.
The main
retrograde event culminated In
hydrothermal alteration as is manifested by the
formation of calcite and chlorite veins and by the growth
of tourmaline. Tourmaline constitutes an early part of
the alteration event and pre-dates calcite and chlorite
veins. The mineral is more abundant in zones of high
gold concentration (10 vol % compared to an average of
2 vol %), which suggests that tourmaline formed part of
the alteration event that accompanied the gold
mineralization event. Calcite represents the latest
mineral phase at Fumani and, like tourmaline, is more
abundant in zones of high gold concentration (6 vol %
compared to 2 vol %). Pyrrhotite, with and without
gold, is found in these veins and appears to be
redistributed from the veins along foliation surfaces in
the GQAS where it replaces magnetite. Initial 87Sr/86Sr
isotopic ratios (Table 2B) for calcite from such veins are
high, greater than 0,727, indicating that Sr, and possibly
the metals in the veins, were derived from an older
crustal source, perhaps the greenstone succession.
Gold mineralization
Gold is concentrated along shear zones near the contacts
with the hanging-, footwall- and eastern-contact
micaceous quartzite (Figure 2). Three main payshoots
occur in the ore body: the hangingwall (HW) ore; the
footwall (FW) ore and the east contact (EC) ore. These
bodies strike at 056° with an average dip of 60°
(parallel to the fold axis) to the north. The HW ore is
characterized by broad bands (1-5 cm) of garnet in a
black matrix of biotite with pyrrhotite (6 vol %) and
minor arsenopyrite (2 vol %). The FW ore is similar to
the HW ore with regard to sulphide mineralization but
garnet is less prominent. The EC ore is richer in
sulphides and is characterized by the absence of garnet
and by the presence of pyrrhotite (13 vol %) and
arsenopyrite (7 vol %) mineralization. The strike length
of payshoots averages 25 m and they seldom reaches a
thickness of more than 14 m.
Sulphide mineralization
Sulphide minerals occur predominantly in the garnet +
biotite + amphibole bands of the GQAS and virtually
obliterate these bands in well-mineralized areas.
Pyrrhotite accounts for about 80% of the sulphide
mineralization at Fumani. It is disseminated throughout
the GQAS and becomes abundant near the contact
between micaceous quartzite and GQAS.
Arsenopyrite comprises only about 15% of the
sulphide minerals, but is very important because it is
associated with gold . It occurs as subhedral to euhedral
grains and is commonly zoned with a core of intergrown
Figure 8 Photomicrograph showing an early core of As-poor
arsenopyrite (A 1) intergrown with early pyrrhotite (PI),
surrounded by later As-rich arsenopyrite (A2). Scale bar = 0,5
mm.
pyrrhotite + arsenopyrite rimmed by a later generation
of arsenopyrite (Figure 8). Very little deformation of
arsenopyrite was noticed apart from a few shattered
grains within small discrete (later reactivated) shear
zones. Minor amounts of magnetite, 1611ingite, and
ilmenite occur. Magnetite is replaced by pyrrhotite both
on mesoscopic and microscopic scales.
Two generations of sulphide mineralization were
recognized in the ore bodies:
1. early As-poor arsenopyrite + pyrrhotite + pyrite; and
2. later As-rich arsenopyrite + pyrrhotite + chalcopyrite
+ 1611ingite.
Early pyrrhotite exhibit very small grain sizes and is
usually included in, and intergrown with, As-poor
arsenopyrite (Table 4). The arsenopyrite in contact with
pyrite is As-poor. Second generation pyrrhotite
Table 4 Microprobe data for the different generations
of arsenopyrite
As-poor arsenopyrite
Fe
Sample
Ni
S
As
Sb
Total
43,61
41,53
43,04
42,74
43,06
0,02
0,12
0,04
0,08
99,44
100,30
99,39
99,55
99,24
AP51
AP78
AP48-1
AP48-2
AP50
35,39
36,06
35,39
35,00
34,78
0,03
0,06
0,18
0,04
20,39
22,65
20,84
21,59
21,28
Average
35,32
0,08
21,35
42,80
0,07
99,61
S
As
Sb
Total
Auriferous As-rich arsenopyrite
Ni
Sample
Fe
AP51
AP46
AP78
AP50
AP61
33,88
34,04
34,31
33,74
34,57
0,25
0,31
0,36
0,46
0,02
19,11
19,27
19,09
18,72
18,73
46,34
46,33
46,53
46,92
46,70
0,02
0,01
0,09
0,01
99,60
99,96
100,38
99,84
100,03
Average
34,11
0,28
18,98
46,56
0,03
99,97
436
surrounds the second auriferous generation As-rich
arsenopyrite (Table 4) which often contains an As-poor
arsenopyrite core. As-poor arsenopyrite is commonly in
contact with chalcopyrite and 1611ingite.
The crystallization temperatures of the different
generations of arsenopyrite were calculated from the Ascontent and associated minerals using the method of
Kretschmar & Scott (1976). The refractory nature of
arsenopyrite prevents homogenization at lower
temperatures and temperatures determined for this
mineral probably reflect those of crystallization. The
early As-poor arsenopyrite crystallized in the
temperature interval 525 - 200°C (curve asp + py + po;
Kretschmar & Scott, 1976), and the later As-rich and
dominantly gold-bearing arsenopyrite, crystallized in the
interval 400 - 250°C (curve asp + loll + po; Kretschmar
& Scott, 1976). The temperature intervals for both
generations of arsenopyrite correspond to the suggested
temperature for gold precipitation (Fyfe & Henley,
1973; Groves et al., 1984), but most of the gold occurs in
the later generation.
Gold occurrence
Microscopic gold grains, ranging in size from 2 - 5
microns, occur in arsenopyrite as well as in silicate
minerals. The sulphide-bound gold constitutes 50 vol %
of the mineralization, of which 45% occurs as inclusions
in As-rich arsenopyrite. All the gold not included in
arsenopyrite is found in silicate minerals and occurs
along cleavage planes in biotite (24%) and amphibole
(6% ), and as inclusions in quartz (20%). Late-stage
minerals (tourmaline and calcite) are always closely
associated with gold in these silicates. The compositions
of 20 individual gold grains were determined by
microprobe analyses and are uniform (3 - 7% Ag),
irrespective of the mode of occurrence of the gold. This
compositional uniformity may indicate a single
mineralizing event. The association between gold and
As-rich arsenopyrite which formed between 400 and
250°C is consistent with post-peak metamorphic
mineralization.
Overview of gold mineralization in the Sutherland
belt
Of the 26 small gold deposits in the Sutherland belt,15
are of the vein type and 11 are of the BIF stratiform
type. Relatively little is written about these deposits and
the only published data are by Weilers (1956) on the
Klein Letaba deposit and Foster (1960) on the Louis
Moore deposit. A short summary of these deposits and
their genetic implications are presented here.
The Louis Moore Mine is hosted by olivine- (and
calcite) and pyroxene-bearing ultramafic rocks. Foster
(1960) believed that the gold was associated with
primary magmatic olivine and calcite and interpreted
Louis Moore as a syngenetic gold deposit. He recognized
a regional metamorphic event followed by a (lower
temperature) hydrothermal event. More detailed
petrographic studies of ultramafic rocks from Louis
Moore indicate that the olivine is metamorphic
S.-Afr.Tydskr.Geo1.1988,91(4)
(Louwrens, 1983) and that gold occurs in bands of
serpentine truncating the olivine. Gold is also found
along cleavage planes in pyroxene and as inclusions in
calcite. The current distribution and occurrence of gold
indicates an epigenetic origin rather than a primary
magmatic origin. Rb-Sr age determinations on primary
biotite from post-ore pegmatite at the mine yield an age
of about 2500 Ma (Table 2A). This age is interpreted to
indicate the time of emplacement of the pegmatite and
indicates that gold mineralization occurred prior to this
event.
Gold at the Klein Letaba Mine occurs in shear zones
in an ultramafic rock consisting of tremolite ± actinolite
(Weilers, 1956). It is found in quartz segregations and in
quartz-sulphide lenses with pyrrhotite, arsenopyrite,
1611ingite, and chalcopyrite (Weilers, 1956). According
to Weilers (1956) the Klein Letaba deposit was formed
during a time of decreasing temperatures, following a
regional metamorphic event, and exhibits a marked
structural control with ore bodies restricted to shear
zones. Rb-Sr age determinations on biotite (Table 2A),
which are easily reset by low-temperature processes,
indicate that mineralization occurred at some time
before 2126 Ma ago. The ore mineralogy at Klein Letaba
constituting arsenopyrite, pyrrhotite, 1611ingite, and
chalcopyrite is in many respects identical to that of
Fumani despite the difference in host rocks. A common
fluid source for Fumani and other deposits in the
Sutherland belt is therefore inferred.
Discussion
Fumani is a stratiform deposit restricted to a specific
lithological unit (the GQAS) and therefore satisfies the
initial syngenetic criterion as listed by Phillips et al.,
(1984). Most of the other characteristics of this deposit,
however, point to an epigenetic origin for the gold:
1. sulphides, although deformed, did not participate in
all of the metamorphic events, unless they were
annealed. Only a small portion of the arsenopyrite
was shattered by later movement along reactivated
shear zones. This probably explains the gold in silicate
minerals which may have migrated from broken
arsenopyrite grains. The refractory nature of
arsenopyrite prevents homogenization at lower
temperatures and temperatures determined for this
mineral probably reflect those of crystallization. In
the case of Fumani being a metamorphosed
syngenetic deposit, one would expect the arsenopyrite
to reflect temperatures near to that of peak
metamorphism. Gold-bearing arsenopyrite, however,
indicates a crystallization temperature of 400 - 250°C
which is well below that of peak metamorphism at
620°C. Further evidence for the post-peak
metamorphic nature of arsenopyrite is suggested by
the absence of arsenopyrite inclusions in the various
generations of garnet. Type P garnet may be part of
the prograde event, whereas types G and S may
belong to the retrograde metamorphic event;
437
S.Afr.J . Geol. ,1988,91 (4)
2. gold mineralization is controlled on the macroscopic
scale by zones of more intense deformation which
occur near the contacts with the micaceous quartzite.
The intensity of deformation in the contact zones is
due to the competency difference between the
GQAS, and the micaceous quartzite and the ore
bodies are found in these zones;
3. transgressive features are not very common at the
Fumani deposit, but on 24 Level crosscutting
pyrrhotite-bearing calcite veins are associated with
the highest gold grades so far recorded in the mine (40
g/t compared to the 4 g/t average);
4. replacement of Fe-oxides (magnetite) by sulphides
(pyrrhotite) occurs on both the mesoscopic and
microscopic scales. This suggests the introduction of
sulphides into the rock by circulating fluids; and
5. the ore-bearing minerals at Fumani and Klein Letaba
are similar and pyrrhotite, arsenopyrite, 1611ingite,
and chalcopyrite occur at both deposits. Weilers
(1956) described the Klein Letaba Mine as an
epigenetic deposit. The coincidence of ore minerals in
different rock types (BIF-GQAS and ultramafic)
suggests similar fluid compositions and depositional
processes.
The mineralizing solutions in the Sutherland belt are
thought to be associated with metamorphism during the
Limpopo orogeny. This event may have resulted from
continent-continent collision about 2700 Ma ago (Van
Reenen et al., 1987). The response to thickened crust in
the area of the Sutherland greenstone belt was the
transport of crustal material southward, away from the
thickened zone, along steep, generally north-dipping
shear zones, such as the one that occurs at the Fumani
Mine. This shearing juxtaposed crust with high-grade
mineral assemblages over crust with lower grade
assemblages. The shear zones also served as the
channelways for the migration of fluids, which were
probably derived from dewatering of the overridden
lower grade crust (Van Reenen & Hollister, 1988). Some
of these fluids presumably were of the correct
composition to be responsible for the retrograde
metamorphic assemblages, the alteration, and the ore
bodies observed at Fumani.
Conclusions
The Fumani gold deposit occurs in a complexly
deformed and metamorphosed BIF (GQAS). The
mineralization was largely controlled by the composition
of the GQAS, which probably supplied Fe for the
formation of sulphides from the hydrothermal fluid, and
ductile shearing allowed infiltration of this fluid. The
competency difference between the GQAS and
micaceous quartzite caused more intense shearing at the
contacts and may also have created suitable channelways
as well as depositional centres for the infiltrating fluids.
The mineralizing solutions contained Au, As, S, and
CO 2 and were possibly derived from the dehydration of
greenstone successions due to thrusting of high-grade
rocks over low-grade rocks during the Limpopo event at
approximately 2700 - 2670 Ma ago ( Van Reenen et al.,
1987; Van Reenen & Hollister, 1988). The solutions
migrated along the shear zone, at the contact between
different rock units, and reacted with the Fe-rich GQAS.
Mineralization is probably post-peak metamorphism
and was introduced along shear zones during late
retrograde metamorphism or hydrothermal activity.
Gold is associated with arsenopyrite which crystallized in
a temperature range of 400 - 250°C. This temperature
interval and the relative abundance of late-stage
minerals in zones of high gold concentration support the
suggested late timing of the mineralizing event.
Acknowledgements
Financial support for this research came through the
Cooperative
Scientific
Programmes,
National
Geoscience Programme of the CSIR. We thank Mining
Corporation (STOK) for granting access to the Fumani
property as well as supplying additional geological
information.
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