Characterization of fluids associated with gold mineralization and with regional high-temperature retrogression of granulites in the Limpopo belt, South Africa

Geochimicaet CosmochimicaActa, Vol. 58, No. 3, pp. 1147-l 159, 1994 Copyright0 1994ElscvierScienceLtd Printed in the USA.All rightsreserved

Pergamon

00 I6-7037/94$6.00+

.OO

Characterization of fluids associated with gold mineralization and with regional hightemperature retrogression of granulites in the Limpopo belt, South Africa* D. D. VAN REENEN,’ A. I. PRETORIUS,’ and C. ROERING’ ‘Department of Geology, Rand Afrikaans University, PO Box 524, Auckland Park 2006, South Africa ‘PO Box 223, Santoy 8491, South Africa (Received September 8, 1992; accepted in revised form July 18, 1993)

Abstract-The Hout River Shear Zone forms the terrane boundary between the granite-greenstone terrane of the Kaapvaal craton and the granulite terrane of the Limpopo belt. It dips steeply northward and flattens to the south and was responsible for allowing hot granulites to spread up and over the cooler greenstone terrane. This shear zone acted as a conduit for deep-seated infiltrating fluids into still hot granulites in the hangingwall producing a regional retrograde orthoamphibole isograd and an associated zone of rehydrated granulites. Thermodynamic constraints on the composition of fluids involved in retrograde anthophyllite formation in pelitic granulites on the isograd, supported by the results of a fluid inclusion study and by direct textural evidence in which olivine in closely associated ultramafic granulites has been partially replaced by hypersthene and magnesite, suggest that the hydrating fluid contained at least 70 mol% COz and that this fluid infiltrated at P-T conditions of about 620°C and 6 kbar pressure. The 6 13Cvalues of CO* extracted from magnesite vary between -5.5 and 6.0% and suggests a deep-seated (possibly mantle) source for the CO*-rich retrograde fluid. Fluid inclusion studies from two lode-gold deposits (Osprey and Louis Moore) in the same high-grade area, and three deposits from the immediately adjacent Sutherland greenstone belt (Fumani, Klein Letaba, and Birthday) at the northern edge of the Kaapvaal Craton, show that CO*-rich fluids with similar characteristics were also associated with epigenetic gold mineralization. All of the ore bodies are located within EW-trending northward dipping, ductile shear zones with an oblique to reverse sense of movement. These relationships suggest that there is a direct link between the source of fluids involved in gold mineralization, the fluids that formed the zone of rehydration in the Southern Marginal Zone of the Limpopo belt, and southward verging faulting during the exhumation of the granulite terrane at about 2670 Ma. INTRODUCTION

In this paper, we present data which characterize the fluids responsible for gold mineralization at high-grade settings in the Limpopo belt and the immediately adjacent Sutherland greenstone belt. We compare this fluid with a postulated deepseated CO*-rich fluid which was responsible for regional hightemperature rehydration of the southern part of the Southern Marginal Zone of the Limpopo belt toward the end of the Limpopo orogeny (ROERING et al., 1992a,b) at about 2670 Ma.

THE ORIGINOF ARCHAEANlode-gold deposits is controversial, and many aspects of this type of mineralization remain problematic. One group of deposits, for example, is epigenetic, and clearly controlled by features such as fractures and/or shear zones, and located in areas of low metamorphic grade. These include deposits such as Giant Golden Mile in Western Australia (e.g., GROVES et al., 1989) and in the Timmins area in Canada ( COLVINEet al., 1988 ). The suggested origin and source of the gold mineralizing fluids is often debatable and speculative, while the deposits themselves have clearly formed in suitable chemical/physical sites. Another clearly distinctive group of deposits are found in high-grade metamorphic environments, e.g., Fraser’s Mine ( BARNICOAT et al., 199 1) and Griffin’s Find (FARE, 1989) in Western Australia. With these deposits a major problem is whether the mineralization is ( 1) epigenetic and related to either the highgrade metamorphic event (e.g., POWELLet al., 199 1)) felsic intrusions (e.g., COLVINEet al., 1988; PERRINGet al., 1987), or mantle degassing (CAMERON,1988; COLVINEet al., 1988), or (2) has pre-existing deposits merely been metamorphosed at high-grade (e.g., PHILLIPS, 1985 ).

GEOLOGICAL SETHNG The Southern Marginal Zone of the Limpopo belt (Fig. I ) is characterized by quartzofeldspathic migmatitic gneisses surrounding highly attenuated enclaves of granulite-facies ultramafic, mafic, and metapelitic rocks and banded iron formation. These enclaves are the dismembered remnants of greenstone successions similar to those in the Sutherland greenstone belt. The granulite terrane is characterized by a number of crustal scale east-west trending, steeply northward dipping shear zones which subdivide the Southern Marginal Zone into discrete crustal blocks which still retain evidence of an earlier deformational pattern (Fig. I; SMITet al., 1992). On a regional scale the shear zones which juxtapose granulites against one another are distinct zones, many tens of kilometers in length and up to several kilometers in width. They are characterized in the field by a change in character of coarse-gained, folded, and banded migmatitic gneisses to fine-grained thinly banded gneisses, in which stretching lineations, boudins, rods, and occasional small-scale sheath folds are all orientated in the same northeastern direction (SMITet al., 1992). The most important shear zone belonging to this group of structures is the Hout River Shear Zone (HRSZ) which forms the terrane boundary

* Presented at the fourth biennial Pan-American Conference on Research on Fluid Inclusions (PACROFI IV), held May 22-24, 1992, at the UCLA Conference Center, Lake Arrowhead, California, USA. 1147

El

( South of HRSZ )

Undifferentiated Gneiss

Amphlbolite Facies

Greenschist Facies

2650 Ma Granite

2450 Ma Granite

}E%--

Roads

Rivers

}

SW

Hout River Shear Zone (HRSZ) Shear Zones Orthoamphibole lsograd

~~~~;~A~~~,_

,

>

Y/ A,,,

0

10

20

SCALE SO

4

FIG. I. Regional geological map of the Southern Marginal Zone (SMZ) of the Limpopo belt and northern part of the Kaapvaal craton (KC) (After SMIT et al.. 1992). Gold I = Fumani. 2 = Klein Letaba. 3 = Birthday, 4 = Louis Moore, 5 = Osprey. Localities 6 and 7 (DR19 and DR54). 8 (DRl57) and 9 (DV43): Samples studied by VAN REI HCILLISTER ( 1988). Localities IO ( DR I9 I ) and I I ( DR 186): Samples for carbon isotopic measurements.

ii El

EJI

d

i$f$ 1957 Ma Schiel Alkaline Complex

Origin of Archaean lode-gold deposits

between the granulite terrane of the Limpopo belt and the granitegreenstone terrane of the Kaapvaal craton (Fig. 1) . The Southern Marginal Zone of the Limpopo belt is also subdivided into a northern granulite subzone and a southern zone of rehydrated granulites by a retrograde ortho-amphibole isograd (VAN REENEN, 1986). The zone of rehydrated granulites is confined to the hanging wall of the crustal terrane bounding HRSZ (Fig. I ), which probably extends down to the mantle ( ROERINGet al., 1992a,b). The isograd in Fig. 1 has been accurately delineated in the field from peliticgneisses of similar chemical composition (VAN REENEN, 1986). It separates garnet + hypersthene + cordierite-bearing lithologies in the granulite subzone in the north, from garnet + orthoamphibole + kyanite-bearing lithologies in the zone of hydrated granulites in the south. The rehydrated pelitic granulites are often characterized by intergrowths of kyanite + gedrite pseudomorphing cordierite. The following two reactions (VAN REENENet al., 1987, Fig. 4c), identified in the same thin section, accurately define the position of the retrograde isograd in pelitic granulites. They also clearly indicate the introduction of an externally derived water-bearing CO+ch fluid phase into rocks still at high grade (T = -C620°C Pk 6 kbar, VAN REENEN, 1986; BAKER et al., 1992). hypersthene + water + quartz = anthophyllite cordierite + water = gedrite + kyanite + quartz Evidence for high temperature retrogression, however, is not confined to rocks of pelitic composition, but can be recognized in all the associated lithologies, including granulitic quartzofeldspathic gneisses, banded iron formation, and mafic and ultramafic granulites. The replacement of olivine in ultramafic granulites (VANSCHALKWYK and VAN REENEN, 1992) according to the reaction

I

30’30’

1149

forsterite + CO* = enstatite + magnesite provides direct textural evidence for the infiltration of an externally derived CO*-rich fluid during the high temperature post-peak metamorphic history of the Limpopo belt. The Sutherland greenstone belt is situated in the Archaean granitegreenstone terrane of the northern Kaapvaal craton, immediately south of the HRSZ (Figs. I and 2). It comprises metavolcanic schists of ultramafic, mafic, and rarely felsic composition together with subordinate banded iron formation, quartzite, and pelitic schist. The outcrop pattern of the Sutherland belt changes, towards the west, from a single ENE-WSW trending belt up to 20 km wide to two narrower belts (the Khavagari belt in the north and the Lwaji belt in the south), which are separated from each other by a 3-5 km body of tonalitic gneisses (Fig. I ). Due to the intense degree of tectonism it is not possible to establish a reliable stratigraphy of the area and, furthermore, there is no evidence for an overall synformal geometry of this greenstone belt (MCCOURT and VAN REENEN, 1992). The Khavagari belt and northern part of the central section are dominated by tremolite-chlorite-anthophyllite-olivine ultramafic schists and the southern parts of the belt by plagioclase-hornblende-bearing mafic schists. The dominant tectonic elements of this belt are northward dipping ductile shear zones which are developed subparallel to the regional schistosity (Fig. 2) (MCCOURTand VAN REENEN, 1992). The mineral lineations in the shear zones are upright to steeply oblique (Fig. 2). GOLD

DEPOSITS

STUDIED

Five gold deposits were studied (Figs. 1 and 2). Fumani and Klein Letaba occur close to or within the HRSZ. Louis

I 30045

23’15’-

. ,&

Strike and dip of bedding

56\

Plunge of mineral elongation lineation

y

Shear zones of thrust sense

I)

Faults

31”

FIG. 2. Structural map of the Sutherland granite-greenstone terrain (after MCCOURT and VAN REENEN, 1992) showing the structural control of the three studies gold deposits in this belt. I, Fumani; 2, Klein Letaba; 3, Birthday.

1150

D. D. Van Reenan, A. I. Pretorius, and C. Roering

Moore and Osprey are located in greenstone remnants in the zone of rehydration of the Southern Marginal Zone and Birthday is situated 6 km south of the HRSZ on the Kaapvaal craton. Birthday, Louis Moore, and Klein Letaba are gold, quartz, and sulphide replacement deposits in shear zones in mafic, ultramafic, and pelitic rocks. Fumani and Osprey occur in shear zones in banded iron formation. Fumani The ore zone at Fumani ( PRETORIUS et al., 1988) is situated within the southward verging HRSZ (MCCOURT and VAN REENEN, 1992). It occurs at the contact of banded iron formation (garnetiferous quartz-amphibole schist) with the surrounding micaceous quartzite. Gold occurs as inclusions in arsenopyrite, biotite, amphibole, and quartz. Competency contrasts during shearing between the banded iron formation and the micaceous quartzite probably created suitable channels for mineralizing fluids. The minimum age of mineralization is given by a 2632 f 53 Ma Rb-Sr age of muscovite from a pegmatite intruding the ore zone ( PRETORIUSet al., 1988). The rare occurrence of ferro-hypersthene + salite + early poikilitic Ca-poor garnet indicates peak metamorphic conditions at upper amphibolite to granulite grade. Retrograde metamorphism still at high temperature and accompanied by shearing is characterized by the assemblage Carich garnet + grunerite + hornblende + biotite + calcite. Osprey At Osprey (formerly New Union; Fig. 1)) gold mineralization occurs in two, subparallel northward dipping shear zones, which are 800 m apart, and oriented subparallel to the regional fabric ( PRETORIUS et al., 1988). In the western shear zone, Au is associated with syntectonic quartz veins in garnetiferous amphibolite (Ga + Hbl + Qz + PI). In the eastern shear zone, it occurs in sheared banded iron formation. The ore bodies average 2 m wide and plunge at 60” to the east. Assemblages in metapelite of garnet + orthoamphibole + biotite + plagioclase + quartz are characterized by intergrowths of kyanite + gedrite pseudomorphing cordierite which demonstrates that these rocks experienced the rehydration phase which established the retrograde orthoamphibole isograd and associated southern rehydrated granulite zone of the Limpopo belt (e.g., VAN REENEN, 1986). The very important conclusion that can be drawn from this deposit is that the gold mineralization is associated with rehydration at amphibolite facies conditions. Louis Moore Gold mineralization at Louis Moore (Figs. 1 and 2; PRETORIUSet al., 1988) is restricted to a shear zone within massive ultramalic granulites, near the contact with the tonalitic to trondhjemitic Klein Letaba gneiss. The ore bodies occur in carbonate and biotite rich, olivine and orthopyroxene-beating partially hydrated ultramafic granulite within chlorite schist. Microscopically gold has been observed in bands of serpentine truncating olivine, along cleavage planes in orthopyroxene and as inclusions in calcite. A Rb-Sr age of approximately 2506 f 50 Ma for biotite from pegmatite intruding the ore

body is a minimum constraint ( PRE~ORIUS et al., 1988).

on the time of mineralization

Klein Letaba Gold mineralization at Klein Letaba (Figs. 1 and 2) occurs in lenses of metapelitic gneiss and quartz-sulphide rock within the HRSZ where it forms the northern boundary of the Sutherland greenstone belt and the southern edge of the Southern Marginal Zone. This shear zone strikes N85”E and dips 82” to the north. Lineations and individual ore bodies plunge 45” to the east along the shear plane. Rehydrated pelitic granulites and prograde olivine-tremolite schists are tectonically intermixed within the shear zone. Muscovite from bodies of pegmatite within this shear zone near the mine yield an Rb-Sr age of -2660 Ma that is believed to reflect a major period of movement (BARTON and VAN REENEN, 1992), possibly associated with mineralization.

Birthday At Birthday, free milling Au occurs in syntectonic quartz veins and in enclosing hornblende-biotite-calcite schists within a ductile shear zone that strikes N75”E and dips at 35” to the northwest. Mineral lineations on the shear plane are generally down dip. The shear zone is developed in medium-grained massive amphibolite consisting mainly of hornblende and plagioclase (An 25 ) with little or no quartz. The same plagioclase composition in both the altered and sheared amphibolite, and in the massive unaltered amphibolite indicates that shearing occurred under P-T conditions very similar to those reflected by the wall rock (amphibolite facies, SIEBER, 199 1).

Ore Mineral

Parageneses

Gold mineralization at Birthday and Louis Moore is not associated with sulphides, but occur almost exclusively as free milling Au in syntectonic quartz veins or as inclusions in silicates ( PRETORIUS et al., 1988; SIEBER, 199 1). Two types of quartz can be distinguished at Birthday: a pale white milky variety, and a black smokey variety, which are believed to represent two different generations of quartz. It is important to note that the highest Au values at this locality are always found in the smokey quartz. At Fumani, Klein Letaba and Osprey Au is present both as free milling Au and as inclusions in sulphides, especially arsenopyrite ( PRETORIUS et al., 1988). The sulphide-hosted Au constitutes 50 ~01% of the mineralization of which 45% occurs as inclusions in As-rich arsenopyrite. The free milling Au occurs on cleavage planes in amphibole (6%) and biotite (24%), and as inclusions in quartz (20%). Arsenopyrite comprises only about 15% of the sulphide minerals but is an important mineral because of its association with Au. Two generations of sulphide mineralization can be recognized at Fumani (PRETORIUS et al., 1988): (1) early As-poor arsenopyrite + pyrrhotite + pyrite, and (2) later As-rich arsenopyrite + pyrrhotite + chalcopyrite + 18llingite. Arsenopyrite occurs as subhedral to euhedral grains and is commonly zoned with a core of intergrown early As-poor

1151

Origin of Archaean lode-gold deposits

arsenopyrite + pyrrhotite rimmed by a later (and Au-bearing) generation of As-rich arsenopyrite. FLUID

INCLUSION

STUDIES

Samples of quartz from which fluid inclusions were analyzed for this study were all obtained from zones of gold mineralization. At Fumani, the quartz was taken from syntectonic veins and from banded iron formation in the mineralized zones. At Klein Letaba and Louis Moore, the quartz occurred as syntectonic boudins and veins within the ore bodies. At Osprey and Birthday, the ore-bodies are syntectonic Au-bearing quartz veins. Analytical Techniques Fluid inclusions were studied in 0.5 mm thick, doubly polished plates and also in normal polished thin sections. The plates are substantially thicker than those normally used for fluid inclusion studies but are optically very clear and breakage is dramatically reduced. Heating and freezing experiments were made using the adapted USGS gas flow stage ( WARRE et al., 1979). The stage was calibrated at low temperatures using the triple points of CO2 (-56.6”C) and HZ0 (0°C) in synthetic inclusions supplied by Synllinc. Individual freezing runs were reproducible to within +0.2”C. Heating and freezing runs were conducted on the same inclusions so that final melting temperatures (Tmf) and homogenization temperatures (Th) could be correlated. Petrography of the Fluid Inclusions Quartz in samples from Fumani exhibits irregular grain boundaries and is invariably deformed. HzO-rich, mixed C02-H20, essentially 100% CO*, and CH,-rich inclusions were identified. Trails of all types of inclusions often cross grain boundaries, suggesting that these inclusions are secondary and were trapped during a post peak metamorphic event. 367 inclusions were studied. In quartz from Osprey, the same types of inclusions were present as at Fumani but the inclusions are randomly distributed and trails rarely cross grain boundaries. They thus may be pseudo-secondary. Furthermore, there is a lower proportion of H*O-rich inclusions than at Fumani. 177 inclusions were studied. In quartz from Louis Moore and Klein Letaba, no HzOrich inclusions were observed and mixed C02-Hz0 inclusions are not as prominent as at either Fumani or Osprey. The inclusions are randomly distributed and trails rarely cross grain boundaries so they might also be pseudo-secondary. Sixty-five and seventy-eight inclusions were studied from Louis Moore and Klein Letaba, respectively. In smokey quartz from Birthday, very few HzO-rich and mixed C02-HI0 inclusions were observed. The inclusions are randomly distributed and only rarely do trails cross grain boundaries, suggesting that they are pseudo-secondary. Ninety-eight inclusions were studied. A Au grain was observed in the plane of a C02-rich fluid inclusion trail. Very few other compositions of inclusions were observed in the vicinity of this grain, suggesting that COz-rich fluids were associated with gold precipitation.

H20-Rich

Inclusions

H,O-rich inclusions range in size between 5 and 10 p and in shape from irregular to slightly elongated. It occurs in trails and in random groups with no obvious planar arrangement. Although inclusion sizes vary, bubble sizes seem to remain relatively constant suggesting the same population, in the case of Fumani supported by Th (Fig. 8). Anisotropic rhombohedral minerals were observed in some of these inclusions from Fumani. These minerals do not occur in all H,O-rich inclusions and do not redissolve upon heating, suggesting that they were heterogeneously trapped (ROEDDER, 1984). A number of these minerals also occur as inclusions in quartz. No minerals were observed in H20-rich inclusions from Osprey. Because H20-rich inclusions were virtually absent from samples from the other occurrences, H20-rich fluids are not considered likely fluids for gold mineralization. Mixed C02-HZ0

Inclusions

Mixed C02-Hz0 inclusions are approximately the same size as HzO-rich inclusions but have a more regular shape. These inclusions (three-phase) at room temperature contain either liquid H20, liquid COz and gaseous COZ, or gaseous COz nucleates on cooling. At Fumani, it was observed that mixed C02-HZ0 inclusions formed at the intersection of CO*rich and H20-rich inclusion trails. Fluid mixing at this deposit is supported by the observation that the composition of the COz in the mixed inclusions is identical to that of the COzrich inclusions. Heterogeneously trapped minerals also occur in the Hz0 portion of the mixed COz-HZ0 inclusions. CO*-Rich Inclusions CO&h inclusions containing no visible H20 phase occur in healed cracks and in randomly distributed clusters in quartz from all the gold occurrences. These inclusions commonly occur as both two-phase, liquid-gas inclusions and as onephase liquid inclusions which nucleate a vapour bubble upon cooling. They vary from 5 to 10 p in size, are more regular in shape than the HzO-rich inclusions, and typically show tabular or negative crystal forms. CH4 Inclusions CH4 inclusions are dark, typically 20 p in size and contain a single, supercritical liquid phase. They contain no detectable Hz0 and can only be distinguished from the C02-rich inclusions by the phases present between -60 and -109’C and form a liquid below 130°C. They are only found at the Klein Letaba and Fumani occurrences. Freezing Experiments Low temperature phase relations in the systems H20-COzNaCl, CO*-CH4 ( SWANENBERG, 1979; BURRUSS, 198 1) and H20-NaCl (CRAWFORD, 198 1) were used in this study. Melting of H,O-rich inclusions occurred between -27 and 0°C with a large temperature difference between initial and final melting and low temperatures for initial melting. Initial melting often occurred below both the eutectics of NaClHz0 and KCI-H20, which together with the temperatures

1152

D. D. Van Reenan,

A. I. Pretorius.

and C. Roering

NaCl at -2l.lOC

KCI at -22.9X

-20

-10

0

10

Final melting “C FIG. 3. Final melting temperature (Tmf) vs. initial melting temperature (tilled squares) and Osprey (open squares). Also shown are the eutectic

offinal melting indicates the presence ofNa+, K+, Ca*+, and Mg*+ in the fluids (Fig. 3). H20-rich inclusions at Fumani (Fig. 4) exhibit final melting temperatures in the range -22 to +l”C with an average value of -3.6”C (5 wt% NaCl equivalent). At Osprey (Fig. 4), final melting occurred between -27 and - 1“C with average values of -23°C (25 wt% NaCl equivalents) and - 12.5 “C ( 16.5 wt% NaCl equivalent ). Apparently the HzO-rich fluids at Osprey were more saline than at Fumani.

0,,,,,,1,,,,,,,,,,,,,,,,,,,,1,, -26

-26

-24

-22

-20

-18

-16

(Tmi) for H*O-rich inclusions from Fumani melting temperatures of various substances.

Clathrate melting was observed only in mixed CO*-Hz0 inclusions. At Fumani and Osprey, melting occurred at +4 and +8.9”C, respectively, compared with 10°C for the pure CO*-H20 system. Melting of solid CO* at temperatures less than -56°C in the presence of vapour and nonaqueous fluid was observed in CO*-rich inclusions and in the CO2 phase of mixed C02Hz0 inclusions at temperatures between -60.5 and -56.6”C, suggesting the presence of varying amounts of other com-

-14

-12

-10

-8

-6

-4

-2

0

Tmf ICE FIG. 4. Final melting temperatures temperature values are averages.

of HzO-rich

fluid inclusions.

Black shading

indicates

areas of overlap.

Labeled

1153

Origin of Archaean lode-gold deposits 120

,f -58.3%

,

-65

-64

-63

-62

-61

-60

-59

-58

-57

-56

-

Tmf COP FIG. 5. Final melting temperatures of C02-rich fluid inclusions from Fumani and Osprey. Black shading indicates areas of overlap. Labeled temperature values are averages.

ponents such as N2 and CH4. At Osprey (Fig. 5) and Birthday, final melting temperatures were close to that of pure CO2 at -56.9”C. C02-rich inclusions from Louis Moore (Fig. 6) melted at -57.3”C. CO*-rich inclusions and CO2 in mixed CO*-Hz0 inclusions from Fumani (Fig. 5) melted at -58.3”C. Final melting temperatures of -56.8 and -57.7”C were observed from Klein Letaba (Fig. 6). Assuming that only the presence of CH4 is responsible for the melting point depression, the melting temperatures indicate, respectively, 10, 11,4, and 2 mol% CH4 in the COz-rich fluids for Fumani, Klein Letaba, Louis Moore, and Birthday ( BURRUSS, 198 1). Because CH4 is strongly partitioned into the gas phase (DONNELLY and KATZ, 1954), the composition of the fluids is a

0 -65

-64

-63

-62

-61

minimum estimate of the CH4 content of the entire inclusion. The densities of CO*-rich inclusions is influenced by CH4 (SWANENBERG, 1979) andare: Osprey (0.766 g/cm’); Louis Moore (0.868 g/cm3); Fumani (0.842 g/cm3); Klein Letaba (0.983 and 0.175 g/cm3); and Birthday (0.988 and 0.872 g/cm3). In CH4 inclusions from Klein Letaba and Fumani, total homogenization into a gas phase occurs in the ranges - 120.5 to -109.2”C and -136.3 to -117.6”C, respectively (Fig. 7). No other phase changes were observed to + 100°C. The homogenization temperatures of these inclusions are an indication of CH4 densities between 0.35 and 0.40 g/cm3 ( ZAGORUCHENKO and ZHUVALEV, 1970).

-60

-59

-58

-57

-56

-!

Tmf CO2 FIG. 6. Final melting temperatures of CO+ich fluid inclusions from Klein Letaba and Louis Moore. Black shading indicates areas of overlap. Labeled temperature values are averages.

D. D. Van Reenan, A. I. Pretorius. and C. Roering

1154

m

’

-128.1%

Klein Letaba (9)

I

0 -153

-149

-145

-141

-137

-133

-129

-125

-121

-117

-113

-109

-105

Th GAS FIG.

7. Homogenization

temperature

temperatures values are averages.

for CH4 fluid inclusions.

Heating Experiments In HzO-rich inclusions, total homogenization to the liquid phase was indicated by the shrinking of the vapor phase. No critical behavior was observed. Inclusions from Fumani homogenized between f89.5 and +239.8”C with an average value of +152”C (Fig. 8). Inclusions from Osprey homogenized between + 115.8”C and +208”C with an average value near + 164°C (Fig. 8). The difference in homogenization temperature may indicate that fluids at Osprey were trapped at a higher temperature than at Fumani. In mixed C02-HZ0 inclusions, homogenization temperatures for COz were identical to those of CO*-rich inclusions. Due to the high CO2 content of the mixed inclusions, they usually developed high internal pressures on heating and de-

Black shading

indicates

areas of overlap.

Labeled

crepitated before homogenization occurred. Therefore, total homogenization of the inclusions was impossible. In COz-rich inclusions, total homogenization was usually into the liquid phase and no critical behavior was observed. At Osprey, homogenization occurred between +IO”C and +3 1“C with an average value of +2O”C (Fig. 9). At Fumani, it occurred in the range -23°C to S16”C with an average value of +2.5”C (Fig. 9). At Louis Moore, homogenization was between -4 and +12’C with an average value of +3.7”C (Fig. 10). At Klein Letaba, it was between -23 and +23”C with average values of - 10 and + 18°C (Fig. 10). Homogenization in the vicinity of + 18°C was into the gas phase. At Birthday, homogenization occurred between -2 1“C and + 16°C with average values of -I I and +6”C (Fig. 10).

8 6

0 90

Th HP0 FIG. 8. Homogenization

temperatures

of H,O-rich

fluid inclusions.

Black shading

indicates

areas of overlap.

I155

Origin of Archaean lode-gold deposits

m

Osprey ( 113)

2.5%

I

20.7%

-21 -17 -13

-9

-5

-1

3

7

11

15

19

23 27 31

Th CO2 FIG. 9. Homogenization temperatures of CO&h fluid inclusions from Fumani and Osprey. Black shading indicates areas of overlap. Labeled temperature values are averages.

inclusions at all of these localities are the same, i.e., they occur either in healed cracks which (with the exception of Fumani) seldomly cross grain boundaries, or in randomly distributed clusters. Data obtained from microthermometric measurements, however, were undistinguishable between smaller and larger inclusions, or between inclusions trapped in healed cracks and those seen in clusters. This is taken as evidence for the presence of essentially similar fluids, but with different densities. The lower density fluids at Klein Letaba and Birthday with Th COZ + 18 and +6”C, respectively (Fig. lo), therefore, could have originated by decrepitation of the earlier denser CO*-inclusions if isochors of successively lower densities were crossed during uplift (as was earlier suggested for the Limpopo belt, VAN REENEN,19d6; VAN REENENand HOLLISTER, 1988).

Discussion of Fluid Inclusion Data The majority of Tm CO2 data from Osprey, Louis Moore, Klein Letaba, and Birthday range between -57.7” to -56.8“C (Figs. 5 and 6), indicating a relatively pure CO2 component. Tm CO2 data for Fumani ( Fig. 5 ) , however, suggest the presence of larger amounts of other gasses such as CH4 at this locality. Histograms for Th COZ L-V(L) for Fumani, Osprey, and Louis Moore (Figs. 9 and 10) are characterized by single maxima, suggesting entrapment under fairly restricted pressure and/or temperature conditions at each of these localities. Klein Letaba and Birthday, however, are characterized by histograms (Figs. 9 and 10) with two well-defined maxima, indicating entrapment at different pressure and/or temperature conditions. The mode of occurrence of the CO*-rich

+18.0X I

1

1-21-17-13

-9

-5

-1

3

7

11 15

19

23

27 31

35

Th CO2 FIG. 10. Homogenization temperatures of COz-rich fluid inclusions from Klein Letaba, Louis Moore, and Birthday. Black shading indicates areas of overlap. Labeled temperature values are averages.

1156

D. D. Van Reenan, A. I. Pretorius, and C. Roering

There also appears to be a trend in the variation of final melting and homogenization temperatures from the retrograde isograd (Osprey) towards the terrane boundary with the Kaapvaal craton (Fumani). Final melting temperatures change from -56.9”C at Osprey through -57.3”C at Louis Moore to -583°C at Fumani (Fig. 1). Homogenization temperatures vary from +2O”C at Osprey through +4”C at Louis Moore to +2”C at Fumani. However, data from Birthday and Klein Letaba differ from this suggested trend and clearly more data will have to be acquired to test the hypothesis. Mixed HzO-CO2 inclusions are most prominent at Fumani where it could be demonstrated that they were trapped at the intersection of CO*-rich with H*O-rich inclusion trails. Saline H*O-rich inclusions are commonly described in many Archaean metamorphic rocks and associated gold deposits, and probably represent younger brines that infiltrated down the Archaean structure (e.g.. TOURET, 198 1). COMPARISON OF FLUIDS INVOLVED IN REGIONAL REHYDRATION AND THOSE INVOLVED IN GOLD MINERALIZATION

The results of this study strongly suggests that the gold mineralization was introduced by a COZ-rich fluid. This is supported by the observation ( 1) that only COz-rich inclusions are common to all the Au deposits studied, and (2) that the highest Au values at Birthday are found in smokey quartz which is characterized by the almost total absence of aqueous inclusions (see also SIEBER, 199 1). The presence of a Au grain in the plane of a COz-rich fluid inclusion trail at Birthday is in agreement with this, as is the observation that the milkey quartz variety at this locality contains very little Au, but numerous aqueous inclusions. VAN REENEN and HOLLISTER ( 1988) characterized the compositions of fluids involved in the regional retrogression of granulite-facies rocks which established the retrograde orthoamphibole isograd and associated zone of rehydrated granulites in the Southern Marginal Zone in an earlier and independent investigation. The investigation of the retrogressed granulites was carried out on samples taken from localities at least 50 km to the west of the gold occurrences described above (Fig. I), and included the study of over 1000 fluid inclusions in pelitic rocks in four samples, representing a traverse across the isograd. These authors identified H20rich inclusions, COz-rich inclusions, and a few mixed COzHz0 inclusions. They found patterns of composition and density of fluid inclusions to be similar to those reported from practically all granulite-facies terranes (e.g., TOURET, 198 1). These comprise a few apparently pure CO2 inclusions with densities appropriate for the P-T conditions of hydration (T = +62O”C, P = +6 kbar), many COz-inclusions with lower densities. and aqueous inclusions of variable salinity containing no detectable COz. The results of their study showed that the hydrating fluid may be represented by secondary COz-rich fluid inclusions in which, because of their small size ( 5- 10 pm), Hz0 contents of up to 0.3 mol fraction may not be detectable ( ROEDDER, 1972 ). The multiple peak histograms (VAN REENEN and HOLLISTER 1988, Fig. 4) for Th CO2 is explained by a clear trapping sequence in which earlier dense inclusions decrepitated with formation of healed

cracks with lower density inclusions. This trapping sequence is to be expected if the host rock experienced a decrease in pressure (during uplift) prior to thermal equilibration (i.e., a clockwise P-T loop). The characteristics of the hydrating fluids (VAN REENEN and HOLLISTER, 1988, Figs. 4, 5, and 6) are very similar to those determined for fluids associated with gold mineralization. The H*O-rich inclusions in both settings contained Ca2+ and Mg2+ besides Na+ and K+ HzO-rich fluids with salinites of 5, 16.5, and 25 wt% NaCl equivalent were established for the Fumani and Osprey occurrences while 4, 17, and 26 wt% NaCl equivalent was determined for the regional metamorphic fluids. Homogenization temperatures between -15 to +3 1“C characterize C02-rich inclusions in the regional study (VAN REENEN and HOLLISTER, 1988, Fig. 4) while homogenization temperatures between -25 and +3l”C were determined from the inclusions associated with gold mineralization. Heating and freezing data for COrrich inclusions from this study (Figs. 5,6,9, and 10) are remarkably similar to those described by VAN REENEN and HOLLISTER, (1988, Fig. 4) from localities which straddles the isograd 50 km to the west (Fig. 1). Calculated C02-isochors, using MacFlinCOR (BROWN, 1992), for Klein Letaba (Th = -11 and +18”C), Birthday (Th = -11 and +6”(J), Louis Moore (Th = +3.7”C), and Osprey (Th = +20.7”C), assuming a pure C02-fluid, are compared (Fig. 11) with CO2-isochors which characterize the regional hydrating fluids and with the published P-Tloop for the Southern Marginal Zone (VAN REENEN and HOLLISTER, 1988, Fig. 5). The range of CO*-isochors for both studies are very similar, but the densist isochores in both cases pass about 1 kbar below M3 (the suggested P-T conditions for the regional hydrating event; VAN REENEN, 1986 ). However, it is impossible to hydrate minerals such as cordierite and hypersthene in the absence ofa HzO-bearing fluid phase. If these inclusions did contain about 0.3 mole fraction optically undetectible Hz0 (as was suggested by VAN REENEN and HOLLISTER, 1988) then the isochores for the densist CO*-

2.;

500

700

800

T.“C FIG. I 1. Representative CO*-isochores, this study (labeled I to 6, excluding Fumani), compared with CO,-isochores representative of the regional hydrating fluid (labeled X, , X2 and X,) (VAN REENEN and HOLLISTER, 1988, Fig. 5). PT-loop after VAN REENEN( 1986). I = Klein Letaba (Th = -I 1°C); 2 = Birthday (Th = -11°C); 3 = Louis Moore (Th = f3.7”C); 4 = Birthday (Th = +6”C); 5 = Klein Letaba (Th = +18’(Z); 6 = Osprey (Th = +20.7”C); X, (Th = -12°C): X2 (Th = f13”C): X, (Th = -3°C).

1157

Origin of Archaean lode-gold deposits rich fluid would pass through the metamorphic for M3.

conditions

NATURE AND SOURCE OF FLUIDS RESPONSIBLE FOR MINERALIZATION AND REGIONAL REHYDRATION

released from dehydration reactions from greenschist to amphibolite facies conditions, moved along shear zones and faults to form greenstone gold deposits. This model has been applied to both epigenetic and syngenetic mineralization. SOURCE OF CO1

The compositional similarities between the CO*-rich fluids associated with regional rehydration and those associated with gold mineralization from a geological terrane that was subjected to the same tectono-metamorphic history suggest that these fluids shared a common source. A knowledge of the regional geological setting of the Au deposits depicted in Fig. 1, therefore, is essential to any discussion of the nature and source of the mineralizing fluids. VAN REENEN et al. ( 1987, 1988) and ROERING et al. (1992a,b) have proposed that the granuhte facies terrane of the Limpopo Belt developed as a result of a south to north collision between the Kaapvaal and Zimbabwe cratons at about 2700 Ma. This period of crustal thickening and granulite formation was followed by uplift and isothermal decompression during which the high-grade rocks were transported upward and outward from the zone of crustal thickening along major shear zones. During this decompressional event the southern part of the Southern Marginal Zone was subjected to the infiltration of a COzrich, HzO-bearing fluid into lithologies still at high grade. This fluid-dominated event established a retrograde isograd which subdivided the Southern Marginal Zone into an unhydrated granulite zone in the north and a zone of hydrated granuhtes in the south (Fig. 1). The CO*-rich nature of the infiltrating fluids, suggested by the results ofa fluid inclusion study (VAN REENEN and HOLLISTER, 1988), is supported by phase equilibrium analysis (BAKER et al., 1992) which showed that the retrograde development of anthophyllite from orthopyroxene in metapelitic granulites along the isograd requires an associated vapour phase to have been relatively low in water (aHzO < 0.7) and temperatures to have been below 620°C. Direct evidence for the interaction of granulite facies assemblages with a COzrich infiltrating fluid, however, is provided by direct textural evidence in associated ultramafic granuhtes where olivine has been partially replaced by magnesite and hypersthene according to the reaction: forsterite + CO2 = enstatite + magnesite (VAN SCHALKWYK and VAN REENEN (1992). Phase equilibrium considerations (VAN SCHALKWYK and VAN REENEN, 1992; Fig. 12) require that the composition of the fluid phase be CO*-rich (XC02 > 0.67) at a temperature less than 670°C and a pressure of 6 Kbar. The results of the fluid inclusion study (VAN REENEN and HOLLISTER, 1988), therefore, is compatible with the results of phase equilibrium analyses of assemblages in both pelitic (BAKER et al., 1992) and ultramafic (VAN SCHALKWYK and VAN REENEN, 1992) granulites. The retrograde isograd (Fig. 1) is clearly spatially related to the north-dipping HRSZ which limits the zone of hydration in the south of the granulite zone. VAN REENEN and HOLLISTER ( 1988) suggested that the rehydrating fluids were produced by prograde devolatilization of the low-grade greenstone lithologies of the Kaapvaal craton in the footwall of the HRSZ. KERRICH and FRYER ( 1979), KERRICH and FYFE (1981), GROVES et al. (1984), and POWELL et al., ( 199 1) also proposed that CO,-rich fluids,

A problem with the greenstone devolatilization model, as far as the source of the CO* is concerned, is that both the hanging wall granulites (Southern Marginal Zone) and footwall greenstones (Sutherland greenstone belt) apparently equilibrated with similar COz-rich fluids at the different ambiant PT-conditions (See also VAN SCHALKWYK, 199 1). The source of the CO*-rich fluids, therefore, may be deep seated as is also suggested by stable carbon-isotope data (VAN SCHALKWYK and VAN REENEN, 1992). These authors extracted CO2 gas for the carbon isotopic analyses from magnesite produced by the high temperature breakdown of ohvine in four samples of ultramafic granulites (two samples from locality DR 19 1 and two from locality DR 186, Fig. I). The two localities which are approximately 15 km apart, are characterized by 6 13C values varying from -5.5 to 6.0%0 (VAN SCHALKWYK and VAN REENEN, 1992, Table 8). Most authors agree that mantle carbon 613C are between -5 and -10%0, although the mantle might not be completely homogenous in isotopic composition (e.g., KYSER, 1986). The available data, therefore, do not exclude the possibility that the COz-rich infiltrating fluid may have been derived from a deep-seated mantle source, although it is recognized that the sparce 6 13C values are equivocal as d 13C values between -8 and -4 are equally consistent with average crust ( FUEX and BAKER, 1973). The near granulite temperature of hydration however, demonstrates that this fluid was introduced into dry rocks still close to granulitegrade which suggests that the source of the infiltrating fluid must at least have been deep-seated. GROVES et al. (1992) recently also pointed out that the occurrence of gold over the entire range of crustal depths (metamorphic grade) for Archaean greenstone belts in Western Australia makes models invoking only metamorphic devolatilization of greenstones, virtually untenable. These authors suggested that deep cntstal or mantle fluid sources and crustal-scale plumbing systems were involved in Archaean lode-gold genesis. SUMMARY The gold deposits in the Sutherland greenstone belt and adjacent Southern Marginal Zone of the Limpopo belt share many common features included below.

1) The structural control on the setting of the Au deposits

2)

is clearly demonstrated by the fact that they are all located within southward verging ductile shear zones associated with a variety of rocks including mafic, pelitic, ultramafic lithologies, and banded iron formation. Most of the ore bodies are oriented parallel to the down-dip mineral elongation lineation, implying a direct geometrical relationship between the present distribution of gold and southward directed thrusting (MCCOURT and VAN REENEN, 1992). The deposits are all epigenetic and mineralization occurred during post-peak metamorphic conditions.

D. D. Van Reenan, A. I. Pretorius, and C. Roering

1158

3) The mineralizing

4)

5)

6)

7)

8)

fluids were introduced into hot wallrock as is suggested by the observation that the associated wallrock alteration is compatible with the metamorphic grade of the adjacent rock. The wallrock alteration at all the deposits is characteristically enriched in CO*, S. and K, an observation common to Archaean lode-gold deposits (e.g., GROVES et al., 1992). The sulphide minerals present are also similar, despite major differences in wallrock lithology, with arsenopyrite often closely associated with gold mineralization. The deposits all appear to have formed during the same time interval (more or less at 2650 Ma) towards the end of the Limpopo orogeny (BARTON and VAN REENEN, 1992; VAN REENEN et al., 1988, 1987; PRETORIUS et al., 1988). The geotectonic scenario for these deposits is suggested to be a collisional environment but the minerahsing plumbing system was only established late in the evolution of this mountain belt. Rehydration under granulite conditions occurred when the granulites were uplifted and brought into contact with the adjacent granite-greenstone craton. The uplift and ultimate exhumation was accomplished in a still active compressional environment so that the deep level granulites of a thickened crust (due to the earlier collision) were thrust up and spread over the adjacent craton (VAN REENEN et al., 1987; ROERING et al., 1992a,b). This contrasts strongly with the tensional environments called on for the exhumation of granulite terranes in core-complex environments. The shear zone separating the Limpopo belt from the Kaapvaal craton is a major crustal terrane boundary fault which is similar to those shear zones which KERRICH ( 1989) suggests are the first order control of greenstone related gold deposits and which represent tectonic melanges between accreted terranes (WYMAN and KERRICH, 1988). Archaean convergent margin tectonics have been considered to be of importance in numerous Australian greenstone deposits (GROVES et al., 1992). The latter environments differ in detail from that of the Limpopo belt. The COz-rich mineralizing fluids were probably derived from a deep-seated (possibly mantle) source which utilized and migrated through crustal scale shear zones. Fluid inclusions with high COz/H20 ratios are commonly related to Archaean lode-gold deposits (e.g., GROVES et al., 1992; GUHA et al., 1991).

Acknowledgments-This work was supported by grants to DDvR from the Foundation for Research Development and the Rand Afrikaans University. We thank the management and staff ofthe Fumani Mine and of GAZGOLD for their logistic support over a number of years. We thank R. Kerrick, E. T. C. Spooner, and J. Touret for their constructive reviews of the manuscript.

Editorial handling: M. A. McKibben

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