Chemical Geology 257 (2008) 129–138

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Chemical Geology

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Stable isotope, fluid inclusion, and mineral chemistry constraints on contaminationand hydrothermal alteration in the Uitkomst Complex, South Africa

Arindam Sarkar a, Edward M. Ripley a,⁎, Chusi Li a, Wolfgang D. Maier b

a Department of Geological Sciences, Indiana University, Bloomington, Indiana, IN 47405, USAb Center for Exploration Targeting, University of Western Australia, Crawley, 6009, Australia

Hydrothermal alteration

⁎ Corresponding author.E-mail address: ripley@indiana.edu (E.M. Ripley).

0009-2541/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.chemgeo.2008.08.026

a b s t r a c t

a r t i c l e i n f o

Article history:

The Uitkomst Complex is a Received 8 May 2008Received in revised form 21 August 2008Accepted 24 August 2008

Editor: R.L. Rudnick

Keywords:Uitkomst ComplexStable isotopesNi-sulfides

satellite intrusion of the Bushveld Complex and contains the only mine currentlyproducing Ni as themain product in South Africa. Previously reported sulfur isotopic data suggest that externallyderived sulfur was important for the generation of sulfide mineralization in harzburgite units that comprise thelower portion of the Complex. We have determined oxygen isotopic compositions of minerals and variations inpyroxenes and plagioclase compositions, and analyzed fluid inclusions in quartz from highly altered zones inorder to evaluate the importance of country rock assimilation and hydrothermal processes in the evolution of thecomplex. Oxygen isotopic compositions of pyroxene and olivine from the sulfide-bearing harzburgite in theComplex range from5.2 to 5.9‰ and suggest that bulk assimilationof high-18O country rocks by parentalmagmasdid not occur or was minimal. Sulfur transfer to magma was accomplished via a fluid which did not perturb theoxygen isotope compositionof themagma.Harzburgite, pyroxenite, andgabbronorite that overlie the ore-bearingharzburgites contain minor amounts of sulfide minerals that are characterized by mantle-like δ34S values of 0±2‰. Low abundances of sulfides in these units are consistent with segregation of cotectic proportions.Hydrothermal alteration is pronounced in the upper and lowermarginal zones of the Complex. Secondary quartzand albite are common in the upper portion of the Main Gabbronorite unit. Measurements of fluid inclusion inquartz indicate that alteration took place at ~500 °C and involved a moderately saline fluid of 20 to 25 eq. wt.%NaCl. Oxygen and hydrogen isotopes of secondary silicate minerals are most consistent with the involvement ofan evolved, long path-lengthmeteoric water that infiltrated the upper and lower contacts zones of the Complex.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

The Uitkomst Complex in the Mpumalanga province of SouthAfrica hosts the only mine currently producing Ni as the main productin South Africa. It is temporally related to the Bushveld Complex(Maier et al., 2004), but intruded ~10 km below the current level of thebase of the Bushveld Complex. Previous studies of the UitkomstComplex by Gauert et al. (1995, 1996) and Li et al. (2002) havesuggested that a large portion of the S involved in ore formation wasderived from sedimentary country rocks, and that the intrusivesequence involved multiple pulses of magma with different S isotopiccompositions. Stratigraphic variations in silicate mineral and wholerock chemical analyses, including the enrichment of incompatibleelements in the upper portion of the Complex, resemble thoseobserved in layered intrusions formed by in situ fractional crystal-lization of a single magmatic pulse, such as the Palisades Sill (Shirley,1987). Magma emplacement history and the nature of magma —

country rock interaction at Uitkomst (including the mechanism of S

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transfer from country rocks) are important issues that must beaddressed to improve our understanding of the genesis of Ni–Cusulfide mineralization in the Uitkomst Complex. The incompatibleelement-enriched portion of the Uitkomst Complex is characterizedby the presence of quartz and abundant secondary minerals such asactinolite, chlorite and albite. These zones could have been producedby interaction with magmatic or externally derived fluids.

The work reported in this communication was initiated with thefollowing goals: (1) better constrain the mechanisms of S assimilation,with particular attention to the relative roles of country rock partialmelting versus S addition via fluids during contact metamorphism, (2)evaluate the importance of multiple magma emplacement eventsrelative to closed system crystallization, and (3) evaluate the effect ofhydrothermal processes in element redistribution in the Complex. Ourinterpretations are based onmineral textures, silicate mineral composi-tions, stable isotopic measurements (S, O, H), and fluid inclusionanalyses.

2. Regional geology and lithology of the Uitkomst Complex

The Uitkomst Complex is located in the Mpumalanga province inSouth Africa, approximately 250 km east of Pretoria (Fig. 1A). The

Fig. 1. Simplified geologic maps and cross-section sketch of the Uitkomst Complex (after Maier et al., 2004).

130 A. Sarkar et al. / Chemical Geology 257 (2008) 129–138

Complex and associated Ni mineralization have been studied by severalinvestigators (e.g., Gauert et al.,1995; Gauert, 2001; deWaal et al., 2001;Li et al., 2002; Maier et al., 2004). In the Uitkomst area, the Nelshoogtegranite of Archean age (~3220 Ma, Anhaeusser, 2001) is overlain inplaces by the Godwan Formation of the Ventersdorp Supergroup. TheGodwan Formation consists of basaltic lavas, polymict quartzites andshales (Hornsey, 1999). In the footwall of the Uitkomst Complex, theArchean basement is overlain by quartzite of the Black Reef Formation,dolomite and quartzite of the Malmani Subgroup (dated at 2449.9±2.6 Ma by U–Pb analyses of zircon; Walraven and Martini, 1995), theBevets conglomerate member of the Rooihoogte Formation, and theTimeball Hill Formation. The upper dolomite of the Malmani Subgroupcontains layers of chert andboth sedimentaryandhydrothermal sulfidesthat are associated with organic matter. The Timeball Hill Formation iscomposed of ~1200 m of graphatic, and locally sulfidic, shale, withirregularly distributed layers of quartzite and ironstone.

The Uitkomst Complex is a tabular body with an exposed areameasuring ~0.8 km inwidth and ~8 km in length (Fig.1B). Geophysicaldata suggest that the Complex may extend for several kilometers atdepth (Gauert, 1998). The Uitkomst Complex intruded the gently-dipping (5°–10°) Late Archean Transvaal Supergroup, near the contactwith the Archean basement, a stratigraphic level ~10 km below thebase of the coeval Bushveld Complex (Maier et al., 2004). TheUitkomst Complex can be divided into seven rock units (Fig. 1C). Frombottom to top these are the Basal Gabbronorite, Lower Harzburgite,Chromitiferous Harzburgite, Main Harzburgite, Pyroxenite, MainGabbronorite and Upper Gabbronorite units. The Basal Gabbronoriteunit is ~3.5 m in thickness and is characterized by medium grainedclinopyroxene, orthopyroxene, and plagioclase, with up to 10 vol.% ofsulfides. It is in direct contact with the Oaktree quartzite of theMalmani Subgroup and is interpreted to be the first magma emplacedat Uitkomst (Gauert et al., 1995). This unit is laterally more extensive

than the overlying Lower Harzburgite unit. Sulfide veins, mainlychalcopyrite and disseminated pyrrhotite, chalcopyrite and pentlan-dite constitute the sulfide assemblages in this unit.

The Lower Harzburgite unit is ~65 m in thickness, is relativelyheterogeneous, and has an intrusive contact with the Basal Gabbro-norite unit. It consists of various ultramafic rocks including poikiliticharzburgite, wehrlite, lherzolite and rare amphibolite. This rock unitalso contains metamorphosed dolomite, chert, and shale xenoliths ofvariable sizes, particularly in olivine wehrlites at the base of the unit(Hornsey, 1999). The xenoliths locally constitute up to a third of thevolume of the unit. Where xenoliths are abundant the unit is rich inmica and amphibole, thought to be related to magma-xenolithinteraction. Disseminated and net-textured sulfides are presentthroughout the unit but are more abundant in olivine wehrlites (upto 30 vol.%).

The Lower Harzburgite unit is overlain by ~55 m of pervasivelyaltered rocks that are characterized by the assemblage serpentine-talc-chlorite-carbonate. The Chromitiferous Harzburgite unit is com-posed of olivine and disseminated chromite set in a matrix ofphlogopite, amphibole, and disseminated sulfides (up to 5 vol.%).Chromitite layers or lenses are also present, particularly in the upperpart of the unit. The Chromitiferous Harzburgite unit is laterally moreextensive than the underlying unit.

The overlying Main Harzburgite unit is ~300 m in thickness andconstitutes the major part of the Uitkomst Complex. This unit ischaracterized by medium-grained poikilitic harzburgite with minordunite and a few thin chromitite layers, concentrated in the lower halfof the unit. Orthopyroxene is the principal interstitial mineral. Thelower half of this unit contains disseminated sulfides (~2 vol.%),mainly pyrrhotite and pentlandite.

The Main Harzburgite unit is overlain by ~70 m of the Pyroxeniteunit. Gauert (1998) described this unit as a transitional zone with rock

131A. Sarkar et al. / Chemical Geology 257 (2008) 129–138

types that vary from olivine pyroxenite at the bottom, throughpyroxenite in the middle to norite-gabbronorite at the top. It's contactwith the overlying unit is transitional. The Pyroxenite unit is relativelyfresh and consists of orthopyroxene and minor clinopyroxene.Disseminated sulfides, mainly pyrrhotite, (up to 1 vol.%) are presentin the upper part of the unit (Gauert et al., 1995).

The overlying Main Gabbronorite unit is ~250 m in thickness andshows a gradational contact with the underlying unit. The lowerpart of the unit is composed of relatively fresh norite, gabbronoriteand olivine gabbro. Xenoliths of sedimentary rocks are present inthis unit (Hornsey, 1999). The upper parts of the unit are alteredleucogabronorite and diorite. Secondary minerals including albite,amphibole and quartz with minor chlorite and epidote are presentin the altered rocks (Fig. 2A and C). Minor calcite veins are present inmicrofractures. The diorite has high concentrations of Zr, P, Nb, Yand LREE (Gomwe and Maier, 2003) and was previously interpretedto be a “sandwich horizon” formed by in situ differentiation (Maieret al., 2004).

At the contact between theMain and the Upper Gabbronorite unitsis a breciated diorite zone of ~5 m in thickness. Rocks in the UpperGabbronorite unit are highly altered, with saussuritized plagioclase,tremolite, chlorite and biotite. The studies of Gomwe andMaier (2003)indicate progressive enrichments of Ni, Cr, andMgOwith height is thisunit. Maier et al. (2004) suggested that this unit represents a distinct,

Fig. 2. (A) Photomicrographs of albite and quartz from the upper part of the Main Gabbronorthe same unit. Sample UP-8, plane polarized light. (C) Photomicrograph showing actinoliteplagioclase and water. Sample UP-10, crossed polars. (D) Histogram of homogenization tem

but older, intrusive phase. A fine-grained (chilled) marginal zoneoccurs in the uppermost 3 m of the unit.

3. Sampling and analytical methods

Forty-nine samples utilized in this study are from drill core SH176(25°36′ N. latitude, 30°28′30″ W. longitude), located in the north-western part of the Complex (Fig. 1B). Petrographic examination wasconducted using a standard transmitted and reflected light micro-scope. Silicate mineral compositions were determined by wavelengthdispersive X-ray analysis using a CAMECA SX-50 electron microprobeat Indiana University, Bloomington. Analytical conditions for majorelements were 15 kV, 20 nA beam current and a peak counting time of20 s. Nickel was analyzed with a 100 nA beam current and a peakcounting time of 100 s. The detection limit for nickel under suchconditions is ~80 ppm.

Fluid inclusion composition and temperature were determinedusing a USGS-style gas flow heating and freezing stage. A heating rateof 2 °C per minute was used. The heating and freezing stage wascalibrated using the critical point of water (370.1 °C), themelting pointof pure CO2 (−56.6 °C), the melting point of carbozole (248.7 °C) andthe melting point of Adipic acid (154.6 °C). Reproducibility oftemperature measurements were within ±3.5 °C at temperaturesabove 300 °C. The salinities of fluid inclusions were estimated by

ite unit. Sample UP-8, crossed polars. (B) Fluid inclusion in quartz from the upper part of, quartz and chlorite produced as a result of the interaction between two pyroxenes,peratures of fluid inclusions in quartz.

Table 1Pyroxene, olivine and feldspar compositions of samples from the Uitkomst Complex

BoreholeSH/176

Rock unit Depth(m)

An Mg#cpx Mg#opx Ni in ol(ppm)

Fo

UP-1 Upper Gabbronorite 135.41 57.6UP-2 Upper Gabbronorite 137.45 70.0 0.49 0.44UP-3 Upper Gabbronorite 152.25 68.4 0.45UP-4 Upper Gabbronorite 163.33 72.7 0.48UP-5 Upper Gabbronorite 181.09 2.6UP-6 Upper Gabbronorite 192.25 33.6UP-7 Upper Gabbronorite 202.29 49.1UP-8 Main Gabbronorite 216.22 3.3UP-9 Main Gabbronorite 217.28 3.2UP-10 Main Gabbronorite 229.21 0.22UP-11 Main Gabbronorite 246.52 4.3 0.27UP-12 Main Gabbronorite 264.52 2.4 0.28UP-13 Main Gabbronorite 278.54 2.2 0.24UP-14 Main Gabbronorite 286.40 2.0 0.30 0.21UP-15 Main Gabbronorite 303.17 56.7UP-16 Main Gabbronorite 313.12 59.5 0.13 259 10UP-17 Main Gabbronorite 329.19 0.23 0.20UP-18 Main Gabbronorite 358.33 65.0 0.28UP-19 Main Gabbronorite 375.42 68.7 0.30 242 28UP-20 Main Gabbronorite 382.48 63.9 0.28UP-21 Main Gabbronorite 402.64 0.48UP-22 Main Gabbronorite 409.04 0.50UP-23 Main Gabbronorite 432.11 0.52UP-24 Main Gabbronorite 450.44 0.41UP-26 Pyroxenite 459.75 76.8 0.69UP-28 Pyroxenite 481.48 66.2 0.72 0.72 528 87UP-29 Pyroxenite 492.04 62.0 3514 89UP-30 Pyroxenite 507.15 0.72UP-31 Pyroxenite 516.08 0.72UP-32 Pyroxenite 527.04 3350 89UP-33 Main Harzburgite 532.17 2901 90UP-34 Main Harzburgite 547.66 3225 88UP-35 Main Harzburgite 567.14 3494 91UP-36 Main Harzburgite 584.22 3382 90UP-37 Main Harzburgite 603.60 0.86 2035 90UP-38 Main Harzburgite 620.50 0.88 3548 88UP-39 Main Harzburgite 639.10 0.86 3686 89UP-40 Main Harzburgite 647.60 0.89 3618 89UP-41 Main Harzburgite 662.34 0.87 3321 89UP-42 Main Harzburgite 673.34 0.87 3593 89UP-43 Main Harzburgite 696.70 3252 89UP-44 Main Harzburgite 716.60 3453 89UP-46 Main Harzburgite 751.42 0.89 1930 88UP-47 Main Harzburgite 775.52 2213 89UP-48 Main Harzburgite 791.06 1966 89UP-49 Main Harzburgite 807.04 2317 89UP-50 Main Harzburgite 828.32 1932 88UP-51 Cmt. Harzburgite 854.06 1690 89UP-53 Cmt. Harzburgite 905.60 1333 86UP-55 Lower Harzburgite 931.50 1910 89

132 A. Sarkar et al. / Chemical Geology 257 (2008) 129–138

freezing point depression (Bodnar, 1993). Trapping temperatures werecalculated following the pressure correctionmethod of Bodnar (1995).

Single grains of olivine, pyroxene, and plagioclase for isotopicmeasurements were drilled from polished sections using a 0.75 mmdiamond drill bit. Intergrowths of biotite, amphibole, chlorite, andepidote could not be separated and were collected as compositesamples for H isotope analyses. Clusters of interstitial grains of sulfideminerals were drilled using a 0.5 mm tungsten carbide drill bit. Quartzwas purified using hydroflurosilicic acid (Sayers et al., 1968). Oxygenwas liberated from pyroxene and plagioclase powders by reactionwith BrF5 at 650 °C (Clayton and Mayeda, 1963). Olivine powders(b100 mesh) were reacted at 700 to 750 °C for a minimum of 16 h.Oxygen yields for olivine varied between 75 and 95%, with triplicateanalyses showing no δ18O variation with yield. Oxygen was thenconverted to CO2 by reaction with a heated graphite disc. Isotopicratios were determined using a Finnigan MAT 252 mass spectrometerwith a both samples and standards showing a reproducibility of±0.2‰ (2 sigma). Oxygen isotopic compositions are reported in

standard δ notation relative to Vienna Standard Mean Ocean Water(VSMOW). NBS-28 quartz has a δ18O value of 9.6±0.2‰ in our lab, andSan Carlos olivine has a value of 5.2±0.2‰ (2 sigma values).

Samples for hydrogen isotope data were analyzed using a hightemperature TCEA method similar to that reported by Sharp et al.(2001). Samples were loaded in silver cups and combusted at ~1400 °Cin a glassy carbon column. Hydrogen was analyzed using a FinniganDelta Plus-XP stable isotope mass spectrometer. Laboratory standardsfrom −60 to −110‰ were calibrated relative to a value of −65‰ forNBS-30 biotite. Analytical reproducibility was within ±0.2‰ andreproducibility of samples analyzed in duplicate or triplicate was ±2‰(2 sigma).

For sulfur isotopic analyses, sample powders and small amounts ofV2O5 were loaded into tin cups and analyzed using ElementalAnalyzer-Continuous Flow Isotope Ratio methodology (Studley et al.,2002). Samples were measured using a Finnegan MAT252 isotoperatio mass spectrometer. Analytical precisionwas better than ±0.05‰,whereas sample reproducibility was ±0.2‰ (2 sigma). NBS-127(BaSO4, δ34S=20.3‰) and IAEA standards (S1=−0.3, S2=20.8) wereused as standards (values on the SO2 scale). Sulfur isotopic composi-tions are reported in standard δ notation relative to Vienna CanonDiablo Triolite (VCDT).

4. Results

4.1. Mineral chemistry

The Mg-number of pyroxenes (MgO/(MgO+FeO), molar), and Ancontent of plagioclase for the Uitkomst Complex are listed in Table 1and illustrated in Fig. 3A. The Fo and Ni contents of olivine from Li et al.(2002) are included in Fig. 3B for comparison. The Mg-numbers oforthopyroxene in the Main Harzburgite unit are rather constant andclose to 0.88. The Mg-numbers of orthopyroxene in the overlyingPyroxenite unit are lower and close to 0.71. The Mg-numbers oforthopyroxene in the overlying Main Gabbronorite unit are signifi-cantly lower and decrease with height from 0.50 at the base of the unitto 0.20 in the middle part of the unit. Orthopyroxene from one samplein the Upper Gabbronorite unit has a Mg-number of 0.42 which issimilar to that of orthopyroxene in the lower part of the MainGabbronorite unit. The stratigraphic variation of Mg-number ofclinopyroxene is in general similar to that of orthopyroxene (Fig. 3A).However, the Mg-numbers of clinopyroxene in the middle part of theMain Gabbronorite unit are significantly lower (~0.20) than in thelower part of the Upper Gabbronorite unit and in the underlyingPyroxenite unit.

The An contents of plagioclase in the Pyroxenite unit and in thelower part of the overlying Main Gabbronorite unit are similar,ranging between 62 and 77. The An contents of plagioclase in thelower part of the Main Gabbronorite unit decrease from 63 at the baseof the unit to 56 is the middle part of the unit. The An contents ofplagioclase in the overlying Upper Gabbronorite increase again up to79. Primary plagioclase is not preserved in the altered zone in theupper part of the Main Gabbronorite unit, but has been replaced tosecondary albite. The stratigraphic variations of pyroxene Mg-numbers and plagioclase An contents described above are similar tothose of olivine Fo contents reported by Li et al. (2002) (Fig. 3B).However, the stratigraphic variation of olivine Ni contents is morecomplicated. Olivine with significantly different Ni contents mayoccur in a single unit, such as the Main Harzburgite where twodifferent olivine populations exist. One population of olivine containsclose to 2000 ppm and the other one close to 3000 ppm.

4.2. Oxygen, S, and H isotopes

Oxygen and S isotopic composition of the Uitkomst Complex arelisted in Table 2. The variations are illustrated in Fig. 4A and B,

Fig. 3. Stratigraphic variations of pyroxene, olivine and feldspar compositions and Ni content in olivine in the Uitkomst Complex.

133A. Sarkar et al. / Chemical Geology 257 (2008) 129–138

respectively. The δ18O values of olivine from the Main Harzburgiteunit show a restricted range between 5.2 and 5.9‰. Several pyroxenesamples from the same unit have δ18O values from 5.4 to 5.9‰ that aresimilar to the values of coexisting olivine. The δ18O values of pyroxenein the Pyroxenite and Main Gabbronorite units vary from 5.4 to 6.9‰.One plagioclase sample from the Basal Gabbronorite unit has a δ18Ovalue of 6.6‰, higher than the δ18O value of coexisting pyroxene(5.4‰). Similarly, the δ18O values of plagioclase in the Pyroxenite,Main Gabbronorite and Upper Gabbronorite units are slightly higherthan those of coexisting pyroxene. Albite from the highly altered zonein the Main Gabbronorite unit shows δ18O values from 8.8 to 9.7‰.Quartz from the same zone has δ18O values between 11.1 and 15.2‰.The δ18O values of the pyroxene and plagioclase from the UpperGabbronorite unit range from 5.6 to 6.9‰ and from 6.2 to 8.9‰,respectively.

Two new S isotopic analyses for the Upper Gabbronorite unit andthree new S isotopic analyses for the Chromitiferous Harzburgite unitare added to the plot of Li et al. (2002) (Fig. 4B). Except for one samplefrom the altered zone, other sulfur-poor samples from the Pyroxeniteunit and the Upper Gabbronorite unit are characterized by δ34S valuesbetween −1.5 and 2‰. Sulfur-poor samples from the upper part of theMain Harzburgite unit have δ34S values from −2.4 to 9.1‰. Sulfur-poor

samples from the lower part of the unit have δ34S values from 0 to 2‰.The sulfide-bearing rocks in the Main and Chromitiferous harzburgiteunits are all characterized by negative δ34S value from −4.5 to −7.1‰. Inthe sulfide-bearing Lower Harzburgite unit δ34S values range from −4 to−2.8‰. The Basal Gabbronorite unit shows a δ34S range of 0.2 to 2.7‰.

The δD values of mixtures of amphibole, biotite, chlorite andepidote are listed in Table 2. The samples fall in a narrow range of δDvalues between −55 and −50‰. The δ18O and δD values of fluidinvolved in hydrothermal alterationwere calculated based on mineralδ18O and δD values and mineral-water fractionation factors (seebelow) at temperatures estimated from fluid inclusions.

The δ18O values of the Malmani dolomite vary between 19 and23‰ (Schiffries and Rye, 1989; Harris et al., 2005). Although δ18Ovalues of the Timeball Hill Shale have not been determined, values ofshale and pelitic hornfels from the Transvaal Supergroup determinedby Schiffries and Rye (1989) and Johnson et al. (2003) range from 11.1to 15.2‰. We assume that samples from the Timeball Hall Shale fall inthe normal shale range of ~8 to 15‰ (e.g., Ripley, 1999). These valuesare much higher than δ18O values of mantle derived magmas (5–6‰;Eiler, 2001) and for this reason bulk assimilation of as little as 5% of a15‰ country rock causes an increase of 0.5‰ in the δ18O value of themagma.

Table 2Oxygen, sulfur and hydrogen isotope values for samples from the Uitkomst Complex

BoreholeSH/176

Rock unit Depth(m)

δ34S δ18Opx δ18Oqt δ18Opl δ18Ool δD

UP-1 Upper Gabbronorite 135.41 0.65 6.9 7.6UP-2 Upper Gabbronorite 137.45 0.71 5.6 6.2UP-3 Upper Gabbronorite 152.25 6.7 8.9UP-4 Upper Gabbronorite 163.33 0.65 6.7 7.1UP-5 Upper Gabbronorite 181.09 7.5 8.7UP-7 Upper Gabbronorite 202.29 6.3UP-8 Main Gabbronorite 216.22 14.5 9 −52UP-9 Main Gabbronorite 217.28 12.4 8.9 −56UP-10 Main Gabbronorite 229.21 15.2 9.7UP-11 Main Gabbronorite 246.52 14.1 8.8 −55UP-12 Main Gabbronorite 264.52 9.5 −50UP-13 Main Gabbronorite 278.54 11.1 9.7UP-14 Main Gabbronorite 286.40 2.61 15.2 9.2UP-15 Main Gabbronorite 303.17 6.1 6.1UP-16 Main Gabbronorite 313.12 0.26 6.4 8.9UP-17 Main Gabbronorite 329.19 1.34 5.9 6.1UP-18 Main Gabbronorite 358.33 1.85 6.2 6.9UP-19 Main Gabbronorite 375.42 0.26 5.9UP-20 Main Gabbronorite 382.48 −0.63 6.7 7.7UP-21 Main Gabbronorite 402.64 −0.64 6.8 7.6UP-22 Main Gabbronorite 409.04 0.34 6.6 7.2UP-23 Main Gabbronorite 432.11 −0.61 6.9 7.6UP-24 Main Gabbronorite 450.44 0.86 5.4 6.2UP-25 Pyroxenite 458.36 1.06 5.6UP-26 Pyroxenite 459.75 1.16 5.7UP-27 Pyroxenite 467.30 0.12UP-28 Pyroxenite 481.48 0.12 6 6.5UP-29 Pyroxenite 492.04 −0.85 5.8 6.3UP-30 Pyroxenite 507.15 −0.56 5.9 6.3UP-31 Pyroxenite 516.08 −0.86 6.3UP-32 Pyroxenite 527.04 −0.02UP-35 Main Harzburgite 567.14 9.10 5.9 5.3UP-36 Main Harzburgite 584.22 4.85 5.4UP-37 Main Harzburgite 603.60 −2.45 5.4 5.2UP-38 Main Harzburgite 620.50 2.86 5.5UP-39 Main Harzburgite 639.10 1.50 5.2UP-40 Main Harzburgite 647.60 1.10 5.2UP-41 Main Harzburgite 662.34 −0.20 5.6UP-42 Main Harzburgite 673.34 1.90 5.9UP-45 Main Harzburgite 741.04 −6.45UP-46 Main Harzburgite 751.42 5.2UP-47 Main Harzburgite 775.52 −4.74 5.5UP-48 Main Harzburgite 791.06 −4.45 5.5 5.2UP-49 Main Harzburgite 807.04 −5.70 5.9 5.9UP-50 Cmt. Harzburgite 828.32 −7.11 5.4UP-51 Cmt. Harzburgite 854.06 −5.50 5.7UP-53 Cmt. Harzburgite 905.60 −6.15 5.9UP-55 Lower Harzburgite 931.50 −2.59 5.8UP-56 Lower Harzburgite 974.61 −5.58UP-57 Lower Harzburgite 974.67 −4.00UP-59 Basal Gabbronorite 987.55 0.25 8.1UP-60 Basal Gabbronorite 990.29 2.70 5.4 6.6

⁎New values from this study; other δ34S values are from Li et al. (2002).

134 A. Sarkar et al. / Chemical Geology 257 (2008) 129–138

4.3. Fluid inclusions

Most quartz grains in the altered zone of the Main Gabbronoriteunit contain both primary and secondary fluid inclusions. The primaryinclusions are either two-phase (vapor–liquid) or, rarely, three-phase(vapor, liquid and daughter minerals). Where present daughterminerals may constitute 10 to 15 vol.% of the inclusions. The shapesof the inclusions are rounded or rectangular (Fig. 2B). They occursolitarily or as clusters in the cores and mantles of quartz grains. Thediameters of the inclusions vary from 1 to 5 µm. Most of the primaryfluid inclusions homogenize to liquid between 310 and 330 °C (Fig. 2D).The salinity of the fluids estimated using freezing temperatures of thetwo-phase fluid inclusions range from 20 to 25 equivalent wt.% NaCl.After application of a pressure correction based on the estimatedemplacement depth of 10 km (Gauert et al., 1995), the trapping

temperatures of the fluid inclusions range between 490 and 510 °C(Bodnar, 1995).

5. Discussion

5.1. Multiple magma inputs, fractional crystallization and cotectic sulfidesegragation

Based on field relations the Basal Gabbronorite has been identifiedas the first intrusive phase in the Uitkomst Complex (Gauert et al.,1995; Maier et al., 2004). The contact of this unit with the overlyingharzburgite is marked by the presence of sedimentary xenoliths.Sulfide assemblages in the lower portion of the Main Harzburgite unitare characterized by strongly negative δ34S values, and relatively low-Ni olivine (Fig. 3B). Oxygen isotopic compositions of olivine andpyroxene from the sulfide-bearing harzburgites show values (5.2 to5.9‰) which indicate that the magmas from which they crystallizedhad experienced little, if any, bulk oxygen contamination involvinghigh-18O rock country rocks. The Δpyroxene–olivine values in the MainHarzburgite unit vary between 0 to 0.3‰ andΔplagioclase–pyroxene valuesin the Pyroxenite range from 0.4 to 0.5‰. These values are consistentwith equilibration at magmatic temperatures in excess of 1000 °C(Chiba et al., 1989). The oxygen isotopic data are in line with thesuggestion made by Li et al. (2002) that S from country rocks wastransported tomagma via a fluid and that bulk assimilation of countryrocks was minimal. We note that the δ18O values of orthopyroxenefrom the Bushveld Complex reported by Schiffries and Rye (1989)range from 6.1 to 6.9‰ and those of olivine and pyroxene reported byHarris et al. (2005) range from 5.6 to 7.6‰. Both Schiffries and Rye(1989) and Harris et al. (2005) conclude that significant amounts ofcontamination by high-18O country rocks are required to explain therelatively high δ18O values of minerals in the Bushveld Complex. Suchhigh δ18O values are not a characteristic of the harzburgite andpyroxenite in the Uitkomst Complex, suggesting that the parentalmagma was less contaminated than that responsible for the BushveldComplex.

The sulfide-poor portion of the Main Harzburgite shows a sharpchange in both S isotope composition and the content of Ni in olivine(Figs. 3 and 4).Maier et al. (2004) also showa pronounced drop in Cu/Pdvalues (b1000) in this unit. Oxygen isotopic values of olivine are withinthe normal mantle range. Our data suggest that this unit represents anew intrusive pulse. The low Ni content in olivine and a δ34S value of−2.5‰ in sample UP-37 are more in line with characteristics of thesulfide-bearing Main Harzburgite. We suggest that this sample is axenolith from the sulfide-bearing Main Harzburgite unit. The twosamples with elevated δ34S values (9.1‰, UP-35 and −4.8‰, UP-36) inthe sulfide-poor Main Harzburgite contain secondary pyrite whichreplaces oxideminerals. The presence of secondary pyrite distinguishesthese samples from those with magmatic pyrrhotite–chalcopyriteassemblages. The sulfur isotope composition of samples from the baseof the sulfide-poor Main Harzburgite unit through the Upper Gabbro-norite unit are 0±2.5‰ and show no evidence for either country rockcontamination or distinct magma inputs. Elevated δ18O values andvariable Δplagioclase–pyroxene values (0–2.5‰) in the Main Gabbronoriteunit are possibly related to post magmatic alteration and are discussedbelow.

The stratigraphic compositional variations of pyroxene, olivine,and feldspar in the sulfide-poor Main Harzburgite, Pyroxenite, andMain Gabbronorite units suggest that these units may be related byfractional crystallization.We have used theMELTS program of Ghiorsoand Sack (1995) and a starting composition of a siliceous, high-MgObasalt to simulate fractional crystallization as a means of producingthe sulfide-poor Main Harzburgite, Pyroxenite, and Main Gabbronor-ite units. Results at 1 kbar pressure and the QFM buffer are listed inTable 3. The crystallization sequence and mineral compositions matchwell with those observed in these units. S isotope compositions of

Fig. 4. Stratigraphic variations of O and S isotopic compositions of the Uitkomst Complex.

135A. Sarkar et al. / Chemical Geology 257 (2008) 129–138

sulfide minerals from the units are consistent with mantle-derived S.We have used the equation of Li and Ripley (2005) to compute thesulfur content of the parental magma at sulfide saturation todetermine if the observed sulfide abundances are consistent withclosed system crystallization. Fig. 5 illustrates that with a startingmagma containing ~1000 ppm sulfur (corresponding to ~20–25% ofmantle partial melting) sulfide saturation is attained at ~23%fractionation. The total amount of sulfides present in the Harzbur-gite–Pyroxenite–Gabbronorite units (Fig. 5) is consistent with the

Table 3Results of fractional crystallization modelling using MELTS modelling at QFM and 1 Kb

Initial magmacomposition

wt.% % crystallization Temperature(°C)

Olivine

SiO2 55.73 20 1295 Fo85TiO2 0.33 25 1265Al2O3 11.34 31 1225MnO 0.18 38 1180MgO 14.05 54 1155FeOT 8.32 55.6 1150CaO 6.38 60 1135K2O 0.89 70 1100Na2O 1.73 74 1070NiO 0.06 75 1065 Fo33P2O5 0.07 78 1040 Fo26Cr2O3 0.16 81 1010 Fo19

84 960 Fo11

attainment of sulfide saturation via fractional crystallization, withmodest enrichment of sulfide in the Pyroxenite unit and slightdepletion in the overlying Main Gabbronorite unit (Fig. 6). Theweighted average S concentration of the sulfide-poor Harzburgite,Pyroxenite, and Main Gabbronorite units is 910 ppm, in line with ourassumed 1000 ppm for the parental magma. Our mineralogic, Sisotopic, and S concentration data are consistent with an origin for theupper portion of the Main Harzburgite unit, the Pyroxenite, and MainGabbronorite units via in situ fractional crystallization.

opx cpx Spinel Feldspar Equivalent unit

En96 Cmt (54) Main HarzburgiteEn95 Cmt (52) ProxeniteEn93 Cmt (47) ProxeniteEn78 Cmt (43) An69.7 ProxeniteEn69 Cmt (42) An64 Main GabbronoriteEn69 En33 An63.5 Main GabbronoriteEn69 En32 Cmt (38) An60 Main Gabbronorite

En29 Cmt (23) An52 Main GabbronoriteEn27 An46 Main GabbronoriteEn26 An45 Main GabbronoriteEn23 An40 Main GabbronoriteEn19 An35 Main GabbronoriteEn14 An5 Main Gabbronorite

Fig. 5. Sulfur content at sulfide saturation (SCSS) for closed system fractionation of asiliceous, high-MgO basaltic magma computed using the equation of Li and Ripley(2005). The crystallization sequence is based on results obtained from MELTS (Ghiorsoand Sack, 1995) at 1 kbar and the QFM buffer. The enrichment of sulfur via fractionalcrystallization is shown for a starting composition of 1000 ppm. Sulfide saturation isreached at ~23% crystallization. The crystallization sequence is similar to that observedin the Main Harzburgite, Pyroxenite and Main Gabbronorite units. Sulfide saturationwould have been attained at the time of pyroxene crystallization.

136 A. Sarkar et al. / Chemical Geology 257 (2008) 129–138

5.2. Hydrothermal alteration

Hydrothermal alteration is intensely developed in the upper partof the Main Gabbronorite unit, the lower part of the UpperGabbronorite unit, and in the Basal Gabbronorite. Alteration ofplagioclase and pyroxene also occur in the lower part of the MainGabbronorite unit, although the overall intensity of alteration is much

Fig. 6. Sulfur concentrations in the sulfide-poor Main Harzburgite, Pyroxenite and MainGabbronorite units. The weighted S concentration is ~910 ppm, and the distribution isconsistent with the attainment of sulfide saturation via fractional crystallization asillustrated in Fig. 5.

reduced. Pyroxene in the altered zone has been converted to mixturesof actinolite, chlorite, and quartz via reactions such as (e.g., Manningand Bird, 1995; Fig. 4A and C):

CaMgSi2O6ðclinopyroxeneÞ þ 9MgSiO3ðorthopyroxeneÞþCaAl2Si2O8ðplagioclaseÞ þ 5H2O ¼ Ca2Mg5Si8O22ðOHÞ2ðactinoliteÞþMg5Al2Si3O10ðOHÞ8ðchloriteÞ þ 2SiO2ðquartzÞ

ð1Þ

Albitization of plagioclase (An2 to An5) occurred via Ca-liberatingreactions similar to (Merino, 1975):

Naþ þ NaAlSi3O8CaAl2Si2O8ðplagioclaseÞ þ H4SiO4 þ 4SiO2ðquartzÞ¼ 2NaAlSi3O8ðalbiteÞ þ Alþ3 þ 4OH− þ Caþ2

ð2Þ

Continued reaction with H2O locally lead to the formation ofepidote (e.g. Bettison-Varga et al., 1995):

3NaAlSi3O8ðalbiteÞ þ Alþ3 þ 8H2O þ 4Caþ2

¼ Ca4Al6Si6O24ðOHÞ2ðepidoteÞ þ 3Naþ þ 3SiO2ðquartzÞ þ 16Hþ

ð3Þ

The homogenization temperatures of fluid inclusions in quartz (e.g.reaction 3), together with high salinity values, are consistent with amagmatic origin for the fluids that were involved in hydrothermalalteration at Uitkomst. This hypothesis is also in line with incompatibleelement enrichment in the rocks, and their late-stage crystallizationnear the top of the Uitkomst magma chamber. However, the elevatedδ18Ovalues of quartz, and to a lesserextent albite, are not consistentwiththe involvement of magmatic fluids derived from mafic magmas. Fluidδ18O values were calculated using temperatures determined from fluidinclusions and the quartz-water fractionation factor of Matsuhisa et al.(1979). Hydrogen isotopic compositions were estimated using petro-graphic modal analyses and a bulk fractionation factor calculated fromthe mineral-water fractionation factors for biotite–water and hornble-nde–water (Suzuoki and Epstein, 1976), chlorite–water (Graham et al.,1984b) and epidote–water (Chacko et al., 1999). The δ18O value of waterin equilibriumwith quartz at 500 °C is estimated to be between 10.3 and13.1‰. The δD value of water in equilibrium with biotite, amphibole,

Fig. 7. δ18OH2O versus δDH2O plot showing the isotopic composition ofwater in equilibriumwith hydrous minerals and quartz from the Uitkomst Complex at 500 °C. The illustratedexchange paths for seawater and both high- and low- latitude meteoric water at variouswater/rock weight ratios were calculated using an example shale with δ18O=13‰, δD=−35‰, 4wt.%water,Δ18Orock–water andΔDrock–water of 2 and −14‰, respectively.MWL is theglobal meteoric water line. Although the initial isotopic composition of meteoric water inthe area of the Uitkomst Complex is poorly constrained, we suggest that our data aremoreconsistent with the involvement of a long path-length meteoric water characterized by alow time-integrated water/rock ratio in the generation of the hydrothermal alteration inthe Main Gabbronorite unit.

137A. Sarkar et al. / Chemical Geology 257 (2008) 129–138

epidote, and chlorite is estimated to be between −16 and −21‰ (Table 2,Fig. 7). The fluid δ18O and δD values are higher than those of typicalmagmatic fluids.

The development of alteration zones at the top and bottom of theUitkomst Complex strongly suggests thatfluids could have derived fromcountry rocks. In Fig. 7 exchange trajectories for meteoric water andseawater are shown that produce fluids with isotope compositionssimilar to those computed for the alteration zones. The analysis suggeststhat long path-lengthmeteoricfluids that exchangedwith country rocks(here represented by an average shale with a δ18O of 13‰ and δD of−35‰), could have been responsible for the hydrothermal alteration.Low time-integrated fluid/rock ratios were generated as a function offluid circulation in country rocks in response to heat associated withmagma emplacement at Uitkomst.

We also modelled the isotope evolution of fluid produced viadehydration reactions in country rocks using a reaction that generatescordierite and biotite (e.g., Ripley et al., 1992):

KAl2ðAlSi3O10ÞðOHÞ2; ðmuscoviteÞ þMg5Al2Si3O10ðOHÞ8ðchloriteÞþ2SiO2ðquartzÞ ¼ðMg; FeÞ2Al4Si5O18ðcordieriteÞþKðMg;FeÞ3AlSi3O10ðOHÞ2ðbiotiteÞ þ 4H2O

ð4Þ

We suggest this reaction could have taken place during contactmetamorphism at Uitkomst. Dehydration was modelled using step-wise increments at 500 °C. Although bulk fluid δD values are similar tothose calculated from the Uitkomst data, δ18O values are moreelevated. Dehydration fluids could have been added to magma beforecrystallization, in line with our premise that this was the principalmechanism for the transfer of S from country rocks to the magma.However, it is unlikely that fluids generated by country rockdehydration reactions would persist through crystallization of themagma. For this reason we favor a model involving the interaction ofcooling igneous rocks andwater of meteoric origin near themargins ofthe intrusion. Fluid/rock ratios were higher in the upper portion of theMain Gabbronorite unit as evidenced by the δ18O values of quartz andalbite. Less elevated, but still anomalous, δ18O values occur locally inthe Upper Gabbronorite unit and lower portion of the MainGabbronorite that flank the intensely altered zones. Plagioclase δ18Ovalues vary from near normal mantle values of 6.1‰ to anomalousvalues as high as 7.7‰ in these units (Fig. 4A). Similarly elevated δ18Ovalues of plagioclase (6.1 to 8.1‰) are found in the Basal Gabbronoriteunit as well, and suggest that hydrothermal fluids responsible for thealteration in this zone were also of meteoric derivation.

6. Conclusions

Oxygen isotopic compositions of olivine and pyroxene from thesulfide-bearing harzburgite unit of the Uitkomst Complex demon-strate that although externally derived sulfur was important for thegeneration of Cu–Ni sulfide mineralization, oxygen contamination viaassimilation of high-18O country rocks did not occur, or was minimal.Interestingly, this contrasts with the elevated δ18O values of olivineand pyroxene in the Bushveld Complex reported by Schiffries and Rye(1989) and Harris et al. (2005) which suggest that Bushveld parentalmagmas may have been contaminated by from 10 to 30% of high-18Ocountry rocks. For the Uitkomst Complex sulfur transfer via a fluidassociated with dehydration reactions in the country rocks is sug-gested; the amount of oxygen added to the magma as H2O in such aprocesswas not sufficient tomarkedly perturbmagma δ18O values, butthe addition of country rock S resulted in a major change in magmaδ34S values.

Sulfide-poor harzburgite, pyroxenite, and gabbronorite that form theupper portion of the Uitkomst Complex are related via in-situ fractionalcrystallization. Sulfur concentrations, aswell as δ18O and δ34S values, areconsistent with cotectic segregation of immiscible sulfide liquid from an

uncontaminated mantle-derived magma. The δD values of hydrousminerals and the strongly-elevated δ18O values of quartz and albite inthe upper portion of the Uitkomst Complex indicate that quartz, albite,actinolite, chlorite andepidotewereproducedasa result of the incursionof evolved meteoric water. Fluid inclusions in quartz suggest thatalteration occurred at a temperature near 500 °C, and the fluid wascharacterized by salinities of 20 to 25 equivalent wt.% NaCl. Fluidinfiltration also occurred along the basal contact of the intrusion, butalteration and isotopic exchangeweremost intense in gabbronorite nearits upper margin.

Acknowledgements

We thank Jim Brophy, Erika Elswick, Chris Harris, and Ed Mathezfor thoughtful reviews of earlier versions of this paper and CraigMoore and Steve Studley for assistance in stable isotopic analyses. Wethank Roberta Rudnick for her assistance as the editor in charge of thismanuscript. This research was supported by a student research grantfrom the Society of Economic Geology to A. Sarkar and by NSF grantsof the United States (EAR-0608645 and EAR-0710910).

References

Anhaeusser, C.R., 2001. The anatomy of an extrusive-intrusive Archaean mafic-ultramafic sequence: the Nelshoogte Schist Belt and Stolzberg Layered UltramaficComplex, Barberton Greenstone Belt, South Africa. S. Afr. J. Geol. 104, 167–204.

Bettison-Varga, L., Schiffman, P., Janecky, D.R., 1995. Fluid-rock interaction in thehydrothermal upflow zone of the Solea Graben, Troodos ophiolite, Cyprus. Thetransition from basalt to metabasalt. GSA Spec. Publ. 296, 81–100.

Bodnar, R.J., 1993. Revised equation and table for determining the freezing pointdepression of H2O–NaCl solutions. Geochim. Cosmochim. Acta 57, 683–684.

Bodnar, R.J., 1995. Experimental determination of the PVTX properties of aqueoussolutions at elevated temperatures and pressures using synthetic fluid inclusions:H2O–NaCl as an example. J. Pure Appl. Chem. 67 (6), 873–880.

Chacko, T., Riciputi, L.R., Cole, D.R., Horita, J., 1999. A new technique for determiningequilibrium hydrogen isotope fractionation factors using the ion microprobe:application to the epidote-water system. Geochim. Cosmochim. Acta 63, 1–10.

Chiba, H., Chacko, T., Clayton, R.N., Goldsmith, J.R., 1989. Oxygen isotope fractionationsinvolving diopside, forsterite, magnetite, and calcite: applications to geothermo-metry. Geochim. Cosmochim. Acta 53, 2985–2995.

Clayton, R.N., Mayeda, T.K., 1963. The use of bromine pentafloride in the extraction ofoxygen from oxides and silicates for isotopic analysis. Geochim. Cosmochim. Acta27, 43–52.

de Waal, S.A., Maier, W.D., Armstrong, R.A., Gauert, C.D.K., 2001. Parental magma andemplacement of the stratiform Uitkomst Complex, South Africa. Can. Mineral. 39,557–571.

Eiler, J.M., 2001. Oxgen isotope variations of basaltic lavas and upper mantle rocks. In:Valley, J.W., Cole, D.R. (Eds.), Stable Isotope Geochemistry. . Rev. Mineral., vol. 43.MSA, pp. 319–364.

Gauert, C.D.K., 1998. The petrogenesis of the Uitkomst Complex, Mpumalanga Province,South Africa: Unpublished Ph.D. Thesis, Pretoria, South Africa, University ofPretoria. 315 pp.

Gauert, C.D.K., 2001. Sulfide and oxide mineralization in the Uitkomst Complex, SouthAfrica: origin in a magma conduit. J. Afr. Earth Sci. 32, 149–161.

Gauert, C.D.K., deWaal, S.A.,Wallmach, T.,1995. Geologyof the ultrabasic to basic UitkomstComplex, eastern Transvaal, South Africa: an overview. J. Afr. Earth Sci. 21, 553–570.

Gauert, C.D.K., Jordaan, L.J., de Waal, S.A., Wallmach, T., 1996. Isotopic constraints on thesource of S for the base metal sulfides of the Uitkomst Complex, Badplass, SouthAfrica. S. Afr. J. Geol. 99, 41–50.

Ghiorso, M.S., Sack, R.O., 1995. Chemical mass transfer in magmatic processes. IV. Arevised and internally consistent thermodynamic model for the interpolation andextrapolation of liquid–solid equilibria in magmatic systems at elevated tempera-tures and pressures. Contrib. Miner. Petrol. 119, 197–212.

Gomwe, T., Maier, W.D., 2003. A geological profile through the Uitkomst Complex on thefarm Slaaihoek, South Africa, with reference to platinum group elements and Sm–

Nd isotopes. Geol. Soc. Am. Abstr. Program 35 (6), 100.Graham, C.M., Atkinson, J., Harmon, R.S., 1984b. Hydrogen isotope fractionation in the

system chlorite–water. NERC 6th Prog. Rep. Res., NERC Publ. Ser. D 25, 139.Harris, C., Pronost, J.J.M., Ashwal, L.D., Cawthorn, R.G., 2005. Oxygen and hydrogen

isotope stratigraphy of the Rustenburg Layered Suite, Bushveld Complex,constraints on crustal contamination. J. Petrol. 46, 1–23.

Hornsey, A.E., 1999. The genesis and evolution of Nkomati mine Ni-sulfide deposit,Mpumalanga Province, South Africa.M.Sc. thesis, University of Natal (Durban). 224 pp.

Johnson, T.E., Gibson, R.L., Brown, M., Buick, I.S., Cartwright, I., 2003. Partial melting ofmetamorphic rocks beneath the Bushveld Complex, South Africa. J. Petrol. 49 (5),789–813.

Li, C., Ripley, E.M., 2005. Emperical equations to predict the sulfur content of maficmagmas at sulfide saturation and applications to magmatic sulfide deposits. Miner.Depos. 40, 218–230.

138 A. Sarkar et al. / Chemical Geology 257 (2008) 129–138

Li, C., Ripley, E.M., Maier, W.D., Gomwe, T.E.S., 2002. Olivine and sulfur isotopiccompositions of the Uitkomst Ni–Cu sulfide ore bearing complex, South Africa:evidence of sulfur contamination and multiple magma emplacements. Chem. Geol.188, 149–159.

Maier, W.D., Gomwe, T., Barens, S.J., Li, C., Theart, H., 2004. Platinum group elements inthe Uitkomst Complex, South Africa. Econ. Geol. 99, 499–516.

Manning, C.E., Bird, D.K., 1995. Porosity, permeability and basalt metamorphism. In:Schiffman, P., Day, H.W. (Eds.), Low-Grade Metamorphism of Mafic Rocks.Geological Society of America Special Paper, vol. 296, pp. 123–140.

Matsuhisa, Y., Goldsmith, J.R., Clayton, R.N., 1979. Oxygen isotopic fractionation in thesystem quartz–albite–anothite–water. Geochim. Cosmochim. Acta 43, 1131–1140.

Merino, E., 1975. Diagenesis in tertiary sandstones from Kettleman North Dome,California. I. Diagenetic Mineral. J. Sediment. Petrol. 45, 320–336.

Ripley, E.M., 1999. Sytematics of sulfur and oxygen isotopes in mafic igneous rocks andrelated Cu–Ni-PGE mineralization. Geol. Assoc. Can. Short Course Notes 13, 133–158.

Ripley, E.M., Butler, B.K., Taib, N.I., 1992. Effects of devolatilization on the hydrogenisotopic composition of pelitic rocks in the contact aureole of the Duluth Complex,northeastern Minnesota, USA. Chem. Geol. 102, 185–197.

Sayers, J.K., Chapman, S.L., Jackson, M.L., Rex, R.W., Clayton, R.N., 1968. Quartz isolationfrom rocks, sediments and soils for determination of oxygen isotope composition.Geochim. Cosmochim. Acta 32, 1022–1025.

Schiffries, C.M., Rye, D.M., 1989. Stable isotopic systematics of the Bushveld Complex: I.constraints on magmatic processes in layered intrusions. Am. J. Sci. 289, 841–873.

Sharp, Z.D., Atudorei, V., Durakiewiez, T., 2001. A rapid method for determination ofhydrogen and oxygen isotope ratios fromwater and hydrous minerals. Chem. Geol.173, 197–210.

Shirley, D.N., 1987. Differentiation and compaction in the Palisades Sill, New Jersey.J. Petrol. 28 (5), 835–865.

Studley, S.A., Ripley, E.M., Elswick, E.R., Dorais, M.J., Fong, J., Finkelstein, D., Pratt, L.M.,2002. Analysis of sulfides inwhole rock matrices by elemental analyzer-continuousflow isotope ratio mass spectrometry. Chem. Geol. 192, 141–148.

Suzuoki, T., Epstein, S., 1976. Hydrogen isotope fractionation between OH-bearingminerals and water. Geochim. Cosmochim. Acta 40, 1229–1240.

Walraven, F., Martini, J., 1995. Zircon Pb-evaporation age determinations of the Oak TreeFormation, Chuniespoort Group, Transvaal Sequence: implications for Transvaal-Griqualand West basin correlations. S. Afr. J. Geol. 98, 58–67.