Formation of the Main Sulfide Zone at Unki Mine, Shurugwi Subchamber of the Great Dyke, Zimbabwe: Constraints From

Research Paper

GEOSPHERE Formation of the main sulfide zone at Unki Mine, Shurugwi Subchamber of the Great Dyke, Zimbabwe: Constraints from

GEOSPHERE, v. 16, no. 2 petrography and sulfide compositions https://doi.org/10.1130/GES02150.1 Jeff B. Chaumba1 and Caston T. Musa2 1Department of Geology and Geography, University of North Carolina at Pembroke, 211 Old Main, 1 University Drive, Pembroke, North Carolina 28372, USA 18 figures; 2 tables; 1 set of supplemental files 2Unki Mines (Pvt.) Limited, Anglo American PLC, P.O. Box 254, Shurugwi, Zimbabwe

CORRESPONDENCE: [email protected] ABSTRACT history of the Great Dyke, whereas others were hosted by sulfides or as discrete platinum group CITATION: Chaumba, J.B., and Musa, C.T., 2020, For‑ mation of the main sulfide zone at Unki Mine, Shurugwi formed late during hydrothermal processes. Low minerals. Pentlandite [(Fe,Ni)9S8], for example, is Subchamber of the Great Dyke, Zimbabwe: Con‑ The major platinum group element (PGE) occur- concentrations of PGEs such as platinum (Pt), palla- characterized by elevated concentrations of Pd and straints from petrography and sulfide compositions: rence in the Great Dyke of Zimbabwe, the main dium (Pd), and rhodium (Rh) in base metal sulfides Rh (Junge et al., 2015). Such investigations of PGE Geosphere, v. 16, no. 2, p. 685–710, https://doi.org /10.1130/GES02150.1. sulfide zone, is a tabular stratabound layer hosted imply that the PGEs in the main sulfide zone and contents of sulfides have routinely been carried in pyroxenites, and it is broadly similar in form Unki Mine are hosted either in silicates and/or out utilizing electron probe microanalysis (EPMA; Science Editor: Andrea Hampel throughout the length of the Great Dyke. We con- platinum group minerals. Very low Co contents in Godel and Barnes, 2008; Osbahr et al., 2013; Zacca- Associate Editor: Susan Eriksson ducted a petrographic and sulfide composition pentlandites in the rocks under investigation are rini et al., 2014). The importance of using the EPMA study on a sulfide-enriched zone from the contact interpreted to imply that very limited Fe substitu- in analyses of PGEs is that it has a spatial resolution Received 11 April 2019 of the mafic sequence–ultramafic sequence through tion by Co, and also of Ni by Co, occurred. Broadly of ~1 μm, with detection limits of tens of parts per Revision received 6 July 2019 Accepted 13 December 2019 the main sulfide zone at Unki Mine in the Shurugwi comparable trends, with minor variations of Fe in million (ppm). Subchamber to its underlying footwall rocks to pyrrhotite, of Co and Ni in pentlandite, and of Cu in Several processes may be responsible for Published online 16 January 2020 place some constraints on the origin of the rocks. chalcopyrite, for example, likely reflect magmatic the origin of the PGE mineralization in layered Pyrrhotite, pentlandite, chalcopyrite, and pyrite processes. The concentrations of these metals in intrusions. Naldrett (1989) suggested that PGEs are the base metal sulfides that were encountered base metal sulfides vary sympathetically, indicat- precipitate from magma and accumulate on the during the study. Pyrrhotite, pentlandite, and chal- ing that their original magmatic signatures were top of a crystal pile. Vermaak and Hendriks (1976) copyrite typically occurred as inclusions in both subsequently affected by hydrothermal fluids. The and Boudreau (2019), in contradistinction, preferred primary (orthopyroxene, plagioclase, and clinopy- spiked pattern displayed by the variations in the an upward infiltration process whereby PGEs are roxene) and secondary (amphibole and chlorite) percent modal proportions of the base metal sul- precipitated and transported from footwall rocks silicate phases, whereas pyrite was observed in fides across the entire investigated stratigraphic by ascending, volatile-rich fluids. Although the only three samples, where it occurred in associa- section is interpreted to reflect remobilization of exact process responsible for the PGE mineral- tion with pyrrhotite. The concentrations of PGEs the sulfides during hydrothermal alteration. Deple- ization remains under debate, the close spatial, in the base metal sulfides were nearly all at or tions in some elements, which occur near the base and possibly genetic, relationship between PGE below minimum detection limits. The intercumulus and at the top of the investigated succession, are mineralization and base metal sulfides has been nature of some of these sulfides in the investigated likely a result of this hydrothermal alteration. documented by other researchers (Viljoen and sequence suggests that they were likely formed Schürmann, 1998). during the crystallization history of these rocks. Base metal sulfides in orthomagmatic Ni-Cu- The occurrence of pyrite, which we interpret to ■■ INTRODUCTION PGE deposits often carry significant concentrations be an alteration phase, suggests that a late-stage of PGEs. Several previous studies have helped shed event, likely formed during hydrothermal alteration, In the Great Dyke of Zimbabwe, the principal some light on the distribution of the PGEs between helped to concentrate the mineralization at Unki platinum group element (PGE)–bearing horizon, various sulfides such as pentlandite, pyrrhotite, Mine. In some cases, however, these sulfides occur termed the main sulfide zone, is hosted in ortho- chalcopyrite, and pyrite (Dare et al., 2010). From partially surrounding some chromite and silicate pyroxenites (Prendergast and Wilson, 1989). PGE experimental investigations, the distribution of This paper is published under the terms of the phases. Thus, some sulfides in the Unki Mine mineralization has been demonstrated to occur PGEs in base metal sulfides is inferred to be a con- CC‑BY-NC license. area were likely formed early in the crystallization bimodally in mafic-ultramafic intrusions, either sequence of sulfide liquid fractionation (Mungall

GEOSPHERE | Volume 16 | Number 2 Chaumba and Musa | Base metal sulfides from Unki Mine, Great Dyke Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/16/2/685/4968602/685.pdf 685 by guest on 30 September 2021 Research Paper

et al., 2005). From such experimental investigations, from the main sulfide zone suggest that magmatic cyclic units (Fig. 1B). A third and relatively small it has been observed that subsequent to the sep- fluids were involved in the alteration (Li et al., 2008). chamber, the Mvuradonha Chamber, occurs at the aration of a Fe-Ni-Cu sulfide liquid from a mafic Further, actinolite has both O and H isotope data extreme north end of the dike (Wilson and Pren- magma, a monosulfide solid solution (MSS) and a ranging from 5.0‰ to 5.6‰ and from 64‰ to 73‰, dergast, 1989). Cu-rich liquid precipitate at ~1000 °C (Kullerud et al., respectively, which Li et al. (2008) interpreted to be The structure of the Great Dyke is synclinal, with 1969; Naldrett, 1989). Os, Ir, Ru, and Rh, based on consistent with magmatic fluids. inwardly dipping layers, where the dips increase experimental work on partition coefficients, pref- Despite numerous studies on mineralized reefs from the central axis toward the margins but erentially partition into the Fe-rich MSS, whereas in layered intrusions documenting the association decrease again near the walls (Worst, 1960; Wilson Cu, Pt, Pd, Ag, and Au are enriched in the fraction- of PGEs with sulfide-enriched horizons, only a and Prendergast, 1989). At depth, the Great Dyke ated liquid rich in Cu. Such partition coefficients tiny fraction of these studies (Brynard et al., 1976; has a dike-like feeder that, in places, has been inter- depend strongly on the Fe/S ratio of a magmatic Todd et al., 1982; Kawohl and Frimmel, 2016) have preted to be connected to deep-seated magma sulfide liquid (Barnes et al., 2001). Li et al. (1996) focused on the composition of the sulfide miner- chambers (Podmore and Wilson, 1987). The trans- concluded that partition coefficients of all metals als themselves. We had three aims in carrying out verse section of the Great Dyke (Fig. 2A) has been increase between MSS and liquid with increasing this base metal sulfide composition study in the interpreted to be trumpet-shaped, with individual S content, both in the sulfide liquid as well as in Unki Mine area of the Shurugwi Subchamber of layers thinning away from the axis and eventually the MSS. The liquid rich in Cu then crystallizes as the Great Dyke. The first one was to carry out a becoming incorporated in the Border Group, which an intermediate solid solution (ISS) at ~900 °C. petrographic study of the sulfides that occur in rests against the Great Dyke walls (Wilson and Pren- At temperatures below 600 °C, the ISS has been the PGE-enriched zone straddling the main sulfide dergast, 1989). The longitudinal section of the layers shown to break down to chalcopyrite and cubanite, zone from the gabbronorites at the base of the of the Great Dyke plunges gently toward the cen- whereas the MSS breaks down to pyrrhotite and mafic sequence to the footwall of the main sul- ter of each chamber to form an overall boat-like pentlandite. Consequently, pyrrhotite and pentlan- fide zone. The second aim was to document the structure (Worst, 1960). Based on U-Pb dating of dite should be enriched in Os, Ir, Ru, and Rh, and concentrations of major and minor elements and zircon and rutile from orthopyroxenites of the P1 chalcopyrite should be enriched in Pd, Pt, Ag, and their distributions in base metal sulfides occurring pyroxenite layer, the age of the Great Dyke is 2575.4 Au, if no discrete platinum group minerals crystal- in this sequence in order to place some constraints ± 0.7 Ma (Oberthür et al., 2002). Further Sensitive lize or no subsolidus reequilibration occurs upon on the origin of the mineralization. Since hydro- high-resolution ion microprobe (SHRIMP) U-Pb further cooling. thermal alteration affected the main sulfide zone studies on the Great Dyke and its satellites by In the Great Dyke, all three models of orthomag- samples under study (Chaumba, 2017), our third Wingate (2000) yielded a comparable Neoarchean matic (Irvine, 1983), hydromagmatic (Boudreau and and final aim was to use MELTS modeling to infer emplacement age of 2574 ± 2 Ma for baddeleyite. McCallum, 1992; Boudreau, 2016), and micronugget the temperature at which this hydrothermal alter- In comparison to other layered intrusions such as models (Tredoux et al., 1995) have been proposed ation likely occurred and determine the Great Dyke the Bushveld Complex, isotope data from the inter- for the origin of the main sulfide zone (Wilson, minerals that likely retained their crystallization val straddling the contact between the ultramafic 2001). Debate on these models continues (Robb, temperatures. and mafic sequences of the Great Dyke indicate a 2005). Stratigraphic offsets in peak concentrations less enriched composition of initial 87Sr/86Sr ratios

of PGEs and base metal sulfides, which occur in (0.7024–0.7028) and εNd (−1 to +1; Maier et al., 2015). the main sulfide zone (Oberthür, 2011), have, in part, ■■ BRIEF OUTLINE OF THE GEOLOGY OF Sulfur isotope values (δ34S) of Great Dyke samples been attributed by Li et al. (2008) to the interac- THE GREAT DYKE from studies carried out by Li et al. (2008) on pyrite, tion between magmatic PGE-bearing base metal pyrrhotite, and pentlandite ranged from 0.1‰ to sulfide assemblages and hydrothermal fluids. Li The Great Dyke of Zimbabwe is one of the 1‰, and Maier et al. (2015) obtained δ34S values on et al. (2008) presented mineralogical and textural world’s largest and better-studied intrusions. It is bulk-rock samples that ranged from −0.3‰ to 0.3‰, evidence from the Hartley Platinum Mine, located a layered mafic-ultramafic intrusion that cuts across which fall within the 0 ± 5‰ range of mantle val- in the Darwendale Subchamber of the Great Dyke, the Zimbabwe craton (Fig. 1A), and it also acts as ues (Ohmoto and Goldhaber, 1997). Thus, relatively which they interpreted to indicate that alteration of host to the second largest resource of PGEs in the moderate amounts of contamination of the Great base metal sulfides and mobilization of metals and world. The Great Dyke is largely composed of two Dyke parent magma must have occurred. S occurred during hydrothermal alteration. Sulfur major chambers, the North and South Chambers The stratigraphic section in the vicinity of Unki isotope data of pyrite, pyrrhotite, and chalcopyrite (Fig. 1B), which Wilson and Prendergast (1989) Mine in the Shurugwi Subchamber, from just above ranging from 0.1‰ to 0.8‰, as well as O isotope further subdivided into subchambers on the basis the mafic sequence–ultramafic sequence contact data for orthopyroxene ranging from 5.1‰ to 6.5‰ of continuity of layering, style, and thickness of to the footwall of the mineralized zone, is shown

o 31o E in Figure 2B. Here, plagioclase pyroxenite is over- 27 E o 32 E Zambezi orogeny lain by a 6-m-thick layer of plagioclase websterite, o 17 S which is capped by a very thin chromitite layer (or MUSENGEZI SUBCHAMBER chromitite stringer; Fig. 2B). Overlying this chromi- Harare tite stringer, there are gabbronorites of the mafic

Mutare sequence (Fig. 2B). A base metal–enriched zone, Approximate limit of Zimbabwean cratonShurugwi called the base metal sulfide zone, occurs ~3 m o Bulawayo Masvingo 17 S beneath the plagioclase websterite–plagioclase Zvishavane Phanerozoic pyroxenite contact (Fig. 2B). cover o 21 S Orogenic belt The main sulfide zone at Unki Mine is a 180 cm Great Dyke Granitoids package of rocks composed of the PGE subzone Greenstone belt 0 50 100km and a base metal subzone (BM subzone; Fig. 2C). East Dyke DARWENDALE SUBCHAMBER

Umvimeela Dyke The peaks in Pt, Pd, Rh, Cu, Au, and Ni occur in a sulfide-enriched horizon—the base metal sulfide HARARE zone (Fig. 2C). The PGE subzone occurs stratigraph- o 18 S ically beneath the BM subzone, with a transitional contact (Fig. 2C). The BM subzone is enriched in Ni and Cu (Fig. 2C). The resource width, the stope width, and the PGE mineralized zone width have thicknesses that vary widely (Fig. 2C). The main sulfide zone, therefore, was encountered toward NORTH CHAMBER the lower part of the investigated succession in this

SEBAKWE work (Figs. 2B and 2C). o SUBCHAMBER 19 S

Unki Mine ■■ SULFIDE MINERALIZATION IN THE GREAT DYKE AND IN THE SHURUGWI SUBCHAMBER

Within the Great Dyke succession, there are sev-

SHURUGWI

SUBCHAMBER eral PGE-enriched layers, which also include most o 20 S of the chromitite layers (Oberthür et al., 2002), as well as several silicate horizons in the upper por- Ma c Sequence tions of the ultramafic sequence (Prendergast and Ultrama c Sequence East Dike Keays, 1989; Prendergast and Wilson, 1989; Wilson

WEDZA

SOUTH CHAMBER Satellite dikes and Tredoux, 1990; Wilson and Prendergast, 2001; SUBCHAMBER Craton & cover rocks Oberthür et al., 2002; Oberthür, 2011). According to

Umvimeela Dike Orogenic belts Oberthür (2002, 2011), up to 2 ppm PGEs (Pt/Pd = Major faults & fractures 0.1) occur in the C1d chromitite and its host rocks. PGEs in the Shurugwi Subchamber, as in the rest of the Great Dyke, occur mainly in a tabular stratabound layer hosted in pyroxenite termed the SOUTHERN 0 km 50 100 o SATELLITES o main sulfide zone. The main sulfide zone is located Limpopo30 orogenyE 31 E in orthopyroxenites close to, and overlapping with, the websterite layer and contains up to 6% sulfide. Figure 1. Map showing location of the Great Dyke within the Zimbabwe Archean craton (after Wilson and Prendergast, 1989). The satellite dikes, major fracture patterns, and faults associated with the Great Dyke are shown together with the Great Dyke Having been discovered over a century ago (Wag- chambers and subchambers. The location of Unki Mine in the Shurugwi Subchamber is also shown. ner, 1914), the main sulfide zone is similar in form

within the main sulfide zone are, thus, defined by Exposed width (km) peak Pd levels that occur near the base of the reef, 4 3 2 1 0 1 2 3 4 whereas peak Pt and Cu levels occur at progres- 14 o dip present day erosion level sively higher stratigraphic levels within the main sulfide zone (Oberthür, 2011). km The main sulfide zone has been classified as 1 Gabbronorite Orthopyroxenite a sulfide-hosted, magmatic PGE deposit (Wilson 2 Dunite-harzburgite et al., 2000). The regular distribution of PGEs in all areas where the main sulfide zone has been inves- 3 Border Group Granitoid country rock Figure 2. (A) Transverse section of tigated has been interpreted to indicate a primary, the Great Dyke showing attitude sulfide-controlled fractionation pattern, whereby Vertical Strat. Lithology Samples of layering and its relationship to the PGEs have been scavenged from the magma scale (m) column wall rocks (after Wilson and Pren- due to their strong partitioning into primary sulfide dergast, 1989). (B) Stratigraphic 10 Gabbronorite section in the Unki Mine area (Naldrett and Wilson, 1990; Prendergast and Keays, MusGa01/02 9 Chromitite showing locations of samples 1989). Other analogues of this process have been used in this study. (C) Assay pro- 8 MusCr01 reported in the Munni Munni intrusion of Western file of the main sulfide zone at Unki MusWebba01a/02a 7 Mine showing the vertical distribu- Australia (Barnes et al., 1990; Barnes, 1993). Plagioclase websterite tion of Pt, Pd, Ni, and Cu as well as According to Coghill and Wilson (1993), plati- 6 the platinum group element (PGE) num group minerals in the Shurugwi Subchamber 5 subzone and the base metal (BM) subzone (BMSZ). The PGE miner- occur in three distinct textural environments: 4 MusWebb01/02 alized zone, the stope width, as (1) at the boundary of sulfides and silicates/hydro- BM 3 MusPxhw01 well as the resource width are silicates, (2) entirely enclosed within sulfides, and Subzone Plagioclase pyroxenite also shown. 2 Peg. plagioclase pyroxenite (3) entirely enclosed within silicate or hydrosilicate Cu MusPeg01 Ni minerals. Wilson (2001) concluded, from orthopy- Stope width Stope Resource width Resource PGE mineralized zone PGE mineralized 1 Plagioclase pyroxenite MusBMSZ01 roxene composition, PGE, and Cu concentration 0 BMSZ Pt MusIRUP01 Plagioclase pyroxenite studies, that PGE-bearing horizons of the Great -1 Pd MusPxaFwt01 Dyke are composed of subzones within which Footwall Fault -2 Plagioclase pyroxenite constant Pd:Pt ratios occur, although significant MusPxbFwt01 PGE variations can occur between the subzones. Subzone

■■ SAMPLING AND ANALYTICAL in all the chambers of the Great Dyke (Worst, 1960). itself is further subdivided into a Pt-dominant upper TECHNIQUES The width of the main sulfide zone varies from 2 to section and a Pd-dominant lower section, with the 8 m, and it can contain up to 5 ppm PGEs and up to two sections being generally separated by an inter- Samples utilized in this study were collected 8% sulfides over 2–3 m (Oberthür, 2011). The main mediate section characterized by moderate Pd/Pt from a stratigraphic zone straddling the main sul- sulfide zone is 1.8 m thick at Unki Mine (Fig. 2B) ratios (Wilson and Prendergast, 2001). fide zone at Unki Mine, currently the only PGE mine and ~2.5 m in the Wedza Subchamber (Prender- A general vertical zonation of minerals occurs in the Shurugwi Subchamber (Fig. 2B), and sample gast, 1991), and it lies close beneath, or overlaps, in the main sulfide zone (Fig. 2C), whereby Fe-Ni descriptions are provided in Table 1. The samples the boundary between the orthopyroxenites and sulfides dominate the lower part, and Cu sulfides were obtained from a borehole that was drilled the overlying websterite (Fig. 2). The vertical dis- dominate the upper part (Coghill and Wilson, toward the axis of the Shurugwi Subchamber at tribution of sulfides (pyrrhotite, pentlandite, and 1993). The sulfides occur as interstitial phases to Unki Mine. Samples were collected from the over- chalcopyrite) and PGEs in the main sulfide zone orthopyroxene, and they show a heterogeneous lying gabbronorites of the mafic sequence (samples displays a pattern wherein the base metals are distribution on a scale of millimeters to centime- MUSGa01/02), stratigraphically above the main dominant in the upper part, termed the BM subzone, ters; in several cases, they are concentrated at the sulfide zone (samples MusCr01, MusWebb01a/02a, and the PGEs are concentrated in the lower part, boundaries of plagioclase oikocrysts (Wilson, 1992). MusWebb01/02, NusPxhw01, and MusPeg01), termed the PGE subzone (Fig. 2C). The PGE subzone These so-called “offset” metal distribution patterns and through the main sulfide zone (samples

TABE 1. IST OF SAMPES FROM MAIN SUFIDE ZONE, UNI MINE, ZIMBABE, AND ADJACENT ROCS Sample ID Stratigraphic location Description MUSGa01/02 Mafic unit, 10 cm above the Medium‑grained, light greenish brown gabbronorite with relative abundances as follows: Pl Op Cp. Up to 1.5 seuences contact sulfides. MUSCr01 Mafic‑ultramafic contact Dark/black chromite stringer with chromium dissemination in places and coarse‑grained sulfides Py, Po, and Ccp. 10 m above BMSZ Up to 3 sulfides. MUSeba01/02 Immediately below the gabbronorite Medium‑grained, green brown acicular Pl websterite, needle‑like and flaky pyroenes, and fine disseminated sulfides contact 10 m above BMSZ up to 3 sulfides. Cp Op Pl. MUSebb01/02 ebsterite/Pl pyroenite gradational Green/gray brown, medium‑grained plagioclase websterite transition with short stubby and elongated Op crystals contact 100 mm above contact and coarse disseminated sulfides up to 5 sulfides, acicular Pl decreasing towards contact. MUSPhw01 Hanging wall of the BMSZ 50 cm, Grayish brown, medium‑grained Pl pyroenite with coarse disseminated sulfides up to 6 sulfides; reef base metal reef subzone. MUSPeg01 6–8 m below the websterite/ Coarse‑grained grayish brown pegmatoidal plagioclase pyroenite coarse Op, Pl, and Cp crystals; grains up to Pl websterite contact 2 cm and coarse net tetured sulfides. Up to 2 sulfides. 0.75–1 m above BMSZ MUSBMSZ01 BMSZ 10 m below websterite Grayish brown, medium‑grained plagioclase pyroenite with coarse disseminated sulfides up to 6 sulfides; reef contact, reef and marker horizon. MUSIRUP01 Below footwall Coarse‑grained, grayish brown pegmatoidal plagioclase pyroenite with large Cp oikocrysts, no hydromagmatic alteration and sulfide mineralization. ess than 1 sulfides. MUSPafwt01 Above the footwall fault Grayish brown, medium‑grained poikilitic Pl pyroenite with fine disseminated and patchy sulfides up to 1.5 sulfides; 50 cm below BMSZ reef, PGE subzone. MUSPbfwt01 Below the footwall fault Grayish brown, medium‑grained, highly poikilitic and granular Pl pyroenite, no hydromagmatic alteration. Up to 1 1.65 m below BMSZ sulfides. Note: BMSZbase metal sulfide zone; PGEplatinum‑group element; Plplagioclase; Oporthopyroene; Cpclinopyroene; Pypyrite; Popyrrhotite; Ccp chalcopyrite.

MusBMSZ01 and MusIRUP) to its footwall (sam- pyroxenite is a very coarse-grained, grayish brown, with both a secondary electron detector and a ples MUSPxaFwt01 and MUSPxbFwt01; Figs. 2B pegmatoidal plagioclase pyroxenite composed of backscattered electron detector, which provided and 2C; Table 1). Sample MUSCr01 came from the coarse-grained orthopyroxene, plagioclase, and compositional information to visualize different chromitite stringer occurring right at the ultramafic clinopyroxene crystals with grain sizes up to 2 cm phases. Compositions were analyzed first by X-ray sequence–mafic sequence contact (Fig. 2B; Table 1). occurring together with coarse net-textured sulfides. energy-dispersive spectroscopy (EDS) for quali- Plagioclase pyroxenite is overlain by a 6-m-thick The PGE subzone and the BM subzone (from which tative and semiquantitative analysis, and then by layer of plagioclase websterite, which is capped by sample BMSZ01 was obtained) have an overlap X-ray wavelength-dispersive spectroscopy (WDS) a very thin chromitite layer (or chromitite stringer; of ~30 cm between them. The BM subzone com- for quantitative analysis using Smithsonian stan- Fig. 2B). The PGE subzone is located below the mences ~20 cm below the Pt peak. In this work, dards. At least three spots were collected for each base metal sulfide zone (sample BMSZ01, which the base metal sulfide zone refers to the sampled phase. Fifteen second counting times were used is part of the main sulfide zone; Figs. 2B and 2C) position within the BM subzone. A sample from on peak and background measurements. For the and the BM subzone; both are distinguished based the BM subzone was obtained ~3 m below the pla- calculation of the oxides, the ZAF matrix correction on their metal profiles, whereby the concentration gioclase websterite–plagioclase pyroxenite contact system of Armstrong (1988) was used. of the PGEs begins to rise to appreciable amounts (Figs. 2B and 2C). Use was made of the rhyolite-MELTS model (ppm levels) as both base metals and sulfide con- Thin sections were first examined under a petro- (v. 1.0.x; Gualda and Ghiorso, 2015) for modeling centrations increase. Sample MusIRUP01 is the graphic microscope. Then, sulfides were analyzed the crystallization temperatures of Great Dyke other sample obtained from the main sulfide zone under a microprobe on polished thin sections. minerals by changing the controlling variable of (Figs. 2B and 2C). Mineral compositions of the sulfide minerals were pressure as equilibrium was repeatedly calculated. The pegmatoidal plagioclase pyroxenite, which obtained using the JEOL JXA-8530F Hyperprobe Isobaric fractional crystallization upon cooling was ranges from a few centimeters to over 2 m in housed at Fayetteville State University, Fayetteville, used to model batches of the parental magma to thickness, was encountered 6–8 m below the North Carolina (Chaumba et al., 2016). The Hyper- the Great Dyke at the liquidus at defined pressures. websterite–plagioclase websterite contact (0.75–1 m probe was operated with a beam current of 20 nA, The parental magma composition to the Great Dyke above the base metal sulfide zone) in the Shurugwi an accelerating voltage of 30 kV, and a minimum utilized in MELTS calculations was that of the East Subchamber. The pegmatoidal plagioclase beam diameter of 1 μm. Images were acquired Dyke (Wilson, 1982).

■■ RESULTS Pl Chr Chr Pl Petrography of the Main Sulfide Zone at Chr Chr Pl Unki Mine Chr Chr Chr Chr Chr Under the petrographic microscope, fine- to Chr Pl Pl coarse-grained chromite crystals from the chromi- Chr Chr tite stringer occur together with either plagioclase Chr Pl or orthopyroxene (Figs. 3A–3F). Where a chromitite Pl stringer occurs at the base of the mafic sequence, Chr such as in the Unki Mine area of the Shurugwi Chr Pl Chr Chr Subchamber, subrounded to idiomorphic chromite 1mm Pl Pl Pl crystals are typically enclosed in coarse-grained, polysynthetically twinned poikilitic plagioclase crystals (Fig. 3A). Almost all chromite crystals from the Chr Chr chromitite stringer under investigation enclose very fine-grained plagioclase crystals (Fig. 3). Anhedral Pl Chr plagioclase crystals from the chromitite stringer Chr are typically enclosed in chromite crystals from the Pl Pl chromitite stringer, and they are not in optical conti- Pl Chr nuity with plagioclase crystals that are not included in chromite (Fig. 3C). In some cases, subhedral chro- Chr Chr Pl mite crystals are oriented across the twin plane of Pl Chr simply twinned plagioclase crystals, in addition to Chr finer-grained chromite crystals from the chromitite stringer, which occur in different twin sets than the Pl simply twinned plagioclase crystals (Fig. 3D). E The boundary between the ultramafic and mafic sequences in the Unki Mine area of the Chr Chr Chr Shurugwi Subchamber is defined by a sharp con- Chr Opx tact between coarse-grained orthopyroxene and Opx Chr Chr plagioclase crystals (Figs. 3E, 3F, 4A, and 4B). At Pl this boundary, subhedral chromite crystals from Chr Opx the chromitite stringer, which in some places Pl completely enclose very fine-grained plagioclase Chr Chr crystals, are also partially or completely enclosed by fine-grained plagioclase crystals (Figs. 3E, 3F, Chr Chr Chr 4A, and 4B). Minute orthopyroxene crystals also Opx occur as inclusions in chromite from the chromitite stringer (Figs. 4A–4F). Very fine-grained pockets of plagioclase crystals commonly occur as inclusions Figure 3. Sample MusCr01. (A) Photomicrograph of poikilitic plagioclase (Pl) enclosing numerous fine- to medium-grained crystals of chromite (Chr); crossed polars. (B) Fine-grained chromite crystals occurring at the contact of plagioclase crystals and also within within the coarse-grained orthopyroxene crystals, some medium-grained plagioclase crystals; crossed polars. (C) Numerous curvilinear crystals of chromite enclosing plagioclase which occur near the contact with the underlying crystal that is in optical continuity with the coarse-grained plagioclase enclosing the chromite crystals; crossed polars. (D) Simple ultramafic rocks (Figs. 4A and 4B). In the upper twinning in coarse-grained plagioclase enclosing numerous fine-grained crystals of chromite; crossed polars. (E–F) Ultramafic sequence coarse-grained pyroxenite at contact with mafic sequence partially surrounds fine-grained chromite crystals (E, plane part of the P1 pyroxenite layer in the underlying polarized light; F, crossed polars); Opx—orthopyroxene. ultramafic sequence, fine-grained chromite crystals from the chromitite stringer tend to be completely

enclosed in very coarse-grained orthopyroxene crystals (Figs. 4C and 4D). Within the sulfide-enriched lower part in the Chr mafic sequence occurring close to the contact with Pl Pl the underlying ultramafic rocks, some plagioclase Pl crystals that enclose numerous fine-grained chro- Opx Pl mite crystals from the chromitite stringer show Opx evidence of alteration (Fig. 4E). Here, the pla- Pl gioclase has minute inclusions of sericite, with Chr Chr Chr even plagioclase crystals that are enclosed within Chr Chr Chr Pl Pl chromite crystals from the chromitite stringer also Pl showing minute inclusions of sericite (Fig. 4E). Pl Approximately 1 cm away from the ultramafic-mafic 1mm Pl sequence contact in the lower mafic succession, Pl coarse-grained clinopyroxene and plagioclase, which display cloudy appearances, comprise the Opx gabbroic rocks (Fig. 4F). Chr Chr Chr Chr Chromite and Sulfide Textural Relationships Revealed by Backscattered Electron Images

Chr Pl Opx Opx At the contact with the overlying lower mafic Opx Opx succession, very fine-grained inclusions of orthopyroxene are also common in chromites from the Chr chromitite stringer (Figs. 5A–5F), and they are Chr Pl included in poikilitic orthopyroxenes (Fig. 5A). Sul- fides rarely occur, partially rimmed by chromite crystals (Fig. 5A). Sulfides in the investigated suc- E Chr Chr Pl cession commonly occur as minute grains, which also occur as inclusions in silicate phases such as orthopyroxenes (Fig. 5B). Coarse-grained orthopy- Chr roxenes wrap around both chromites and sulfides Chr Pl (Fig. 5B). Cpx In the lowermost part of the lower mafic succes- Pl sion, relatively coarser-grained sulfide crystals tend to occur in interstices between chromite crystals Pl Pl from the chromitite stringer and plagioclase crystals, Chr Chr Pl and they also appear to partially surround chromite crystals from the chromitite stringer (Fig. 5C). Numerous very fine-grained sulfide crystals occur Figure 4. Sample MusCr01. (A–B) Photomicrographs of fine-grained chromite (Chr) enclosing very fine-grained plagioclase (Pl) crys- scattered within the coarse-grained orthopyroxene tals (A, plane polarized light; B, crossed polars). (C) Coarse-grained orthopyroxene (Opx) crystals enclosing numerous fine-grained (Fig. 5B) and plagioclase (Fig. 5C), as well as at the chromite and plagioclase crystals; crossed polars. (D) Interstitial chromite in orthopyroxene, which also encloses orthopyroxene in places; crossed polars. (E) Interstitial chromite in plagioclase, which also encloses plagioclase in places; crossed polars. (F) Coarse- boundary of chromite crystals from the chromitite grained clinopyroxene and plagioclase crystals in gabbronorite; crossed polars. stringer and orthopyroxene crystals (Fig. 5C). One sulfide crystal and two chromite crystals displayed a triple junction (Fig. 5D); the sulfide crystals typically

Chr Opx Chr Chr Opx Pl Pn Opx Sulf Chr Pn Chr Chr Po Sulf Opx Chr See (b) Pl below Chr Pl Chr Pn Opx Chr Chr Chr Chr Chr Opx Pl Opx Chr Opx Pl Chr Chr Pl Chr Pl Chl Sulf Sulf Chr Sulf Pn

Sulf Chr Chl Chr Sulf Opx Opx Opx

Figure 5. Backscattered electron (BSE) images, sample MusCr01. (A) Chromite (Chr) crystals that enclose very fine-grained Figure 6. Backscattered electron (BSE) images, sample MusCr01. plagioclase (Pl) crystals and partially enclose sulfide (Sulf) crystals. Orthopyroxene (Opx) and plagioclase are the other crys- (A) Numerous sulfide crystals occurring as inclusions in mostly tals. (B) Micron-size sulfide crystals enclosed in orthopyroxene. Chromite crystals are enclosed in orthopyroxene crystals. orthopyroxene (Opx) crystals, but fewer sulfides occur in (C) Intercumulus sulfide crystal occurring at a triple junction between plagioclase and two chromite crystals. Micron-size chromite (Chr) and plagioclase (Pl) crystals. Po—pyrrhotite. sulfide crystals are also enclosed by plagioclase, with some sulfide crystals occurring at the contact between chromite (B) Close-up of sulfide crystal in A, showing chlorite (Chl) oc- and orthopyroxene crystals. (D) Chromite crystals occurring in association with orthopyroxene and plagioclase crystals. curring in association with pentlandite (Pn).

occur either partially rimming chromite crystals or very fine-grained (Figs. 6A and 6B). Chlorite tends Pyrrhotite, pentlandite, and chalcopyrite can all as inclusions in orthopyroxene crystals in the P1 to occur together with pentlandite inclusions in occur as inclusions in some silicate phases such pyroxenite layer. chromite, forming a rim that partially surrounds as plagioclase (Fig. 7B). Some fine-grained sulfide crystals occur as the pentlandite (Fig. 6B). In some patches within the chromitite stringer, inclusions in relatively coarser-grained chromite Both pyrrhotite and pentlandite occupy inter- intergranular and fine-grained crystals (~60μ m) crystals from the chromitite stringer, whereas other stices between silicate and chromite crystals from of pyrrhotite, chalcopyrite, and pyrite (Fig. 8A) fine-grained sulfide crystals occur at the boundary the chromitite stringer, with other finer-grained have a tendency to form sulfide clusters that are between chromite crystals (Fig. 6A). The base metal pyrrhotite crystals occurring within silicate miner- relatively coarser grained (can exceed 1000 μm sulfide pentlandite typically occurs as inclusions als and at the boundaries between chromite and in size) than sulfides that occur as inclusions or within both chromite crystals from the chromitite orthopyroxene crystals (Fig. 7A). Chalcopyrite between silicate minerals. Besides the pyrite crys- stringer and orthopyroxene crystals (Figs. 6A and crystals occur wholly enclosed within plagioclase tals occurring within the chromite stringer, the only 6B). Pyrrhotite, another common base metal sulfide crystals, with the latter also containing a cluster of other lithologies that contain pyrite crystals are the encountered in the chromitite stringer, is typically both pentlandite and pyrrhotite crystals (Fig. 7B). gabbronorite from the mafic sequence (sample

MusGa02) and the pegmatoids occurring just above Also, in the base metal sulfide zone, crystals of pyrite and chromite crystals occur as inclusions in one or the BM subzone (sample MusPeg01; Tables 1 and 2; exceeding 100 μm in size occur at the center, with the other, and both sulfides and chromite occur as Supplemental Table1). All three base metal sulfides pentlandite again occurring at the edges of the pyrite inclusions in both orthopyroxene and plagioclase of pyrrhotite, chalcopyrite, and (subhedral) pyrite crystals (Fig. 9B). Chlorite occurs in association with crystals. Further, some sulfides partially wrap were observed occurring together only in the con- these base metal sulfides (Figs. 9A and 9B). around both chromite and some silicate crystals. tact zone chromitite sample MusCr01 (Fig. 8B). In the plagioclase websterite, which occurs Within the base metal sulfide zone, some sulfide stratigraphically below the chromite stringer, grains are slightly coarser grained, often >200 μm both pyrrhotite and chalcopyrite commonly form Unki Mine Main Sulfide Zone Sulfide Chemistry across and 400 μm in length, with pyrrhotite, which elongated crystals (Fig. 10A). Anhedral crystals of tends to occupy the center of the sulfide cluster, pyrrhotite and pentlandite typically occur together Representative pyrrhotite, pentlandite, chalco- occurring together with chalcopyrite crystals occur- in the plagioclase websterite, often surrounded by pyrite, and pyrite analyses from the main sulfide ring near the margins of pyrrhotite crystals (Fig. 9A). chlorite crystals (Fig. 10B). In summary, both sulfides zone at Unki Mine are shown in Table 2, with the

Chr Chr Chl Pl Chl Chl Chr Amp Chl Pl Po Chr Chr Po Chl Opx Chr Ccp Chl Pn Py Opx Opx Ccp Chr Po Po Po Chl Po See (b) below Chr Chr Po Pn Pn Chl Chr Chl Po Chr Opx Chr Po Pl Chr Opx Opx Opx Chr

Chl Opx Po Ccp Ccp Amp Pn Po Chl Pl Ccp Py Pn Po Po Po Pn Pn Pn Pn Opx Pn Py

Figure 7. Backscattered electron (BSE) images of samples MusCr01 Figure 8. Backscattered electron (BSE) images of sample Figure 9. Backscattered electron (BSE) images of sample (A) and MusPxbfwt01 (B). (A) Close-up of Figure 5C showing MusCr01. (A) Close-up of Figure 5D showing pyrrhotite (Po), MusBMSZ01. (A) Pyrrhotite (Po) and pentlandite (Pn) crystals 1 Supplemental Table. Results of analyses of standards pyrrhotite (Po) and pentlandite (Pn) crystals. Chr—chromite. pentlandite (Pn), and pyrite (Py) crystals occurring in association occurring as inclusions in amphibole (Amp). Chlorite (Chl) also and results of sulfides from Unki Mine, Zimbabwe. (B) Chalcopyrite (Ccp) crystal as well as an aggregate of pyrrho- with orthopyroxene (Opx) and plagioclase (Pl) crystals. Chr— occurs in association with amphibole. (B) Pyrrhotite and pent- Please visit https://doi.org/10.1130/GES02150.S1 or tite and pentlandite crystals occurring as inclusions in plagioclase chromite; Chl—chlorite; Amp—amphibole. (B) Close-up of area landite crystals occurring in association with amphibole and access the full-text article on www.gsapubs.org to (Pl from sample MusPxbfwt01). In this view, no sulfides occur in indicated in A, showing intercumulus pentlandite, pyrrhotite, chlorite. Opx—orthopyroxene; Ccp—chalcopyrite. view the Supplemental Table. the orthopyroxene (Opx) crystals occurring with the plagioclase. and pyrite crystals and chalcopyrite (Ccp) crystal.

TABE 2. REPRESENTATIE PYRRHOTITE, PENTANDITE, CHACOPYRITE, AND PYRITE ANAYSES IN T FROM THE MAIN SUFIDE ZONE AT UNI MINE, ZIMBABE Point Sample S Fe Cu Co Ni Ca Pd Ag Rh Zn Pb Au Mn Pt Ir Total MD 0.003 0.003 0.005 0.003 0.00 0.003 0.023 0.01 0.028 0.005 0.027 0.023 0.003 0.017 0.016 Po Pyrrhotite P1 Po Ga02 P1 39.329 57.126 0.030 0.032 0.39 0.133 0.023 0.01 0.028 0.005 0.109 0.023 0.003 0.017 0.030 97.1 P2 Ga02 P2 .861 5.837 0.02 0.027 0.381 0.100 0.023 0.01 0.028 0.005 0.12 0.023 0.01 0.017 0.016 100.39 P3 Ga02 P3 3.952 52.321 0.007 0.032 0.3 0.236 0.025 0.01 0.028 0.012 0.159 0.023 0.003 0.017 0.052 97.15 P17 Ga02 P17 39.157 56.76 0.005 0.028 0.336 0.359 0.023 0.01 0.028 0.005 0.158 0.023 0.003 0.017 0.06 96.85 P18 Ga02 P18 1.28 55.066 0.277 0.02 0.298 0.75 0.023 0.05 0.028 0.008 0.105 0.023 0.003 0.017 0.016 97.76 P19 Ga02 P19 38.939 57.136 0.013 0.030 0.368 0.15 0.023 0.01 0.028 0.005 0.027 0.023 0.007 0.017 0.03 96.69 P28 Ga02 P28 0.100 56.08 0.005 0.00 0.502 0.322 0.023 0.01 0.028 0.005 0.027 0.023 0.009 0.017 0.016 97.03 P51 Ga02 P1 39.093 57.213 0.023 0.01 0.539 0.105 0.023 0.01 0.028 0.005 0.035 0.023 0.003 0.017 0.016 97.09 P2 Ga02 P2 0.97 57.138 0.032 0.032 0.575 0.079 0.023 0.01 0.028 0.005 0.09 0.023 0.011 0.017 0.016 98.89 P3 Po Ga02 P3 39.139 57.091 0.062 0.037 0.56 0.02 0.023 0.01 0.028 0.005 0.027 0.023 0.006 0.017 0.016 96.9 Pn Pentlandite 1 Pn Ga02 P1 38.291 2.202 0.073 0.17 3.231 0.123 0.023 0.01 0.028 0.005 0.162 0.05 0.00 0.026 0.016 97.35 15 Ga02 P15 33.172 2.773 0.077 0.178 36.0 0.098 0.023 0.066 0.028 0.005 0.068 0.023 0.007 0.017 0.017 9.87 25 Ga02 P25 33.06 26.867 0.239 0.019 3.99 0.100 0.023 0.020 0.028 0.005 0.069 0.023 0.003 0.017 0.060 95.6 35 Ga02 P35 32.683 27.030 0.075 0.06 3.53 0.111 0.023 0.01 0.028 0.005 0.062 0.023 0.003 0.017 0.078 9.63 Ga02 P 0.636 55.620 0.018 0.03 1.23 0.169 0.023 0.01 0.028 0.005 0.053 0.023 0.012 0.017 0.07 97.82 16 Ga02 P16 55.213 1.52 0.012 0.130 1.658 0.273 0.023 0.01 0.028 0.005 0.069 0.023 0.013 0.017 0.030 98.86 23 Ga02 P23 5.951 3.08 0.036 0.029 1.075 0.085 0.023 0.01 0.028 0.005 0.125 0.023 0.008 0.02 0.058 99.51 31 Pn Ga02 P31 57.130 38.595 0.021 0.138 2.675 0.10 0.023 0.090 0.028 0.005 0.120 0.023 0.003 0.017 0.02 98.95 Ccp Chalcopyrite 11 Ccp Ga02 P11 35.979 26.552 31.951 0.010 0.003 0.296 0.023 0.08 0.028 0.010 0.027 0.023 0.005 0.017 0.016 9.86 12 Ga02 P12 0.359 25.320 31.397 0.022 0.033 0.177 0.023 0.01 0.028 0.02 0.12 0.023 0.008 0.017 0.016 97.7 13 Ga02 P13 35.802 26.50 31.959 0.020 0.053 0.131 0.023 0.01 0.028 0.017 0.052 0.023 0.015 0.017 0.016 9.56 20 Ga02 P20 35.920 26.352 30.8 0.015 0.577 0.196 0.023 0.551 0.028 0.005 0.027 0.023 0.003 0.017 0.016 9.11 27 Ga02 P27 33.937 27.32 32.822 0.007 0.12 0.171 0.023 0.02 0.028 0.007 0.027 0.023 0.015 0.017 0.016 9.5 36 Ga02 P36 3.679 26.863 32.599 0.009 0.050 0.115 0.023 0.033 0.028 0.022 0.179 0.023 0.003 0.017 0.016 9.55 37 Ccp Ga02 P37 3.192 27.622 32.851 0.013 0.023 0.082 0.023 0.01 0.028 0.025 0.178 0.023 0.00 0.017 0.016 9.99 Py Pyrite 6 Py Ga02 P6 55.156 3.305 0.211 0.055 0.067 0.165 0.023 0.020 0.028 0.008 0.137 0.023 0.012 0.017 0.025 99.18 7 Ga02 P7 55.63 3.315 0.115 0.056 0.081 0.19 0.023 0.01 0.028 0.005 0.106 0.023 0.007 0.017 0.07 99.35 22 Ga02 P22 5.301 3.975 0.010 0.025 0.627 0.118 0.023 0.01 0.028 0.005 0.156 0.023 0.011 0.017 0.030 99.27 2 Ga02 P2 5.31 2.70 0.005 0.071 0.35 0.27 0.023 0.01 0.028 0.009 0.273 0.023 0.00 0.017 0.02 98.1 30 Py Ga02 P30 57.395 1.903 0.019 0.189 0.096 0.121 0.023 0.030 0.028 0.005 0.123 0.023 0.005 0.017 0.029 99.91 Note: MDminimum detection limit.

pyrrhotite analyses, 163 of these analyses fell in concentration occurring in the base metal sulfide Amp Po the 38.1–40 wt% S range, representing the bulk zone, followed by those in the mafic rocks above Opx of pyrrhotite analyses (Fig. 11A). The bulk of pyr the top of the studied interval (Fig. 12C). The con- Amp rhotite S analyses fell within the 36.1–42 wt% range, centration of Ni in pentlandite (Fig. 12D) broadly Ccp Po Chl with very few analyses falling below 36.1 wt% S mimicked that of Co in pentlandite (Fig. 12C; sam- Ccp and greater than 42 wt% S (Fig. 11A). Average Co ple MusPxaFwt01). Opx contents in pyrrhotite, which were low, occurred at even lower concentrations in the sample from Pn gabbroic rocks at the top of the main sulfide zone Chalcopyrite Ccp Amp (Fig. 12A). Samples from within the main sulfide zone footwall were characterized by the lowest Co in Chalcopyrite analyses with the highest fre- Chl Opx pyrrhotite values compared to those from the main quency (62) fell in the 32.1–34 wt% S range sulfide zone. Samples from areas stratigraphically (Fig. 11C). Next in frequency (38) were chalcopy- above the main sulfide zone, such as those from rite analyses that plotted in the 34.1–36 wt% S the chromitite stringer (sample MusCR01) as well range (Fig. 11C). With minor deviations, average as in plagioclase pyroxenite (sample MusPxhw01), Cu concentrations in chalcopyrite across the main were also characterized by low concentrations of sulfide zone (Fig. 12D) broadly mimicked the vari- Co in pyrrhotite (Fig. 12A). ation displayed by both Co and Ni in pentlandite Chl (Figs. 12C and 12D, respectively), although average Pn Po Co contents in chalcopyrite occurred in much lower Pentlandite concentrations than those in the latter. Chl Sulfur concentrations in pentlandite were concentrated in the 28.1–38 wt% range, with the 32.1–34 Pyrite wt% range constituting the bulk of the analyses with a frequency of 125 (Fig. 11B). Pentlandite S concen- Only 11 pyrite analyses were obtained, rep- trations typically ranged between 30 and 40 wt% resenting a mere 0.02% of all sulfide analyses. S

Figure 10. Backscattered electron (BSE) images of sample for the lower portions of the sampled stratigraphic concentrations in pyrite fell within the 52.1–58 wt% MusWebb02/02a. (A) Pyrrhotite (Po), chalcopyrite (Ccp), and interval, and only in the upper part did these S range, with seven of these falling in the 54.1–56 pentlandite (Pn) occurring in association with amphibole (Amp) concentrations show a wider range from 28 to 57 wt% range (Fig. 11D). and orthopyroxene (Opx). (B) Pentlandite and pyrrhotite occur- wt% (Fig. 12B). Average values of S concentrations ring wholly included in chlorite (Chl). in pentlandite across the sampled stratigraphic interval were ~33 wt%. Cobalt concentration in pent- ■■ DISCUSSION rest of the analyses shown in the Supplemental landite across the investigated interval showed a Table (see footnote 1). With the exception of Ir spiked pattern, ranging from a low of 16.2 wt% just The base metal sulfides that were observed concentrations in pyrrhotite and pentlandite, con- above the footwall of the main sulfide zone (sam- within the main sulfide zone at Unki Mine, as well as centrations for metals such as Pt, Pd, Rh, Au, Ag, ple MusIRUP01) to a high of 33.4 wt% in the base stratigraphically below and above it, were pyrrho- and Zn were almost always at, or just above, detec- metal sulfide zone of the main sulfide zone (sample tite, pentlandite, chalcopyrite, and pyrite, in order of tion limits in all three sulfides observed during this MusBMSZ; Fig. 12B). The chromitite stringer near decreasing abundance. Similar base metal sulfides study (Table 2; Supplemental Table). Consequently, the top of the main sulfide zone had the second have been reported from elsewhere in the main no useful plots of these elements could be obtained. highest Co concentration in pentlandite (33 wt%), sulfide zone in other subchambers of the Great with the sample above the main sulfide zone hav- Dyke (Prendergast and Wilson, 1989; Wilson and ing the second lowest Co concentration of 24 wt% Prendergast, 2001). These sulfides occur both as Pyrrhotite (Fig. 12B). inclusions in primary silicate phases such as ortho- Average Co concentrations in pentlandite also pyroxene, clinopyroxene, and plagioclase, as well Pyrrhotite analyses (240) constituted ~39% of showed a spiked pattern with stratigraphic height as in association with secondary silicate phases all sulfide analyses, totaling 614. Out of the 240 across the investigated section, with the lowest Co such as amphibole and chlorite. This suggests

180 140 160

120 140

120 100 y y c c n n 100 80 e e u u q q e e 80 r r f f 60 60 40 40

20 20

0 0 <30 30.1-32 32.1-34 34.1-36 36.1-38 38.1-40 40.1-42 42.1-44 44.1-46 >46 <29 29.1-32 32.1-35 35.1-38 38.1-41 41.1-44 44.1-47 47.1-50 50.1-53 53.1-56 >56.1 Pyrrhotite S (wt%) Pentlandite S (wt%) 8 60 7 50 6

40 5 y y c c n n e e u u 30 4 q q e e r r f f 3 20 2

10 1

0 0 <24 24.1-26 26.1-28 28.1-30 30.1-32 32.1-34 34.1-36 36.1-38 38.1-40 >40 <52 52.1-54 54.1-56 56.1-58 >58 Chalcopyrite S (wt%) Pyrite S (wt%)

Figure 11. (A) Bar diagram showing frequency vs. pyrrhotite S (wt%) compositions. Most samples have compositions that fall in the 31–43 wt% S range. (B) Bar diagram showing frequency vs. pentlandite S (wt%) compositions. Most samples have compositions falling in the 32–36 wt% S range. (C) Bar diagram showing frequency vs. chalcopyrite S (wt%) compositions. Most samples have compositions falling between 32 and 36 wt% S. (D) Bar diagram showing frequency vs. pyrite S (wt%) compositions.

2 2 ) ) ( m ( m t t c

1 c a 0 a 0 t t n Co In Po 0 n o o c c

Co in Ccp -1 c c fi -2 fi -2 a -2 a m - -3 u- m u / / -4 -4 c c -4 fi fi a a -5 Ca02 m m

Cr01

-6 w w -6 -6 Webb01a/02a o o l l e e -7 Webb01/02 b b

/ Pxhw01 / e

e -8 -8 -8 Peg01 v v o o -9 BMSZ b b

a IRUP01

t -10 -10 Pxafwt01 h -10 h g g i Pxbfwt01 i -11 e e H H -12 -12 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 25 30 35 40 45 50 55 60 Co in Po and Co in Ccp wt% Pentlandite S wt%

2 2 ) ) ( m

1 t

( m 1 c

t a 0 c 0 0 t

a 0 n t Fe in Po o n

-1 c

o Cu in Ccp -1 c c Ni in Pn ﬁ c -2 -2 -2

a -2 ﬁ a m

-3 - -3 m -

Co in Pn u u / -4 c / -4 -4 -4 c ﬁ ﬁ a -5 a -5 m

m -6 -6 -6 w

w -6 o o l l

e -7

e -7 b b / / -8

e -8 e -8 -8 v v o o -9 b

b -9 a a

t -10 -10 -10 h -10 h g g i

i -11 e

-11 e H H -12 -12 15 20 25 30 35 15 25 35 45 55 Co in Pn wt% Fe in Po, Cu in Ccp, and Ni in Pn wt%

Figure 12. (A) Graph showing variation of average Co in pyrrhotite (Po) and average Co in chalcopyrite (Ccp) with stratigraphic height across the investigated section; u—ultramafic. (B) Plot of stratigraphic height of the investigated section (which includes the main sulfide zone at Unki Mine) vs. pentlandite S (wt%) concentration. (C) Graph showing variation of average Co in pentlandite (Pn) with stratigraphic height across the investigated section. (D) Graph showing variation of average Fe in pyrrhotite (Po), average Cu in chalcopyrite (Ccp), and average Ni in pentlandite (Pn) with stratigraphic height across the investigated section.

the occurrence of both primary magmatic sulfide of PGEs from the magma by early-formed sulfides, transported via chloride and bisulfide, which are mineralization as well as remobilization of some of subsolidus annealing of sulfide resulting in both the most important ligands for Pd and Pt transport that primary mineralization. It must be pointed out, remobilization and concentration of PGEs, and later in hydrothermal fluids (Wood et al., 1989; Ridley, though, that coexistence does not imply cogene- remobilization of PGEs by hydrothermal fluids at 2013), although hydroxyl and mixed species (van

sis, as sulfide liquid is denser than silicate liquid elevated fS2/fO2 (pyrite), leading to the formation of Middlesworth and Wood, 1999) are also thought (Murase and McBirney, 1973). However, if sulfide discrete platinum group minerals. to have contributed to the mobility PGEs in hydro- segregation and silicate crystallization occurred Unlike in other areas of the Great Dyke, the thermal fluids in a minor way. The PGE-enriched concurrently, they are likely to both have accumu- concentration of sulfides in the Unki Mine area main sulfide zone, consequently, is likely to have lated together and formed disseminated sulfide increases gradually from the base of the PGE sub- been concentrated by hydrothermal alteration, as ores, as is the case in the main sulfide zone. It is also zone upwards, whereas Pt and Pd (and other PGEs) evidenced by the alteration of the associated silicate possible that late-stage mobilization of high-density, rise sharply (Wilson and Prendergast, 2001). An phases (Chaumba, 2017). low-viscosity sulfide-rich melts, which may range inverse relationship was reported by Wilson and from sulfide-rich silicate magma pulses to coherent Prendergast (2001) between Pt and Pd, with Pd pulses of pure sulfide liquid (Saumur et al., 2016; being concentrated at the base and Pt at the top of Textural Observations Saumur and Cruden, 2017), may have been respon- the PGE subzone. Based on metal profiles, Pt and sible for the concentration of the PGEs at Unki Mine. Pd appear to be decoupled from each other, but Some sulfides, such as those occurring in Evidence for this comes from the iron-rich ultra- this relationship is not supported by interelement association with chromite, plagioclase, and ortho- mafic pegmatites in the Unki Mine area (Chaumba, plots (Wilson and Prendergast, 2001). The main sul- pyroxene (Fig. 7), appear to be primary, whereas 2017). The altered nature of the silicate minerals in fide zone in the Unki Mine area has been shown to others, which occur in the presence of either chlorite the Unki Mine area suggests that at least some of be unusually uniform due to its location, which is or both chlorite and chromite (Figs. 6, 9, and 10), the PGEs may have been concentrated by hydro- entirely in the axis of the Great Dyke. Results from appear to be secondary. Chlorite occurring in asso- thermal fluids. the present study, however, tend to suggest a more ciation with chromite crystals (Fig. 6) is interpreted Coghill and Wilson (1993), Evans and Buchanan pronounced effect of hydrothermal alteration on the to have been formed as a reaction product of the

(1991), Evans et al. (1996), Oberthür et al. (1997), mineralization in the main sulfide zone at Unki Mine reaction MgAl2O4+ 4·MgO + 3SiO2 + 4·H2O = chlorite

Prendergast (1990), and Wilson and Tredoux (1990) than previously envisaged, as discussed below. (Mg5Al2Si3O10[OH]8) (Evans and Frost, 1975; Kimball,

all favored a primary magmatic origin for the PGE Barnes and Liu (2012) interpreted the correla- 1990), where MgAl2O4 occurs in the solid solution mineralization in the Great Dyke but allowed for tion between Pt and Pd in sulfide-poor Australian state in the spinel phase. The timing of this reaction local-scale remobilization of the PGEs. These komatiites to be controlled by original crystallization is interpreted to have been during hydrothermal authors based their interpretations on the close and accumulation of olivine, whereas in dissemi- alteration that accompanied sulfide mineralization, correlation of PGEs and primary silicate geochem- nated ores, the correlation was controlled by the due to the presence of sulfides such as pentlandite, istry, as well as on the textures and distribution abundance of sulfide. In massive ores, Barnes and which occur together with chlorite (Figs. 9A and 9B). of interstitial sulfides in the mineralized horizons Liu (2012) attributed the Pt and Pd concentrations of the Great Dyke. These authors argued that the in komatiite-hosted massive ores to fractional main sulfide zone, for example, provides evidence crystallization of the sulfide liquid (Barnes and Elemental Abundances in Sulfide Minerals that the sulfides were segregated from the magma Naldrett, 1987; Barnes, 2004). Such fractionation, and accumulated, with minor redistribution, within however, involves the high-temperature MSS phase, The observations in this study—that concen- the partly consolidated silicate crystals. Wilson which is characterized by low partition coefficients trations of PGEs such as Pt, Pd, Rh, and (to some and Prendergast (2001) observed that the hetero- for both Pt and Pd (Barnes et al., 1997; Ebel and extent) Ir in the main sulfide zone occur at or just geneous small-scale distribution of sulfides in the Naldrett, 1996), resulting in an inability to produce above detection limits in the base metal sulfides main sulfide zone formed due to the growth of order-of-magnitude variations in Pt and Pd. Results encountered during this study—lend support to an plagioclase oikocrysts within the pyroxene crystal by Barnes (2004) from modeling the fractionation of earlier study by Evans et al. (1996), who concluded framework, which forced the droplets to concen- the MSS of the Silver Swan massive sulfide orebody that in the Darwendale Subchamber of the Great trate around their margins. The distribution of at Black Swan in Western Australia showed that the Dyke, the PGEs are not hosted in the single chromi- platinum group minerals and the varying PGE concentrations of Pt and Pd would not be accounted tite layer they investigated (the sulfide-enriched C1d content of sulfide mineralization were inferred by for by accumulation of MSS orthocumulates, which chromitite layer), which hosts anomalous but spo- Wilson and Prendergast (2001) to be the result of they ascribed to hydrothermal alteration and remo- radic PGEs. This is unlike in the Bushveld Complex, multiple processes, which involved the scavenging bilization. PGEs in the Great Dyke were probably where Merkle (1992) observed that some platinum

group minerals containing Pt, Pd, and Rh are con- the main sulfide zone except for the topmost and correlation is interpreted to indicate magmatic centrated in the intercumulus silicates and are also lowermost samples (Fig. 12D). Co and Ni in pent- processes that are preserved in the main sulfide frequently associated with base metal sulfides. It landite vary in a similar manner across the main zone samples at Unki Mine. For pyrrhotite from the must be noted that even in the Merensky reef, most sulfide zone (Figs. 12B and 12C, respectively). Cu main sulfide zone at Unki Mine, however, Ni and Co of the PGEs are hosted in silicates (Merkle, 1992). in chalcopyrite behaves in a similar manner to both (which, along with Cu, are the most common ele- Backscattered electron images from within, as well Co and Ni in pentlandite, except in the lower part ments that substitute for Fe in pyrrhotite; Vaughan, as stratigraphically below and above, the main sul- of the investigated succession (Figs. 12B and 12C). 2011) show a positive correlation (Fig. 14B). This fide zone (see Figs. 2C and 5–10) show some close Variations in Fe, Co, and Ni in pentlandite have been suggests that Ni and Co in pyrrhotite substituted association of hydrosilicate phases and base metal shown to display linear trends (Hem et al., 2001). In for Fe in roughly equal proportions, implying that sulfides, suggesting that scavenging of the PGEs by other layered intrusions, Ni contents in pentlandite the original magmatic signatures are still retained hydrothermal fluids likely helped concentrate the have been shown to correlate negatively with Fe in some sulfides despite later hydrothermal remo- mineralization in the main sulfide zone. The origi- and Co (Cogulu, 1993). In the Crystal Lake intru- bilization. This observation is supported by the nal source of the mineralization, however, is likely sion, Thunder Bay, Ontario, Canada, Cogulu (1993) behavior of elements such as Ni and Co in pent- to have been magmatic (Prendergast and Keays, observed that Co contents increase with depth due landite, which behave sympathetically (Figs. 12B 1989; Wilson and Prendergast, 2001). Prendergast to contamination of the magma by assimilation and 12C). and Wilson (1989), for example, observed that, of sedimentary xenoliths. Co and Ni in the Crystal Pentlandites are typically characterized by two regardless of the extent of alteration in the main Lake intrusion have been shown to be antipathetic, different trends of Co substitutions (Riley, 1977). sulfide zone, the uniform vertical profiles of both correlating negatively with each other. Unlike in the One trend is restricted to serpentinites and hydro- Cu + Ni and Pt + Pd throughout the Great Dyke are Crystal Lake intrusion, where Co and Ni in pent- thermal veins, where Co tends to substitute for in agreement with an “entirely” magmatic origin. landite are ascribed to contamination by country Fe, whereas the other trend is observed in meta- However, results from this study, as well as those rocks, Co and Ni in pentlandite in the investigated morphosed massive sulfide ores in Scandinavia, of Li et al. (2008), support an interaction between succession are interpreted to reflect magmatic where Co tends to replace Ni (Riley, 1977). In the primary magmatic PGE-bearing base metal sulfide processes, because they vary sympathetically and main sulfide zone at Unki Mine, a plot of atomic % assemblages and late-stage hydrothermal fluids were likely to have been subsequently affected by Co versus atomic ratio Ni/(Ni + Fe) shows no clear of magmatic origin, based on both petrographic the same hydrothermal processes. The depletion in trend of Co replacing Ni (Fig. 14C). This is unlike observations and sulfide compositions. Oberthür both these elements in sample MusIRUP01 near the the upper zone of the Bushveld Complex, where et al. (2003), from a study of primary distribution base of the main sulfide zone, as well as in sample Co-rich pentlandite follows a trend of preferential patterns in the main sulfide zone in the Darwen- MusGa01/02 at the top (Figs. 12B and 12C), may be replacement of Ni (Merkle and von Gruenewaldt, dale Subchamber of the Great Dyke, also concluded due to hydrothermal processes. 1986), suggesting different mineralizing processes that hydrothermal alteration helped concentrate Figure 13A shows plots of the variation of both in the two intrusions. mineralization. Cu and Ni in pyrrhotite versus stratigraphy from the The Unki Mine main sulfide zone pentlandites Somewhat mimicking the trends displayed by investigated succession. Both Cu and Ni in pyrrho- are characterized by very low Co contents (Table 2; S concentrations in pyrrhotite, S concentrations tite have high concentrations in the main sulfide Supplemental Table [footnote 1]), an indication that in pentlandite across the main sulfide zone tend zone, although a spiked pattern is noticeable over Fe substitution by this element was very limited. to vary widely only in the uppermost part, as com- the section. Since Cu in particular has low abun- On a ternary plot of Co-Ni-Fe, Unki Mine main sul- pared to the middle and lower parts (Fig. 12A). dances in silicates due to its chalcophile nature, its fide zone samples plot in the low-Co part of the Average concentrations in pentlandite across the abundance is correlated with the abundance of sul- field of natural pentlandites (Fig. 14D). According main sulfide zone are ~34 wt% S (Fig. 12A). Across fides. The spiked pattern of both Cu and Ni across to Merkle and von Gruenewaldt (1986), pentlan- the main sulfide zone, average concentrations of the investigated section (Fig. 13A) suggests that dite in the upper zone of the Bushveld Complex Co in pentlandite display a spiked pattern (Fig. 12B). some of these metals, as well as sulfur, were remo- is consistently Co-rich, with the lowest observed With the exception of samples from the top and bilized by hydrothermal fluids. Figure 13B shows value being 8.2 atomic %, a highest value of 46.4 near the bottom of the investigated section, Fe in the percent relative abundances of the sulfides, and, atomic %, and a mean of 28.7 atomic %, values pyrrhotite tends to not always mimic the variations in general, pyrrhotite is the most dominant sulfide, that are much higher than those from the main of both Co and Ni in pentlandite and Cu in chalcopy- followed by pentlandite, chalcopyrite, and pyrite. sulfide zone at Unki Mine. In the upper zone of the rite (Figs. 12B and 12C). Although occurring in lower A good positive correlation is displayed Bushveld Complex, very high Fe/Ni ratios ensure concentrations, the variation of Co in both chalco- between Cu and Fe in chalcopyrite from the main that the samples plot toward higher Fe/Ni values pyrite and pyrrhotite shows similar patterns across sulfide zone at Unki Mine (Fig. 14A). This positive (Fig. 14D) than typical pentlandites (Merkle and von

Gruenewaldt, 1986). However, it must be noted that 2 2 ) Cu in Po Ni in Po Bushveld Complex pentlandites from the Merensky m

( 1 Reef are comparable to those from sulfide-bearing t c

a 0 0 0 zones from both the Great Dyke and Stillwater Com- t

n plex, probably indicating comparable processes

o -1 c (likely magmatic) in early stages that were respon- c i -2 -2 -2 f sible for the formation of the mineralized horizons a -3 m in these two large layered mafic intrusions. - u

/ -4 -4 -4 c i

f -5 a S-Fe-Ni Phase Relations m -6 -6 -6 w o l -7 Pentlandite compositions from the main sulfide e

b zone at Unki Mine are plotted on an S-Fe-Ni ter- / -8 -8 -8 e

v nary diagram in Figure 15A, and these samples plot -9 o MSZ above and below the pentlandite “field” (Kullerud b a -10 -10 t -10 et al., 1969). Most Unki Mine main sulfide zone h

g -11 pentlandites do not define a trend of pentlandites i

e that were exsolved from pyrrhotite, but rather they H -12 -12 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 0.1 0.2 0.3 0.4 0.5 0.6 define pentlandite compositions that are consistent with an origin between MSS nickel-poor (mono- Cu and Ni (wt%) in Po sulfide-poor, MSS1) and MSS nickel-rich (MSS2) pentlandites (e.g., Guo et al., 1999). Bushveld Com- 2 2 2 2

) plex pentlandites, for example, are nickel-rich and Ccp Py m Po Pn

( are MSS2 pentlandites (Fig. 15A). At temperatures

t 1 c of 900–1100 °C (Fig. 15A), main sulfide zone pent-

a 0 0 0 0 t 0 landites plot in the field of MSS plus liquid, likely n o

c indicating that the pentlandites crystallized from -1 c i

f -2 -2 -2 -2 magma (Kullerud et al., 1969; Guo et al., 1999). This

a -2 is consistent with the sulfides that formed early in m

- -3

u the crystallization history of the main sulfide zone.

/ -4 -4 -4 -4

c -4

i Later pentlandite crystals likely were formed during f

a -5 hydrothermal alteration, likely at temperatures of m -6 -6 -6 -6 ~300 °C, since no pyrrhotite was observed in the

w -6

o samples investigated, which would be consistent l

e -7

b with pentlandite being exsolved from pyrrhotite / -8 -8 -8 -8 e -8 (Kullerud et al., 1969; Guo et al., 1999). The occur- v

o -9 rence of pentlandite in the main sulfide zone at Unki

b MSZ a

Mine indicates that the final stage of mineralization

t -10 -10 -10 -10 -10

h in this part of the Great Dyke occurred at tempera- g

i -11

e ture below 610 °C, as pentlandite is only stable H -12 -12 -12 -12 below this temperature (Vaughan and Craig, 1978). 0 10 30 50 70 0 10 30 50 70 0 10 30 50 70 0 10 30 50 70 Phase relations in the Fe-Cu-S ternary system Percent relative sulfides at 700 °C and 300 °C (Barton and Skinner, 1979; Vaughan and Craig, 1997) are shown in Figure 15B Figure 13. (A) Plot of Cu (wt%) and Ni (wt%) in pyrrhotite (Po) across the stratigraphic height investigated section, which includes the main sulfide zone (MSZ) at Unki Mine, displaying spiked patterns; u—ultramafic. (B) Plots showing the relative percentages to deduce information from crystallization of chalco- of pyrrhotite (Po), pentlandite (Pn), chalcopyrite (Ccp), and pyrite (Py) with stratigraphic height in the investigated section. pyrite from magma. Melts were entirely crystallized

35 30 33

25 31 Ga02 29 ) 20 Cr01 ) % % t Webb01a/02a t w w 27 ( (

o Webb01/02 p c

P 15

n 25 i

Pxhw01 n

i i

u N

Peg01 C 10 23 BMSZ01 IRUP01 21 5 Pxafwt01 19 Pxbfwt01

0 17 0 0.2 0.4 0.6 0.8 1 1.2 18 23 28 33 38 Co in Po (wt%) Fe in Ccp (wt%) 0.025 Ca02 Co Cr01 Webb01a/2a 0.020 Bushveld Complex Webb01/02 Upper Zone Pxhw01 pentlandites Peg01 ) BMSZ01

% 0.015

c i IRUP01 m

o Pxafwt01 t a (

Pxbfwt01

o 0.010 Natural C pentlandites Bushveld C Stillwater C

0.005

0.000 0.00 0.10 0.20 0.30 0.40 0.50 0.60 Fe Bushveld Complex MR pentlandites Ni Ni/(Ni+Fe) atomic ratio

Figure 14. (A) Plot of Ni in pyrrhotite (Po) vs. Co in pyrrhotite (Po), showing a weak negative correlation between the two. (B) Plot of Cu in chalcopyrite (Ccp) vs. Fe in chalcopyrite (Ccp), showing a positive correlation between the two. (C) Plot of the variation Co concentration (in atomic %) vs. Ni/(Ni + Fe) atomic ratio in pentlandite from the main sulfide zone at Unki Mine. (D) Plot of the Fe-Co-Ni cation plot for pentlandite samples from the main sulfide zone at Unki Mine. Field of pentlandites from the upper zone of the Bushveld Complex pentlandite is also shown. The shaded region shows the limits of known compositions of pentlandite (e.g., Brynard et al., 1976; Merkle and von Gruenewaldt, 1986; Harney and Merkle, 1992). Note that Bushveld Complex Merensky Reef (MR) pentlandites (Kawohl and Frimmel, 2016) are comparable to those from sulfide-bearing zones from both the Great Dyke and Stillwater Complex. Stillwater Complex pentlandite data are from Todd et al. (1982) and Godel and Barnes (2008).

Ga02 Pn S Cr01 (a) 60% 60% Webb01a/2a Py o Pxhw01 900 -1100 C

o Peg01 MSS 1000 o C MSS 900 C BMSZ01 o IRUP 01 MSS 1100 C Pxafwt01 Pxbfwt01 Pn BushveldC Figure 15. (A) Pentlandite (Pn) composi- 20% 20% tions (wt%) from the main sulfide zone at Unki Mine plotted on S-(Ni + Co)-Fe ter-

20% 30% 40% 60% nary diagram. Labeled dashed fields and 10% 50% 70% Fe Ni+Co shaded field are high-temperature monosulfide solid solution (MSS) fields of Fe-Ni-S diagram (wt%) S Kullerud et al. (1969), Py—pyrite. (B) Sim- plified phase relations for chalcopyrite (Ccp) samples from main sulfide zone at Unki Mine in the S-Fe-Cu system at 300 °C (Bar- ton and Skinner, 1979). Abbreviations for Fe Ni+Co phases are: bn—bornite, ISS—intermediate or Cu or Fe solid solution, py—pyrite, po—pyrrhotite, Ga02 Cpy S cc—chalcocite, ccp—chalcopyrite, and cv— CR01 covellite. The composition of an ideal Ccp (b) 60% 60% is also shown. Stillwater Complex Ccp data Webb01a/02a are from Todd et al. (1982) and Godel and Barnes (2008). Pxhw01 Py Peg01 BMSZ01 Po IRUP01 Cv Ideal Ccp Pxafwt01 Pxbfwt01 ISS 300 o C Stillwater C Cc bn 20% 20% Cu Fe S-Fe-Cu diagram (wt% )

at 700 °C with two extensive solid solution fields sulfidation reactions can be represented by the fol- been the case for some hydrous phases observed (Vaughan and Corkhill, 2017). The first of these, the lowing equations (Peregoedova et al., 2006): in samples from the main sulfide zone at Unki Mine intermediate solid solution (ISS), includes chalco- (Figs. 5, 8, and 10B). SO2(fluid) +FeO, NiO, CuO(silicate) Fe–Ni–Cu sulfide + 3/2 O2 pyrite and other phases that are close to CuFeS2 in At a temperature of 600 °C, equilibrium phase

composition, whereas the secondS solidO2(fluid )solution+FeO, N iO, CuO(silicate) Fe–Ni–Cu sulfide + 3/2 O2 , (1) relations in the central portion of the ternary Fe-Ni-S field occurs around bornite (Fig. 15B). Unki Mine system are dominated by MSS and pentlandite main sulfide zone chalcopyrite compositions plot or (Fig. 16). A wide range in composition is exhibited close to the stoichiometric composition of chalco- by the pentlandite, which coexists with the MSS pyrite but define a trend from high (31%) to low S H2S(fluid) +FeO, NiO, CuO(silicate) Fe–Ni–Cu sul(Fig.fide +16),H2O with(fluid) pyrite and/or vaesite being stable with

(28%), i.e., between ~29 and ~34 wt%H2S Cu(flui d(Fig.) +Fe 15B).O, Ni O, C uO(silicate) Fe–Ni–Cu sulfide + H2O(fluid) . (2) the MSS along the S-rich boundary of the MSS (Hill, A few samples (MusIRUP, MusPeg01, MusBMSZ) 1984). A narrowing of the MSS field occurs with have somewhat lower Cu concentrations below At Unki Mine, the main sulfide zone samples falling temperature (Fig. 16). Naldrett and Kullerud 29 wt% (Fig. 15B). On further cooling to 300 °C, may be explained by precipitation of sulfide after an (1967) investigated the isotherms that limit the sol- these fields shrink, with the associated separation introduced S-rich fluid has stripped Fe, Ni, and Cu vus between the S-rich and S-poor limits of the by solid-state diffusion that produces exsolution from the silicate assemblages, as described in the solvus shown in Figure 16. The temperature of the textures (Vaughan and Craig, 1997). These samples equations above. Several sulfides from the main breakdown of the MSS is interpreted to occur above were probably affected more by later processes sulfide zone at Unki Mine occur in close associa- 260–300 °C, and this temperature determines the than those that plot closer to the stoichiometric tion with hydrous silicates such as amphibole and stable coexistence of pyrite and pentlandite (Fig. 8B) composition of chalcopyrite. Since no exsolution chlorite (Figs. 6, 9, and 10B). Ballhaus and Stumpfl in the Fe-Ni-S system (Naldrett and Kullerud, 1967). textures were observed in the samples under study, (1986) also documented sulfides that occur in asso- This 260–300 °C breakdown temperature of the MSS we conclude that sulfide precipitation occurred at ciation with hydrous silicate phases like biotite and is thought to constrain the upper limits on the for- temperatures higher than 300 °C. phlogopite in the Merensky Reef of the Bushveld mation of the coexistence of pyrite and pentlandite As can also be observed in Figure 15B, samples Complex, which they interpreted to have formed (Hill, 1984), probably during hydrothermal alteration

from the Stillwater Complex are comparable to a from the liberation of H2O during the sulfidation in the case of the main sulfide zone at Unki Mine. number of Unki Mine main sulfide zone samples, reaction in Equation 2 above. A comparable sce- In the main sulfide zone at Unki Mine, only a cou- probably indicating a similar origin in the miner- nario is likely to also have occurred in the Great ple of samples were observed with both pyrite and alized zones in these two intrusions. Compared to Dyke, but with amphibole and chlorite (Chaumba, pentlandite, a likely indication that the bulk of the the Unki Mine main sulfide zone samples, the J-M 2017) rather than biotite being the phases that samples must have been formed at temperatures

reef of the Stillwater Complex is characterized by were formed from the liberation of H2O during the above 300 °C, resulting in the lack of the coexis- more samples that have compositions close to the sulfidation reaction in Equation 2. Further, Lorand tence of pyrite and pentlandite. The paucity of pyrite stoichiometric composition of chalcopyrite, proba- and Gregoire (2006) also attributed the sulfides in in the main sulfide zone Unki Mine samples lends bly indicating less alteration in this intrusion during some phlogopite-rich peridotite xenoliths from the support to the formation of the main sulfide zone mineralization in its J-M Reef than in the main sul- Bultfontein kimberlite pipe from the Kaapvaal cra- at temperatures above 300 °C. fide zone of the Great Dyke. ton in South Africa to the reaction in Equation 2 Pentlandite compositions from the main sulfide The formation of sulfides in crystalline rocks above. Some of the chlorite in main sulfide zone zone at Unki Mine are plotted on an S-Fe-Ni ter- may be explained in two ways: (1) formation from samples probably originally crystallized as biotite nary diagram in Figure 15A, and these samples plot

metasomatic S-rich (SO2 or H2S) fluids that pre- and/phlogopite that was later altered. Biotite and above and below the pentlandite “field” (Kullerud cipitated sulfides by the leaching of Fe, Ni, and phlogopite, which occur together with sulfides in et al., 1969). The pentlandites under study range in S Cu from the silicate matrix; and/or (2) formation the Bushveld Complex, were similarly thought have from 39.5% to 46.7% and have near-constant Fe and from the introduction of an immiscible sulfide melt. crystallized in the vicinity of sulfides from a fraction- Ni proportions (Fig. 16), with no major differences The formation of immiscible sulfide melts in the ated intercumulus melt that was highly enriched in in the concentrations of primary and secondary Bushveld Complex was described by Ballhaus and volatiles (Ballhaus and Stumpfl, 1986). In the Bush- pentlandites. Most Unki Mine main sulfide zone Stumpfl (1986), who concluded that precipitation veld Complex, Ballhaus and Stumpfl (1986) further pentlandites (both primary and secondary) do not of sulfides can occur as a result of the interaction observed that amphibole, chlorite, and talc occur as define a trend of pentlandites that were exsolved

of an H2S fluid with silicate minerals. Peregoedova later alteration phases of cumulus minerals such as from pyrrhotite, but rather they define pentland- et al. (2006) showed that MSS can be produced via pyroxene, as well as intercumulus phases like pla- ite compositions that are consistent with an origin

a reaction between silicate phases and SO2. Such gioclase. It is also possible that this may also have between Ni-poor MSS (monosulfide-poor, MSS1)

and nickel-rich (MSS2) pentlandites (Guo et al., S 1999). Bushveld Complex pentlandites, for example, 60% 60% Pyrite are nickel-rich and are MSS2 pentlandites (Fig. 15A). MSS o 600 500o It has already been noted from experimental solvus o solvus 400 o investigations that the distribution of PGEs in base 300 o metal sulfides is inferred to be a consequence of sul- 300 50% 400o fide liquid fractionation (Peregoedova et al., 2004; 500o Pentlandite Naldrett, 2004; Mungall et al., 2005). The occurrence Pn 600 o solvus at or below detection limits of PGEs in the main sulfide zone samples investigated here probably MSS solvus indicates that PGEs that may have crystallized 4200%% 40% S together with base metal sulfides during the sulfide Fe Ni liquid fractionation stage in the main sulfide zone S 60% 60% Fe Ni were probably redistributed/remobilized during a o Pyrite MSS 600 o 500 o solvus later hydrothermal alteration event(s). solvus 400 300o Ca02 Cpy The relative shrinkage rates of the S-poor and 300 o 50% 400o CR01 S-rich limits of the MSS with temperature, as well 500o Pentlandite Pn 600 o solvus Webb01a as the position of the bulk compositions between MSS Webb01/02 the S-rich and S-poor stability limits (Fig. 16), solvus Pxhw01 determine the order of appearance of pentlandite Peg01 and pyrite (Naldrett and Kullerud, 1967). For the BMSZ01 S-poor and Cu-rich majority of samples from the IRUP01 main sulfide zone at Unki Mine, which plot below Pxafwt01 or within the pentlandite solvus (Fig. 16), pentlan- Pxbfwt01 20% 20% dite would have crystallized from temperatures as MReefKF16 Fe Ni high 610 °C (Kullerud, 1963) to temperatures as low Fe-Ni-S diagram (At% ) J-MReef as the breakdown of MSS at 260–300 °C. In this

case, pentlandite would have exsolved from the Figure 16. Plot showing the central portion of the Fe-Ni-S system (in atomic %) for samples from the main sulfide zone at Unki melt, making pyrite an unstable phase until the Mine. Isotherms limiting solvus of the Fe-rich portion of monosulfide solid solution (MSS) are also shown (after Naldrett et al., establishment of pyrite-pentlandite tie lines at tem- 1967). The majority of main sulfide zone samples plot in low low-S pentlandite (Pn) solvus, with few samples plotting in the pyrite solvus. Bushveld Complex Merensky Reef (MReef) pentlandite data are from Kawohl and Frimmel (2016), whereas Stillwater peratures below the 260–300 °C breakdown of the Complex chalcopyrite data are from Todd et al. (1982) and Godel and Barnes (2008). MSS (Fig. 16). This scenario is likely to have been the more widespread one given the abundance of pentlandite and the paucity of pyrite in the main sul- (orthomagmatic processes; Junge et al., 2015), base metal sulfides with alteration phases such as fide zone samples at Unki Mine. In certain localized the low concentrations of PGEs in pentlandite in amphibole and chlorite. The plagioclase webster- portions, however, the primary bulk composition the main sulfide zone at Unki Mine (Table 2; Sup- ite, which occurs at the top of the P1 pyroxenite would have fallen in the S-rich side of the MSS, plemental Table [footnote 1]) may lend support to layer of the ultramafic sequence, and which is over- which would have enabled pyrite to exsolve from remobilization of the PGEs by hydrothermal fluids. lain by gabbroic rocks of the mafic sequence, may the melt, resulting in pentlandite being an unstable Consequently, the reported occurrences of microm- have been formed due to compaction (Meurer and phase until the establishment of pyrite-pentland- eter-sized discrete platinum group minerals in ores Boudreau, 1996). From studies of concentrations ite tie lines at temperatures below the 260–300 °C of the Bushveld Complex (Junge et al., 2014) are of trace elements such as P and Zr that are weakly breakdown of the MSS (Fig. 16). consistent with a dominantly orthomagmatic origin compatible in cumulate minerals, Meurer and Unlike in the PGE-enriched UG-2 chromitite for the PGEs in the Bushveld Complex, which is not Boudreau (1996) observed that when a sharp drop layer of the Bushveld Complex, which is charac- what we observed in the main sulfide zone at Unki in the density of cumulus minerals occurs, such terized by higher concentrations of both Pd and Rd Mine. Chaumba (2017) also presented evidence of as that encountered at the ultramafic sequence– in pentlandite and where the PGEs are interpreted hydrothermal alteration in the main sulfide zone at mafic sequence boundary of the Great Dyke, the to have formed due to magmatic differentiation Unki Mine in the form of the close association of interstitial liquid distributions closely mimic those

predicted by compaction models. The position of Dyke likely occurred at pressures ranging from 1 indicate hydrothermal alteration of the plagioclase. the feldspathic websterite is comparable to that of to 5.5 kbar. Results of MELTS modeling that were Although no olivine crystals occur in the P1 layer, as the mineralized zone of the Munni Munni Complex obtained for pressures ranging from 1 to 6 kbar for this mineral crystallized stratigraphically lower than (with a stratigraphic section that is broadly compa- orthopyroxene (Fig. 17A) showed that the average the main sulfide zone, calculated olivine composi- rable to the Great Dyke’s) that was investigated by orthopyroxene Mg# (100 × Mg/[Mg + Fe2+]) com- tions are consistent with formation at temperatures Meurer and Boudreau (1996). position of Unki Mine main sulfide zone samples of 1150 °C and higher, and at pressures ranging From their study of the origin of mineralization (=81.8) was likely to have been attained at tempera- from 1 to 5.5 kbar (Fig. 18B). However, it is also in several important layered intrusions, Peach and tures of at least 1150 °C and pressures on the order interesting to note that some Great Dyke olivines Mathez (1996) made broadly comparable observa- 2–2.5 kbar (Fig. 17A; Chaumba, 2017). Orthopyrox- may have been crystallized at temperatures of 800– tions, i.e., that mineralization in the main sulfide ene compositions for the ultramafic sequence of 930 °C and pressures of 2–4 kbar (Fig. 18B). These zone of the Great Dyke was not a single event. the Great Dyke (Wilson, 1982) would have been olivines, which crystallized at these relatively lower Peach and Mathez (1996) argued that the metal crystallized at temperatures higher than 1150 °C temperatures, are also interpreted to have been profiles in the main sulfide zone are not consis- (Fig. 17A). affected by the hydrothermal fluids that resulted in tent with a model of continuous, single-process For clinopyroxene, the average Mg# obtained the concentration of PGEs. The pressures obtained sulfide fractionation. They suggested that the min- for Unki Mine main sulfide zone samples of 87.4, from MELTS modeling of 1–5.5 kbar are comparable

eralization in the main sulfide zone originated from which is higher than the modeled Mg#CPX (Fig. 18B; with pressures of 1–4 kbar that were obtained by processes that postdated the initial sulfide liquid Chaumba, 2017), may imply that this mineral Chaumba (2017) from thermobarometry calcula- or that concentration of the PGEs took place by an was likely to have been hydrothermally altered tions, lending support to the conclusion that the entirely different mechanism (Peach and Mathez, (Fig. 17A). Clinopyroxene compositions reported by Great Dyke must have been emplaced at depths 1996). Observations from this work lend support Wilson (1982) are likely to have been crystallized at of 6.1–12.6 km (Chaumba, 2017). to that hypothesis. temperatures ranging from 800 °C to almost 950 °C The lack of anomalous enrichment of PGEs in at pressures of 2–4 kbar (Fig. 17B). In addition to base metal sulfides from the Shurugwi Subchamber the orthopyroxene having crystallized earlier at of the Great Dyke is consistent with observations Inferences from MELTS Modeling higher temperatures than those at which clinopy- from other “layered intrusions” such as the Noril’sk roxenes crystallized, the Great Dyke clinopyroxenes (Barnes et al., 2006), the Bushveld Complex (Godel Mineral compositions of the Great Dyke can likely crystallized over a wider temperature range et al., 2007), and the Sudbury Complex (Dare et al., be modeled by fractional crystallization processes (Figs. 17A and 17B). This observation is supported 2010), for example, which also lack such PGE enrich- through the MELTS software (Gualda and Ghiorso, by the first appearance of cumulus orthopyroxene ments in their base metal sulfides. These findings 2015) by using the parental magma composition rather than clinopyroxene in the Great Dyke (Worst, lend support to the involvement of postmagmatic of the Great Dyke (Wilson, 1982). Isobaric crystal- 1960; Wilson, 1982; Wilson and Prendergast, 1989). processes in the redistribution of magmatic sul- lization was modeled at pressure intervals of 500 Further, the lower crystallization temperatures fide ores (the hydromagmatic model). However, bars from 1 to 10 kbar as temperature was lowered shown in Figure 17B are also consistent with either in previous studies on different Ni-Cu-PGE depos- at 50 °C intervals starting at ~1200 °C. All initial the postcumulus nature of this mineral (clinopyrox- its (Godel et al., 2007), distinct enrichments of Pd models were performed as equilibrium crystalli- ene is a postcumulus phase in the P1 layer) in the and Pt in chalcopyrite were not observed. Thus, zation simulations at anhydrous water contents, a ultramafic sequence or the temperature at which postmagmatic processes are thought to play an pressure of 2 kbar, a temperature of 1300 °C, and hydrothermal alteration occurred (Chaumba, 2017). important role in the redistribution of PGE in mag- oxidation states of QFM +0.6 and QFM +0 (QFM For plagioclase, the anorthite content (An = 100 matic sulfide ores (Dare et al., 2010). For example, is quartz-fayalite-magnetite). Mineral composition × Ca)/[Na + Ca]) of plagioclase in the main sulfide Barnes et al. (2006) and Dare et al. (2010) suggested results were then compared with those obtained zone at Unki Mine was 68.5, which is higher than that the enrichment of Pd in pentlandite is due to from the present investigation to determine if the modeled values (Fig. 18A; Chaumba, 2017). diffusion during its exsolution from the MSS. Small Unki Mine mineral compositions were affected by Both Unki Mine measured An contents and mod- quantities of Pd, however, may originate from Pd hydrothermal alteration. Calculations for tempera- eled An contents are consistent with crystallization in the original MSS structure, whereas most Pd ture by assuming isobaric fractional crystallization at high temperatures in excess of 1150 °C at rela- can be derived from the nearby ISS by diffusion upon cooling were conducted from 5.5 to 1 kbar. tively low pressures of 1–2 kbar (Fig. 18A). Some (Dare et al., 2010). No mineral composition results of MELTS model- of the Unki Mine measured An contents are higher Unlike in the Platinova Reef of the Skaergaard ing were obtained at pressures of ≤1 kbar and ≥6 than those obtained from MELTS modeling pres- intrusion, where zones of PGE enrichment and pre- kbar, suggesting that crystallization of the Great sures greater than 2 kbar, which we interpret to cious metal minerals are intimately associated with

1200

1150

1100

) 1050 C o (

r 1000 u t a r

e 950 2 kbars p 2.5bars m

e 900 3.5 kbars T 4 kbars Wilson (1982, 1992) Wilson (1982, 850 4.5 kbars 5 kbars

800 5.5 kbars Figure 17. (A) Plot of MELTS modeling Unki Opx crystallization temperature vs. orthopyroxene (Opx) Mg#. Unki Mine plagioclase 750 compositions would have been formed at 76 77 78 79 80 81 82 temperatures of 1150 °C. (B) Plot of MELTS modeling crystallization temperature vs. Orthopyroxene Mg# clinopyroxene (Cpx) Mg#. Unki Mine clinopyroxene compositions would have been 1200 formed at temperatures ranging from 1 kbars 800 °C to 950 °C. This is consistent with 1.5 kbars the postcumulus nature of this mineral, and 1150 its intercumulus nature in the ultramafic 2 kbars sequence, or it reflects temperatures of 2.5 kbars 1100 hydrothermal alteration at temperatures 3 kbars of 800–950 °C. 3.5 kbars )

C 1050 4 kbars o ( 4.5 kbars e r 1000 5 kbars u t 5.5 kbars a r Unki Cpx e 950 p m e

T 900

850 Wilson 1992) (1982,

800

750 76 78 80 82 84 86 88 Clinopyroxene Mg#

Anorthite (An) Cu sulfide globules mostly located at, or close to, silicate and oxide boundaries (Holwell et al., 2016), 0 20 40 60 80 in the main sulfide zone of the Great Dyke at Unki 1200 Mine, this is not the case, possibly pointing to different origins of mineralization in the two layered 1150 intrusions. Although both the Great Dyke and the Unki Skaergaard intrusion fall under the “offset reefs” 1100 classification, the mineralized horizons are hosted

) in different lithologies (ultramafic rocks in the case C

o of the main sulfide zone of Great Dyke; mafic rocks

( 1050

e in the Platinova Reef of the Skaergaard intrusion). r u

t In the Platinova Reef, precious metal minerals have

a 1000 r been shown to be intimately associated with Cu e

p 1 kb sulfide globules, mostly located at, or close to, sil-

m 950 1.5 kb Wilson (1982) e icate and oxide boundaries (Holwell et al., 2016),

T 2 kb 3 kb whereas no elevated PGE concentrations in base 900 3.5 kb metal sulfides were observed in this study. 4 kb 4.5 kb 850 5 kb 5.5 kb ■■ CONCLUSIONS 800 Forsterite (Fo) Pyrrhotite, pentlandite, chalcopyrite, and pyrite were the sulfides encountered during the present 76 78 80 82 84 86 88 90 1200 study in the main sulfide zone at Unki Mine. Some of these base metal sulfides, which occur as inclu- 1150 sions in silicates and chromite, were most likely formed early in the crystallization history of the 1 kb 1100 main sulfide zone. Other sulfides, which occur in 1.5 kb association with hydrosilicates, were likely formed 2 kb o 1050 later during a late-stage event associated with

( C) ( 2.5 kb hydrothermal alteration. 3 kb 1000 3.5 kb As is the case of the C1d chromitite layer in the 4 kb Darwendale Subchamber of the Great Dyke, low 950 4.5 kb concentrations of PGEs such as Pt, Pd, and Rh in 5 kb base metal sulfides may imply that the PGEs in the Temperatur e 5.5 kb main sulfide zone at Unki Mine are hosted in sili- 900 Wilson (1982) cates and/or platinum group minerals. The broadly 850 comparable trends, with minor variations across the main sulfide zone, of Fe in pyrrhotite, of Co and Ni in 800 pentlandite, and of Cu in chalcopyrite, for example, are interpreted to reflect magmatic processes. Very 750 low Co contents in Unki Mine main sulfide zone pentlandites indicate very limited Fe substitution by Figure 18. (A) Plot of MELTS modeling crystallization temperature vs. plagioclase An content. Unki Mine pla- Co, and substitution of Ni by Co. The concentrations gioclase compositions would have been crystallized at temperatures of 1150–1200 °C. (B) Plot of MELTS modeling of metals such as Co, Ni, and Cu in sulfides tend to crystallization temperature vs. olivine Fo content. Great Dyke olivine compositions would have been formed at temperatures above 1100 °C. The temperatures below 950 °C represent Great Dyke olivine compositions that vary sympathetically, implying that they either are were hydrothermally altered. primary magmatic signatures, or they were likely

to have been subsequently affected by the same and the Bulletin of the Society of Economic Geologists, v. 82, Dare, S.A.S., Barnes, S.-J., and Prichard, H.M., 2010, The distribu- p. 165–183, https://doi .org /10 .2113 /gsecongeo .82 .1 .165. tion of platinum-group elements (PGE) and other chalcophile hydrothermal processes. Depletions in some ele- Barnes, S.-J., Makovicky, E., Makovicky, M., Rose-Hansen, J., elements among sulfides from Creighton Ni-Cu-PGE sulfide ments, which occur near the base and at the top of and Karupmoller, S., 1997, Partition coefficients for Ni, Cu, deposit, Sudbury, Canada, and the origin of palladium in the main sulfide zone, however, suggest that these Pd, Pt, Rh, and Ir between monosulfide solid solution and pentlandite: Mineralium Deposita, v. 45, p. 765–793, https:// metals (and mineralization in the main sulfide zone) sulfide liquid and the formation of compositionally zoned doi.org /10 .1007 /s00126 -010 -0295 -6. Ni–Cu sulfide bodies by fractional crystallization of sulfide Ebel, D.S., and Naldrett, A.J., 1996, Fractional crystallization of were likely to have been subsequently affected by liquid: Canadian Journal of Earth Sciences, v. 34, p. 366–374, sulfide ore liquids at high temperature: Economic Geology hydrothermal processes. Positive correlations of https://doi.org/10.1139/e17-032. and the Bulletin of the Society of Economic Geologists, v. 91, elements like Cu and Fe in chalcopyrite from the Barnes, S.-J., van Achterbergh, E., Makovicky, E., and Li, C., p. 607–621, https://doi.org/10.2113/gsecongeo.91.3.607. 2001, Proton microprobe results for the partitioning of plati- Evans, B.W., and Frost, B.R., 1975, Chrome-spinel in progres- main sulfide zone at Unki Mine, for example, indi- num-group elements between monosulphide solid solution sive metamorphism—A preliminary analysis: Geochimica cate preservation of magmatic processes in the and sulphide liquid: South African Journal of Geology, et Cosmochimica Acta, v. 39, p. 959–972. main sulfide zone samples at Unki Mine. v. 104, p. 275–286, https://doi.org/10.2113/gssajg.104.4.275. Evans, D.M., and Buchanan, D.L., 1991, Application of petro- Barnes, S.-J., Cox, R.A., and Zientek, M.L., 2006, Platinum-group graphic studies to MSZ platinum-group element and element, gold, silver and base metal distribution in com- base metal mineralisation at Zinca prospect, Great Dyke, positionally zoned sulfide droplets from the Medvezky Zimbabwe: Transactions of the Institution of Mining and ACKNOWLEDGMENTS Creek Mine, Noril’sk, Russia: Contributions to Mineralogy Metallurgy, Section B, Applied Earth Science, v. 100, p. B216– Chaumba gratefully acknowledges funding for this work from and Petrology, v. 152, p. 187–200, https://doi.org/10.1007 B226. the University of North Carolina–Pembroke Office of Graduate /s00410-006-0100-9. Evans, D.M., Buchanan, D.L., and Parry, S.J., 1996, Böhmke Reef: Studies and Research that covered the cost of microprobe anal- Barton, P.B., Jr., and Skinner, B.J., 1979, Sulfide mineral stabili- Platinum-group element mineralisation associated with C1d yses. Thin sections costs were covered by a Summer Research ties, in Barnes, H.L., ed., Geochemistry of Hydrothermal Ore chromitite of the Great Dyke, Zimbabwe: Transactions of Fellowship at University of North Carolina–Pembroke, which Deposits (2nd ed.): New York, J. Wiley and Sons, p. 278–403. the Institution of Mining and Metallurgy, Section B, Applied is also gratefully acknowledged. Anglo American Corporation Boudreau, A., 2019, Hydromagmatic Processes and Plati- Earth Science, v. 105, p. B82–B88. Zimbabwe supported this research and contributed financially num-Group Element Deposits in Layered Intrusions (1st Godel, B., and Barnes, S.-J., 2008, Platinum-group elements in several ways. We are extremely grateful to two anonymous ed.): Cambridge, UK, Cambridge University Press, 286 p. in sulfide minerals and the whole rocks of the J-M Reef reviewers for their careful and helpful reviews of this article. Boudreau, A.E., 2016, The Stillwater Complex, Montana— (Stillwater Complex): Implications for the formation of the Overview and the significance of volatiles: Mineralogical reef: Chemical Geology, v. 248, p. 272–294, https://doi.org Magazine, v. 80, p. 585–637, https://doi .org /10 .1180 /minmag /10.1016/j.chemgeo.2007.05.006. .2016.080.063. Godel, B., Barnes, S.-J., and Maier, W.D., 2007, Platinum-group REFERENCES CITED Boudreau, A.E., and McCallum, I.S., 1992, Concentration of elements in sulphide minerals, platinum-group minerals, Armstrong, J.T., 1988, Quantitative analysis of silicate and oxide platinum-group elements by magmatic fluids in layered and whole-rocks of the Merensky Reef (Bushveld Complex, minerals: Comparison of the Monte Carlo, ZAF and Φ(ρZ) intrusions: Economic Geology and the Bulletin of the Soci- South Africa): Implications for the formation of the reef: procedures, in Newbury, D.E., ed., Microbeam Analysis: ety of Economic Geologists, v. 87, p. 1830–1848, https://doi Journal of Petrology, v. 48, p. 1569–1604, https://doi .org/10 San Francisco, California, San Francisco Press, p. 239–246. .org/10.2113/gsecongeo.87.7.1830. .1093/petrology/egm030. Ballhaus, C.G., and Stumpfl, E.F., 1986, Sulfide and platinum Brynard, H.J., De Villiers, J.P.R., and Viljoen, E.A., 1976, Miner- Gualda, G.A.R., and Ghiorso, M.S., 2015, MELTS_Excel: A Micro- mineralization in the Merensky Reef: Evidence from hydrous alogical investigation of the Merensky Reef at the Western soft Excel–based MELTS interface for research and teaching silicates and fluid inclusions: Contributions to Mineralogy Platinum Mine, near Marikana, South Africa: Economic of magma properties and evolution: Geochemistry Geo- and Petrology, v. 94, p. 193–204, https://doi.org/10.1007 Geology and the Bulletin of the Society of Economic Geol- physics Geosystems, v. 16, p. 315–324, https://doi.org/10 /BF00592936. ogists, v. 71, p. 1299–1307, https://doi .org /10 .2113 /gsecongeo .1002/2014GC005545. Barnes, S.J., 1993, Partitioning of the platinum group elements .71.7.1299. Guo, J., Griffin, W.L., and O’Reilley, S.Y., 1999, Geochemistry and gold between silicate and sulphide magmas in the Chaumba, J.B., 2017, Hydrothermal alteration in the main sulfide and origin of sulphide minerals in mantle xenoliths: Qilin, Munni Munni Complex, Western Australia: Geochimica et zone at Unki Mine, Shurugwi Subchamber of the Great Dyke, southeastern China: Journal of Petrology, v. 40, p. 1125–1149, Cosmochimica Acta, v. 57, p. 1277–1290, https://doi.org/10 Zimbabwe: Evidence from petrography and silicates mineral https://doi.org/10.1093/petroj/40.7.1125. .1016/0016-7037(93)90064-4. chemistry: Minerals (Basel), v. 7, no. 7, p. 127, https://doi.org Harney, D.M.W., and Merkle, R.K.W., 1992, Sulfide mineralogy Barnes, S.J., 2004, Komatiites and nickel sulfide ores of the Black /10.3390/min7070127. at the main magnetitite layer, upper zone, eastern Bushveld Swan area, Yilgarn craton, Western Australia. 4. Platinum Chaumba, J.B., Mundalamo, H.R., Ogola, J.S., Cox, J.A., and Complex, and the effect of hydrothermal processes on pent- group element distribution in the ores, and genetic impli- Fleisher, C.J., 2016, Petrography, sulfide mineral chemistry, landite composition: European Journal of Mineralogy, v. 4, cations: Mineralium Deposita, v. 39, p. 752–765, https://doi and sulfur isotope evidence for a hydrothermal imprint on p. 557–570, https://doi.org/10.1127/ejm/4/3/0557. .org/10.1007/s00126-004-0440-1. Musina copper deposits, Limpopo Province, South Africa: Hem, S.R., Makovicky, E., and Gervilla, F., 2001, Compositional Barnes, S.J., and Liu, W., 2012, Pt and Pd mobility in hydrothermal Evidence for a breccia pipe origin?: Journal of African trends in Fe, Co and Ni sulfarsenides and their crys- fluids: Evidence from komatiites and from thermodynamic Earth Sciences, v. 120, p. 141–159, https://doi.org/10.1016 tal-chemical implications: Results from the Arroyo de la modelling: Ore Geology Reviews, v. 44, p. 49–58, https://doi /j.jafrearsci.2016.05.003. Cueva deposits, Ronda peridotite, southern Spain: Cana- .org/10 .1016 /j .oregeorev .2011 .08 .004. Coghill, B.M., and Wilson, A.H., 1993, Platinum group minerals in dian Mineralogist, v. 39, p. 831–853, https://doi.org/10.2113 Barnes, S.J., McIntyre, J.R., Nisbet, B.W., and Williams, C.R., the Selukwe Subchamber, Great Dyke, Zimbabwe: Implica- /gscanmin.39.3.831. 1990, Platinum-group element mineralisation in the Munni tions for PGE collection mechanisms and post-formational Hill, R.E.T., 1984, Experimental study of phase relations at 600˚C Munni Complex, Western Australia: Mineralogy and Petrol- redistribution: Mineralogical Magazine, v. 57, p. 613–633, in a portion of the Fe-Ni-Cu-S system and its application ogy, v. 42, p. 141–164, https://doi.org/10.1007/BF01162688. https://doi.org/10.1180/minmag.1993.057.389.06. to natural sulphide assemblages, in Buchanan, D.L., and Barnes, S.-J., and Naldrett, A.J., 1987, Fractionation of the plat- Cogulu, E.H., 1993, Mineralogy and Chemical Variations of Jones, M.J., eds., Sulfide Deposits in Mafic and Ultramafic inum-group elements and gold in some komatiites of the Sulphides from the Crystal Lake Intrusion, Thunder Bay, Rocks: Institute of Mining and Metallurgy Special Publi- Abitibi greenstone belt, northern Ontario: Economic Geology Ontario: Geological Survey of Canada Open-File 2749, 21 p. cation, p. l4–21.

Holwell, D.A., Barnes, S.J., Le Vaillant, M., Keays, R.R., Fisher, L., Complex: Indications for collection mechanisms and post- at Hartley Platinum Mine, Zimbabwe. Part 1: Primary dis- and Prosser, R., 2016, 3D textural evidence for the formation magmatic modification: Canadian Journal of Earth Sciences, tribution patterns in pristine ores of the main sulfide zone of ultra-high tenor precious metal bearing sulfide microdrop- v. 29, p. 209–221, https://doi.org/10.1139/e92-020. of the Great Dyke: Mineralium Deposita, v. 38, p. 327–343, lets in offset reefs: An extreme example from the Platinova Merkle, R.K.W., and von Gruenewaldt, G., 1986, Compositional https://doi.org/10.1007/s00126-002-0337-9. Reef, Skaergaard intrusion, Greenland: Lithos, v. 256–257, variation of Co-rich pentlandite: Relation to the evolution Ohmoto, H., and Goldhaber, M.B., 1997, Sulfur and carbon p. 55–74, https://doi .org /10 .1016 /j .lithos .2016 .03 .020. of the upper zone of the western Bushveld Complex, South isotopes, in Barnes, H.L., ed., Geochemistry of Hydrother- Irvine, T.N., 1983, Observations on the origins of Skaergaard Africa: Canadian Mineralogist, v. 24, p. 529–546. mal Ore Deposits (3rd ed.): New York, J. Wiley and Sons, layering: Carnegie Institution of Washington Yearbook, Meurer, W.P., and Boudreau, A.E., 1996, Compaction of density p. 517–611. v. 82, p. 284–289. stratified cumulates: Effect on trapped liquid distributions: Osbahr, I., Klemd, R., Oberthür, T., Brätz, H., and Schouwstra, R., Junge, M., Oberthür, T., and Melcher, F., 2014, Cryptic variation The Journal of Geology, v. 104, p. 115–120, https://doi.org 2013, Platinum-group element distribution in base-metal of chromite chemistry, platinum-group element and plati- /10.1086/629805. sulfides of the Merensky Reef from the eastern and west- num-group mineral distribution in the UG-2 chromitite: An Mungall, J.E., Andrews, D.R.A., Cabri, L.J., Sylvester, P.J., and ern Bushveld Complex, South Africa: Mineralium Deposita, example from the Karee Mine, western Bushveld Complex, Tubrett, M., 2005, Partitioning of Cu, Ni, Au, and plati- v. 48, p. 211–232, https://doi.org/10.1007/s00126-012-0413-8. South Africa: Economic Geology and the Bulletin of the num-group elements between monosulfide solid solution Peach, C.L., and Mathez, E.A., 1996, Constraints on the forma- Society of Economic Geologists, v. 109, p. 795–810, https:// and sulfide melt under oxygen and sulfur fugacities: Geo- tion of platinum-group element deposits in igneous rocks: doi.org/10.2113/econgeo.109.3.795. chimica et Cosmochimica Acta, v. 69, p. 4349–4360, https:// Economic Geology and the Bulletin of the Society of Eco- Junge, M., Wirth, R., Oberthür, T., Melcher, F., and Schreiber, doi.org/10.1016/j.gca.2004.11.025. nomic Geologists, v. 91, p. 439–450, https://doi.org/10.2113 A., 2015, Mineralogical siting of platinum-group elements Murase, T., and McBirney, A.R., 1973, Properties of some com- /gsecongeo.91.2.439. in pentlandite from the Bushveld Complex, South Africa: mon igneous rocks and their melts at high temperatures: Peregoedova, A., Barnes, S.J., and Baker, D.R., 2004, The for- Mineralium Deposita, v. 50, p. 41–54, https://doi .org /10 .1007 Geological Society of America Bulletin, v. 84, p. 3563–3592, mation of Pt-Ir alloys and Cu-Pd–rich sulphide melts by /s00126-014-0561-0. https://doi.org/10.1130/0016-7606(1973)84<3563:POSCIR>2 partial desulfurization of Fe-Ni-Cu sulphides: Results of Kawohl, A., and Frimmel, H.E., 2016, Isoferroplatinum-pyrrho- .0.CO;2. experiments and implications for natural systems: Chem- tite-troilite intergrowth as evidence of desulfurization in the Naldrett, A.J., 1989, Magmatic Sulfide Deposits: New York, ical Geology, v. 208, p. 247–264, https://doi.org/10.1016/j Merensky Reef at Rustenburg (western Bushveld Complex, Oxford University Press, 186 p. .chemgeo.2004.04.015. South Africa): Mineralogical Magazine, v. 80, p. 1041–1053, Naldrett, A.J., 2004, Magmatic Sulfide Deposits: Geology, Geo- Peregoedova, A., Barnes, S.J., and Baker, D.R., 2006, An experi- https://doi.org/10.1180/minmag.2016.080.055. chemistry and Exploration: Berlin, Springer, 727 p., https:// mental study of mass transfer of platinum-group elements, Kimball, K.L., 1990, Effects of hydrothermal alteration on the doi.org/10.1007/978-3-662-08444-1. gold, nickel and copper in sulfur-dominated vapor at mag- composition of chromian spinels: Contributions to Miner- Naldrett, A.J., and Kullerud, G., 1967, A study of the Strathcona matic temperatures: Chemical Geology, v. 235, p. 59–75, alogy and Petrology, v. 105, p. 337–346, https://doi.org/10 mine and its bearing on the origin of the nickel-copper ores https://doi.org/10.1016/j.chemgeo.2006.06.004. .1007/BF00306543. of the Sudbury district, Ontario: Journal of Petrology, v. 8, Podmore, F., and Wilson, A.H., 1987, A reappraisal of the structure, Kullerud, G., 1963, Thermal stability of pentlandite: Canadian p. 453–531, https://doi.org/10.1093/petrology/8.3.453. geology and emplacement of the Great Dyke, Zimbabwe, in Mineralogist, v. 7, p. 353–366. Naldrett, A.J., and Wilson, A.H., 1990, Horizontal and vertical Halls, H.C., and Fahrig, W.F., eds., Mafic Dyke Swarms: Geo- Kullerud, G., Yund, R.A., and Moh, G., 1969, Phase relations in variations in noble metals in the Great Dyke of Zimbabwe: logical Association of Canada Special Paper 34, p. 433–444. the Fe-Ni-S, Cu-Fe-S and Cu-Ni-S systems, in Wilson, H.D.B., A model for the origin of PGE mineralization by fractional Prendergast, M.D., 1990, Platinum-group minerals and hydro- ed., Magmatic Ore Deposits: Society of Exploration Geo- segregation: Chemical Geology, v. 88, p. 279–300, https:// silicates ‘alteration’ in Wedza-Mimosa platinum deposit, physicists Geophysical Monograph 4, p. 323–343, https:// doi.org/10.1016/0009-2541(90)90094-N. Great Dyke, Zimbabwe: Transactions of the Institution of doi.org/10.5382/Mono.04.23. Naldrett, A.J., Craig, J.R., and Kullerud, G., 1967, The central Mining and Metallurgy, Section B, Applied Earth Science, Li, C., Makovicky, E., Rose-Hansen, J., and Makovicky, M., 1996, portion of the Fe-Ni-S system and its bearing on pentlan- v. 99, p. B91–B105. Partitioning of Ni, Cu, Ir, Rh, Pt, Pd between MSS and dite exsolution in iron-nickel ores: Economic Geology and Prendergast, M.D., 1991, The Wedza-Mimosa platinum deposit, sulphide liquid: Effects of composition and temperature: the Bulletin of the Society of Economic Geologists, v. 62, Great Dyke, Zimbabwe: Layering and stratiform PGE Geochimica et Cosmochimica Acta, v. 60, p. 1231–1238, p. 826–847, https://doi.org/10.2113/gsecongeo.62.6.826. mineralization in a narrow mafic magma chamber: Geo- https://doi.org/10.1016/0016-7037(96)00009-9. Oberthür, T., 2002, Platinum-group element mineralization of logical Magazine, v. 128, p. 235–249, https://doi.org/10.1017 Li, C., Ripley, E.M., Oberthür, T., Miller, J.D., and Joslin, G.D., the Great Dyke, Zimbabwe, in Cabri, L.J., ed., The Geol- /S0016756800022081. 2008, Textural, mineralogical and stable isotope studies of ogy, Geochemistry, Mineralogy and Mineral Beneficiation Prendergast, M.D., and Keays, R.R., 1989, Controls of plati- hydrothermal alteration in the main sulphide zone of the of Platinum-Group Elements: Canadian Institute of Mining, num-group element mineralization and the origin of the Great Dyke, Zimbabwe, and the precious metals zone of the Metallurgy and Petroleum Special Volume 54, p. 483–506. PGE-rich main sulphide zone in the Wedza Subchamber Sonju Lake Intrusion, Minnesota, USA: Mineralium Deposita, Oberthür, T., 2011, Platinum-group element mineralization of of the Great Dyke, Zimbabwe: Implications for the genesis v. 43, p. 97–110, https://doi.org/10.1007/s00126-007-0159-x. the main sulfide zone, Great Dyke, Zimbabwe: Reviews in of, and exploration for, stratiform PGE mineralization in Lorand, J.-P., and Gregoire, M., 2006, Petrogenesis of base Economic Geology, v. 17, p. 329–349. layered intrusions, in Prendergast, M.D., and Jones, M.J., metal sulphide assemblages of some peridotites from the Oberthür, T., Cabri, L.J., Weiser, Th., McMahon, G., and Müller, P., eds., Magmatic Sulphides—The Zimbabwe Volume: London, Kaapvaal craton (South Africa): Contributions to Mineralogy 1997, Pt, Pd and other trace elements in sulfides of the main Institution of Mining and Metallurgy, p. 43–69. and Petrology, v. 151, p. 521–538, https://doi.org/10.1007 sulfide zone, Great Dyke, Zimbabwe—A reconnaissance Prendergast, M.D., and Wilson, A.H., 1989, The Great Dyke /s00410-006-0074-7. study: Canadian Mineralogist, v. 35, p. 597–609. of Zimbabwe II. Mineralisation and mineral deposits, in Maier, W., Määttää, S., Yang, S., Oberthür, T., Lahaye, Y., Huhma, Oberthür, T., Davis, D.W., Blenkinsop, T.G., and Höhndorf, A., Prendergast, M.D., and Jones, M.J., eds., Magmatic Sulphi- H., and Barnes, S.-J., 2015, Composition of the ultramaf- 2002, Precise U-Pb mineral ages, Rb-Sr and Sm-Nd sys- des—The Zimbabwe Volume: London, Institution of Mining ic-mafic contact interval of the Great Dyke of Zimbabwe tematics for the Great Dyke, Zimbabwe—Constraints on and Metallurgy, p. 21–42. at Ngezi mine: Comparisons to the Bushveld Complex and late Archean events in the Zimbabwe craton and Limpopo Ridley, J., 2013, Ore Deposit Geology: Cambridge, UK, Cam- implications for the origin of the PGE reefs: Lithos, v. 238, belt: Precambrian Research, v. 113, p. 293–305, https://doi bridge University Press, 398 p., https://doi.org/10.1017 p. 207–222, https://doi.org/10.1016/j.lithos.2015.09.007. .org/10.1016/S0301-9268(01)00215-7. /CBO9781139135528.

Merkle, R.K.W., 1992, Platinum-group elements in the middle Oberthür, T., Weiser, Th.W., Gast, L., and Kojonen, K., 2003, Geo- Riley, J.F., 1977, The pentlandite group (Fe,Ni,Co)9S8: New data group of chromitite layers at Marikana, western Bushveld chemistry and mineralogy of the platinum-group elements and an appraisal of structure-composition relationships:

Mineralogical Magazine, v. 41, p. 345–349, https://doi.org Vaughan, D.J., and Craig, J.R., 1978, Mineral Chemistry of Metal Wilson, A.H., and Prendergast, M.D., 2001, Platinum-group /10.1180/minmag.1977.041.319.05. Sulfides: London, Cambridge University Press, 493 p. element mineralisation in the Great Dyke, Zimbabwe, and Robb, L., 2005, Introduction to Ore-Forming Processes: Malden, Vaughan, D.J., and Craig, J.R., 1997, Sulfide ore mineral stabili- its relationship to magma evolution and magma chamber Massachusetts, Blackwell Science, 384 p. ties, morphologies, and intergrowth textures, in Barnes, H.L., structure: South African Journal of Geology, v. 104, p. 319– Saumur, B.M., and Cruden, A.R., 2017, Ingress of magmatic Ni-Cu ed., Geochemistry of Hydrothermal Ore Deposits (3rd ed.): 342, https://doi.org/10.2113/gssajg.104.4.319. sulphide liquid into surrounding brittle rocks: Physical and New York, Wiley-Interscience, p. 367–434. Wilson, A.H., and Tredoux, M., 1990, Lateral and vertical vari- structural controls: Ore Geology Reviews, v. 90, p. 439–445, Vermaak, C.F., and Hendriks, L.P., 1976, A review of the miner- ation of the platinum-group elements, and petrogenetic https://doi.org/10.1016/j.oregeorev.2017.06.009. alogy of the Merensky Reef, with specific reference to new controls on the sulphide mineralisation in the P1 pyrox- Saumur, B.M., Cruden, A.R., and Boutelier, D., 2016, Sulfide data on the precious metal mineralogy: Economic Geology enite layer of the Darwendale Subchamber of the Great liquid entrainment by silicate magma: Implications for the and the Bulletin of the Society of Economic Geologists, v. 71, Dyke, Zimbabwe: Economic Geology and the Bulletin of the dynamics and petrogenesis of magmatic sulfide deposits: p. 1244–1269, https://doi.org/10.2113/gsecongeo.71.7.1244. Society of Economic Geologists, v. 85, p. 556–584, https:// Journal of Petrology, v. 56, p. 2473–2490, https://doi .org/10 Viljoen, M.J., and Schürmann, L.W., 1998, Platinum-group met- doi.org/10.2113/gsecongeo.85.3.556. .1093/petrology/egv080. als, in Wilson, M.G.C., and Anhaeusser, C.R., eds., Mineral Wilson, A.H., Murahwi, C.Z., and Coghill, B.M., 2000, Stratig- Todd, S.G., Keith, D.W., Le Roy, L.W., Schissel, D.J., Mann, E.L., Resources of South Africa (6th ed.): Pretoria, South Africa, raphy, geochemistry and PGE mineralization of the central and Irvine, T.N., 1982, The J-M platinum-palladium reef of Council for Geoscience, p. 532–568. zone of the Selukwe Subchamber, Great Dyke: Journal of the Stillwater Complex, Montana: I. Stratigraphy and petrol- Wagner, P.A., 1914, The geology of a portion of the Belingwe African Earth Sciences, v. 30, p. 833–853, https://doi.org ogy: Economic Geology and the Bulletin of the Society of District of southern Rhodesia: Transactions and Proceedings /10.1016/S0899-5362(00)00055-5. Economic Geologists, v. 77, p. 1454–1480, https://doi .org /10 of the Geological Society of South Africa, v. XVII, p. 39–54. Wingate, M.T.D., 2000, Ion-microprobe U-Pb zircon and bad- .2113/gsecongeo.77.6.1454. Wilson, A.H., 1982, The geology of the Great Dyke, Zimbabwe: deleyite ages for the Great Dyke and its satellite dykes, Tredoux, M., Lindsay, N.M., Davies, G., and McDonald, I., 1995, The ultramafic rocks: Journal of Petrology, v. 23, p. 240–292, Zimbabwe: South African Journal of Geology, v. 103, The fractionation of platinum group elements in magmatic https://doi.org/10.1093/petrology/23.2.240. p. 74–80, https://doi.org/10.2113/103.1.74. systems, with the suggestion of a novel causal mechanism: Wilson, A.H., 1992, The geology of the Great Dyke, Zimbabwe: Wood, S.A., Mountain, B.W., and Fenlon, B.J., 1989, Thermody- South African Journal of Geology, v. 98, p. 157–167. Crystallization, layering, and cumulate formation in the P1 namic constraints on the solubility of platinum and palladium van Middlesworth, J.M., and Wood, S.A., 1999, The stability of pyroxenite of cyclic unit 1 of the Darwendale Subchamber: in hydrothermal solutions; reassessment of hydroxide, bisul- palladium (II) hydroxide and hydroxy-chloride complexes: Journal of Petrology, v. 33, p. 611–663, https://doi .org/10 fide, and ammonia complexing: Economic Geology and the An experimental solubility study at 25–85 degrees C and 1 .1093/petrology/33.3.611. Bulletin of the Society of Economic Geologists, v. 84, p. 2020– bar: Geochimica et Cosmochimica Acta, v. 63, p. 1751–1765, Wilson, A.H., 2001, Compositional and lithological controls 2028, https://doi .org /10 .2113 /gsecongeo .84 .7 .2020. https://doi.org/10.1016/S0016-7037(99)00058-7. on the PGE-bearing sulphide zones in the Selukwe Sub- Worst, B.G., 1960, The Great Dyke of Southern Rhodesia: South- Vaughan, D.J., 2011, Sulphides, in Bowles, J.F.W., Howie, R.A., chamber, Great Dyke: A combined equilibrium-Rayleigh ern Rhodesia Geological Survey Bulletin 47, 234 p. Vaughan, D.J., and Zussman, J., eds., Rock-Forming fractionation model: Journal of Petrology, v. 42, p. 1845– Zaccarini, F., Garuti, G., Fiorentini, M.L., Locmelis, M., Kolleger, Minerals Volume 5A: Non-Silicates: Oxides, Hydroxides 1867, https://doi.org/10.1093/petrology/42.10.1845. P., and Thalhammer, O.A.R., 2014, Mineralogical host of and Sulfides (2nd ed.): London, The Geological Society, Wilson, A.H., and Prendergast, M.D., 1989, The Great Dyke of Zim- platinum group elements (PGE) and rhenium in the mag- p. 627–892. babwe I. Tectonic setting, stratigraphy, petrology, structure, matic Ni-Fe-Cu sulfide deposits of the Ivrea Verbano zone Vaughan, D.J., and Corkhill, C.L., 2017, Mineralogy of sulfides: emplacement and crystallization, in Prendergast, M.D., and (Italy): An electron microprobe study: Neues Jahrbuch für Elements, v. 13, p. 81–87, https://doi .org /10 .2113 /gselements Jones, M.J., eds., Magmatic Sulphides—The Zimbabwe Vol- Mineralogie, Abhandlungen, v. 191, p. 169–187, https://doi .13.2.81. ume: London, Institution of Mining and Metallurgy, p. 1–20. .org/10.1127/0077-7757/2014/0255.