Halogen Geochemistry of the Stillwater and Bushveld Complexes: Evidence for Transport of the Platinum-Group Elements by Cl-Rich Fluids

Boudreau, A. E.; Mathez, E. A.; McCallum, I. S.

doi:10.1093/petrology/27.4.967

Cited by 300 publications

(112 citation statements)

References 29 publications

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“…These are the coarsegrained nature of the reef (e.g., Nicholson & Mathez 1991, Mathez et al 1997, the REE-rich nature of the clinopyroxene in the reef (Mathez 1995), and the Clrich nature of the apatite associated with the reef (Boudreau et al 1986).…”

Section: Redistribution Of Pge By Fluidsmentioning

confidence: 99%

Variations in the Nature of the Platinum-Group Minerals in a Cross-Section Through the Merensky Reef at Impala Platinum: Implications for the Mode of Formation of the Reef

Prichard

Barnes

Maier

et al. 2004

The Canadian Mineralogist

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A study of the abundance, size, distribution and composition of platinum-group minerals in samples from a section of the Merensky Reef at Impala Platinum, on the farm Reinkoyalskraal, in the western Bushveld Complex, South Africa, has shown that melanorite, leuconorite and anorthosite contain a PGM assemblage that consists almost exclusively of Pt and Pd bismuthotellurides, predominantly moncheite and merenskyite. In the chromite-rich lithologies, this assemblage of Pt-Pd-Bi telluride PGM is joined by a Pt-Pd-Rh sulfide PGM assemblage of cooperite, braggite and an unnamed Cu-Pt-Rh sulfide, with laurite and rare Sn-bearing PGM. This additional assemblage tends to be Pd-poor. The PGM are rarely enclosed by chromite. All the PGM are predominantly associated with base-metal sulfides, either as euhedral PGM or laths forming an exsolution texture within the base-metal sulfides. Rhodium, present as the unnamed Cu-Pt-Rh sulfide, is associated with pyrrhotite and pentlandite. Throughout this section of the reef, the PGM are commonly located at the edge of base-metal sulfides adjacent to serpentine, chlorite and amphibole that form on the edges of silicate grains. In the chromite-poor samples, Pt-Pd-Bi tellurides and their associated base-metal sulfides are located commonly within silicates, including plagioclase and quartz. The chromite-bearing rocks in this section of the Merensky Reef are enriched in Os, Ir, Ru, Rh and Pt. We test three models for the formation of the PGM: coprecipitation of PGM and chromite, crystallization of PGM from a sulfide liquid, and redistribution of PGE and base metals by hydrous intercumulus fluid. The strong association of PGM with base-metal sulfides suggests that the PGE were collected by an immiscible base-metal sulfide liquid. This liquid crystallized as Mss, with Rh being concentrated in the Mss, and then as Iss. These exsolved to pyrrhotite, pentlandite and chalcopyite and PGM. In the chromite-rich layer, we note a lack of minerals containing Pd in the PGM assemblage. No one model satisfactorily explains the PGM distribution. Rather, the PGM observed are likely to result from late, low-temperature processes superimposed on the magmatic ones.Keywords: platinum-group minerals, Merensky Reef, Impala Platinum mine, chromitite, sulfide, telluride, Bushveld Complex, South Africa. SOMMAIRENotre étude de l'abondance, la taille, la distribution et la composition des minéraux du groupe du platine (MGP) dans les échantillons prélevés d'une section du banc de Merensky à la mine Impala Platinum, sur la ferme Reinkoyalskraal, dans la partie occidentale du complexe de Bushveld, en Afrique du Sud, démontre que la mélanorite, la leuconorite et l'anorthosite contiennent un assemblage de MGP fait presqu'exclusivement de bismuthotellurures de Pt et Pd, surtout monchéite et merenskyite. Dans les § E-mail address: prichard@cardiff.ac.uk 424 THE CANADIAN MINERALOGIST roches riches en chromite, un assemblage de sulfures de Pt-Pd-Rh, soit cooperite, braggite et un sulfure à Cu-Pt-Rh sans nom, avec lau...

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Section: Redistribution Of Pge By Fluidsmentioning

confidence: 99%

Variations in the Nature of the Platinum-Group Minerals in a Cross-Section Through the Merensky Reef at Impala Platinum: Implications for the Mode of Formation of the Reef

Prichard

Barnes

Maier

et al. 2004

The Canadian Mineralogist

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“…The formation of PGE mineralization in large mafic-ultramafic intrusions, such as the Great Dyke, Bushveld Complex in South Africa, the Stillwater Complex in Montana, USA, and the Noril'sk Ni-Cu-(PGE) deposits in Siberia is therefore commonly thought to be magmatic [3,4]. However, there is also evidence in many PGE deposits in layered intrusions that secondary hydrothermal activity can remobilize base metals and PGEs [5,6].…”

Section: Introductionmentioning

confidence: 99%

Hydrothermal Alteration in the Main Sulfide Zone at Unki Mine, Shurugwi Subchamber of the Great Dyke, Zimbabwe: Evidence from Petrography and Silicates Mineral Chemistry

Chaumba

2017

Minerals

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Abstract:The main platinum-group element (PGE) occurrence in the Great Dyke of Zimbabwe, the Main Sulfide Zone (MSZ), is a tabular stratabound layer hosted in pyroxenites. A petrographic and silicate composition study across the MSZ at Unki Mine in the Shurugwi Subchamber was conducted to help place some constrains on the origin of the mineralization. The PGE-enriched zone at Unki Mine is a~10 m thick package of rocks ranging from gabbronorites, a chromitite stringer, plagioclase websterite, plagioclase pyroxenite (pegmatitic in one narrow zone), a base metal sulfide zone and it is largely located below the contact of the Mafic and Ultramafic Sequences. Pyroxenes have been partially hydrothermally altered to amphibole and chlorite in most lithologies. In addition, sulfides tend to occur as cumulus phases or as inclusions in all the silicate phases. Two generations of sulfide mineralization likely occurred at Unki Mine with primary sulfides occurring in association with cumulus phases, and the relatively finer-grained, often lath-like, sulfides that occur in association with alteration phases of chlorite and amphibole that were likely formed later during hydrothermal alteration. Chlorite thermometry yields temperatures ranging from 241 to 390 • C, and from 491 to 640 • C, and they are interpreted to be temperatures recording the hydrothermal event(s) of magmatic origin which affected the mineralization at Unki Mine. Two-pyroxene thermometry yields temperatures that range from 850 to 981 • C, and these temperatures are interpreted to indicate a hydrothermal imprint on the minerals that constitute the MSZ.

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“…Evidence for these interactions include the presence of laterally-continuous, pegmatitic stratiform horizons (e.g., the Merensky and Howland Reefs) and more local discordant bodies (Raedeke & McCallum, 1984;Viljoen & Scoon, 1985); postcumulus chlorine-rich apatite, biotite, and hornblende (Boudreau et al, 1986); and abundant graphite in some of the pegmatitic and sulfide-rich rocks (Volborth & Housley, 1984;Ballhaus & Stumpfl, 1985). The hightemperature vapor is of interest because it may be an important agent for geochemical transport.…”

Section: Introductionmentioning

confidence: 99%

“…The hightemperature vapor is of interest because it may be an important agent for geochemical transport. For example, it has been speculated that magmatic vapor was in part responsible for the concentration of platinum group elements (PGEs) in the Merensky and Howland Reefs (e.g., Lauder, 1970, Vermaak, 1976Boudreau et al, 1986).…”

Section: Introductionmentioning

confidence: 99%

Carbon Distribution in the Stiliwater Complex and Evolution of Vapor During Crystallization of Stillwater and Bushveld Magmas

Mathez

Dietrich²,

Holloway

et al. 1989

Journal of Petrology

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The occurrence and distribution of carbon in the Stillwater Complex have been investigated. In mineralized troctolite and associated rocks of olivine-bearing zone I (OB I), carbon is present as graphitic material and calcitc. The assemblage forsterite-antigorite-calcite-graphite and the petrographic relations indicate equilibration of the carbon-rich phases during serpentinization. Typical OB I troctolite contains 500-1100 ppm wt. carbon, 40-70% of which is in calcite, whereas troctolite from higher stratigraphic positions generally contains <400 ppm carbon. Due to the metamorphism, it is not possible to deduce the extent to which enrichment of carbon in the ore zone is inherited from magmatic processes. In contrast, there is good evidence for magmatic graphite in parts of the Bushveld Complex.The C-O-H-Cl system has been investigated for conditions of Stillwater and Bushveld crystallization. In alkali-poor fluids over a wide range of igneous and metamorphic conditions, the important chlorine species are HO and CH 3 C1. The addition of chlorine to a C-O-H fluid in equilibrium with graphite leads to a quantitative increase in HO + CH 3 C1 and corresponding decrease in H 2 O contents, and, when Cl/H exceeds 1, to a CO 2 + CO-rich fluid with little H 2 O. Similarly, in more reduced fluids, CH* contents arc depressed by the formation of CH 3 O.From consideration of volatile solubilities and abundances in mafic magmas and the nature of the C-O-H-O system, it is hypothesized that the first fluid to exsolve from Bushveld and Stillwater intercumulus melt was composed of a mixture of CO 2 , CO, and HO with minor amounts of sulfur species and H 2 O. A model is developed for the evolution of such a fluid with cooling. The model assumes that graphite began to precipitate from the fluid at supersolidus temperature and that the system cooled down a T-f Ol path parallel to and > 2 log units below that of the Ni-NiO oxygen buffer. Upon the appearance of graphite, the fluid evolved to a more hydrogen-rich composition by graphite precipitation and loss of oxygen to the surrounding silicate-oxide assemblage. Cooling of fluid to 25 °C below the first appearance of graphite resulted in reduction in the fluid mass by >70%, thus concentrating chlorine, sulfur and other residual species in the intercumulus fluid and melt. The model explains the presence of chlor-apatite and the enrichment of graphite in the Bushveld Critical Zone and predicts that chlor-apatite-bearing Stillwater rocks were similarly enriched in graphite during crystallization.

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Halogen Geochemistry of the Stillwater and Bushveld Complexes: Evidence for Transport of the Platinum-Group Elements by Cl-Rich Fluids

Cited by 300 publications

References 29 publications

Variations in the Nature of the Platinum-Group Minerals in a Cross-Section Through the Merensky Reef at Impala Platinum: Implications for the Mode of Formation of the Reef

Variations in the Nature of the Platinum-Group Minerals in a Cross-Section Through the Merensky Reef at Impala Platinum: Implications for the Mode of Formation of the Reef

Hydrothermal Alteration in the Main Sulfide Zone at Unki Mine, Shurugwi Subchamber of the Great Dyke, Zimbabwe: Evidence from Petrography and Silicates Mineral Chemistry

Carbon Distribution in the Stiliwater Complex and Evolution of Vapor During Crystallization of Stillwater and Bushveld Magmas

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