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Chapter 9: Anhydrite Assimilation by Ultramafic Melts of the Bushveld Complex, and Its Consequences to Petrology and Mineralization

By
Marina A. Yudovskaya
Marina A. Yudovskaya
1
School of Geosciences, University of Witwatersrand, Wits 2050, South Africa
2
Institute of Geology of Ore Deposits, Mineralogy, Petrography and Geochemistry RAS, Staromonetny 35, Moscow 119017, Russia
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Sergey F. Sluzhenikin
Sergey F. Sluzhenikin
2
Institute of Geology of Ore Deposits, Mineralogy, Petrography and Geochemistry RAS, Staromonetny 35, Moscow 119017, Russia
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Gabi Costin
Gabi Costin
3
Department of Earth, Environmental and Planetary Sciences, Rice University, 6100 Main Street, Houston, Texas 77005
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Konstantin N. Shatagin
Konstantin N. Shatagin
2
Institute of Geology of Ore Deposits, Mineralogy, Petrography and Geochemistry RAS, Staromonetny 35, Moscow 119017, Russia
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Elena O. Dubinina
Elena O. Dubinina
2
Institute of Geology of Ore Deposits, Mineralogy, Petrography and Geochemistry RAS, Staromonetny 35, Moscow 119017, Russia
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Danie F. Grobler
Danie F. Grobler
4
Exploration & Geology, Ivanplats (Pty.) Ltd., Mokopane, South Africa
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Henriette Ueckermann
Henriette Ueckermann
5
Department of Geology, University of Johannesburg, Auckland Park 2006, South Africa
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Judith A. Kinnaird
Judith A. Kinnaird
1
School of Geosciences, University of Witwatersrand, Wits 2050, South Africa
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Published:
January 01, 2018

Abstract

Sulfate assimilation by mafic to ultramafic melt is thought to be an important process in the genesis of magmatic PGE-Ni-Cu deposits. We consider petrological indicators and possible mechanisms of anhydrite assimilation by ultramafic melts of the northern limb of the Bushveld Complex. On farm Turfspruit, an anhydrite-bearing sedimentary raft of the Duitschland Formation separates the Platreef from underlying Lower zone peridotites. The proportion of anhydrite across the raft increases from negligible in corundum-sillimanite-magnetite hornfels at the base to 95 to 100% in anhydrite marble at the top. Underlying Lower zone peridotites lack anhydrite, whereas overlying Platreef pyroxenites contain both widespread interstitial to euhedral anhydrite as well as spherical to irregularly shaped anhydrite inclusions in association with olivine chadacrysts inside oikocrystic orthopyroxene. Olivine chadacryst compositions (Mg# 79–81 and 0.33–0.46 wt % NiO) support their pristine liquidus origin, although an association of Al-enriched orthopyroxene and interstitial anorthite indicates exchange reactions involving anhydrite and aluminosilicates from hornfels.

Plagioclase from the anhydrite-contaminated rocks has an Sr isotope initial ratio (Sri) of 0.7047 to 0.7063, similar to the compositions of Bushveld early primitive magmas, in agreement with a relatively nonradiogenic signature of the anhydrite-bearing contaminant with Sri of 0.7057 to 0.7094. The range of Sri of plagioclase from the underlying Lower zone peridotites (0.7040–0.7067) and from the Turfspruit platinum reefs just below the Main zone contact (0.7068–0.7084) supports their correlation and synchronous emplacement with the Lower zone and the top of the Upper Critical zone in the western and eastern limbs of the Bushveld.

The δ34S values of anhydrite (12.2–14.5‰) and a coexisting pyrrhotite-millerite-chalcopyrite sulfide assemblage (6.2–7.8‰) in a hornfelsed raft and overlying pyroxenites are interpreted to have resulted from open-system isotopic exchange, indicating closure temperatures of 750° to 820°C. The assimilation of sedimentary anhydrite is interpreted to be an important component of contact-style mineralization of the Platreef at Turfspruit that took place through the erosion and disintegration of footwall rocks by dynamic pulses of hot magmas. Chemical dissolution, thermal decomposition, and melting of sulfate-bearing rafts or xenoliths are viable assimilation processes that result in the saturation of silicate melt with sulfate, exsolution of immiscible sulfate melts, crystallization of cumulus and interstitial anhydrite, and precipitation of contact-style sulfide mineralization at the base of the intrusion. Reef-style mineralization at the top of the Platreef shows contrastingly negligible compositional and isotopic evidence of sulfate assimilation.

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Contents

Metals, Minerals, and Society

Society of Economic Geologists (SEG)
Volume
21
ISBN electronic:
9781629496405
Publication date:
January 01, 2018

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