Platinum-group Mineral (PGM) and Base-metal Sulphide (BMS) Inclusions in Chromitites of the Zedang Ophiolite, Southern Tibet, China and their Petrogenetic Significance

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• 作者：GUO Guolin ; YANG Jingsui ; Paul T. ROBINSON ; LIU Xiaodong ; XU Xiangzhen and XIONG Fahui
• 刊名：Acta Geologica Sinica - English Edition
• 出版年：2016
• 出版时间：June 2016
• 年：2016
• 卷：90
• 期：3
• 页码：900-912
• 全文大小：10218K
• ISSN：1755-6724

文摘

Voluminous platinum-group mineral (PGM) inclusions including erlichmanite (Os,Ru)S₂, laurite (Ru,Os)S₂, and irarsite (Ir,Os,Ru,Rh)AsS, as well as native osmium Os(Ir) and inclusions of base metal sulphides (BMS), including millerite (NiS), heazlewoodite (Ni₃S₂), covellite (CuS) and digenite (Cu₃S₂), accompanied by native iron, have been identified in chromitites of the Zedang ophiolite, Tibet. The PGMs occur as both inclusions in magnesiochromite grains and as small interstitial granules between them; most are less than 10 μm in size and vary in shape from euhedral to anhedral. They occur either as single or composite (biphase or polyphase) grains composed solely of PGM, or PGM associated with silicate grains. Os-, Ir-, and Ru-rich PGMs are the common species and Pt-, Pd-, and Rh-rich varieties have not been identified. Sulfur fugacity and temperature appear to be the main factors that controlled the PGE mineralogy during crystallization of the host chromitite in the upper mantle. If the activity of chalcogenides (such as S, and As) is low, PGE clusters will remain suspended in the silicate melt until they can coalesce to form alloys. Under appropriate conditions of fS₂ and fO₂, PGE alloys might react with the melt to form sulfides-sulfarsenides. Thus, we suggest that the Os, Ir and Ru metallic clusters and alloys in the Zedang chromitites crystallized first under high temperature and low fS₂, followed by crystallization of sulphides of the laurite-erlichmanite, solid-solution series as the magma cooled and fS₂ increased. The abundance of primary BMS in the chromitites suggests that fS₂ reached relatively high values during the final stages of magnesiochromite crystallization. The diversity of the PGE minerals, in combination with differences in the petrological characteristics of the magnesiochromites, suggest different degrees of partial melting, perhaps at different depths in the mantle. The estimated parental magma composition suggests formation in a suprasubduction zone environment, perhaps in a forearc.