Abstract

This paper reports the discovery of carbonic CH4-dominated fluid inclusions in two carbonate-hosted zinc-lead deposits in Griqualand West, South Africa. The primary carbonic inclusions were identified in coarse crystalline quartz of the main mineralisation stages at the Pering and Bushy Park deposits, hosted by Neoarchean dolostones of the Campbellrand Subgroup of the Transvaal Supergroup. The composition of the carbonic inclusions was determined from their low temperature phase behaviour during microthermometry and was confirmed with Raman microspectometry. It provides evidence for the presence of a highly reactive reducing agent (CH4) that may have been important during the mineralisation process. The carbonic inclusions and their coexistence with saline aqueous fluid inclusions allow the determination of the fluid temperature (200 to 240°C) and pressure (0.8 to 1.5 kbar) during mineralisation, suggesting that ore formation took place at 2.8 to 4.8 km depth under lithostatic pressure conditions. This is deeper than widely believed for Mississippi Valley-Type deposits, with which the Pering and Bushy Park deposits show many similarities.

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