Nature of ore-forming fluid and formation conditions of BIF-hosted gold mineralization in the Archean Amalia Greenstone Belt, South Africa: Constraints from fluid inclusion and stable isotope studies

摘要

Graphical abstractDisplay OmittedHighlights?Orogenic gold mineralization hosted in Archean banded iron formation.?Alteration mineral assemblage suggest relatively oxidizing conditions at deposit site.?Simple gold-sulfide association, no sulfates, restricted stable isotope values, lowfO2, T<400°C.?Ore-forming fluid has low salinity H2O-CO2composition.?Physico-chemical conditions suggest transport of reduced gold-bisulfide complex in the fluid.?Decrease in total sulfur content and increase infO2resulted in gold deposition.AbstractOrogenic gold mineralization in the Amalia greenstone belt is hosted by oxide facies banded iron-formation (BIF). Hydrothermal alteration of the BIF layers is characterized by chloritization, carbonatization, hematization and pyritization, and quartz-carbonate veins that cut across the layers. The alteration mineral assemblages consist of ankerite-ferroan dolomite minerals, siderite, chlorite, hematite, pyrite and subordinate amounts of arsenopyrite and chalcopyrite. Information on the physico-chemical properties of the ore-forming fluids and ambient conditions that promoted gold mineralization at Amalia were deduced from sulfur, oxygen and carbon isotopic ratios, and fluid inclusions from quartz-carbonate samples associated with the gold mineralization.Microthermometric and laser Raman analyses indicated that the ore-forming fluid was composed of low salinity H2O-CO2composition (~3wt%NaCl equiv.). The combination of microthermometric data and arsenopyrite-pyrite geothermometry suggest that quartz-carbonate vein formation, gold mineralization and associated alteration of the proximal BIF wall rock occurred at temperature-pressure conditions of 300±30°C and ～2kbar. Thermodynamic calculations at 300°C suggest an increase infO2(10?32–10?30bars) and corresponding decrease in total sulfur concentration (0.002–0.001m) that overlapped the pyrite-hematite-magnetite boundary during gold mineralization. Although hematite in the alteration assemblage indicate oxidizing conditions at the