Original Article Gold Mineralization in Banded Iron Formation in the Amalia Greenstone Belt, South Africa: A Mineralogical and Sulfur Isotope Study

摘要

The Blue Dot gold deposit, located in the Archean Amalia greenstone belt of South Africa, is hosted in an oxide (± carbonate) fades banded iron formation (BIF). It consists of three stratabound orebodies;;Goudplaats, Abelskop, and Bothmasrust. The orebodies are flanked by quartz-chlorite-ferroan dolomite-albite schist in the hanging wall and mafic (volcanic) schists in the footwall. Alteration minerals associated with the main hydrothermal stage in the BIF are dominated by quartz, ankerite-dolomite series, siderite, chlorite, muscovite, sericite, hematite, pyrite, and minor amounts of chalcopyrite and arsenopyrite. This study investigates the characteristics of gold mineralization in the Amalia BIF based on ore textures, mineral-chemical data and sulfur isotope analysis. Gold mineralization of the Blue Dot deposit is associated with quartz-carbonate veins that crosscut the BIF layering. In contrast to previous works, petrographic evidence suggests that the gold mineralization is not solely attributed to replacement reactions between ore fluid and the magnetite or hematite in the host BIF because coarse hydrothermal pyrite grains do not show mutual replacement textures of the oxide minerals. Rather, the parallel-bedded and generally chert-hosted pyrites are in sharp contact with re-crystallized euhedral to subhedral magnetite ± hematite grains, and the nature of their coexistence suggests that pyrite (and gold) precipitation was contemporaneous with magnetite-hematite re-crystallization. The Fe/(Fe+Mg) ratio of the dolomite-ankerite series and chlorite decreased from veins through mineralized BIF and non-mineralized BIF, in contrast to most Archean BIF-hosted gold deposits.