Abstract

Low-salinity, high-temperature (>200 degrees C), reducing fluids are poorly represented in ore-forming processes, yet appear to account for 90 percent of the primary gold mined to date. Such fluids have been implicated in the formation of many Archcan greenstone, Witwatersrand, slate belt, and epithermal gold deposits. These low-salinity auriferous fluids occur in variable time, space, lithological sequence, and tectonic settings and are most easily explained by metamorphic devolatilization of mafic and/or graywacke successions.Metamorphism of chlorite-calcite-albite-quartz assemblages (e.g., mafic or graywacke) at the greenschist-amphibolite facies boundary occurs around 480 degrees + or - 20 degrees C (for 3-5 kbars) and produces large volumes of low-salinity, H 2 O-CO 2 fluid similar in composition to those recorded in many gold deposits. Widespread pyrite in the above assemblage typically leads to elevated levels of dissolved sulfur. Such conditions are ideal for gold transport as a molecular Au-S complex and can lead to deposits rich in gold relative to silver and base metals.Deposition of gold occurs in the temperature range of 250 degrees to 400 degrees C and can be facilitated by lower temperature, lower sulfur activity, and changes in oxygen activity. Lower sulfur activity is most readily achieved by fluid interaction with Fe-rich host rocks with the formation of pyrite or pyrrhotite, whereas lower oxygen activity is readily achieved by interaction with carbonaceous metasediments.Low-salinity fluids produced by metamorphic devolatilization provide a common theme embracing the formation of many Archcan greenstone, slate belt, Witwatersrand, and potentially epithermal gold deposits. It involves the common features of high geothermal gradient, low-salinity fluid, high Au/Ag, high Au/base metals, and broad synchroneity with metamorphism. It also accounts for the considerable variability in hosting structures, detailed timing relations, host rocks, alteration assemblages, and geologic age of different deposits. Interestingly, the similarities are explained by deeper processes, the differences by processes at or near the depositional sites.

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