Low-salinity, high-temperature (>200øC), 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, Witwa-tersrand, 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 ex-plained 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 ø _+ 20øC (for 3-5 kbars) and produces large volumes of low-salinity, HzO-COz fluid similar in composition to those re-corded 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 ø to 400 øC and can be facilitated by lower temperature, lower sulfur activity, and changes in oxygen activity. Lower sulfur
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Phillips et al. (1993) studied this question.