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Abstract

The McLaughlin Mine is a hot-spring type gold-mercury deposit located in the Coast Ranges of northern California. The “sheeted vein complex” is the center of the hot-spring system that formed the McLaughlin deposit. The sheeted vein complex merges from a subaerial siliceous sinter into a bilaterally symmetric, subparallel, multistage vein swarm. The precious metals are well zoned with gold largely restricted to the upper 200 m of the deposit. Silver can dominate anywhere in the system but is always more abundant than gold below 200 m. Below 350 m, silver is rare, gold has not been observed and mineralization is dominated by base metal sulfides. Fluid inclusion analysis suggest that the ore forming fluids were low salinity NaCl dominated, low CO2 fluids. The deepest samples (> 800 m below the paleosurface) have an average temperature of 235°C. The ascending hydrothermal fluid intersected the hydrostatic boiling curve at ~400 m below the surface and paralleled the hydrostatic boiling curve to the surface. Boiling of an ascending hydrothermal fluid accounts for the metal zoning observed in the sheeted vein complex. During boiling CO2 is partitioned into the vapor phase faster than H2S, resulting in the deeper portion. 3f the ore body being enriched in silver with respect to gold and the shallow portions of the ore body enriched in gold with respect to silver. On the basis of the physical and chemical conditions of the ore forming fluids, gold grade, as well as silica and temperature gradients the hydrothermal fluid is undersaturated with respect to gold prior to the onset of boiling. There is a strong trend for increasingly light δ18Oqtz with depth. The most isotopically enriched samples are from the subaerial sinter and the lightest sample are from the deepest samples. This trend is a temperature effect and is the result of increasing fractionation with decreasing temperature. The oxygen isotopic composition of the hydrothermal fluid remained fairly constant at ~93%o. The oxygen and deuterium composition of the hydrothermal fluids are consistent with a meteoric water origin. The hydrothermal fluids have a pronounced oxygen shift due to water/rock interaction but do not have a deuterium shift. The water/rock ratios are low but similar to other geothermal systems in the Coast Ranges and other epithermal deposits emplaced within sedimentary sequences.

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