Abstract Silica carbonate alteration of serpentinite is ubiquitous in the Coast Ranges of northern California, occurring in barren, Hg-rich and auriferous hydrothermal systems. The alteration is formed by the low temperature reaction of CO 2 -rich fluids with serpentinite minerals. The alteration is considered to be an exchange of cations with little net gain or loss of oxygen. The major element flux is characterized by the addition of silica and CO2 and a depletion in all other cations. The trace element flux is different for each suite examined. Barren silica carbonate assemblages are not elevated in any of the trace elements analyzed. Mercury-rich suites are elevated in Hg and the auriferous suites are elevated in Au, As, Sb and Hg. The mineralogical changes resulting from the alteration is a halo of magnesite around a core of silicified serpentinite. The variation in alteration minerals may reflect variations in fluid temperature. Oxygen isotopes suggest that the alteration is low temperature around 20°C, and that the mineral-springs were likely active at the site of the McLaughlin deposit prior to and after the hot-spring activity, that formed the McLaughlin ore body. The sulfur isotopic composition from a variety of mercury deposits and active hydrothermal systems show fairly consistent values of about 0%o, indicating a magmatic source.

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 CO 2 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 CO 2 is partitioned into the vapor phase faster than H 2 S, 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 δ 18 Oqtz 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.