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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 CO2-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.

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