The Plio-Pleistocene McLaughlin mine is a large hot-spring gold–silver–mercury deposit, located in the northern Coast Ranges of California. Two distinctly different suites of volcanic rocks host mineralization: an Upper Jurassic basalt, part of the Coast Range ophiolite; and an upper Cenozoic basaltic andesite, part of the Clear Lake Volcanic Field. The two suites of volcanic rocks range from least altered rocks showing primary igneous mineralogy, to intensely altered rocks composed of an assemblage of quartz, montmorillonite, sericite, and adularia. Mass balance calculations indicate almost total loss of Na, Ca, Mg, and Sr and large gains in Κ and Ba as a result of the hydrothermal alteration. The mass flux corresponds to the replacement of plagioclase feldspar with sericite and adularia, and mafic minerals with montmorillonite. Trace elements show no evidence for mobility. Rocks adjacent to the hydrothermal system have the greatest δ18O values and the highest calculated mass flux, suggesting the greatest degree of chemical exchange with the hydrothermal fluid. Rocks peripheral to the hydrothermal system have lower δ18O values that are a hybrid of primary magmatic values and values imposed by the hydrothermal alteration assemblage. These rocks also have calculated mass fluxes that reflect only partial exchange with the hydrothermal fluid.