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The Geysers geothermal area and associated Clear Lake Volcanics have aroused much scientific interest and have been subjected to many sorts of geophysical investigations. Summaries of these studies have been published by Isherwood (1981) and Majer and others (1992). In this Paper we provide a broad tectonic interpretation of the Clear Lake region and the relationship to the local gold mineralization, by interpreting magnetic and gravity data together with other available geologic and geophysical information. These results stem from work in progress and should be considered preliminary.


A late Pliocene and Pleistocene hot-spring deposit consisting of sinter terraces underlain by veins is the site of the McLaughlin Mine in northern California. The deposit is localized along the contact between hanging wall mudstone of the Upper Jurassic Knoxville Formation (basal formation of the Upper Jurassic and Cretaceous Great Valley sequence) and serpentinized ultramafic and mafic rocks of the Middle Jurassic Coast Range ophiolite in the footwall. The Stony Creek fault separates the two units, and dips moderately northeasterly. Pliocene basaltic andesite and volcaniclastic rocks unconformably overlie the sedimentary and ophiolitic rocks in the mine area. These rocks intruded as sills to shallow depths or erupted after phreatomagmatic explosions from small volcanic centers. Magmatism was also localized along the Stony Creek fault. The volcanic rocks are part of the older phase of the Clear Lake Volcanics, which formed in a Pliocene to Holocene volcanic field that lay mostly to the west and northwest.

The deposit consists of two epithermal orebodies, the north and south orebodies, which are connected by a narrow 2one of mineralization. The oldest hydrothermal event recognized in the deposit was adularization of the hanging wall mudstone and silicification of the footwall serpentinite. Alteration formed a roughly tabular body that is discontinuous along the Stony Creek fault. Basaltic andesite sills, plugs, and flows are propylitized and locally argillized. Precious metals are present in opal, chalcedony, and quartz veins that cut brittle rocks, including altered rocks, basaltic andesites, and tabular lithons present in the footwall serpentinite. The south

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