Quicksilver deposits of the Mount Diablo district, Contra Costa County, California

Abstract

The Mount Dlablo district, which lies about 30 miles northeast of San Francisco, is one of the few Californian quicksilver districts that have only recently come into prominence. It has been actively productive from 1936 through 1939, and during most of that time its single operating property has been among the leaders of the industry. The lodes are in fracture zones near the footwalls of inclined, more or less tabular serpentine masses in Franciscan rocks. They are thought to have been formed by hot-spring action so recent that It is still giving rise to sulphurous gases and methane. The lodes are unique in that metacinnabar is an abundant primary ore mineral. The ore shoots are in zones of intense brecciation and are controlled in part by cross fractures. When this district was visited in August 1939 work had been temporarily suspended because of labor difficulties and little ore was in sight in accessible workings except lowgrade material in open cuts; but from September 1939 at least through January 1940 the production gradually increased. The geologic setting is such as to encourage the hope that other ore shoots will be found. INTRODUCTION Location The Mount Diablo quicksilver mining district is in Contra Costa County, northern California (fig. 2), low on the eastern side of the north peak of Mount Diablo. All of the workings so far productive are in the SE£ sec. 29, T. IN., R. 1 E , Mount Diablo base and meridian, and are now held by the Bradley Mining Co. Prospects outside of this company's property were