Aggregate mining on Mount Zion, Clayton, California

Two construction aggregate companies, Cemex and Hanson Aggregates, operate respective crushed stone quarries on the east and west slopes of Mount Zion in Clayton, California. These sidehill quarries utilize a single highwall and mine Jurassic diabase of the Coast Range ophiolite that formed as a sheeted dike complex. Hydrothermal veins, some containing 20%-30% disseminated pyrite and chalcopyrite, cut the diabase. The east quarry, operated by Cemex, was started by the Harrison-Birdwell Company in 1947. The west quarry, operated by Hanson, was started by the Henry J. Kaiser Sand and Gravel Company in 1954. The Cemex quarry highwall is visible as you come into the city of Clayton on Marsh Creek Road, with a height of approximately 280 m (920 ft). The height of the highwall at the Hanson quarry is approximately 215 m (700 ft). Both operations remove weathered diabase overburden to expose fresh diabase, which is drilled, blasted, and hauled to the plant for processing. To ensure aggregate is suitable for construction, quality assurance testing is conducted in accordance with the specifications of various agencies. These quarries supply the surrounding area with aggregate for hospitals, schools, highways, dams, and other buildings. Noteworthy projects supplied by the Clayton quarries include the Concord BART Station, Interstate-680, Interstate-580, Calaveras Dam, Sherman Island Levee, Highway 4, Highway 24, and Bay Bridge epoxy asphalt. Before aggregate was mined, Mount Zion was the site of a copper rush from 1862 to 1864. Gold and silver were also reported in various assays from the Clayton district. Although prospecting created excitement around Clayton, no productive orebodies were ever discovered.