Mercury

Mercury has been mined on the northeast flank of Mt. Diablo off and on since its discovery in 1863. Prior to that, Indians used the colored mineral for ceremonial purposes. The mercury (also referred to as quicksilver) occurs in the form of cinnabar (red mercury sulfide) and metacinnabar (a black mercury sulfide). The host rock for ore is silica-carbonate rock, itself formed from the hydrothermal alteration of serpentine, lying in the boundary fault zone that separates Franciscan from Great Valley rocks. The silica-carbonate rock is made up of varying quantities of silica (chalcedony and opal) together with magnesian carbonates and stained rusty red by alteration of iron sulfide minerals. The rock is commonly spongy in appearance. Topographically, silica-carbonate rock forms resistant outcrops. It is believed that the mercury minerals were deposited from hydrothermal solutions which formed mostly in fractures in the silica-carbonate rock.. Ryne Mine produced most of the cinnabar while metacinnabar was produced at the Mt. Diablo Mine.

A man named Welch discovered cinnabar at what is known as the Ryne Mine in 1863. It operated for about 10 years before becoming uneconomic. In 1933 it was discovered that black metacinnabar in the area known as the Mt. Diablo Mine also contained mercury and was more abundant than cinnabar. The demand for mercury during the second world war resulted in an expansion of operation that was to continue until 1958 when mining operations again ceased. It is estimated that about $1,500,000 worth of mercury was extracted from the mines. Unfortunately a continuing legacy of the mining is the acid mine water emptying into and contaminating Marsh Creek.

Building Stones and Rip-rap

The diabase quarries on the northside of the mountain (Zion Peak) are currently being excavated for crushed rock and rip rap material. There were several excavations in graywacke on the northside of the mountain for the same purposes, but they are now abandoned. Blue schist from the Franciscan rocks on Mt. Diablo yielded good dimension stone and was popular for building construction due to its color.

Copper and Precious Metals

About 40,000 pounds of copper was produced from the mines in the diabase in the 1860's, but there is no activity now. Minor amounts of gold and silver associated with the copper were also produced. It was rumored that the best area to discover gold or silver was in the Back Canyon area (unfortunately inside the park boundary).

Travertine

Travertine, a finely crystalline massive calcium carbonate deposit frequently associated with hot springs, was quarried along the northside of Mt. Diablo (Lime Ridge) for many years by the Cowell Cement Company. (Additional information on the Cement Industry).

Coal and White Sand

North of Mt. Diablo and outside the park in the Black Diamond Mines area, lignite coal beds in the Domengine Formation were the largest known and most extensively mined coal deposits in California. From the 1860's to the beginning of this century, the Mount Diablo coal field supplied coal to the rapidly expanding urban and industrial centers of the San Francisco Bay area. Finally closed as newer and cheaper energy sources were discovered, during its lifetime the mines produced approximately 4,000,000 tons of coal valued between $15 and $20 million.

At the base of the Domengine Formation exposed in the Black Diamond Mines Regional Park, there is a thin section of white sands called the "Ione" sands, a description carried across the Central Valley from major white sand deposits in the Ione Formation on the east side of the Valley. The sands appear to be continuous across the valley subsurface and of equivalent age. The white sands, that were used for making glass, were mined from two deposits in the area from 1920 until 1949 when they ceased operation.

Gas and Oil

The Domengine Formation also acts as a reservoir for natural gas and the Martinez Formation produces oil in the subsurface northeast of Mt. Diablo.

The above article was excerpted from "Geology of Mt. Diablo - A Training Manual" by Roi Peers (1996)

COWELL CEMENT INDUSTRY
The following was excerpted from the article "Cowell and It's Cement Industry" by Craig Lyon (1997)

As one drives easterly on Ygnacio Valley Road across Lime Ridge northwest of Mt. Diablo, scars from open pit quarry operations can be seen to the right of the road, To the left is a tall smokestack in an area that in past years was called Cowell but which is now part of Concord. This was the site of the Cowell Lime and Cement Company that produced portland cement from 1905 to 1946. The cement plant was one of the largest employers in the area in the early 1900's.

The rock was quarried in the Lime Ridge area and carried on a narrow gauge railroad, or by truck, to the Cowell plant where it was crushed, mixed with clay, and then converted to cement in roasting kilns. Sacked cement, which was marketed as "Mt.Diablo Cement", left Cowell daily on the company's standard gauge Bay Point and Clayton Railroad, At Bay Point, the cars went on their way to other areas via Southern Pacific, Western Pacific, or Santa Fe railroads.

The rock quarried at Lime Ridge and used to make cement is called travertine, It is the same composition as limestone - almost pure calcium carbonate. The rock is hard, sometimes shows banding, and is colored a light tan by the inclusion of a small amount of iron oxide, Cave formations are formed from the same kind of material, The travertine at Lime Ridge formed, probably in fairly recent geologic time, from circulating waters carrying calcium carbonate in solution which came to the surface through a very porous sandstone called, by geologists, the Domengine formation of Eocene age (about 50 million years old). The water evaporated, leaving behind the travertine which formed a layer on top of the sandstone that originally covered an area 2,5 miles long and 0.5 miles wide and varied in thickness up to a maximum of 20 feet, The Domengine sandstone is exposed in the light-colored quarry faces and can also be seen in the large roadcut just east of the entrance to Cal. State Hayward .

You may also want to look at A Collection of Articles on Mount Diablo Mining - early writings on mining activities during the late 1800's compiled by Chris Valle-Riestra.