Almaden: Home of the Richest Gold Rush Era Mines

Hikers and bikers using the extensive trails at Santa Clara Quicksilver Park in south San Jose may not know that they are on top of the most lucrative mines in California history. During the California Gold Rush of the mid-19th Century it wasn’t gold on top of the earnings charts – it was mercury, also called quicksilver, mined from the hills of New Almaden. Over 300 miles of tunnels as deep as a half mile weave underground where cinnabar ore, made up of mercury and sulfur, was mined until the 1970s. Mercury baked out of the ore was used to separate gold from sand and debris by prospectors who swarmed to the Sierra Nevada mountains and foothills.

A Mexican army officer, Andrés Castillero, recognized the mineral in 1845 when he saw it used for decoration and for trading by Ohlone Indians living in the area. His interest in pursuing a claim was interrupted by the threat of war between the U.S. and Mexico, and Castillero (for whom San Jose’s Castillero Middle School is named) only mined on a very small scale before returning to Mexico. By 1846 his interest in the mines had been assumed by a company, and large-scale quicksilver mining was underway to help feed the demands of the gold fields.

There was a downside to the mercury bonanza: the mineral is dangerous to humans. The mining in New Almaden (named after the Almaden mines of Spain) released so much mercury into the south bay watershed that fish caught in the area even today are considered too contaminated to be eaten. The mines have been safely sealed and the scenic area has been converted to recreational use.

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