I recently visited California and my friend took me to this old mining area near Grass Valley / Nevada City, CA. The Empire Mine site was one of the largest, richest, and longest operating gold mines of California from 1850-1956. It produced more than eight billion dollars in gold. Today the area is a historic state park.
Gold deposits in this mining district occurred in quartz veins deep underground. Granite bodies called plutons were formed when molten rock slowly cooled below the surface of the earth. During the final stages of cooling, liquids with dissolved silica and gold pushed their way into the fractures, shear zones, and fault lines of older rocks and hardened; forming the solid veins of gold found in quartz. Tradition called for water to wash gold deposits from the sand or stream beds, but it was ineffective in this area. Instead, the miners were lowered in buckets into deep shafts to chip and drill through the rock. After filling the drill holes with black powder, they denotated it, loaded the rock into ore cars, and took it to the mine headframe for crushing. At the stamp mill, the crushed ore mixed with water was placed on copper plates coated with mercury. Then it was combined with free gold to form an amalgam. Water washed away any impurities. The cleaned amalgam went to the refinery for further processing. In 1905, cyanide was used to dissolve gold while it was still embedded in the quartz. The gold was leached out into a liquid form.
Until mules were introduced, miners moved the ore-laden cars manually. When the mules were introduced, the miners lowered them into the shafts. The mules lived underground until they became too old to work. The mules pulled the ore cars. The miners fed and took care of their mules. When the mules were brought to the surface, they had to wear blinders.