Existing comment:
Geology
Placer gold originated in quartz veins formed deep beneath the earth's surface. Through uplifting and exposure to surface weathering, the gold was separated from the surrounding rock material.
The freed gold was gradually deposited along stream channels in the form of nuggets and dust, gold, being one of the heaviest materials, was deposited whenever the current slowed, settling in cracks and crevices along the river bottom.
About 60 million years ago, large wide rivers similar to the rivers of the Sierra Nevada today, concentrated the freed gold in layers of placer deposits hundreds of feet deep. Volcanic activity covered much of this network of streams, trapping and preserving these gold-bearing deposits below layers of basalt.
As the landscape and climate changed, newly formed rivers cut across these older channels transporting the depositing the placer gold. Faulting raised some of the older river beds, exposing the gold-bearing layers.
Early mining operations concentrated primarily on the accessible placer deposits of rivers only 1 to 2 million years old. Hydraulic miners used the force of water to work the older placer deposits. In the valley where rivers spread out and slowed, dredges worked the old channels as well as the existing river beds.
Over 100 million years ago semi-liquid rock called magma was forced up beneath the region that is now the Sierra Nevada. Changing pressure and temperature fractured the overlying rock material, forcing hot, mineral-filled water into the cracks and fissures. As the hot water solution cooled, quartz was deposited in veins, and with it gold. Is is these underground quartz veins containing lode gold that are sought by hard-rock miners. |