Geology
Geological forces operating over hundreds of millions of years resulted in the large concentration of gold in California. Only concentration makes gold economically recoverable.
Some 400 million years ago, California lay at the bottom of a large sea; underwater volcanoes deposited lava and minerals (including gold) onto the sea floor. Beginning about 200 million years ago, tectonic pressure forced the sea floor beneath the American continental mass. As it sank, or subducted, below today's California, the sea floor melted into very large molten masses (magma). This hot magma forced its way upward under the future California, cooling as it rose, and as it solidified, veins of gold formed within fields of quartz. These minerals and rocks came to the surface of the Sierra Nevada, and eroded. Water carried the exposed gold downstream and deposited it in quiet gravel beds along the sides of old rivers and streams. The forty-niners first focused their efforts on these deposits of gold, which had been gathered in the gravel beds by hundreds of millions of years of geologic action.
Read more about this topic: California Gold Rush