Sierra Nevada, California
McGee Mountain and Mount Morrison of the Sherwin Range: © TheDailyNathan
The Sierra Nevada is a mountain range in California and Nevada and forms part of the North American Cordillera, a chain of mountains that extends down the west coast of North America. The Sierra Nevada foothills are famous for their high concentrations of gold. They are host to around 13,000 gold mines and have yielded more than 4000 tonnes of gold from lode (veins of metal ore) and placer (found in gravels and sands) deposits. In 1848 the discovery of placer gold in sand and gravel stream beds sparked the ‘California Gold Rush’ the largest voluntary migration of people in American history.
Small quantities of gold occur in many different rocks however this is rarely enough to be worth mining. The abundance of gold found in the Sierra Nevada is a direct result of the region’s particular tectonic history in which chemical and physical processes acted to increase gold concentrations over hundreds of millions of years.
The origin of the Sierra Nevada gold deposits is related to subduction and the related addition of material onto the western edge of the North American continent. From Late Paleozoic to Jurassic times a series of subduction zones developed on the western margin of the North American plate. Subduction of oceanic crust under the North American plate initially generated a magmatic arc on the continent. Hot magma from the mantle upwelled and erupted as a chain of volcanoes on the continent and crystallised below the surface as huge granitic plutons and batholiths. Later stages of subduction saw the development of a series of oceanic volcanic arcs which were eventually accreted (added) on to the continent. During this accretion, sediment and crustal material between the North American Plate and the incoming arcs was also forced onto the continent through thrusting. These collisional processes are known as ‘orogenies’, and are responsible for the formation of the so-called ‘orogenic gold’ deposits of the Sierra Nevada, like the ones found in the Sperrin Mountains in Northern Ireland. Owing to this complex tectonic history of island arc accretion and magmatic intrusion, the Western Metamorphic Belt of the Sierra Nevada foothills contains a diverse array of Paleozoic (544 to 245 million years ago) and Mesozoic (245 to 55 million years ago) sedimentary and igneous rocks which have undergone varying degrees of deformation and metamorphism. During the orogeny, circulating hydrothermal fluids became enriched in gold, which rose up through fractures in the continental crust mainly following the weak and shattered rocks along the major fault zones of the Western Metamorphic Belt. As these fluids cooled, they precipitated (deposited) gold-bearing veins of milky white quartz.
The origin of these hydrothermal fluids is unclear – whether they were derived from dehydration of hydrous minerals during metamorphic processes at depth, or were formed by the deep circulation and heating of water from the surface, or perhaps the fluids came out of solution from subduction-related magmas.
More recent uplift and titling, initiating in the Miocene around 20 million years ago, has subjected the rocks of the Sierra Nevada to enhanced weathering and erosion. Gold, uplifted and exposed on the surface, has been eroded and transported by rivers, resulting in pockets of placer gold in the sand and gravel beds of ancient rivers and streams.
Further reading:
Hill, M. (2006). Geology of the Sierra Nevada: Revised Edition. University of California Press.