Active and Fossil Hydrothermal Mineralization in the Salton Trough Rift
The Salton Trough rift, manifested geomorphically as the Imperial Valley in southern California and the Mexicali Valley in northern Baja California (Figure 1), possesses an abundance of natural resources. These valleys are major areas for agriculture, game fish, and waterfowl migration and management. Major nonrenewable resources include metallic minerals (primarily gold) and industrial minerals (primarily gypsum and aggregate). Finally, two of the world’s largest liquid–dominated geothermal energy fields are located in the rift: the Salton Sea field in California and the Cerro Prieto field in Mexico. The rift is therefore of major economic importance to the population of southwestern North America. The purpose of this field trip is to introduce economic geologists to the diversity of hydrothermal mineralization in this modern continental rift environment.
Viewed in the simplest way the Salton Trough consists of an active continental rift, underlain by a fragmented oceanic ridge spreading system, into which has been deposited the delta of the Colorado River. Deposition of the delta has significantly influenced the character of economic mineralization in the rift. The combination of magmatic heat sources, thick porous sediments, tectonic activity, and saline lakes have provided a unique environment for the accumulation and movement of metalliferous hydrothermal brines. Before considering these aspects, a summary of the tectonic evolution of the Gulf of California and Salton Trough is presented.
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The Diversity of Mineral and Energy Resources of Southern California
The Kramer borate deposit is located in the northwestern Mojave Desert, about 90 air miles northeast of Los Angeles and 3 miles north of the town of Boron (figure 1). The deposit derives its name from the mining district in which it lies. The Kramer deposit, presently being mined from the Boron open pit, has been a world-class source of sodium borates since mine startup in 1926 and continues to be the largest source of borates in the world.
The Kramer ore body is a roughly lenticular sedimentary sequence of borax (Na2B4O7 • 10H2O) and kernite (Na2B4O7 • 4H2O) containing interbedded claystone. This central crystalline facies is successively enveloped by facies consisting of ulexite (Na,CaB5O9 • 8H2O) -bearing claystone, colemanite (Ca2B6O11 • 5H2O) -bearing claystone, and barren claystone. Studies indicate the Kramer borates were deposited in a small structural, nonmarine basin, associated with thermal (volcanic) spring activity during Miocene time.
The Kramer deposit does not crop out. It was discovered accidentally in 1213 by Dr. John Suckow, a homesteader, who struck colemanite while drilling a water well (figure 2). Exploratory drilling and shaft sinking after World War I by Pacific Coast Borax Company, the predecessor of U.S. Borax, led to the discovery of borax and kernite in 1925. In 1926 PCB went into large-scale, underground sodium borate mining in the Baker mine, located nearly 2 miles east of Suckow's discovery well. The company soon closed all its calcium borate operations near Ryan in Death Valley in favor of the more easily processed sodium borates at Boron.
U.S. Borax ships mostly bulk, refined sodium borate products and boric acid to both domestic and world markets from Boron. Principal uses for these products are in the manufacturing of glass and fiberglass, herbicides, ceramics, soaps and detergents, fluxes, fertilizers, and fire retardants.