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The Monte Blanco borate deposits are located along the southern margin of Death Valley’s Furnace Creek Wash, south of Twenty Mule Team Canyon road in California. Topographic and geologic mapping by S. Muessig and F.M. Byers, Jr., in 1954 documented these deposits’ geologic settings, geometries, mineralogies, and chemical characteristics. They estimated borate resources at the time to be in excess of 550,000 tons B2O3. The borate bodies are composed of predominantly ulexite and colemanite. They lie beneath Monte Blanco itself and along a northwest-trending series of conspicuous, white hills and mounds formed by northeasterly dipping, fine-grained sedimentary beds and basaltic volcanic rocks of the Miocene and Pliocene Furnace Creek Formation. Steeply dipping beds of ulexite (NaCaB2O3·8H2O) and interbeds of shale form the main mass of the principal Monte Blanco deposit; colemanite (Ca2B6O11·5H2O) occurs peripherally in tabular masses of limestone that are 10 to 50 feet thick. The deposit is fault-bounded on the north and grades laterally into shales and tuffs on the south. Satellite borate deposits occur to the northwest of the principal Monte Blanco deposit. They are composed of colemanite and ulexite and are steeply dipping to flat-lying tabular bodies, 10 to 60 feet thick. The bodies pinch out in the shale that forms both their hanging walls and their footwalls. Shale beds occur in both the colemanite and the ulexite masses. Most of the satellite bodies form hogbacks. Geologic data suggest that in Miocene and Pliocene time, fine-grained sediments, volcanic debris and flows, and volcanically associated, boron-rich fluids gradually filled a fairly flat playa-like environment. At times, thick beds of felty crystals of ulexite developed and were interlayered as lenses in a thick series of mudstones as is seen today at the Eagle Borax works. After burial, the exterior of the ulexite deposit was altered to massive colemanite by ground water, which produced the “shell” of colemanite that typically surrounds the presently outcropping ulexite bodies.

Geology of the Monte Blanco borate deposits, Furnace Creek Wash, Death Valley, California