13.8: Geological Resources of the Desert Provinces

Evaporite Deposits

The Mojave and Colorado Desert Provinces are the home to extensive evaporite deposits, as well as an important rare earth element mine. This section will discuss these resources. Gold and copper mines are also found throughout the region, generally associated with Mesozoic and Cenozoic igneous rocks; refer to Sierra Nevada for more about these types of resources.

A by-product of the development of playa lakes is the accumulation of evaporite deposits which can be mined. Evaporites are minerals that form by precipitation directly from water. They are typically water soluble, so when lakes refill, the minerals dissolve. The catalyst for their formation is the high salinity levels that develop as the lakes evaporate. Evaporite minerals are often referred to as “salts”, and indeed, table salt (halite) is an example of one such evaporite mineral (Figure \(\PageIndex{1}\)), as is sylvite (KCl). Other well-known evaporite minerals include gypsum and calcite.

Figure \(\PageIndex{1}\): Halite from Searles Lake is an example of an evaporite mineral from this region. In this image of a specimen collected by someone, clusters of pink Halite cubes grown on a finer substrate “Halite-Nahcolite-118814” by Rob Lavinsky, is licensed under CC BY-SA 3.0.

Halite is mined as a food additive, chemical agent, water softener, or road de-icer. Gypsum is used as a building material; it is the main component in dry wall and is also used as a fertilizer. Other evaporites include sylvite-potassium chloride-which is used in agriculture, medicine, food processing, and other applications.

The mineral hanksite is another evaporite mineral found in this area (Figure \(\PageIndex{2}\)). It was first identified at Searles Lake in 1888 and named for an American geologist (Henry Garber Hanks). It is a sulfate mineral (Na₂₂K(SO₄)₉(CO₃)₂Cl) that can be found in dry lakes, typically buried in mud.

Figure \(\PageIndex{2}\): A cluster of dark colored doubly terminated hexagonal hanksite crystals collected from Searles Dry Lake. “Hanksite crystal step from Searles Dry Lake, California” by Alisha Vargas, is licensed under CC BY 2.0.

Sodium borates are another type of salt which accumulate in the California dry lakes. Borates are hydrous minerals built on the Borate anion (BO₄). These include ulexite (Figure \(\PageIndex{3}\)), borax, colemanite, and kernite. These minerals were the basis for the borax industry that developed initially in the Death Valley region. Over time, borate mining moved away from the Death Valley region, and focused on the Searles Lake region in the Mojave where mining continues at present (Figure \(\PageIndex{4}\)). The origin of the boron (B) in these minerals is thought to be nearby volcanic materials. As hot water associated with young volcanic centers leached these rocks, it carried boron into the playa lakes, where it precipitated as borate minerals.

Figure \(\PageIndex{3}\): Ulexite specimen of “TV rock” collected from near Boron, CA. Elongate fibrous Ulexite crystals in this specimen occur as columnar masses on a calcite base. “Ulexite-Calcite-40062”by Rob Lavinsky, is licensed under CC-BY-SA-3.0

Figure \(\PageIndex{4}\): Trona, CA, which abuts Searles Lake, is the location of borate mining. In this desert landscape, the large flat white playa is the source of borate minerals mined here. “01-2007-Trona-CA-onSearlesLake” by Bobak Ha'Eri, is licensed under CC BY-SA 2.5.

Box \(\PageIndex{1}\): Searles Lake Borate Mining

Searles Lake is known for the abundance of rare elements and evaporate minerals, such as trona, hanksite, and halite formed within its sediments. Evaporites are minerals that are left behind when saltwater evaporates. During the Pleistocene Epoch (beginning approximately 2 million years ago), Searles Lake was one of a chain of lakes fed by stream flow from the Sierra Nevada to the west. Lake levels rose and fell depending on glacial outwash from the Sierra Nevada as climate shifted. Successive layers of sediment were deposited as lake levels fluctuated, preserving an important record of regional climate change. The lakes gradually dried up completely as climatic conditions became hotter and drier (as today), forming a string of playas—enclosed basins with no outlets.

Borates that are mined at Searles Lake are chemical compounds that include the element boron (B) and are important as providers of an essential plant micronutrient, for metallurgical applications, and as components of specialized types of glass, anticorrosive coatings, fire retardants, and detergents.

The Rio Tinto mine is one of Earth’s richest borate deposits (Inset Figure X.2). Together with mines in Argentina, they produce almost 40 percent of the world’s supply of industrial borate minerals. The mining complex in Searles Lake playa includes tailings piles, open pit, processing facilities and settling ponds spread across approximately 54 square kilometers (21 square miles).These mining operations extract sodium and potassium-rich minerals (primarily borax and salt) for industrial use.

This mine is the largest open-pit mine in California and is among the largest borate mines in the world (Inset Box Figure \(\PageIndex{1}\)). Concentric benches along the pit wall are accentuated by shadows and mark successive levels of material extraction. Mine tailings are visible as stacked terraces along the northern boundary of the mine. Ore processing facilities occupy a relatively small percentage of the area, and are located directly to the west of the open pit.

Inset Box Figure \(\PageIndex{1}\): One of Earth’s richest borate deposits is at the center of the largest open-pit mine in California. This aerial image of an open pit mine reveals the large pits surrounded by mining infrastructure in the desert landscape. Settling ponds can be seen along with the terraced levels of the open pit. “Rio Tinto Borax Mine” by NASA, is licensed under public domain.

Borate minerals such as borax, kernite, and ulexite are found in the deposits at the Rio Tinto mine. The first mining claim in the area was filed in 1913 following the discovery of boron-bearing nodules during well drilling. Much of the mine workings were underground until 1957, when U.S. Borax changed to open-pit mining.

Query \(\PageIndex{1}\)

Lithium Mining

A massive underground brine pool known as The Salton Sea Geothermal Reserve, located about 90 miles south of Palm Springs has been identified as a potentially important source of the element lithium. This region, which is now referred to as Lithium Valley, has one of the worlds largest deposits of lithium. Lithium (Li) is used in many modern batteries, such as those used in electric cars, smart phones and grid energy storage. This very light weight metal allows batteries to hold their charge for a long time. This type of energy storage is critical to reducing the use of fossil fuels.

Lithium is widely occurring, but does not tend to occur in its elemental form; rather, it occurs as ions in fluids, or as a minor element in granitic rocks or pegmatites. High concentrations of this element are relatively rare, but large deposits can be found in dry lakes where years of evaporation have concentrated lithium and other elements found in briney waters on the dry lake beds. In most places on earth, Lithium is mined by flooding these dry lakes or by pumping brines rich in lithium to the surface and allowing brine pools to evaporite. However, the Salton Sea deposit is contained in brines that are already pumped through geothermal systems that generate power for southern California. The Li-rich brines were considered to be waste products until recently, but with growing dependence on this element, companies are looking at technology that can be used to extract Lithium from the brines without relying on evaporative processes.

Video How the troubled Salton Sea could become the worlds largest Lithium supplier

This video discusses the production of lithium at the Salton Sea and it's potential value.

Rare Earth Mining

In the eastern edge of the Mojave Province, the largest concentration of rare earth elements in the western hemisphere is currently mined. The Mountain Pass Mine (Figure \(\PageIndex{5}\), Figure \(\PageIndex{6}\)), located on the border of California and Nevada is a strategically important source of the rare earth elements Lanthanum, Cerium, Neodymium, Samarium, Europium, and Gadolinium. These materials are used as catalysts for metallurgy and for manufacturing of glass and ceramics, as well as magnets.

The large, high grade reserves of rare earth elements here (REE’s) are hosted in carbonatite which intrudes 1.7 Ga gneisses. Carbonatite is an unusual Ca-rich igneous intrusive rock. The most important rare earth elements, Cerium and Lanthanum, are concentrated in a rare carbonate mineral called bastnasite (Ce,La)CO₃F.

Figure \(\PageIndex{5}\): The Mountain Pass Mine, imaged by the Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite on May 28, 2011. In this image, the Mountain Pass mine, adjacent to US15, appears as a light colored disturbance to the desert landscape. ”Mountain Pass 2011” by NASA, is licensed under public domain.

Figure \(\PageIndex{6}\): The Mountain Pass Rare Earth Mine & Processing Facility is the only active and scaled rare earth mining and processing facility in the United States. The facility is owned by MP Materials. The mine contains a large terraced pit, large tailings piles and numerous buildings associated with the processing of material that has been extracted. ”Mountain Pass Rare Earth Mine and Processing Facility” by Tmy350, is licensed under CC BY-SA 4.0.

Video (\PageIndex{1}\): Rare earths crunch? Why we need them and who has them

This short video discusses the value of geologic deposits that contain high concentrations of rare earth metals.

Query \(\PageIndex{2}\)

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