Rio Tinto Borax Mine, Kramer Borate deposit, Boron, Kern County, California, USAi

Regional Level Types
Rio Tinto Borax Mine	Pit
Kramer Borate deposit	Deposit
Boron	Town
Kern County	County
California	State
USA	Country

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The primary mineralization is borax, precipitated in a permanent shallow lake fed by thermal (volcanic) springs rich in sodium and boron. Kernite formed later during deep burial (+1,500 ft / ca. 457 m) at temperatures above 53 °C (Christ and Garrels, 1959). The sodium borates, together with claystone, occur as a core facies within the Middle Miocene (16 mybp) Kramer beds (Barnard and Kistler, 1967) and are completely enveloped by ulexite-bearing shales (Gale, 1946). Stratigraphic and structural studies indicate deposition in a small, non-marine structural basin elongated east–west and bounded to the south by the Western Borax fault (Joe Siefke, 2010). Zeolites occur in basalt underlying the borate deposit.

Borate mineralization in the Kramer area was first recognized in 1913, when O. Suckow registered a claim after identifying colemanite nodules during well drilling. The claim was later acquired by the Pacific Coast Borax Company, whose exploration drilling discovered the sodium-borate deposit in 1925. Initial exploitation was underground via the Baker Mine shaft near the eastern end of the orebody. During 1926–1927, Suckow Borax Mines and Western Borax Co. began operations farther west; both were subsequently taken over by Pacific Coast Borax (Western Borax in 1933). The Jenifer Mine, located between the Western Borax and Suckow workings, is now included within the property.

In the late 1940s the former Western Borax property was purchased by the California Borate Co. In 1956 Pacific Coast Borax merged with United States Potash Corporation to form U.S. Borax, which converted from underground to open-pit mining in 1957. In 1968 U.S. Borax became part of the Rio Tinto Group. The property was formerly owned and operated by U.S. Borax (a subsidiary of RTZ Borax Ltd., RTZ Corporation PLC, London; home office Los Angeles until 1992) and is now owned by Rio Tinto.

The mine is located north of the town of Boron in secs. 14, 19, 23, and 24, T11N, R8W (SBM/MDM), approximately 7 miles (11 km) NW of Kramer and 4 miles (6.44 km) NW of Boron. Additional names applied to the locality include Osborne shaft, Discovery shaft, West Baker Mine, and the U.S. Borax Mine.

Workings began underground (1927–1957) and were later subhumed by a large open pit. The open pit measures approximately 2,000 m in length, 1,600 m in width, and 182.88 m in depth. In 1984 the operation removed about 10,000 tons per day. In 1989 production averaged 3,000–3,500 short tons per day (O’Driscoll, 1990). U.S. Borax operates the open pit, refinery, and boric acid plant. The waste-to-ore ratio is about 7:1 with almost 100% ore recovery and is expected to increase as the pit expands to the SSW. On average, one train of 30–35 one-hundred-ton cars serves the mine. Operations are energy-intensive. Approximately 50% of the kernite is stacked in the pit, wetted, and allowed to hydrate to borax over several weeks. Processing is described by O’Driscoll (1990).

Reserve and resource data are given by Kistler and Smith (1975) and Roskill Information Services Ltd. (1993). In-situ resources were estimated in 1975 at 36,281,000 metric tons of ore at 25 wt.% B₂O₃, and in 1992 at 127,000,000,000 metric tons at 27.7 wt.% B₂O₃. At 1990 mining rates, reserves were considered adequate for 30–40 years (O’Driscoll, 1990).

Commodity List

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Mineral List

Mineral list contains entries from the region specified including sub-localities

41 valid minerals. 4 (TL) - type locality of valid minerals.

Rock Types Recorded

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