Summary:
RTB Boron is an open pit borate mining operation located within the Kramer Borate deposit in Boron, California.
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 (NaCaB5O9.8H2O) and colemanite (Ca2B6O11.5H2O) - bearing claystone, and barren claystone. Studies indicate the Kramer borates were deposited in a small structural, non-marine basin, associated with thermal (volcanic) spring activity during Miocene time.
The Miocene Kramer beds are divided into three distinct members, the Saddleback basalt member, the Shale member, and the Arkose member, in ascending order. The Saddleback basalt comprises up to 600 feet of olivine basalt flows and is the only Kramer member forming surface outcrops – as ridges northwest, north, and northwest of Boron open pit. The basalt is overlain by the Shale member, which consist of up to 400 feet of borate-bearing and barren claystones and shale. The Shale member is overlain by the Arkose member, which comprises up to 800 feet of arkosic sandstones, which are locally silty and interbedded with tuffaceous clays.
The sodium borate facies is divided into seven stratigraphic units:
- Four high-grade units: Upper ore, Middle ore, Lower ore, and Basal ore, typically contain over 75-percent borax.
- Three generally low-grade units, A-zone, B-zone, and C-zone, which typically contain less than 60-percent borax.
- The Basal ore is the thinnest and least extensive of the high-grade units; the Lower ore is the thickest and most extensive highgrade unit. Only in the thick central portion of the sodium borate facies, are all the units present. Stratigraphic control is maintained by use of a number of clay-tuff and claystone marker beds within the sodium borate facies.
The sodium borates are roughly elliptical-shaped in plan, 2 miles in length (E-W), 1 mile in width, and range to a greatest thickness of 300 feet in the south-central portion of the deposit.
Calcium borates are far more extensive, with thin beds extending more than 1 mile to the west of the pit, and some related stratigraphy as far as several miles away.
Borate mineralization occurs from about 50 ft to 1,200 ft below surface.