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Boron, Kramer |
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California, USA |
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B
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Boron, or Kramer borate deposit is located in the upper Mojave Desert of California, 145 km NE of Los Angeles (#Location: 35° 2' 25"N, 117° 40' 42"W).
It lies near the edge of a large Tertiary basin, the basement of which is Jurassic and Cretaceous granites and meta-volcanics. The Lower Tertiary sequence consists of continental sediments, primarily arkosic sands and silts, with occasional fresh-water limestones and volcanics. The overlying host sequence is immediately underlain by Tertiary basalt flows, and is overlain by a >60 m thick series of continental sands similar to those below the basalt. Fossil evidence suggests an age of middle Miocene for the sediments above the borates (Kistler and Smith, 1983).
The host sequence is ~130 m thick and comprises an upper and lower band of claystones that are 35 and 20 m thick respectively, sandwiching around 80 m of borax [Na2B4O7.nH2O - n=10 or 5]. The lower claystone contains ulexite [NaCaB5O9•nH2O - n=8 or 5], with associated shale and bentonitic clay, while the upper claystone is calcareous with colemanite [Ca2B6O11•5H2O] bearing claystone and tuffaceous claystone with ulexite. The borax layer contains kernite [Na2B4O7•4H2O] in its lower section, and has lower grade intervals (Kistler and Smith, 1983).
The boron deposit, including the upper and lower claystones, is lenticular with areal dimensions of approximately 1600 x 800 m and averages 100 m in thickness. Laterally and vertically, from the central borax core, the deposit exhibits facies changes to ulexite and clay. The deposit is thought to have formed in a shallow 'permanent' lake, and to have been related to late stage volcanic activity.
The deposit was subsequently buried by 750 m of Miocene and Pliocene sediments, with the deeper portions being 'metamorphosed' to kernite. Subsequent uplift and erosion exposed and eroded section of the orebody and caused portion of the kernite to be rehydrated to borax (Kistler and Smith, 1983).
Colemanite mineralisation was discovered in a water bore in 1913, although the Na borates were not encountered until 1925. Production of kernite and borax commenced in 1927 by underground methods. From 1957 the ore was exploited by open pit (Kistler and Smith, 1983).
Published resource figures include:
Original reserve - 87 Mt @ 25% B2O5 (Kistler and Smith, 1983),
Proved + probable reserves, December, 2015 - 23 Mt of B2O5 (Rio Tinto Annual Report, 2016).
Production in 2015 was 476 000 tonnes of B2O5 (Rio Tinto Annual Report, 2016).
The most recent source geological information used to prepare this decription was dated: 1983.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Boron
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