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Geology, Alteration, and Mineralization at the America Mine Gold Deposit, San Bernardino County, California

Joe Wilkins, Jr.
Joe Wilkins, Jr.
Exploration Consultant, Tucson, AZ
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Frank L. Hillemeyer
Frank L. Hillemeyer
La Cuesta International Inc., Kingman, AZ
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January 01, 1996


A heap-leachable, open pit and underground gold reserve of about 173,000 ounces has been delineated by drilling at the America Mine, San Bernardino County, California. The reserve is contained within a 346,000 ounce geologic resource.

The principal control for the gold mineralization is the ENE-striking, 20° to 40° NNW -dipping America Mine shear zone. The shear zone is up to 150 feet thick, is marked by mylonitic fabric in footwall rocks, and is overprinted by brittle fracturing and shearing. Ore-grade gold mineralization is up to 110 feet thick and occurs inN60°E- to N70°E-trending ore shoots which are up to +3000 feet long and up to 600 feet wide. These elongate ore shoots are axial-parallel to sub-horizontal flexures developed within the shear. The America Mine shear zone juxtaposes assemblages of rocks with contrasting origins and metamorphic grades, suggesting a terrain boundary. Footwall rocks are amphibolite-grade granitic gneisses with sparse marble lenses. Hanging wall rocks are metadiorite and metagabbro which intruded greenschist facies volcanic and sedimentary rocks.

The shear contains tectonic slices (or is perhaps intruded by) a leucocratic granite and is intruded by a laterally extensive hypabyssal quartz-eye dacite. Both intrusives, confmed to the footwall side of the shear zone, are sheared, altered, and gold-mineralized. Post-mineral, NW -striking, NE-dipping, dacite dikes related to a regionally developed dike swarm in the Bullion Mountains, cut the shear, but are also slightly offset by the shear. Mineralization consists of micron-sized gold associated with intense quartz-sericite-pyrite alteration which overprints all shear zone lithologies. Accessory minerals include galena, sphalerite, tetrahedrite, chalcopyrite, and their oxidation products. Gold-free, carbonatebarite- pyrite fissure veins were emplaced along NNW-striking listric normal faults. The listric faults, which dip steeply west, cut, offset, tilt and rotate the ore shoots. The listric faults generally have minor offset (<200 feet), and related post-mineral rotation is estimated at 20°, consistent with the weak-tomoderate mid-Tertiary extension related to the Mojave Rift structural province.

The gold deposit at the America Mine is interpreted as mesothermal mineralization hosted by ductile-brittle shearing along a late Mesozoic or early Tertiary thrust or a rotated subvertical (60°-70°) shear zone. The deposit is structurally complicated by mid-Tertiary extension and rotation, postmineral dacite dike emplacement, and Tertiary-Quaternary transpressional faulting.

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Figures & Tables


Society of Economic Geologists Guidebook Series

Tertiary Extension and Mineral Deposits, Southwestern U.S.

William A. Rehrig
William A. Rehrig
I. Low-Angle Tectonic Features of the Southwestern United States and Their Influence on Mineral Resources
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James J. Hardy
James J. Hardy
II. Dismemberment of Porphyry Copper Mineralization in the Rosemont-Helvetia District, Arizona
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Society of Economic Geologists
ISBN electronic:
Publication date:
January 01, 1996




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