Geology, Alteration, and Mineralization at the America Mine Gold Deposit, San Bernardino County, California
Published:January 01, 1996
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Joe Wilkins, Jr., Frank L. Hillemeyer, 1996. "Geology, Alteration, and Mineralization at the America Mine Gold Deposit, San Bernardino County, California", Tertiary Extension and Mineral Deposits, Southwestern U.S., William A. Rehrig, James J. Hardy
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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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Tertiary Extension and Mineral Deposits, Southwestern U.S.
Starting in Las Vegas, we will traverse through many of the geometric elements and complexities of hanging wall deformation above the regional detachment systems of the Colorado River extensional terrane. We will study the interaction of normal faults as arranged in regional, crustal-scale mega-domains and the bounding structures that separate these tilt domains. As we progress through the classic Eldorado Mountains-Hoover Dam region, where many of the ideas of listric normal faulting were first popularized, we will see both the real rocks and the historic rationale for their deformation. By examining the listric versus domino models for normal faulting, we will utilize different geometric techniques for determining the depth to the detachment structures and percent extension. Continuing further south toward southernmost Nevada, we will cross the accommodation zone that separates the Lake Mead and Whipple dip domains and further descend to deeper structural levels to examine lower levels of the major normal faults and their tilting of upper-crustal blocks and associated offset along the regional detachment faults. Fluid flow within the shattered fault zones and its relationship to the 3-D geometries of the fault surfaces will be studied both along the faults and within the hydrothermally altered and mineralized wallrocks.
(From Las Vegas proper, drive east on Tropicana to Pecos, south to Russell, then continue east to Southern Nevada Vocational Technical Center. Follow signs to its entrance then drive around edge of campus to dirt road on northern edge, which provides elevated view toward the north to Frenchman Mountain. View is much better in the afternoon than in the morning.)
Stop 1-1: Frenchman Mountain Tilt Block
(Return to Russell Road, turn right to freeway (1.4 miles) and south along freeway to Boulder City. Take truck route, Buchanan Blvd., around Boulder City and turn off highway at “Lakeview Overlook,” about 5 miles pastBoulder City for view and discussion of Lake Mead shear zone.)
Stop 1-2: Lake Mead Shear Zone and Associated Alteration by Hydrothermal Fluids
(Return to Highway 93 and continue descent to Hoover Dam. Park in new parking structure and walk to gorge overlook about 100 meters up the road on the Nevada side for best view of structural features.)
Stop 1-3: Extension Within the Volcanic Rocks Exposed at Hoover Dam
(Return to Boulder City along Highway 93 and to turnoff to Highway 95 south toward Searchlight. Note extreme alteration at junction of Highways 93 and 95. Continue south