James J. Hardy, 1996. "Society of Economic Geologists Post-Meeting Field Trip II: Dismemberment of Porphyry Copper Mineralization in the Rosemont-Helvetia District, Arizona", Tertiary Extension and Mineral Deposits, Southwestern U.S., William A. Rehrig, James J. Hardy
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The purpose of this trip is to demonstrate the manner in which Laramide and middle Tertiary faults controlled the original emplacement of, and subsequent disruption of porphyry copper mineralization. Two copper skarn deposits are present in the district. The larger of these, the Rosemont deposit of ASARCO Inc. contains approximately 400 million tonnes of ore with grades of 0.53% to 0.61% copper (Anzalone, 1995). The smaller Peach-Elgin deposit contains about 42 million tonnes of ore with grades of 0.58% copper (Anzalone, 1995). The field trip will examine both Laramide faults that controlled the original emplacement of ore-related intrusions, as well as middle Tertiary faults that subsequently tilted, truncated and reorganized the original mineral system.
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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