Tectonostratigraphic Framework and Timing of Mineralization at the Relief Canyon Mine, Pershing County, Nevada

摘要

The Relief Canyon mine exploited a carbonate-hosted, disseminated Au-Ag-(F) deposit located at the intersection of the Mesozoic Fencemaker thrust zone and late Tertiary range-bounding fault zone in the southern Humboldt Range. A metamorphosed, tectonically thickened section of multiply deformed Middle-Late Triassic platform carbonate, volcanic, and siliciclastic rocks intruded by gabbro dikes and sills comprise the mine section. Collapse of a back-arc basin and development of the Luning-Fencemaker fold- and thrust belt produced compressional deformation features in these rocks. D, formed during NE-SW shortening and created a S_0/S_1, planar fabric and small isoclinal folds during greenschist metamorphism in the Early to Middle Jurassic. D_2 documents a shift to N-S shortening and the generation of thrusts, duplexes, folds of varying geometry and scale, boudins, mullions, lineations, crenulations, spaced cleavage, and a S_2 planar fabric. D3 structures control barren orogenic quartz±tourmaline±carbonate±sulfide veins that formed in shear zones and flatter extensional fractures. D4 structures are Miocene and younger normal faults of NNW, NNE, and NE trends, including the range-front fault zone. Two episodes of mineralization produced distinct hydrothermal assemblages that are spatially superimposed at the deposit. The dominant style consists of multiple generations of jasperoidal quartz-illite-pyrite±fluorite (QIPF) with trace amounts of gold, electrum, chalcopyrite, sphalerite, galena, cinnabar, stibnite, tetrahedrite, native silver, and hessite(?). Illite from the QIPF assemblage yielded a 40Ar/39Ar date of 23.51±0.11 Ma. Later gold mineralization generated quartz-calcite±adularia±kaolinite veins that exhibit bladed and lattice calcite, suggestive of boiling fluids at low temperatures (~≤225°C) and shallow depths (~<270 m). Adularia from this assemblage produced a ~(40)Ar/~(39)Ar date of 14.92±0.05 Ma. Igneous rocks coeval with either mineralizing episode have not been identified in the deposit area. QIPF replacement and open-space filling took place in three structurally and stratigraphically controlled zones. Most of the current (and historical) resources are, in Main Zone, variably silicified, polyphase solution collapse breccias, and cave-fill sediments developed in thick-bedded limestone (Cane Spring Formation) immediately below a D_2 thrust of the Fencemaker thrust system. Allocthonous slates, phyllites, siltites, and quartzites of the Grass Valley Formation cap these breccias. Mineralizing fluids in the Lower Zone followed thrusts, shears, and tectonic-solution collapse breccias in foliated to tectonized limestone interleaved with a pale green, mafic volcanic rock and stretched pebble conglomerate. The Jasperoid Zone mineralized interval occurs below and in the same rock package as the Lower Zone, but is characterized by intense silicification along limestone tectonites and steeply dipping, sheeted fractures striking N 30°-35°E. Sheared contacts of foliated, fine-grained, gabbro sills/ dikes were common sites of mineralization. The sedimentological and deformation history of the Relief Canyon mine area created a favorable tectonostratigraphic framework where reactive carbonate rocks were juxataposed against impermeable rocks (gabbros, siliciclastics, volcanics) and the thrusts, shears, breccias, and extensional structures that served as fluid conduits. Regional data imply that extension and/or rotation possibly created faults or opened older structures during both mineralizing episodes. The mid-Miocene mineralizing episode at Relief Canyon may have derived mantle-sourced heat, metals, and volatiles from the Northern Nevada Rift system.