Fortitude mine, Battle Mountain Mining District, Lander County, Nevada, USAi

Regional Level Types
Fortitude mine	Mine
Battle Mountain Mining District	Mining District
Lander County	County
Nevada	State
USA	Country

Structure: Virgin Fault Golconda Thrust

Alteration: Skarn formation began with the alteration of clastic rich sedimentary rocks to biotite/skarn hornfels. Introduction of metasomatic fluids further extended the biotite halo, flooding the system with k and fe, and adding orthoclase to the biotite hornfels assemblage. This assemblage replaced much of the protolith regardless of original composition. Continued metasomatism at high temperature (300C - 550C) formed garnet and pyroxene in calcareous rocks and overprinted the biotite-orthoclase hornfels. Sulfide minerals, dominated by pyrrhotite, began to precipitate with the garnet-pyroxene skarn and continued as temperatures declined. Gold mineralization began at 300C-400C and continued to lower temperatures along with minor arsenopyrite, pyrite, marcasite, sphalerite, galena, bismuth, and tellurides. During cooling of the hydrothermal system, the garnet-pyroxene skarn was altered to actinolite, prehnite, and chlorite.

Commodity: Ore Materials: native gold, electrum, pyrrhotite, pyrite, chalcopyrite, arsenopyrite, marcasite, galena, sphalerite, native bismuth, hedleyite, joesite, volnyskite Gangue Materials: hedenbergite, andradite garnet (with grossularlite rims), diopside, actinolite, chlorite, epidote, prehnite, calcite, quartz

Deposit: The Fortitude Deposit is a world-class gold-silver skarn deposit in the main Battle Mountain District. The deposit was comprised of an upper and a lower ore zone, formed in place, but separated by the N-striking, W-dipping Virgin Fault and by a granodiorite porphyry dike intruded along the fault. The upper ore zone in and east of the footwall of the fault formed in host rocks calcareous siltstone and conglomerate of the Battle Formation. The larger lower ore zone that formed the major portion of the deposit formed in host Antler Peak Limestone west of and in the hanging wall of the Virgin Fault. Discontinuous upper zone ore consisted of selective sulfide replacement of thin calc-silicate pods and lenses concentrated along faults or at fault interesections. Lower zone ore was a more continuous stratiform and stratabound orebody elongated northeast, up to 600 meters long, averaging 150 meters wide and 25-30 meters thick. Native gold is common and is most frequently associated with arsenopyrite, native bismuth, and tellurides. The highest gold concentrations occur in the pyrrhotite-dominant sulfide zones which contain only minor chalcopyrite.

Deposit type: Skarn Cu

Development: The Fortitude Deposit was discovered in late 1980 north of the West orebody at Battle Mountain Gold?s operations. After three years of pre-production striping and miing of the upper Fortitude ore zone, the larger, higher-grade lower Fortitude ore zone began production in late 1984. Mining ceased in 1993 after the reserves were depleted. In November, 1988 Dighem flew a test line over the exposed sulfide body at Fortitude at a bird height of 30-50 m. The magnetic anomaly over ore in the pit was 500 nt. The massive sulfide ore also gave a strong inphase em response with a high inphase-to-quadrature ratio. The apparent resistivity of ore measured 0.1 ohm-m in contrast to 10-100 ohm-m for limestone host rock. ; econ.com: production costs per equivalent ounce of gold after byproduct credits and including depreciation were $206 in 1985, $172 in 1986, and $156 in 1987. Battle Mountain Gold Co. pays no royalties on production from the Fortitude deposit.

Geology: Skarn minerals are zoned in abundance and composition relative to the granodiorite porphyry stock. Proximal skarn near the pluton contact is garnet and copper rich, relatively oxidized, and formed at high temperatures. Distal skarn, up to 1.5 km from the pluton contact, is pyroxene and gold-rich, relatively reduced, and formed at lower temperatures. Average garnet-pyroxene ratios and pyroxene iron-manganese contents are useful guides to position within the skarn system and to gold and copper grades. The reduced nature of the Copper Canyon porphyry, indicated by very low ferric/ferrous iron ratio (Fe +3/Fe +2 < 0.5), is an important characteristic of this gold-enriched system and distinguishes it from other gold-poor igneous/hydrothermal systems. Doebrich notes that contouring of blast hole assay maps shows strong N-S and NNE- linear trends in gold concentration. These higher grade gold zones correspond to garnetiferous skarn within an overall pyroxene skarn zone. Conglomerate hosts ore in the upper zone.

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Link to USGS MRDS:	10310329

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