Carlin-type Gold Deposits and Current Mining Activities at Jerritt Canyon, Elko County, Nevada

摘要

The Jerritt Canyon (JC) mine property is located in the Independence Mountains mining district, 81 km north of Elko in northeast Nevada. From 1981 to December 31, 2014, 256,640 kg (8.25 Moz) of gold have been produced from 22 Carlin-type gold deposits (CTGDs) at JC. Deposits include 13 separate open pits and 9 underground (UG) mines averaging 7.0 grams per tonne (g/t) Au. Early gold production totalling -156 tonnes (~5 Moz) from 1981 to 1999 was primarily from open pits. UG mining started in 1993 and since 1998 has contributed the majority of the refractory ore feed to the on-site process plant that includes two roasters and a carbon-in-leach circuit (1.36 M tonnes/year capacity). As of January 2015, four UG mines (Smith, SSX-Steer, Starvation Canyon, and Saval 4) produce~2,950 tonnes/day at an average grade of 6.51 g/t. The primary UG mining methods include underhand cut and fill and long-hole stoping. The 2014 gold production from JC ores was 5,061 kg (157,422 oz). Geology of the Independence Mountains range consists of a N-trending antifor-mal horst block dominated by Paleozoic marine sedimentary rocks. The CTGDs at JC are primarily hosted in interbedded carbonaceous micritic limestone and argillaceous dolomitic limestone of the Silurian to Ordovician Hanson Creek Formation (SOhc) upper subunits 1 through 3. Mineralization is also hosted by variably calcareous and carbonaceous laminated siltstone of the basal part of the overlying Devonian to Silurian Roberts Mountains Formation (DSrm). Between the SOhc and DSrm is the Saval Discontinuity (SD), a paleokarst horizon. The SD increased permeability and created a zone of weakness, which focused subsequent deformation and directed ore fluids toward this part of the stratigraphic section. The SOhc and DSrm lie beneath the Roberts Mountains thrust fault and are exposed in erosional and tectonic windows through the siliciclastic units of the Roberts Mountains allochthon (upper plate). Additional repetition of the lower plate Paleozoic stratigraphy resulted from imbricate duplex and related lateral ramp structures below some deposits. Basaltic dikes cut the sedimentary rocks typically along NW-striking faults in the mine areas and have two distinct ages (~324 Ma and 35-41 Ma). Both dike groups locally host mineralization and the younger ones are contemporaneous with it. Jerritt Canyon experienced at least three pre-mineral fold and thrust events, syn-mineral extension with dike emplacement, and some post-mineral faulting. Structural controls on mineralization include limbs and hinge zones of anticlines, low-angle thrusts, E-, NE- and NW-striking normal faults, and fault intersections. Ore controls also include a swarm of NW-striking variably listric normal faults, some filled by both dike groups which transect the entire range from east to west, paralleling the primary fold trend. Gold distribution across the district appears closely related to the presence and/or intensity of this structural phase. The gold-bearing hydrothermal fluids migrated along all of the faults as well as both dike groups. The combination of large-scale folds, thrust faults, Mississippian dikes, listric normal faults and Eocene gold mineralization indicate several long-lived, deep-seated NW-trending structural zones in this part of the Independence Mountains. The mineralized faults are markedly influenced by pre-existing fold and fault geometry at the deposit scale resulting in complex and sometimes compartmentalized ore bodies and deposit clusters. Arsenian pyrite/marcasite are the main gold host minerals with associated orpiment and realgar. Dolomitization, silicification, decalcification, carbon enrichment, and argillization are the main alteration types. Gold mineralized rock is enriched with As, Hg, T1, Sb, Te, and Ba similar to other CTGDs in the region.