Big Springs Mine (Sammy Creek; Mac Ridge; Mesona claim; Jack Creek project; Bull Run Mine), Independence Mountains Mining District, Elko County, Nevada, USAi

Regional Level Types
Big Springs Mine (Sammy Creek; Mac Ridge; Mesona claim; Jack Creek project; Bull Run Mine)	Mine
Independence Mountains Mining District	Mining District
Elko County	County
Nevada	State
USA	Country

The Big Springs Mine is composed of seven deposits developed by Freeport-McMoran Gold Co. (later Independence Mining Co., now Anglo Gold): Mac Ridge, M242936; North Sammy Creek, W700398; South Sammy Creek, W700399; 701 deposit, 131 deposit, 401 deposit, and 601 deposit.

Structure: Deformation related to the Antler Orogeny is the earliest tectonic event recorded in the Big Springs rocks, during which siliceous western assemblage rocks were emplaced over eastern assemblage carbonate and siltstone along the Roberts Mountain thrust fault. Compressional deformation of the Sonoma Orogeny occurred in the late Permian, during which the foredeep accumulated Schoonover Sequence was thrust over both the allochthonous western and autochthonous eastern assemblage rocks. Sonoma tectonism in the Big Springs area is evidenced by SE-verging overturned folds and imbricate thrust faults, all of which have been cut by later normal faults. A strong N75W fabric is manifested in folds and shears throughout the Independence Range and may be related to the Wells fault. This structural zone may have been reactiviated during the formation of the NW-striking Northern Nevada Rift Zone and Basin and Range extensional faulting. Deposits are structurally controlled at intersections of NE-trending and E-W-trending fault zones and fold axes. The E-W structure is part of a large ore-controlling shear zone that also truncates the northern mineralized zone at Big Springs. Many of the dikes occur along the NE-trending and E-W-trending fault intersections. A strong N75W fabric is manifested in folds and shears throughout the Independence Range and may be related to the Wells fault. This structural zone may have been reactiviated during the formation of the NW-striking Northern nevada Rift Zone and Basin and Range extensional faulting.

Alteration: Propylitic alteration is common throughout the volcanic rocks of the Dorsey Creek member. Gold mineralization occurs locally when the propylitically altered rocks are overprinted by quartz-sericite-pyrite-dolomite alteration

Tectonics: Antler foredeep basin

Commodity: Ore Materials: gold Gangue Materials: pyrite, marcasite,arsenopyrite, arsenical pyrite, sphalerite, chalcopyrite, stibnite

Deposit: The Big Springs mine consists of seven distinct deposits (Mac Ridge, North Sammy Creek, South Sammy Creek, 701 deposit, 131 deposit, 401 deposit, and 601 deposit) plus many additional resource areas of anomalous gold mineralization which were not mined. The deposits are characterized by disseminated, near-surface gold mineralization within 400 feet of the surface. Mineralization is structurally controlled and occurs primarily in allochthonous rocks of the Dorsey Creek and Mikes Creek members of the Schoonover Sequence. Host rocks include thin-bedded to cherty argillite, chert, conglomerate, siltstone, arenite, massive limestone, and fossiliferous limestone. Deposits are structurally controlled at intersections of NE-trending and E-W-trending fault zones and fold axes. The E-W structure is part of a large ore-controlling shear zone that also truncates the northern mineralized zone at Big Springs.

Deposit type: Sediment-hosted Au

Development: The area was first explored in the early 1980s by Freeport- McMoRan Gold Company. Extensive drilling, mapping, sampling, and construction of access roads were completed by Freeport by 1982. Mining of the Big Springs deposit was initiated by a joint venture between Freeport- McMoRan Gold Company (later Independence Mining Company) and Bull Run Gold Mines, Ltd. The mine was placed into production in September, 1987 and was fully operational by October, 1987. Initial production was from heap-leaching oxidized ore, but second phase construction provided a fluid-bed roasting system and conventional ore-milling facilities. Initially, Freeport planned to process 400,000 tons/year from three deposits at Big Springs to produce about 60,000 oz/year gold. The deposit was mined out in 1994, having produced a total of 386,000 troy ounces of gold from seven separate deposits on the property. Most of the gold was contained in refractory sulfide ore requiring a pre-oxidizing roasting method to enable conventional CIL recovery. Big Springs Project. In 2004, Gateway Gold Corp. continued work on its Big Springs project. A new geological model was developed based on the results of last year?s drilling and the company expects drilling this year will validate the model. In 2006, Gateway Gold Corp. announced that based on recent drill results at the Big Springs Project, resources aggregate 914,000 tons grading 0.345 ounces of gold per ton inferred using a 0.2 opt Au cutoff. (was 904,200 tons grading 0.288 ounces of gold per ton measured+indicated prior to the new round of drilling.)

Geology: The deposits occur primarily in allochthonous rocks of the Devonian to Permian Schoonover Sequence, which are cut by diorite and quartz monzonite intrusives which also host ore.

Ore(s): Mineralization at Big Springs was controlled by an E-W shear zone that also truncates the northern mineralized zone .Ore deposits are localized at intersections of NE-trending faults with this E-W-trending fault zone and fold axes.

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Link to USGS MRDS:	10310525

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