Olinghouse Mine, Olinghouse Mining District (White Horse Mining District), Pah Rah Range (Pah Rah Mts), Washoe County, Nevada, USAi

Regional Level Types
Olinghouse Mine	Mine
Olinghouse Mining District (White Horse Mining District)	Mining District
Pah Rah Range (Pah Rah Mts)	Mountain Range
Washoe County	County
Nevada	State
USA	Country

Structure: Ne-trending faults of the Olinghouse Fault Zone The NE-trending Olinghouse Fault runs through the property, a regional left-lateral strike-slip structure with dip-slip component, and recent movement (probably the site of a major earthquake in 1869). The north end of the Walker Lane fault zone passes just east of the Olinghouse district.

Alteration: With the exception of a broad zone of propylitic alteration, the alteration features at Olinghouse are unusually subtle. On the surface, the quartz veins are narrow and there is essentially no color anomaly. The replacement of plagioclase by K-feldspar surrounding the veins can be determined only in thin section, and the alteration halo is small.

Tectonics: Situated near the western boundary of the Basin and Range tectonic province

Commodity: Commodity Info: Gold fineness is approximately 700. Ore Materials: free gold, electrum Gangue Materials: chalcopyrite, galena, pyrite, and sphalerite

Deposit: Gold occurs as small wires and free grains in generally open veins, along with quartz, calcite, goethite, epidote, stilbite, heulandite, hydrothermal K-feldspar, and sparse adularia. The adularia has been dated at 10.46 Ma (Larry Garside, personal communication). The pyrite content is less than 2% and is generally oxidized. Open space structures are common. Traces of chalcopyrite, galena, and sphalerite are associated with the ore, with higher base-metal concentrations within the high-grade ore shoots. Ore grade mineralization is most closely associated with K-feldspar, chlorite, and the zeolite minerals stilbite and heulandite. Orebodies occur in a series of closely spaced, NE-trending, near-parallel structures with mineralization occurring along faults, in shear zones and adjacent to dikes. The silver content of the ore seldom exceeds 0.1 oz/ton, with most silver occurring in electrum.Geochemical values of arsenic, antimony and mercury in the ore are generally low.

Deposit type: Epithermal vein, Comstock

Development: Prospecting began in the Olinghouse area in 1860, with district production peaking from 1901-1903, and minor production continuing intermittently through recent years. Modern exploration began in 1986 when Western Goldfields secured a small land position in the eastern part of the district and drilled 21 holes. Although gold mineralization was encountered in most of these holes, their land position did not allow testing the heart of the district, and the property was dropped. Phelps Dodge secured a land position in the central part of the district in 1991 and by 1993 had drilled 37 reverse circulation drill holes and 7 core holes. In 1993 Phelps Dodge elected to sell their interest in the property, whereupon Alta Gold purchased the property in 1994. By the end of 1994, Alta had submitted a plan for exploration to the BLM calling for drilling of 550 holes, trenching, and bulk sampling of adits, to begin in 1995. The 1995 drilling program confirmed earlier ore estimates and increased reserves. In early 1996, Alta Gold began its third phase of drilling on the Olinghouse gold project, where preliminary results had confirmed the presence of high-grade mineralization. The program, consisting of 42,000 ft. of core and reverse-circulation drilling, began with the twinning of a previous hole to verify assay values. This first core hole cut two different ore zones and contained significant visible gold. At a 330-ft depth, the hole cut 64 ft grading 1.97 oz Au/ton, including a 31.5 ft interval at 3.9 oz Au/ton. The same hole cut 61 ft grading 0.049 oz Au/ton. The second core hole 400 ft west also contained visible gold. Following 1996 drilling results, Alta began the permitting process to mine the deposit and submitted a draft EIS for the Olinghouse Mine Project to the BLM in 1997. The final EIS was issued in February 1998, and the mine began production in September 1998. The mine is scheduled to produce 100,000 ounces/year over its projected 6-year mine life, beginning in 1999. The mine is operated by conventional open pit methods, with the Green Hill pit the first to produce ore, to be followed by the Payback pit with possible later mining of the Keystone and Sunbeam pits. The best exploration indicators for the Olinghouse deposits have been abundant gold veins and old mine workings, the abundance of placer gold, the volume of rock affected by alteration, and the concentration of epidote in the center of the district. Soil sampling has also been effective at locating new deposits.

Geology: The Olinghouse deposit is underlain by a sequence of Oligocene to Miocene volcanic rocks, intrusions, and volcaniclastic sediments. Most units strike NNW and dip SW. The oldest rock units in the mine area are a sequence of Oligocene to Miocene ignimbrites composed of volcanic ash, ash flow tuff, rhyolite flows and lahars dated at 22.7 Ma. Although altered and mineralized, relatively minor production has come from these rocks. These tuffs are overlain by the Pyramid Sequence, the principal host rock for Olinghouse mineralization. The Pyramid Sequence consists of vesicular basalt, andesite, flow breccias, lahars, and volcanic sediments, all ranging in age from 12 Ma to 16 Ma. This sequence is unconformably overlain by the Kate Peak Formation, a thick sequence of post-mineral andesitic to rhyolitic flows, flow breccias and tuffs. The volcanic rocks are cut by a series of NE-trending structures, some of which show recent strike-slip movement. The structures are intruded by basaltic to dacitic dikes. Gold mineralization occurs in veins and vein swarms within and adjacent to the dikes following these structures. The principal productive structures are referred to as the Number 1, Number 2, Number 3, Blue Dike, Sunbeam, and Standard structures. The Number 1, 2 and 3 structures and the Blue Dike occur close together with moderate intervening waste, and comprise the open pit deposit. The veins occur within a broad, NE-trending, propylitic aureole occupying several square miles and roughly encircling the central Olinghouse district. The volcanic rocks exhibit nearly complete chloritization of the original mafic minerals and groundmass, and the plagioclase is altered to calcite, sericite, epidote, and albite. Epidote increases towards the central part of the district. As the veins are approached, the intensity of groundmass alteration increases and plagioclase feldspar is replaced by K-feldspar and sericite. Mafic phenocrysts are replaced by chlorite, epidote, K-feldspar, and goethite.

Ore(s): Mineralization is both lithologically and structurally controlled, with mineralization hosted by the Pyramid sequence of andesitic to basaltic volcanic and volcaniclastic rocks, and controlled by NE-trending structures.

Other Databases

Link to USGS MRDS:	10310550
Link to USGS MRDS:	10198320

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