Aurora Mine Area, Aurora Mining District, Mineral County, Nevada, USAi

Regional Level Types
Aurora Mine Area	Deposit
Aurora Mining District	Mining District
Mineral County	County
Nevada	State
USA	Country

Structure: A young, less than 3 Ma old, broad regional upwarp extends form the southwest of the Bodie Hills through the Bodie District to the area of the Aurora District. This upwarp may continue to the northeast, south of the Borealis mine. Steep N-S striking faults offset the mineralized NE-striking faults.

Alteration: Progressive weak to strong propylitic alteration with quartz-illite alteration surrounding silicification in the mineralized faults.

Commodity: Commodity Info: Average. Au:Ag ratio of 1:5. Higher gold values found near surface decreased as silver values increased with depth. Ore Materials: native gold, argentiferous tetrahedrite, electrum, acanthite, naumannite, bromargyrite, chalcopyrite, pyrite. Gangue Materials: quartz, adularia, calcite

Deposit: The Aurora silver-gold deposits are for the most part epithermal fissure vein deposits. Veins consist mostly of finely granular white quartz, which in some places has a milky-white porcelain-like appearance. The veins are commonly composed of layers of quartz of different grain sizes, and all the veins contain cavities lined with clear quartz crystals. The veins are a series of NE-striking to east-striking anastomosing quartz-adularia veins in pre- Esmeralda volcanic rocks that are exposed in a window in an area of predominantly barren younger rocks. A few ounces of placer gold (.408 fine) were recovered from Bodie Creek in 1940-41. Anastomosing quartz-adularia veins vary in width from less than an inch to as much as 60 feet (one reference states up to 130 feet). Average width is 4 feet. Most veins are wider at intersecting faults, with horsetailing of the veins at their extremes.

Deposit type: Epithermal vein, Comstock

Development: The first camp in the early 1860s was at Esmeralda, but was soon relocated to Aurora, which reached a population of 10,000 by 1864. There were 17 active amalgamation mills in 1864, the largest of which had 30 stamps. The total amount of ore handled is estimated at 670,000 tons. From 1861 to 1869, nearly $29,500,000 in gold and silver were produced from the district, mostly by the Aurora Mines Co. and the Aurora Consolidated Mining Co. The district began to decline after 1869, although it was productive intermittently until the early 1900s. From 1914 to 1918 the district enjoyed a brief revival when $1,850,000 in gold and silver were produced, chiefly by the Goldfield Consolidated Mines Co. Much of this production was probably from reworking of tailings and dump material. As late as 1940 as small production was recorded from the district, but in 1956 there was no activity. In 1973-74, St. Joe explored for porphyry copper-moly deposits in the area and inferred possible deep (5,000-15,000 ft.) mineralization. In the late 1970s Homestake drilled the Prospectus and Humboldt veins and on Last Chance Hill but failed to find a large bulk tonnage deposit. In 1981 Hanna drilled the Juniata Mine area and the new Esmeralda area, identifying only small reserves. In 1983 Hanna drilled the Prospectus Vein, intersecting ore grades at depths of 50-100 ft. Follow-up drilling by Hanna and later by Nevada Goldfields outlined reserves of 350,000 tons of ore grading/0.185 opt gold. Electra Resources was drilling the property in 1980-1981 and in 1982, Electra combined with Pacific Northwest to form Electra Northwest Resources, Ltd. to develop the mine and conducted drilling and engineering programs. Plans were to process an initial 3000 tons of ore. Electra Northwest formed a joint venture with Centennial Minerals, and began production in July 1983 at 1000-1500 tpd at what was called the Aurora Partnership Mine, developed on the Humboldt Vein. Initial recoveries averaged 0.08 opt gold from material grading 0.12 opt gold. Full scale production was planned for 1984. Mine life projected at least five years. Production at the adjacent Aurora Mine (developed on the Prospectus Vein) began in 1987 by Nevada Goldfields. As the Aurora Mine production continued, Minerex Resources, Ltd. took over operation of the Aurora Partnership Mine in 1990, followed by Electra Mining Corp in 1994. The Aurora Partnership Mine appears to have ceased production in 1996, leaving just Nevada Goldfields mine in production, and it too closed by 1999. Precious metals exploration in the Aurora area has resumed with the recent increased gold prices. In January 2004, Metallic Ventures Gold Inc. reported the start-up of its 350 ton-per-day mill at its Esmeralda property at Aurora, and poured its first precious metal bullion on February 16. However, in the third quarter of 2004, Metallic Ventures suspended all production and exploration activities at this project in the Aurora district. The project was not providing consistent positive cash flow and, in order to ensure the company has adequatefunding for its other Nevada projects, Esmeralda was placed on care and maintenance status.

Geology: Aurora has been is the most productive district in Mineral County in historic times, accounting for 40% of the total county production. In general the ore was of low grade, though some shoots ran as high as $1000/ton. The gold: silver ratio may have been as low as 1:14. The mineralized area of the district is approximately 2 miles long. Host rocks are pre-Esmeralda andesitic to latitic agglomerates and flows.

Ore(s): NE-striking faults are occupied by quartz-adularia-gold-silver veins. Three stages of veining are recognized. Precious metal deposition is associated with the second stage of veining, probably during boiling of the hydrothermal fluid. Ore was deposited by hydrothermal fluids in open space fillings.

References

Other Databases

Link to USGS MRDS:	10310395