Hansen Creek Deposit, Potosi Mining District, Osgood Mountains, Humboldt County, Nevada, USAi

Regional Level Types
Hansen Creek Deposit	Deposit
Potosi Mining District	Mining District
Osgood Mountains	Mountain Range
Humboldt County	County
Nevada	State
USA	Country

Structure: Gold mineralization is generally found at the intersection of a number of high-angle and low-angle fault sets. The low-angle faults and associated folds are the result of Devonian and Permian-age compressional events and the higher angle faults and fracture sets formed during Tertiary extension. Mineralization is both structurally and stratigraphically controlled. The Getchell fault is a zone of overlapping fractures which have an overall strike of N10W. Hotz and Willden (1964) offer evidence for up to 3500 feet of left lateral strike slip displacement and only a relatively small amount of dip slip movement along the Getchell fault. McCollum and McCollum (1991) indicate that the sense of movement on the Getchell fault is right lateral. thrust faults to the north and NNW-trending faults.

Alteration: Alteration comments: there is a metamorphic aureole around the Osgood Mountains granodiorite which has produced in the surrounding shaly rocks a mineral assemblage consisting of cordierite-, biotite-, and andalusite-hornfels. Locally limy beds are recrystallized and calc-silicate minerals are developed. Hydrothermal alteration consists chiefly of decarbonatization accompanied by silicification in the limestone beds. Cordierite, andalusite, and biotite of the metamorphic aureole are altered to sericite and/or chlorite. Igneous dikes and portions of the main stock are altered such that plagioclase is altered to sericite and kaolinite and biotite is altered to sericite, chlorite, and pyrite.

Commodity: Ore Materials: native gold, native silver, electrum Gangue Materials: realgar, orpiment, pyrite, scheelite, pyrrhotite, arsenopyrite, marcasite, magnetite, stibnite, ilsemmanite, cinnabar, hubnerite, calcite, chabazite, sericite, chlorite, barite, gypsum, fluorite, getchellite, galkhaite, laffittite, arsenolite, guerinite, haidingerite, pharmacolite, weilite, coloradoite, bismuthinite, cassiterite, molybdenite, ferrimolbdite, galena, sphalerite, covellite, chalcocite, garnet, epidote

Deposit: The known gold deposits within the Getchell Trend are Carlin- type, sediment-hosted, replacement deposits containing micron gold. Gold mineralization is found in a number of different rock types generally at the intersection of a number of high-angle and low-angle fault sets. The low-angle faults and associated folds are the result of Devonian and Permian-age compressional events and the higher angle faults and fracture sets formed during Tertiary extension. Mineralization is both structurally and stratigraphically controlled. Gold is associated with arsenic, mercury, and to a lesser extent antimony, and commonly with pervasive decalcification, silicification and carbonaceous alteration. Gold is micron-scale generally intergrown with arsenical pyrite, which in turn, is encrusted in barren, diagenetic pyrite. Late stage realgar and orpiment are commonly associated with high-grade ores. The main deposit is confined to a zone nearly 7000 ft. long at the northern end of the Getchell fault zone. Deep exploration shows that the mineralization persists at least 1 km down-dip on the Getchell fault system and also occurs along the parallel Village fault. Maximum width of ore is 200 ft., with an average width of 40 ft. Within ore zones, gold occurs as native grains that range in size from <1 micron to nearly 1 mm, with smaller grains more abundant than larger grains. Most of the gold is intimately associated with the fine grained quartz-carbon matrix of the altered rock termed "gumbo" by Joralemon (1951). Of the sulfides, pyrite and marcasite are principal hosts to gold. As of 1951, the gold:silver ratio in bullion ranged from 2:1 to 134:1 and averaged 10:1 for the entire bullion production to that date. Joralemon (1951) observed microscopic metallic grains in the Getchell ore that he concluded were native silver, although the particles were so small that conclusive chemical tests were not possible. No other silver minerals have been recognized except for very rare grains of electrum. Geochemical work at the Getchell mine and vicinity has demonstrated that As-W-Hg anomalies occur in rocks and soils over the arsenic-gold deposits and that these anomalies are not broad haloes but are restricted to the mineralized area. The highest metal contents are found in oxidized iron-rich material along fractures and bedding planes in barren bedrock, lesser values in caliche coatings on exposed bedrock, and lowest but still anomalous values in soil.

Deposit type: Sediment-hosted Au

Development: Prospectors Edward Knight and Emmet Chase discovered gold in 1933 and located the first claims in 1934. With the financial backing of Noble Getchell and George Wingfield, the Getchell Mine, Inc. was organized in 1936 and was brought into production in 1938. In 1938, the mining rate was about 500 tpd of oxide ore and 150 tpd of sulfide ore. Sulfide ore was roasted at 1500 degrees Fahrenheit for one hour and fifteen minutes preparatory to cyanidization. In 1941, a Cottrell electric precipitating unit was installed to save the arsenic that was liberated by roasting the sulfide ore, and in 1943-45, when government wartime restrictions forced the shutdown of many gold producers, Getchell mine was permitted to continue operations as a producer of "strategic" arsenic. In 1943, arsenious oxide was being produced at the rate of 10-25 tpd from furnace fume. Also in 1942, a 227 tonne scheelite flotation plant was built to recover tungsten from Getchell ore. A slack labor supply, and high supply costs forced the gold operations to cease at the end of World War II. The US Bureau of Mines developed a carbon recovery process on site and the mine reopened in 1948 with expanded mill capacity and more underground development, but closed again in mid-1950 when known oxide reserves were exhausted. Gold production was suspended in 1951. From 1951-56, the mill processed tungsten ores mined from throughout the district. Tungsten production ceased in 1957. in 1960, Goldfield Consolidated Mines Co. purchased the interests in Getchell Mine, Inc. from the estates of Wingfield and Getchell. Gold production resumed in June 1962 and continued to December, 1967, when the mine was closed and the mill dismantled. Cyprus Mines formed a joint venture with Goldfield in 1970, with Cyprus as operator. Cyprus dropped the property at the end of 1971. Conoco leased the property from Goldfield in 1972 and completed exploration including over 300 drill holes. Metallurgically difficult sulfide reserves were identified during this program. Conoco subleased the property from 1975 to 1978 to General Electric Co. who conducted tungsten exploration along the margins of the Osgood Stock. In 1981, Conoco purchased the property from Goldfield Corp., but by 1983 had sold the property to First Mississippi for $5 million. At that time the property consisted of 14,100 acres of fee land and almost 5000 acres of unpatented claims, and reserves at the time of purchase were in excess of 750,000 ounces of gold. Mining feasibility and metallurgical studies were initiated in 1984. Heap leaching of waste rock dumps from previous mining operations commenced at the end of fiscal 1985, producing 91 ounces of gold in that fiscal year. By mid-1985, the Getchell property had increased the area of unpatented claims to 13,900 acres. In May, 1987, the board of First Mississippi Corp. authorized open pit mine development and construction of a new mill utilizing autoclave technology to process 3000 tons of ore per day. The mill was completed and production resumed in 1989 combining a traditional cyanide leach circuit with pressure oxidation. The mill started up on oxide ore in February, 1989. Sulfide ore was run through the first pressure oxidation autoclave in April, 1989 followed by the start up of the other two autoclaves in May and June, 1989. By the end of fiscal year 1989, project capital costs stood at $90.3 million, 14% over the June 1987 feasibility study estimate. In fiscal year 1989, overall gold recovery for combined oxide and sulfide mill ores was 89.8%. Heap leaching of waste rock from previous mining operations was completed in fiscal year 1989. Heap leaching continued beyond this date using oxide reserves from the Summer Camp orebody discovered in 1985. Production of oxide open pit ore commenced at the nearby Turquoise Ridge mine in 1991 and in the same year, an underground orebody adjacent to the pit area. This ore was to be mined when the pit level was deep enough to provide lateral access. In 1995, FirstMiss Gold changed its name to Getchell Gold. Underground production commenced at Turquoise Ridge Mine in May 1998. On May 27, 1999 Placer Dome completed a merger with Getchell Gold Corporation, resulting in Placer Dome owning 100% of the Getchell gold property. Gold production has been suspended since July 1999 and the property is on care and maintenance. Production from approximately 58% of the property is subject to a 2% net smelter return royalty payable to Franco Nevada Mining Corporation Ltd. Placer Dome wrote off the carrying value of the property in 2001. On October 25, 2001, Newmont Mining Corporation and Getchell Gold Corporation signed a letter of intent under which Newmont would buy ore from the Getchell mine for processing at Newmont's adjacent Twin Creeks mine.

Geology: Geology comments: Bagby and Cline (1991) offer preliminary results from research which indicate that confining pressures on the Getchell ore system varied from approximately 370-430 bars either during, or at some time subsequent to mineralization. These fluid pressures are greater than those which are normally accepted as epithermal.

Ore(s): Economic amounts of gold are restricted to tabular sheet-like zones (termed "veins" by Joralemon) within the Getchell fault zone and within favorable calcareous lithologies.

Other Databases

Link to USGS MRDS:	10310490

Other Regions, Features and Areas containing this locality

References