Getchell Mine, Adam Peak, Potosi Mining District, Osgood Mountains, Humboldt County, Nevada, USAi

Regional Level Types
Getchell Mine	Mine
Adam Peak	Peak
Potosi Mining District	Mining District
Osgood Mountains	Mountain Range
Humboldt County	County
Nevada	State
USA	Country

A former Au-Ag-As-W-Sb-Hg-Ba(baryte)-Mo-F-Tl-Te-Bi-Sn-Pb-Zn-Cu occurrence/mine located in secs. 4 & 9, T38N, R42E, and in secs. 28, 29, 32 & 33, T39N, R42E, MDM, 7.4 km (4.6 miles) NNE of Adam Peak (coordinates of record), on private land within a Bureau of Land Management administered area. Discovered by Edward Knight and Emmet Chase in 1933. The overall life of the mine is 1938-present. Owned & operated by the First Miss Gold Corporation. Owned & operated by Newmont Gold Mining Company (2003) & Placer Dome Gold (2003). Operated during the periods 1938-1945, 1948-1950, 1962-1967, 1985-1999. MRDS database stated accuracy for this location is 10 meters.

The Getchell property consists of the Getchell, Turquoise Ridge and N Zone deposits. The Main pit has now encompassed the earlier Central and South pits. This is the same location as the old MRDS record M055410.

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 tons per day of oxide ore and 150 tons per day of sulfide ore. Sulfide ore was roasted at 1,500 degrees Fahrenheit (ca. 815° C) for one hour and fifteen minutes preparatory to cyanidation. In 1941, a Cottrell electric precipitating unit was installed to save the arsenic that was liberated by roasting the sulfide ore, and in 1943-1945, 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 tons per day from furnace fume. Also in 1942, a 227-tonne scheelite flotation plant was built to recover tungsten from Getchell ore. Slack labour 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 the mid-1950s 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 Company 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 was dismantled. Cyprus Mines formed a joint venture with Goldfield in 1970, with Cyprus as the 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 Corporation, 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 5,000 acres of unpatented claims, and reserves at the time of purchase were more than 750,000 ounces (ca. 21.2 tonnes) 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 the fiscal year 1985, producing 91 ounces (ca. 2.6 kg) 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 Corporation authorized open pit mine development and construction of a new mill utilizing autoclave technology to process 3,000 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 startup of the other two autoclaves in May and June 1989. By the end of the fiscal year 1989, project capital costs stood at $90.3 million, 14% over the June 1987 feasibility study estimate. In the fiscal year 1989, the overall gold recovery for combined oxide and sulfide mill ores was 89.8%. The heap leaching of waste rock from previous mining operations was completed in the fiscal year 1989. Heap leaching continued beyond this date using oxide reserves from the Summer Camp ore body 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 ore body adjacent to the pit area. This ore was to be mined when the pit level was deep enough to provide lateral access. In 1995, First Miss 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.

Mineralization is a polymetallic deposit (Mineral occurrence model information: Model code 173; USGS model code 26a.1; Deposit model name: Sediment-hosted Au; Mark3 model number 17), hosted in shale & limestone of the Preble Formation (Middle to Late Cambrian). The individual ore bodies are roughly tabular, strike NNW and dip 40-60E at a thickness of 1,000 meters, a width of 60.96 meters and a length of 2,133.6 meters. Controls for ore emplacement involved economic amounts of Au restricted to tabular, sheet-like zones (termed "veins" by Joralemon) within the Getchell fault zone and favourable calcareous lithologies.

The known gold deposits within the Getchell Trend are Carlin-type, sediment-hosted, replacement deposits containing micron gold. Gold mineralization at Getchell is associated with a curvilinear fault system that strikes NNW and dips 40° to 75° E, on the eastern flank of the Cretaceous Osgood granodiorite stock. The mineralized fault zone and the Cretaceous granodiorite both cut Paleozoic sediments of the Cambrian Preble and upper Cambrian to lower Ordovician Comus Formations which both belong to the Transition Assemblage and the Ordovician Valmy Formation of the Western Assemblage. Thermal metamorphism along the intrusive contact formed tungsten-bearing garnet-diopside skarns, passing outwards into wollastonite calc-silicate rock and marble. In the southern parts of the Getchell Mine area, the skarn is about 30 meters wide adjacent to the granodiorite contact, passing out into marble. Pelitic shales of the Preble and Comus Formations are thermally metamorphosed to cordierite-andalusite bearing hornfels nearest the contact, grading outwards into a biotite-cordierite-andalusite interval, to an outer biotite zone. The Osgood Stock and associated hornfels and skarns are found in both the footwall and hanging wall of the mineralized fault zones. Gold mineralization is found in several different rock types generally at the intersection of several high-angle and low-angle fault sets. The low-angle faults and associated folds are the results 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 with 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 7,000 feet 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 feet, with an average width of 40 feet. 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 the principal hosts of 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 the soil.

Regional alteration: 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.

Associated rocks include Late Cretaceous granodiorite and porphyry of the Osgood Mountains pluton. Local rocks/geologic units include alluvial deposits.

Geology: 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 after mineralization. These fluid pressures are greater than those which are normally accepted as epithermal.

Regional geologic structures: Regional thrust faults to the north and NNW-trending faults.

Local geologic structures: 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 3,500 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. The Getchell Fault Zone is a complex system of sub-parallel, high-angle faults which is at least 500 meters wide. The zone is made up of several fault planes, separated by brecciated gouge and characterized by intense clay alteration, and by brecciation in the hanging wall.

The main Getchell deposit within the fault has been drilled to a depth of 600 meters down dip from the original surface and remains open down dip. There is a 'Main Vein' which is a dominant structure with a distinct footwall, complexed by several conjugate veins to the west. Sub-parallel, mineralized structures have also been found up to 200 meters into the footwall of this main structure, while alteration, fault gouge, and mineralization occur up to 500 meters to the east into its hanging wall (First Miss Gold Inc., 1993).
Movement on the Getchell Fault has been both normal and dextral strike-slip (McCollum & McCollum 1990). Based on the relative displacement of the Paleozoic sediments and the Cretaceous of the Osgood Mountain Stock, it is believed that the Getchell Fault is a reactivated older structure (D. Bond, personal communication, 1993). The most recent displacement has taken place during the Miocene to present Basin and Range movement, representing further reactivation of an older structure. The fault cuts all three main stratigraphic units found within the pit, as well as the Osgood Mountain Stock. Altered blocks of granodiorite, rimmed by the skarn assemblage, are faulted downwards along the footwall structure into the Getchell Fault Zone and subsequently mineralized with gold (First Miss Gold Inc., 1993).

Workings included surface and underground openings. The mine has been developed by both underground and surface workings at various times during its production history. The Getchell deposit was developed by the North, Center, South, and Hansen Creek Pits. The Getchell underground is fully developed and is accessed from the Getchell open pit via two portals. The Getchell underground has a relatively short remaining mine life based on current proven reserves, although its life may be extended with lower processing costs, additional exploration drilling, and engineering analysis. Mining methods for the Getchell underground are currently 100% drift-and-fill, as the last of the longhole ore was produced in 2005.

Analytical data results: Average ore grade 0.18-0.23 ounces per ton Au.

Reserve/resource data: August 1997 - proven and probable reserves at the Getchell property, not including Turquoise Ridge, are 14.9 million tons grading 0.3 ounces Au/ton. Including Turquoise Ridge, the total mineral inventory is 10.9 million ounces of Au. ⓘ Cassiteriteⓘ 'Chabazite' ⓘ Chaidamuiteⓘ Chalcociteⓘ Chalcopyriteⓘ 'Chlorite Group' ⓘ Christiteⓘ Cinnabar ⓘ Cobaltiteⓘ Coloradoite ⓘ Copiapite ⓘ Cordieriteⓘ Covellite ⓘ Crandallite ⓘ Dolomiteⓘ Epidote ⓘ Epsomiteⓘ Ferrimolybditeⓘ Fluorapatite ⓘ Fluorapatite var. Carbonate-rich Fluorapatite ⓘ Fluorapophyllite-(K) ⓘ Fluorapophyllite-(Na)ⓘ Fluoriteⓘ Galenaⓘ Galkhaiteⓘ 'Garnet Group'ⓘ Getchellite (TL) ⓘ 'Gismondine Subgroup' ⓘ Goethite ⓘ Graphiteⓘ Guériniteⓘ Gypsumⓘ Haidingerite ⓘ Halotrichite ⓘ 'Heulandite Subgroup'ⓘ Hübneriteⓘ Ilsemanniteⓘ Jarosite ⓘ Jordaniteⓘ Kaolinite ⓘ Kermesiteⓘ Laffittite ⓘ Langite ⓘ Laumontite ⓘ Loránditeⓘ Magnetiteⓘ Malachite ⓘ Mansfielditeⓘ Marcasite ⓘ Massicotⓘ Melanterite ⓘ Meta-autunite ⓘ Metacinnabar ⓘ Metastibniteⓘ Molybdeniteⓘ Muscovite ⓘ Muscovite var. Illiteⓘ Muscovite var. Sericite ⓘ Native Arsenic ⓘ Native Copperⓘ Native Goldⓘ Native Gold var. Electrumⓘ Native Silver ⓘ Native Sulphurⓘ Orpiment ⓘ Palygorskiteⓘ Pararealgar ⓘ Pascoiteⓘ Pharmacolite ⓘ Pickeringiteⓘ Picropharmacolite ⓘ Piemontite ⓘ Polhemusiteⓘ Pyrite ⓘ Pyrite var. Arsenic-bearing Pyrite ⓘ Pyrolusiteⓘ Pyrrhotiteⓘ Quartz ⓘ Quartz var. Amethyst ⓘ Quartz var. Chalcedony ⓘ Rauenthaliteⓘ Realgar ⓘ Rozeniteⓘ Scheeliteⓘ Scoroditeⓘ Sphalerite ⓘ Stibiconiteⓘ Stibniteⓘ 'Stilbite Subgroup' ⓘ Symplesite ⓘ Tvalchrelidzeite ⓘ Uytenbogaardtite ⓘ Wakabayashiliteⓘ Weilite ⓘ Wollastonite