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Getchell, Turquoise Ridge |
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Nevada, USA |
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Au
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Super Porphyry Cu and Au
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The Getchell and Turquoise Ridge gold mines are located about 50 km to the north-east of the town of Winnemucca, Humboldt County, in north-central Nevada, USA. The ore deposits occur on the eastern flank of the Osgood Range within the Potosi Mining District. It is about 85 km to the WNW of the Carlin orebody (#Location: Getchell - 41° 12' 48"N, 117° 15' 22"W; Turquoise Ridge - 41° 12' 58"N, 117° 14' 38"W).
The Getchell and Turquoise Ridge mines are developed on the western and eastern sections of the main Gethchell body respectively, and in 2007 were both underground operations.
For details of the geological setting of the Getchell Trend see the "Getchell Gold Trend - Geology" record.
Initial mining interest in the district dates back to 1883, directed at the skarn related copper, lead and silver associated with the contact of the Cretaceous Osgood Mountain Stock. In 1916 tungsten was found to be present in the skarn and was mined sporadically until 1955. Only limited mining was carried out until gold was discovered at Getchell in 1934 by two prospectors. Within a year they had sold the deposit to Nobel Getchell and his partner, George Wingfield (Nanna, 1987).
Published reserves and production at Getchell and Turquoise Ridge include:
8.5 Mt @ 5.82 g/t Au = 49.5 t Au (Total Reserve, 1993).
1.25 Mt @ 9.6 g/t Au (Underground Reserve, 1993, included in total).
45 t Au (Reserve 1992, Getchell Handout, 1993).
5.9 Mt @ 6.6 g/t Au (Proven+Probable Reserve, 1994, AME, 1995).
3.6 Mt @ 8.6 g/t Au = 31 t Au (Production, to 1983, Tooker, 1985).
0.75 t Au (Production, 1983-87, Nanna, et al., 1993).
3.3 Mt @ 5.52 g/t Au = 18 t Au (Mill Production 1987-92, Getchell Handout, 1993).
4.2 Mt @ 1.18 g/t Au = 5 t Au (Leach Production 1987-92, Getchell Handout, 1993).
14 Mt @ 6.6 g/t Au (Production + reserves + potential, 1983, Bagby and Berger, 1985).
6.8 Mt @ 5.8 g/t Au = 40 t Au (Reserve, 1985, Nanna, et al., 1987).
2.15 Mt @ 18.1 g/t Au = 39 t Au (Proved + probable reserve, 2003, Newmont, 2007).
1.8 Mt @ 10.4 g/t Au = 18.7 t Au (Measured + indicated resources, 2003, Newmont, 2007).
1.2 Mt @ 15 g/t Au = 18 t Au (Inferred resources, 2003, Newmont, 2007).
Geology
The gold mineralisation at Getchell is associated with a curvi-linear fault system that strikes at around 345 to 350°, dips at 40 to 75° east, and occurs on the eastern flank of the Cretaceous Osgood Stock granodiorite. The mineralised fault zone and the Cretaceous granodiorite both cut Palaeozoic 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 (Nanna, et al., 1987).
Thermal metamorphism along the intrusive contact formed tungsten bearing garnet-diopside skarns, passing outwards into wollastonite calc-silicates and marble (Nanna, et al., 1987). In the southern parts of the Getchell Mine area the skarn is about 30 m 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 (Bagby & Cline, 1990). The Osgood Stock and associated hornfels and skarns are found in both the footwall and hangingwall of the mineralised fault zones (Nanna, et al., 1987).
The stratigraphy encountered within the Getchell mine area is as follows, from the base:
Cambrian, Preble Formation - composed of "typical deep water marine sediments", comprising alternating bands of thin to thick carbonaceous shales, phyllites and thin bedded limestones (FirstMiss Gold Inc., 1993; Stolberg & Dunning, 1985). In the open pits it comprises a basal argillite rich unit with thin limestone beds; a middle limestone with thin argillite beds which hosts most of the mineralisation of the formation; and an upper unit similar to the basal member. All of these are dark and carbonaceous and host ore over a 12 to 30 m width within and adjacent to the Getchell Fault. In the mine area the Preble Formation is predominantly within the footwall of the east dipping Getchell Fault Zone.
Unconformity
Upper Cambrian to Lower Ordovician Comus Formation - composed of thin to thick bedded, intercalated dolomitic limestones, shales, siltstones and minor amounts of chert (FirstMiss Gold Inc., 1993; Stolberg & Dunning, 1985). In the mine area the Comus Formation is mainly within the hangingwall of the Getchell Fault Zone.
Ordovician Valmy Formation - which consists of chert, shale, sandstone, minor limestone, greywacke, basalt and greenstone. At Getchell the presence of massive chert, volcanics and greenstone are the criteria that distinguish the Valmy from the Comus Formation (FirstMiss Gold Inc., 1993). In the mine area the unit is mainly basalt, with chert, sandstone and some thin limestone. It is separated from the other two major units by normal fault contacts.
All of these units are cut by the:
Cretaceous, Osgood Mountain Stock, dated at 90 Ma - This body is composed of hornblende granodiorite and granodiorite porphyry, with associated dykes and sills. It is generally medium to coarse grained, equigranular, and except on the margins, very uniform. The late stage sills and dykes include aplite dykes in the outer sections of the stock, while porphyritic dacite and andesite dykes also cut the intrusive and sediments. These dykes have been dated at 90 Ma, the same age as the main granodiorite stock. The intrusive complex, is made up of two connected lobes forming a dumb-bell shape with overall dimensions of 10 km in length north-south, and a maximum width of 3 km. It is cut and displaced by the Getchell Fault and is found in both the footwall and hangingwall of the ore and fault system, separated by Palaeozoic sediments (FirstMiss Gold Inc., 1993; Bagby & Cline, 1990).
Structure
The Getchell gold deposit lies within a structurally prepared fault zone, the Getchell Fault, which is controlled by 345 to 350° trending faulting which dips at 40 to 75° to the east. Ore grade gold has been found at three locations within the fault zone, namely the original Getchell deposits, Hansen Creek and Summer Camp. These are distributed over a strike length of approximately 5 km. The Getchell Fault swings to a more north-south trend near Summer Camp, the most southerly of the Getchell deposits, and curves to a 20° trend near Pinson, some 6 km further to the south. This curvature follows the margin of the Osgood Mountain Stock (Nana, et al., 1987).
A fourth deposit, Turquoise Ridge, is located approximately 1 km to the east of the Main Pit on a north-east trending fault system. This latter fault system extends from Turquoise Ridge to the Main Pit where it intersects the main Getchell Fault Zone. To the north-east it heads towards the Chimney Creek orebody. The NNW trending faulting of the Getchell Fault Zone post dates east-west crossing structures, but pre-dates, or is contemporaneous with, north-east and north-west trending structures and joint swarms. These north-east and north-west cross-structures tend not to be displaced, but commonly coalesce with the Getchell Fault Zone, favouring the interpretation that they are largely contemporaneous. They also appear to exert a major control on ore deposition (Nanna, et al., 1987). The older east-west fault and joint sets produce graben structures throughout the mine area, some of which drop the Valmy Formation down to become juxtaposed with the Comus Formation.
Movement on the Getchell Fault has been both normal and dextral strike-slip (McCollum & McCollum 1990). On the basis of the relative displacement of the Palaeozoic sediments and the Cretaceous granodiorite of the Osgood Mountain Stock it is believed that the Getchell Fault is a reactivated older structure. 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 mineralised with gold (FirstMiss Gold Inc., 1993).
The Getchell Fault Zone is actually a complex system of sub-parallel, high angle faults which is at least 500 m wide. The Getchell Fault Zone is made up of a number of fault planes, separated by brecciated gouge. It is characterised by intense clay alteration, and by brecciation in the hangingwall. The main Getchell deposit within the fault has been drilled to a depth of 600 m 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, mineralised structures have also been found up to 200 m into the footwall of this main structure, while alteration, fault gouge and mineralisation occur up to 500 m to the east into its hangingwall (FirstMiss Gold Inc., 1993).
The intrusion of the Osgood Mountain Stock has formed an antiform/dome within the surrounding sedimentary sequence, with all of the sediments on its eastern margin being strongly faulted and folded, but generally dipping back to the east.
Mineralisation and Alteration
Ore grade mineralisation is known in a number of main zones in the Getchell area. These are i). the Getchell Mine itself which is hosted in carbonaceous limestone, silty limestone and calcareous mudstone breccias and extends over a length of approximately 2000 m along the Getchell Fault Zone; ii). Hansen Creek, 600 m to the south along the Getchell Fault; iii). Summer Camp, which is another 1500 m to the south, again on the Getchell Fault; iv). Turquoise Ridge which is 1000 m to the east of Getchell and is located on a north-east trending fault which intersects the Getchell Fault Zone in the Main Getchell Pit, hosted by hornfelsed mudstone, limestones, calcareous mudstones and some pillow basalts (FirstMiss Gold Inc., 1993); and v). North Zone, which is hosted within Ordovician interbedded carbonaceous mudstones and limestones, and calcareous mudstone breccias, comprises 15 distinct mineralised shoots, 12 of which are sub-parallel, NW trending, NE dipping stratigraphically controlled and three with strong structural control (Barrick Gold, 2006).
The Getchell Mine itself was originally three pits, i). the North Pit; ii). Centre Pit; and iii). South Pit (Nanna, et al., 1987). The latter two were subsequently amalgamated into the single Main Pit. Within the North Pit the orebody is 12 to 30 m thick, within and adjacent to the Getchell Fault, while in the Main Pit it averages 30 m in thickness, ranging from 15 m at the thinnest to a maximum of around 40 m. Spurs of ore branch off of the main orebody where cross-cutting/coalescing hangingwall faults intersect the main Getchell Fault Zone. These may locally be up to 15 m thick.
The gold mineralisation occurs along the fault systems outlined above. It is found in carbonaceous fault gouge zones within thin bedded limestones, carbonaceous shales, hornfels and poorly sorted siltstones of the Preble, Comus and Valmy Formations, and in altered granodiorite of the Osgood Mountain Stock. Mineralisation is also found within cherts, shales and hornfels of the Valmy Formation. Gold deposition is controlled by both structural preparation and by lithology, and it appears that any rock type may be mineralised, if structurally favourable, with some being better hosts than others (Bagby & Cline, 1990; FirstMiss Gold Inc., 1993). The bulk of the low grade ore is found within the gouge zone, while narrower high grade veins occur within faults cutting the more competent rocks in the footwall of the main Getchell Fault Zone.
The Getchell Fault Zone is composed of a number of fault planes, separated by brecciated gouge. The clasts are made up of the sediments that form the wall rocks of the fault zone, and of granodiorite near the contact with the Osgood Mountain Stock. Gold mineralisation occurs both within matrix gouge and the breccia fragments of the fault zone. The gouge is a clayey mass enclosing soft fragments from 1 mm up to several cm's across. Bright red realgar and yellow orpiment staining is patchily developed throughout the fault zone. The granodiorite in the footwall of the fault zone is strongly argillised and is partly mineralised.
Host rocks apparently influence the localisation of ore by both their porosity and chemical composition. The least favourable lithologies are granodiorite, basalt, massive beds of limestone and skarn. The best hosts are the thin bedded, dirty limestones, carbonaceous shales and siltstones of the Preble and Comus Formations (FirstMiss Gold Inc., 1993). Better grade tends to be hosted by the siltier units with associated dolomite or calcareous material, while an increase in the content of clay corresponds to a decrease in grade. Similarly relatively impermeable rocks may form barriers. Unexpected high grade shoots have been found in embayments of sediments into the hangingwall granodiorite, while hangingwall sills cap ore zones that spread along their undersides (FirstMiss Gold Inc., 1993).
The general trend of mineralisation follows the strike of the main Getchell Fault Zone, and if not influenced by crossing structures, forms a relatively consistent ore zone in suitable host rocks. Apart from the North Pit however, this latter case is not typical of the orebody as a whole, where cross faults render the structure more complex (FirstMiss Gold Inc., 1993). The highest grade mineralisation is generally found where the later north-east fault set cuts the main NNW to north-south Getchell Fault Zone. The exception is at Turquoise Ridge where the ore is within a north-east fault removed from the main Getchell Fault Zone. Cross faults that pre-date mineralisation shift blocks of un-favourable or favourable host rocks into the structurally prepared zone of mineralisation. Cross faults and shear zones also create shoots of greater structural porosity and in that way influence grade distribution (FirstMiss Gold Inc., 1993).
In the Main Pit a series of gold bearing, near vertical hangingwall faults approach the main Getchell structure from the south-east. As the two structural sets approach, the intervening rock mass is often mineralised. The hangingwall structures intersect the main fault zone to produce high grade, near vertical shoots raking slightly to the south. At this juncture the main Getchell Fault Zone steepens locally. The hangingwall structures were also highly mineralised in a wedge formed by the main Getchell Fault Zone to the west, and the granodiorite below and to the north, producing the oxidised ore that was initially mined in the 1930's and 1940's. The Turquoise Ridge Fault has similarly influenced the localisation of high grade shoots where it intersects the Getchell Fault Zone in the Main Pit. This mineralisation extends out along the north-east trending fault towards Turquoise Ridge. Normal fault movement and sinistral strike slip movement has caused the ore zones to pinch and swell along strike and down dip. Post ore movement has resulted in brecciation and the displacement of mineralised blocks by up to 100 m (FirstMiss Gold Inc., 1993).
Folding of the host rocks prior to mineralisation also appears to have influenced the distribution of ore. Mineralisation may be concentrated along the axes of folds on both the small scale in folds that are a metre or so across, and by larger anticlines. In the Main Pit the footwall limestones and shales of the Preble Formation are sub-parallel to the strike and dip of the Getchell Fault Zone. In the North Pit however the strike of the beds wraps to the north-west, forming a steeply plunging anticline. Secondary porosity along the nose of the anticline is interpreted to have been partially responsible for the formation of the North Pit orebody (FirstMiss Gold Inc., 1993).
The gold at Getchell is interpreted to have been deposited into the settings described above by several stages of Tertiary hydrothermal activity. The bulk of the ore contains refractory mineralisation, with the gold being associated with several phases of pyrite. The host rocks had been subjected to deformation and thermal metamorphism, ranging from sericite to biotite stable, prior to mineralisation. Deposition of diffuse gold bearing pyrite began during an early stage of hydrothermal activity, accompanied by brecciation, silica flooding (forming jasperoids), silicification and clay alteration of the pre-existing sericite, feldspar and biotite. Later hydrothermal events are interpreted to have remobilised and coarsened the diffuse gold and introduced additional gold, arsenic and mercury, forming the present deposit. The gold associated with the first phase of disseminated pyrite is in the 0.1 to 1 µm size range, while the later stages contain gold in the 1 to 10 µm range, also associated with pyrite (Nanna, et al., 1987). Alteration within the granodiorite is characterised by the development of sericite, although the subsidiary eastern mass of granodiorite within the eastern sections of the pit is strongly argillised.
The predominant mineralogy of the ore includes pyrite, arsenopyrite, quartz, calcite, realgar and orpiment. The arsenic minerals predominate, with associated Hg, Ag, Sb, Zn, Cu, Ce, Tl and Au. Molybdenite and scheelite are found within the mine areas also. The minor ore minerals include cinnabar, stibnite, chalcopyrite and sphalerite. In addition to quartz and calcite, the gangue minerals include marcasite, pyrite, arsenopyrite, magnetite, barite, fluorite and chabazite Secondary minerals resulting from local weathering include gypsum, picropharmacolite and ilsemannite (Stolburg & Dunning, 1985).
Mercury occurs as cinnabar and within stibnite, being much more common in the southern part of the Main Pit, relative to the North Pit. Cinnabar occurs as small, generally <1 mm crystals, associated with quartz, realgar and stibnite. Pyrite often nucleates on cinnabar crystals. Fluorite is one of the most common gangue minerals and occurs as simple cubes in the carbonaceous quartz rock associated with realgar and orpiment. In the North Pit late stage calcite has coated the fluorite, stibnite, orpiment and realgar. Orpiment and realgar are the two most abundant arsenic sulphides. The major portion of orpiment occurs as veins of foliated, columnar or fibrous masses, and as cavity fill in quartz rich fracture fillings. Orpiment and realgar were apparently a late stage product following the gold and quartz mineralisation. In rocks with abundant carbonaceous matter, these arsenic minerals may be surrounded by dense mattes of remobilised carbon (Stolburg & Dunning, 1985).
Gold occurs as sub-micron sized particles of the native metal, associated with fine grained carbonaceous quartz and shales, as inclusions within sulphides, particularly marcasite and pyrite and as fine particles within and between quartz and clay grains. Gold is only visible in polished section under magnification, although some isolated grains have been reported under low magnification (Stolburg & Dunning, 1985). No published data apparently reveals the relationship between the gold mineralisation and the tungsten bearing skarn. At Pinson however, gold occurs in fractures that cut calc-silicate rocks of the Comus Formation (Bagby & Cline, 1990).
Pyrite occurs in a number of forms. These include fine pyrite cubes, possibly syn-thermal metamorphism, which are disseminated throughout the host rock, while large cubes and irregular masses of pyrite are strung out along laminae. The coarser pyrite tends to occur in fracture filling veinlets, often brecciated and then re-healed. Some pyrite preceded marcasite which is found attached to pyrite, or as coatings on pyrite, while some veinlets contain a jumble of marcasite prisms and coarse pyrite cubes in a quartz gangue. Pyrite is also found replacing marcasite. Fine grained pyrite and realgar fill interstices and occur in a breccia matrix with coarse granular quartz (FirstMiss Gold Inc., 1993).
Petrographic analysis of the carbon at Getchell has shown that original organic carbon occurred within the Palaeozoic sediments. Subsequent metamorphism and alteration has silicified and recrystallised the sediments to a cryptocrystalline state with carbon remobilised as dustings on the bedding planes. Carbon has also locally migrated out of the bedding planes into cleavage faces and into the interstices between grain boundaries in highly silicified rocks. After at least one stage of brecciation and hydrothermal alteration the carbon has been remobilised out of crystal interstices and is found in veinlets and fractures, and as colloidal carbon in micro-vugs. Carbonaceous matter is found throughout the orebody in a succession of ever heavier hydrocarbons ranging from light organic hydrocarbon to graphite. The form appears to be influenced by the original organic content and by the subsequent metamorphic and hydrothermal influences. No direct relationship has been found, however, between the gold grade and the carbon content or carbon type in the ore (FirstMiss Gold Inc., 1993). Graphite, amorphous carbon and organic complexes occur in silicified zones of the orebody and give it a dark grey colour. The graphite is very fine grained and occasionally coats the ore minerals (Stolburg & Dunning, 1985). In general the more highly altered Preble Formation, which is black in its un-altered form, is changed to grey, corresponding in part to the remobilisation of carbon. In some instances however, there is an increase in carbon towards the faults.
All of the deposits at Getchell are a combination of oxide and sulphide mineralisation. The Main, Hansen Creek and Summer Camp pits all contain both oxide and sulphide ore, while Turquoise Ridge had only yielded minable oxide ore to late 1993. Exploration in the sulphide mineralisation below Turquoise Ridge had not revealed any ore at that stage. The gold in the oxide ore usually occurs as discrete micron size grains in pseudomorphs after pyrite. It is, as such, associated with an immature oxide assemblage and has not started to migrate or coarsen. The major sulphide phases (pyrite and realgar-orpiment) have combined to produce goethite, arsenian iron oxides and arseniosiderite, all of which replace pyrite in situ (FirstMiss Gold Inc., 1993).
Fluid inclusion studies on the gold mineralisation, as distinct from the skarn related tungsten, has indicated temperature of mineralisation of 286° to 268°C and pressures of 370 to 430 bars (Bagby & Cline, 1990).
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2006.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Getchell Turquoise Ridge
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Selected References:
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Cail T L, Cline J S 2001 - Alteration associated with Gold deposition at the Getchell Carlin-type Gold deposit, north-central Nevada: in Econ. Geol. v96 pp 1343-1359
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Cline J S 2001 - Timing of Gold and Arsenic Sulfide mineral deposition at the Getchell Carlin-type Gold deposit, north-central Nevada: in Econ. Geol. v96 pp 75-89
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Cline, J.S., Hofstra, A.H., Muntean, J.L., Tosdal, R.M. and Hickey, K.A., 2005 - Carlin-Type Gold Deposits in Nevada: Critical Geologic Characteristics and Viable Models: in Hedenquist, J.W., Thompson, J.F.H., Goldfarb, R.J. and Richards, J.P. (eds.), Economic Geology, 100th Anniversary Volume, Society of Economic Geologists, pp. 451-484.
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Groff J A, Heizler M T, McIntosh W C, Norman D I 1997 - 40Ar/39Ar dating and mineral paragenesis for Carlin-type Gold deposits along the Getchell Trend, Nevada: evidence for Cretaceous and Tertiary Gold mineralization: in Econ. Geol. v92 pp 601-622
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Nanna D, Baumann M, Berentsen E, Steinman G 1987 - Getchell deposit: in Johnson J L (Ed.), 1987 Bulk Mineable Precious Metal Deposits of the Western United States - Guidebook for Field Trips Geol. Soc. Nevada pp 353-356
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Silberman M L, Berger B R and Koski R A, 1974 - K-Ar Age Relations of Granodiorite Emplacement and Tungsten and Gold Mineralization near the Getchell Mine, Humboldt County, Nevada : in Econ. Geol. v. 69 pp. 646-656
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Tosdal R M, Cline J S, Fanning C M, Wooden J L 2003 - Lead in the Getchell-Turquoise Ridge Carlin-type gold deposits from the perspective of potential igneous and sedimentary rock sources in Northern Nevada: Implications for fluid and metal sources: in Econ. Geol. v98 pp 1189-1211
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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