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Getchell Gold Trend - Geology |
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Nevada, USA |
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Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Getchell Trend of gold deposits is located within the Great Basin geologic province of Nevada, USA. The rocks within the trend include sediments ranging in age from Cambrian to Permian and in lithologic type from shelf sandstones and carbonates to oceanic clastics and greenstones. These are overlain and/or cut by Mesozoic intrusives, Tertiary volcanics, and Quaternary gravels and alluvium (McCollum & McCollum , 1990).
Four separate successions are represented within and adjacent to the Getchell Trend. Each is separated by either a major structural feature which has juxtaposed sequences from different tectonic settings, or by angular unconformities. The principal successions or rock groupings within the trend comprise the following (from Madrid & Roberts, 1991; McCollum & McCollum, 1990; Hoover, et al., 1990; Foster & Kretschmer, 1990; Osterberg & Guilbert, 1990):
i). Western, or Siliceous Assemblage - This succession is mainly found to the west of the Getchell Trend and comprises silici-clastics and cherts with minor greenstones. It is interpreted as having been deposited in a deeper, oceanic setting on the western margin of the North American Craton. The Western Assemblage is interpreted to have been thrust eastwards onto the Transition Assemblage above the Roberts Mountains Thrust during the Devono-Carboniferous Antler Orogeny.
ii). Transition Assemblage - comprising shelf sediments whose depositional setting was transitional between the main 'Eastern, or Carbonate Assemblage' and the 'Western, or Siliceous Assemblage'. The Eastern Assemblage is found within the Carlin and Cortez-Battle Mountain Trends, both of which are further to the east, but is not seen in the Getchell Trend. The Devono-Carboniferous Roberts Mountains Thrust, which separates the autochthonous Eastern Assemblage from the allochthonous Western Assemblage further to the east, is also not seen within the Getchell Trend. The Transition Assemblage exposed on the Getchell Trend may well be allochthonous, above the Roberts Mountains Thrust. Minor exposures interpreted as belonging to the Ordovician Valmy Formation have been mapped within the Transition Assemblage however, representing eastward projecting fingers of the Western Assemblage. The Western and Transition Assemblages were isoclinally folded and subjected to low grade regional metamorphism during the Devono-Carboniferous Antler Orogeny.
iii). Overlap Assemblage - composed of Carboniferous to Permian rocks, mainly clastics and carbonates, which unconformably overlie both the Transition and Western Assemblage rocks. The deposition of this sequence followed the Antler Orogeny and associated movement on the Roberts Mountains Thrust. The sediments of the Overlap Assemblage were derived from the Antler Highland which was formed as a result of the imbrication of the Roberts Mountain Allochthon.
iv). Havallah Sequence - made up of silici-clastics and interbedded mafics which are separated from the two assemblages above on the Getchell Trend by a major suture, the Osgood Mountains Suture. This sequence was deposited during the same time interval as the Overlap Assemblage, but to the west of the Antler Highland. Tectonic activity on the Osgood Mountains Suture has juxtaposed the time equivalent, but lithologically dissimilar, Havallah Sequence and the Overlap Assemblage.
These four sequences are cut and/or are overlain by:
v). Mesozoic and Tertiary Rocks - represented by Jurassic clastics, Cretaceous granodiorite (90 Ma), Miocene basalts, clastics, tuffs and rhyolites, Pliocene basalt, and Quaternary alluvium and gravels.
Of the key deposits of the trend, Getchell is hosted by the Transition Assemblage and by rocks which some authors have interpreted as belonging to the Valmy Formation of the Western Assemblage. Preble and Pinson fall wholly within the Transition Assemblage, while the Twin Creeks - Chimney Creek and Rabbit Creek orebodies are embraced by the Overlap Assemblage. All exhibit a strong structural control of ore. The Getchell and Pinson orebodies are found on the margin of a dumb-bell shaped mass of Cretaceous granodiorite, along whose margins a series of tungsten bearing skarns are also distributed.
Stratigraphy
The stratigraphy on the Getchell Trend may be summarised as follows, divided into the main assemblages, and later cover rocks and intrusives, as outlined above. Many of these units are also found within the Cortez-Battle Mountain Trend. For detail of the units common to both trends, see the 'Battle Mountain Eureka Gold Trend - Geology' record.
Western, Siliceous Assemblage. The sequences of this assemblage are mainly found to the west of the Getchell Trend deposits, although rocks interpreted as belonging to the Valmy Formation, a member of this assemblage, are present within the western sections of the trend near Getchell. The main units are:
- Cambrian, Harmony Formation - of upper Cambrian age, composed of feldspathic sandstone and siltstone, with shale, minor limestone and minor pebble conglomerate. The sandstone has 50 to 80% quartz, 10 to 30% feldspar and minor amounts of mica, heavy minerals and lithic clasts. Plagioclase and K-feldspar occur together. The sandstone layers are graded and are considered to be mid-fan turbidites (Stewart, 1980).
- Ordovician, Valmy Formation - described in this area as comprising cherts, greenstones and argillites (Hoover, etal., 1990). Rocks interpreted as being of the Valmy Formation at Getchell are composed of chert, shale, sandstone, minor limestone, greywacke, basalt and greenstone. At Getchell the presence of massive chert, volcanics and sandstone are the criteria for distinguishing Valmy Formation rocks from the Ordovician Comus Formation of the Transition Assemblage (FirstMiss Gold Inc., 1993). The Valmy Formation has been described more fully in the Cortez-Battle Mountain Trend section.
Transition Assemblage. This assemblage represents the oldest and structurally lowest rocks recognised within the Getchell Trend. The Western Assemblage has apparently been thrust eastward over the Transition Assemblage during the Devono-Carboniferous Antler Orogeny. The main units are as follows, from the base:
- Cambrian, Osgood Mountain Quartzite - is mainly composed of quartzite with minor amounts of conglomerate, phyllitic siltstone, limestone and dolomite (Stewart & Carlson, 1978; Stewart, 1980; Madrid & Roberts, 1991). To the north it outcrops as a massive quartz-arenite which forms prominent knobbly exposures (McCollum & McCollum, 1990). It also outcrops extensively in the southern sections of the Osgood Range to the west of the mineralised trend. According to Osterberg & Guilbert (1990) it is composed of quartz-arenites and litharenites that grade upwards into the fine grained sandstones, siltstones, shales and limestones of the Cambrian Preble Formation, with the proportion of limestone increasing upwards.
- Cambrian, Preble Formation - lithologically this is a thinly interbedded unit of shale or phyllite and silty carbonates. Thicker bedded carbonate units are present, some locally silicified to black cherts or replaced by barite (McCollum & McCollum, 1990). The Preble Formation can be broken into three distinct units, namely, a: i) Lower Unit of sandy shale, quartzitic sandstone and phyllitic shale; ii) Middle Unit, consisting of a sequence of limestone, carbonaceous shale and calcareous shale, along with phyllitic shale and quartzitic sandstone; and iii) Upper Member which comprises a monotonous grouping of phyllitic shale, with some sandy shale and carbonaceous beds (Kretschmer, 1987). The unit is generally tightly folded, metamorphosed to marble and hornfels and is locally silicified along the ridge crest where it has been intruded by the Osgood Mountain Stock (McCollum & McCollum, 1990).
- Unconformity.
- Upper Cambrian to Lower Ordovician, Comus Formation - which is a thin bedded carbonate and shale unit with dolomite, siltstone and some chert and volcanics. Rhythmic inter-bedding of carbonate and shaly rocks is common, sometimes occurring as fine laminations. Intra-formational conglomerate occurs in the section near the intercalated limestone and cherty beds. The rocks are distinctively darker and somewhat more thickly bedded than the underlying Preble Formation and have been locally largely metamorphosed to quartz-amphibole hornfels (Kretschmer, 1987; McCollum & McCollum, 1990). At Rabbit Creek the main host to ore is mapped as the Comus Formation. At this location it has a strong basaltic component, with the lower member containing 25% and the upper 50% basaltic lava flows, with associated tuffs, cherts and calcareous and carbonaceous shales, mudstones and siltstones (Bloomstein, et al., 1990).
Overlap Assemblage. The sediments of this assemblage were deposited following the Devono-Carboniferous Antler Orogeny and unconformably overlie both the Transition and Western Assemblages. The main units are as follows, from the base:
- Upper Carboniferous (Pennsylvanian), Battle Formation - which contains locally derived clasts as large as boulders and has a variable thickness, having been deposited on a surface with considerable topographic relief. At least some of the conglomerate is marine with fossils, while other sections are marginal marine, or locally continental (Stewart, 1980).
- Upper Carboniferous (Pennsylvanian) to Lower Permian, Etchart Formation - which consists of sandy and conglomeratic silty carbonates and rests either on the Battle Formation Conglomerate, or directly on the Preble and Comus Formations (McCollum & McCollum, 1990). At Chimney Creek, where it is the main host to ore, it is up to 850 m thick and is composed of a basal pebbly litharenite, overlain by coarse grained sandy dolomite with intercalated siltstones, calcareous sandstone, sandy bio-sparite, calcitic conglomerate, dolomitic siltstone and argillite (Osterberg & Guilbert, 1990). The Etchart Formation is equivalent to the Antler Peak Limestone described in the Cortez-Battle Mountain Trend.
Havallah Sequence. This succession is found to the west of the Osgood Mountains Suture of McCollum & McCollum, 1990 [or the Farrel Canyon Thrust of Osterberg & Guilbert (1990), a member of the Golconda Thrust package] and comprises the following, from the base:
- Lower Carboniferous (Mississippian), Farrel Canyon Formation, at least 1400 m thick - comprising a heterogeneous assemblage of interbedded lithic quartz-arenite, shale, siltstone, radiolarian chert and andesitic volcanic rocks. It may be divided into two litho-types, i) a sandstone which consists predominantly of quartz-arenite, argillite, siltstone and minor ribbon chert, and thin limestone turbidites; and ii) a chert litho-type composed predominantly of submarine andesitic volcanics, overlain by radiolarian cherts, with argillitic interbeds and occasional beds of chert pebble conglomerate (McCollum & McCollum, 1990).
- Lower Carboniferous (Mississippian), Goughs Canyon Formation, approximately 1000 m thick - which includes a basaltic volcanic hyaloclastite, and a carbonate debris flow assemblage which is largely devoid of quartz (McCollum & McCollum, 1990). In the Dry Hills section of the northern Osgood Mountains, at Chimney Creek, meta-basalt lavas, with interflow chemical sediments which carry upper Palaeozoic radiolaria, have been assigned to the Goughs Canyon Formation. At this location these volcanics lie conformably below the Overlap Assemblage Etchart Limestone and non-conformably overly the Ordovician Valmy Formation which is in turn interpreted to have been thrust eastward above the Roberts Mountains Thrust. This implies that the Goughs Canyon Formation near Chimney Creek is an equivalent of the main Goughs Canyon Formation of the Havallah Sequence, interfingering eastward into the Overlap Assemblage (Osterberg & Guilbert, 1990).
- Upper Carboniferous (Pennsylvanian), Harmony Formation, up to 800 m thick - This unit, where it occurs some 5 km to the west of the Pinson Mine, was earlier correlated with the Cambrian Harmony Formation. Here it is composed of conglomeratic debris flows, shale, quartz-feldspathic sandstone, radiolarian chert, dolomite and dolomitic sandstone. The lower 50 m includes chert, argillite and silty limestone. These are overlain by 600 m of argillite, reworked feldspathic sandstone, minor limestone turbidites and thick submarine gravity debris flows of pebble to boulder sized conglomerates. The debris flow conglomerates comprise clasts of quartzo-feldspathic sandstone and limestone or minor chert set in a pelitic matrix containing detrital feldspar and quartz sand grains. Although most clasts average around 1 cubic metre, matrix supported mega-clasts of both sandstone and limestone with volumes of up to 500 cubic metres are present. Much of the 'Harmony Formation' in this area is made up of material shed westwards from the Antler Highland, primarily clasts of well lithified quartzo-feldspathic sandstone, lithologically identical to the Cambrian Harmony Formation. Cambrian fossils are found in the clasts and Permian in the matrix (McCollum & McCollum, 1990).
- Permian, Adam Peak Formation - including a predominantly clastic sequence of calcareous and phosphatic sandstone, siltstone and shale, overlying a basal quartz-pebble conglomerate. Spicular ribbon cherts, dolomite and limestone turbidites and fusulinid debris are also present. It unconformably overlies the "Harmony Formation" and is made up of clastics derived from the Antler Highland, which at this stage were predominantly fine grained (McCollum & McCollum, 1990).
Intrusives, represented principally by:
- Cretaceous, Osgood Mountain Stock, dated at 90 Ma - which is of granodioritic composition and is present as a dumb-bell shaped pair of connected plugs, overall some 10 km long and up to 3 km wide. It comprises hornblende granodiorite and granodiorite porphyry and has associated apophyses, dykes and sills of granodiorite, andesite porphyry, dacite porphyry and aplite (FirstMiss Gold Inc., 1993; Kretschmer, 1987). The granodiorite of the stock is generally medium to coarse grained, equigranular, and except on the margins, very uniform (Kretschmer, 1987). This stock corresponds to an aero-magnetic high of around 600 nT from a survey flown at 135 to 150 m above the surface. Magnetic images indicate that its margins dip shallowly outwards on all sides. At about 1 to 3 km from the mapped contact the margins of the stock appear to change dip and become steep to vertical. Other interpreted shallow intrusions are also found along the Getchell Trend, near Chimney Creek, between Pinson and Preble and between Chimney Creek and Getchell. Apophyses are found adjacent to the stock margins, while a shallow, narrower extension is indicated to the NNW for several kilometres parallel to the long axis of the stock (Grauch & Bankey, 1990).
Younger Cover, comprising Tertiary and Quaternary volcanics and lacustrine to alluvial sediments includes: Miocene, basalt and basaltic-andesite; clastic sediments and tuffs; rhyolite flows and tuffs; Pliocene, basalt; Quaternary, gravel; older alluvium; younger alluvium.
For details of mineralisation in the trend see the "Getchell, Turquoise Ridge" record.
The most recent source geological information used to prepare this decription was dated: 1996.
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.
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Selected References:
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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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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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