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Jerritt Canyon Geology and Structure - Enfield Bell, Marlboro Canyon, Alchem, Generator Hill |
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Nevada, USA |
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The Jerritt Canyon district embraces a number of mines in the Independence Mountains of northern Elko County, north-eastern Nevada, USA.
The Jerritt Canyon Project, which involves the operation of the Enfield Bell Mine, which is located in the Jerritt Canyon Window of the Roberts Mountains Thrust. It is approximately 80 km to the north of the town if Carlin and 60 km NNE of the Carlin mine. The Jerritt Canyon deposits are part of a generally north-south aligned belt of sediment hosted gold deposits that extend over a distance of around 50 km along the spine of the Independence Range. This range was formed by Basin and Range block faulting.
The Enfield Bell Mine exploits four separate orebodies within an area of approximately 3 x 2 km. These orebodies, plus another five mineable reserves are distributed over a NNE elongated area of around 12 x 5 km, while another five geological resources extend the zone to approximately 20 x 10 km. The ore mined to 1990 was taken from three of the four major mineable reserves that had been outlined at that stage. By 1990 production was under way on all four. These were the Marlboro Canyon, Alchem, North Generator Hill and the West Generator Hill orebodies. The other six mineable reserves are Winters Creek, Saval/Steer, Mill Creek, Burns Basin and Pattani Springs. The geological resources that were known in 1990 were Wright Window, California Mountain, Pie Creek, Waterpipe and Starvation. The four main orebodies are located within a single window of Eastern Assemblage rocks formed by the Roberts Mountains Thrust, while the other reserves and resources are found within one other larger and four smaller immediately adjacent windows separated by narrow widths of intervening upper plate rocks (Bratland, 1990; Weideman, et al., 1990; Birak & Hawkins, 1986.
The Jerritt Canyon deposit was discovered in 1972 and first gold production from the property occurred in 1981. Open pit mining was conducted from early 1981 until late 1999, with the mining carried out in the areas of Marlboro Canyon, Alchem, Lower North Generator Hill, Upper North Generator Hill, West Generator, Burns Basin, Mill Creek, Pattani Springs, California Mountains, Dash, Winters Creek, Steer Canyon, and Saval Canyon. The annual production from these areas ranged from approximately 1.2 to 43 tonnes of gold during this period. Underground operations commenced in 1997 at SSX, and continued until 2008 with production from the Steer, Murray, MCE, Smith, West Generator and Saval deposits. In 2009, a new mine plan was prepared. Underground mining from the Smith deposit recommenced in late January 2010 and underground mining at SSX recommenced in early October 2010. From the start of mining in 1980 to the end of December 2020, ~301 tonnes (~9.7 Moz) of gold were produced from ~44.5 Mt ore mined at an average grade of 6.2 g/t Au.
Published production and reserve figures include:
District Resource, 1989 - 39 Mt @ approx. 4 g/t Au in 10 deposits for 155 t of Au, (Hofstra, et al., '90)
Enfield Bell
Reserve, 1988 - 17.4 Mt @ 5 g/t Au= 87 t Au
Production to 1989 - 9 Mt @ 7 g/t Au = 63 t Au (Weideman, et al., 1990)
Reserve, 1989 - 13.7 Mt @ 3.5 g/t Au (Weideman, et al., 1990)
Jerritt Canyon District
Other mineable Reserves, 1990 - 40 t (Weideman, et al., 1990)
Production+Reserve, 1982 -12.75 Mt @ 8.3 g/t Au (Bagby & Berger, 1985)
Production+Reserve, 1984 - 12 Mt @ 6.7 g/t Au (Romberger, 1986)
Proved+Probable Reserve, 1994 - 22 Mt @ 5.1 g/t Au (AME, 1995)
Proved+Probable Underground Reserve, 31 December 2021 - 2.529 Mt @ 5.41 g/t Au (First Majestic Silver Corp 2021)
Measured+Indicated Underground+Open pit Resource, 31 December 2021 - 8.55 Mt @ 5.84 g/t Au (First Majestic Silver Corp 2021)
Inferred Underground+Open pit Resource, 31 December 2021 - 6.927 Mt @ 5.61 g/t Au (First Majestic Silver Corp 2021).
It is assumed that the 155 t Au resource stated for the district by Hofstra, et al., (1990) includes all of the mineable reserves detailed above and associated geological resources not proven at that stage. The reserve at 1989 and production to 1989 quoted in the tabulation above from Weideman, et al., (1990), only relate to the four main orebodies of Marlboro Canyon, Alchem, North Generator Hill and the West Generator Hill and, they say, only constitutes about 55% of the Jerritt Canyons district's mineable reserve. The 40 t Au in the 'other mineable deposits of the district' is calculated from that statement.
Economic interest in the Independence Mountains originated with antimony prospecting and minor production which dates back to at least 1918. Barite prospecting began in the late 1950's, although significant beneficiation of barite did not occur until many years later. In the southern part of the area a few small pods of massive sulphides, mainly pyrite-marcasite and lesser base metals, occur in pyritised silici-clastics of the Western Assemblage within the Roberts Mountains Allochthon. These pods are small, generally less than 2 m in length (Daly, et al., 1990).
Although the original geochemical anomaly that led to the discovery in the north fork of Jerritt Canyon was a strong "bulls-eye" target, the bulk of the current reserves were found beneath a cover of colluvium and soil that was geochemically barren (Birak & Hawkins, 1986). Geochemical soil and rock samples were assayed for Au, As and Sb. Because of the concealed nature of the deposit, it was necessary to preferentially sample fractures and follow up small leakage anomalies along faults ( I M Clementson et al., Mine visit, 1988).
Geology
The geology of the Independence Mountains incorporates most of the major elements of the Carlin and Cortez-Battle Mountain. The Independence Mountains are a generally north-south oriented range produced by Tertiary Basin and Range block faulting. The lowest sequence exposed in the range is the autochthonous Ordovician to Devonian Eastern, or Carbonate Assemblage which is structurally overlain by the Ordovician Western, or Siliceous Assemblage which constitutes the Roberts Mountains Allochthon. These allochthonous rocks were thrust eastward over the Roberts Mountains Thrust during the Devono-Carboniferous Antler Orogeny. The leading edge of the Roberts Mountains Thrust is well to the east of the Independence Mountains, although windows of lower plate rocks are exposed by erosion of antiformal folds and up-faulted blocks which has brought the thrust plane to the surface. Some of the folding in the Roberts Mountains Thrust surface is related to the Antler Orogeny (Daly, et al., 1990).
In the northern sections of the Independence Mountains, both allochthonous and autochthonous rocks are unconformably overlain by Carboniferous to Permian sediments of the Overlap Assemblage, which were eroded from the Antler Highland. The Antler Highland was formed by the imbrication associated with the emplacement of the Roberts Mountains Allochthon. Rocks of the Overlap Assemblage are not exposed in the immediate Jerritt Canyon district. All of these rocks, the Eastern, Western and Overlap Assemblages, were in turn overthrust by the Golconda Allochthon during the Permo-Triassic Sonoma Orogeny. The Golconda Allochthon comprises a western assemblage of Devonian to Permian silici-clastics, known locally as the Schoonover Sequence (generally equivalent to the Havallah Sequence described in sections 5.6 and 5.10) which were thrust eastward over the Golconda Thrust. The leading edge of this structure, which trends generally NNE, is mapped in the northern Independence Range, north of the Jerritt Canyon District, although deformation associated with the orogeny is evident at Jerritt Canyon (Daly, et al., 1990).
Other structures in the area, dated at late Jurassic to early Cretaceous, may be related to the early stages of thrusting of the late Jurassic to middle Tertiary Sevier Orogeny. Thermal maturation of carbonaceous sediments in much of northern Nevada may also be related to this event, as was the development of igneous and metamorphic core complexes in the Ruby Mountains 100 km to the south-east. A granodiorite to tonalite batholith of possible Cretaceous age is found within the district and is probably related to the same period of tectonism. Regional Cenozoic structural and tectonic features include both intrusive and ash-flow magmatism that ranges from Oligocene to mid-Miocene in age, as well as the detachment extensional event of the Oligocene and the Basin and Range uplift of the Miocene. Intermediate to silicic ash-flow magmatism and related intrusive activity has been dated at around 40 Ma in the Independence Mountains. East-north-east striking normal, transform faults and bi-modal volcanism are apparently related to the 17 to 14 Ma extension event which produced the north-west trending North Nevada Rift which is broadly coincident with the Cortez-Battle Mountain Trend, 120 km to the south-west. The same volcanism and faults cut across the northern Independence Mountains (Daly, et al., 1990).
The stratigraphy of the Jerritt Canyon District may be summarised as follows, from the structurally lowest rocks (Daly, et al., 1990, Weideman, et al., 1990, Birak and Hawkins, 1986):
Ordovician, Eureka Quartzite, 310 m thick - comprising a lower and upper quartzite, separated by a carbonate sequence. The quartzites are typically massive, white and composed of medium grained, sub-rounded quartz in a silica cement, with scattered grains of pyrite. A few black siltstone interbeds up to 6 m thick occur within it as do some black quartzite bands. The massive quartzites are usually present as beds up to 3 m thick, although cross bedding is evident locally. Silica ±barite boxworks occur along fractures and within breccia zones. The Eureka Quartzite is only found within the southern parts of the Jerritt Canyon Window in the Jerritt Canyon district.
Ordovician to Silurian, Hanson Creek Formation, up to 230 m thick - which has been sub-divided into 5 members, as follows, from the base:
• Basal Member - chert, limestone and laminated calcareous siltstone. This unit is a poor gold host and is typically poorly exposed, occurring as a discontinuous sequence of laminated, silty limestone to calcareous siltstone with lesser interbeds of black chert and carbonaceous micrite. It generally ranges from 0 to 38 m thick, but may locally be up to 120 m.
• Member 4 - medium bedded (about 5 cm thick beds), medium to coarse grained carbonaceous limestone with abundant irregular shaped pods and lenses of black chert which may be up to 30 cm thick. This member commonly overlies the Eureka Quartzite directly. It is 30 to 40 m thick at Enfield Bell, averages 76 m in thickness regionally, but is locally up to 138 m. Member 4 is only a minor gold host at Jerritt Canyon.
• Member 3 - a sequence of interbedded 2.5 to 5 cm thick, black, carbonaceous, micrite beds, and laminated, very carbonaceous, argillaceous dolomitic limestone beds which are generally <2.5 cm thick. This unit is known as the banded limestone. It locally contains chert beds. Member 3 is one of the major hosts to gold mineralisation at Jerritt Canyon. The micrite is very fine grained and consists of 70 to 90% calcite, up to 15% dolomite, minor organic carbon and accessory pyrite. The argillaceous dolomitic limestone is commonly laminated and contains significant amounts of organic carbon and pyrite which help define the laminations. Member 3 is poorly exposed and has a variable thickness. It is at least 90 m thick, although it is commonly stratigraphically or structurally truncated by the Roberts Mountains Formation along the Saval Discontinuity in the Jerritt Canyon district. In the Enfield Bell mine area the upper sections of this member have been intensely silicified.
• Member 2 - a medium bedded, dark grey, fine grained limestone, which may be thick bedded, wavy thin bedded to nodular, wavy thin bedded, or, rarely oolitic. It is approximately 33 m thick, but usually does not reach this thickness. This member may be entirely truncated by the Saval discontinuity, but otherwise generally averages 12 to 18 m in thickness. Throughout much of the district the contact with the underlying member 3 is gradational. Member 2 has suffered varying degrees of silicification and acid leaching and is usually altered to light grey, 'sandy' dolomite. It is fossiliferous and contains a variety of fauna and intra-clastic beds with well rounded peloids up to 1 cm in diameter. The base of the member is usually well laminated. Member 2 is a minor host to gold mineralisation at Jerritt Canyon.
• Upper Member - alternating interbedded black chert and fine grained, carbonaceous limestone. This unit is widely exposed throughout the district, preserved because of its pervasive silicification. It varies in thickness from 0 to 37 m and unconformably overlies member 2 with a slight angularity. Bedding thicknesses average 5 cm, although individual chert beds may be up to 30 cm thick, and limestone 15 cm. Locally the upper sections contains silty, laminated limestone beds similar to those of the overlying Roberts Mountains Formation. Lensing and stretching of chert beds is common, and intra-formational truncation surfaces are well preserved in silicified rocks. The Upper Member is a minor host to gold mineralisation at Jerritt Canyon.
Two distinct sequences of Hanson Creek Formation are mapped in the Independence Mountains. These are the Burns Creek Sequence which is characterised by abundant secondary silicification and development of jasperoids, while rocks of the relatively un-altered Smith Creek sequence are typically more clastic rich and argillaceous rocks verging on calcareous shale.
Silurian to Devonian Roberts Mountains Formation, 100 to 200 m thick regionally, but averaging 75 m at Enfield Bell - exposures are generally limited to thin, silicified outcrops that occur directly above the Hanson Creek Formation, and minor lenses of laminated limestone known locally as the "resistant unit". The lower contacts of the Roberts Mountains Formation with the Hanson Creek Formation is either a disconformity or a fault, or both. This contact is known as the Saval discontinuity. The upper contact is commonly either the Roberts Mountains Thrust, a younger thrust, or an unconformity with younger rocks. The Roberts Mountains Formation is generally a mono-lithologic unit that comprises laminated calcareous siltstone, which is tan or grey when oxidised or weathered. Intercalations of laminated silty limestone (the "resistant unit") occur at various levels throughout the formation. The laminations in this lithology are 0.5 to 1 mm thick and are marked by increased iron oxides. It tends to break into 2.5 to 5 cm blocks.
Un-oxidised parts of the formation are black, pyritic and have less well defined laminations.
The calcareous siltstone consists of varying amounts of quartz (20 to 60%) and nearly equal quantities of calcite and dolomite, 5 to 15 % sericite-illite, pyrite-iron oxide, stringers of "non-carbonate" carbon, feldspar, chlorite, a rather mature heavy mineral suite and minor calcareous cement. Quartz grains are sub-angular to sub-rounded, moderately well sorted and 0.05 to 0.35 mm in size. Calcite grains are generally subhedral and average 0.07 mm. Dolomite is considerably coarser and commonly occurs as euhedral rhombohedrons dispersed throughout the rock. Sericite flakes are as much as 0.13 mm in length and wrap around quartz grains. Pyrite occurs as 0.05 to 0.1 mm cubes distributed along laminations in un-oxidised rocks, while when oxidised, hematite or goethite are the dominant Fe species. Carbon occurs as stringers and tendrils concentrated along laminae and around grain boundaries.
The silty limestone of the "resistant member" differs from the calcareous siltstone in the relative amounts of carbonate versus quartz and the increased content of calcareous cement.
The Roberts Mountains Formation is one of the major hosts to gold mineralisation at Jerritt Canyon.
Devono-Carboniferous Roberts Mountains Thrust. The overlying allochthonous Western, or Siliceous Assemblage, which locally comprises the Ordovician Snow Canyon Formation, McAfee Quartzite and Jacks Peak Formation, was thrust eastward over this structure during the Devono-Carboniferous Antler Orogeny.
Ordovician, Snow Canyon Formation, up to 1900 m thick regionally, but around 300 m at Enfield Bell - this is the dominant upper plate unit in the Jerritt Canyon district. It comprises a basal limestone-greenstone complex, variegated chert, argillite, siltstone, shale, bedded barite and poorly bedded lenses of quartzite. In sections of the Independence Mountains it has been subdivided into a lower member of mudstone, sandstone and chert; a middle member composed primarily of various basaltic rocks; and an upper member of 'turbidite facies'. At Enfield Bell it is described as being a very carbonaceous, slightly calcareous, attenuated shale and fissile siltstone with lesser amounts of dolomitic siltstone, varicoloured cherts and propylitically altered mafic lavas and scattered coeval, basaltic dykes. Minor quartzite occurs near the top of the formation. The poorly preserved bedding of the rocks, the ubiquitous minor shears and small discontinuous carbonate and quartz veinlets are the result of diagenesis, low grade metamorphism and tectonism.
Ordovician, McAfee Quartzite, 300 m thick - which conformably overlies the Snow Canyon Formation. It is composed dominantly of thick bedded, massive quartzite and graptolitic shale.
Ordovician, Jacks Peak Formation, 110 to 150 m thick - comprising a lower chert member and an upper quartzite. It apparently conformably ovals the McAfee Quartzite. Some authors consider this unit to be a thrust sheet of McAfee Quartzite.
Possibly Tertiary, Dykes and Sills - composed of olivine basalt and monzodiorite.
Tertiary Extrusives.
The Eureka Quartzite, Hanson Creek Formation and Roberts Mountains Formation belong to the autochthonous Eastern, or Carbonate Assemblage, while the Snow Canyon Formation, McAfee Quartzite and Jacks Peak Formation are members of the Western, or Siliceous Assemblage. The latter correlate with the Valmy and Vinini Formations of the Carlin, Cortez-Battle Mountain and Getchell Trends described respectively in sections 5.2, 5.6 and 5.10.
Structure
Faulting is common in the Enfield Bell mine area and is the most important structural feature related to mineralisation. Folding may have been contemporaneous with a pre-mineral set of faults. There are four sets of faults in the Enfield Bell mine area. Movement on these structures ranges in age from Devonian to Tertiary. The oldest are the Devono-Carboniferous Roberts Mountains Thrust and its associated sympathetic low angle normal and reverse faults which are found cutting both the upper and lower plate assemblages (Birak & Hawkins, 1986).
The next younger major set within the mine area are the east-west faults. These are cut in turn by younger north-west and north-east trending faults. The ages of these three sets is equivocal, although the east-west set seems to have first been formed during the Mesozoic with a Tertiary over-print. One such east-west structure is the Snow Canyon Fault which occurs immediately to the north of the mine. Smaller scale east-west structures within the mine influence the shape and disposition of the orebodies. The main examples are the Marlboro Canyon, Bell and Lower North Generator Hill faults. These faults commonly juxtapose rocks of both the Hanson Creek and Roberts Mountains Formation in the footwall against lithologies of the Roberts Mountains Formation in the hangingwall. In general the dip of these faults flattens with depth (Birak & Hawkins, 1986).
After the development of east-west faulting, the high angle north-west and north-east sets were formed during Basin and Range tectonism, with the north-east set possibly being the youngest. However, locally, north-west faults also cut both other sets. Initial activity on all of the faults appears to predate mineralisation. Ore coincides with faults and blossoms at fault intersections (Birak & Hawkins, 1986).
The MAP Anticline, a south-west plunging antiform between the North Generator and Marlboro Canyon orebodies is believed to have formed as a result of thrusting. Its western limb strikes WNW and dips SSW, whereas the eastern limb strikes NNE and dips ESE. Dips on each limb range from 25 to 65, with the eastern being the steeper. The fold limbs are delineated by dipping beds of jasperoid that are clear on aerial photographs (Birak & Hawkins, 1986).
See also the second part of this description, Jerritt Canyon - Mineralisation.
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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Hofstra A H, Snee L W, Rye R O, Folger H W 1999 - Age constraints on Jerritt Canyon and other Carlin-type Gold deposits in the western United States - relationship to mid-Tertiary extension and magmatism: in Econ. Geol. v94 pp 769-802
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Peters S G, Armstrong A K, Harris A G, Oscarson R L, Nobel P J 2003 - Biostratigraphy and structure of Paleozoic host rocks and their relation to Carlin-type Gold deposits in the Jerritt Canyon Mining District, Nevada: in Econ. Geol. v98 pp 317-337
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Robbins E I, DAgostino J P, Haas Jr. J L, Larson R R and Dulong F T, 1990 - Palynological assessment of organic tissues and metallic minerals in the Jerritt Canyon gold deposit, Nevada (U.S.A.): in Ore Geology Reviews v5 pp 399-422
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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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