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Cortez, Pipeline, South Pipeline, Crossroads, Cortez Hills, Cortez Pediment, NW Deeps, Deep South, Gold Acres |
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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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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Cortez, Pipeline, South Pipeline, Crossroads, Cortez Hills, NW Deeps, Deep South and Cortez Pediment gold deposits are located within Lander County in north-central Nevada, some 55 km to the south-east of the town of Battle Mountain, 113km SW of Elko and 95 km to the SSW of the Carlin gold mine. Pipeline, South Pipeline and Crossroads are approximately 8 km NW of the original Cortez pit. The Cortez mine and mill is 13 km to the south-east of the Gold Acres deposit and 5.8 km to the north-west of the Horse Canyon orebodies. Cortez Hills and Cortez Pediment are around 1.5 km SE of the original Cortez pit (Radtke, et al., 1987).
(#Location: Pipeline - 40° 14' 54"N, 116° 42' 58"E; Cortez - 40° 11' 33"N, 116° 36' 52"E; Cortez Hills - 40° 10' 10"N, 116° 36' 27"E)
These orebodies are located in a corridor within the Cortez Window, an exposure of autochthonous Eastern, Carbonate Assemblage rocks some 13 x 2.5 to 4 km in area, surrounded on three sides by upper plate, allochthonous Western, Siliceous Assemblage rocks. The ore is hosted by the Eastern Assemblage carbonates which are bounded by the Devono-Carboniferous Roberts Mountains Thrust above and on three sides. To the north a major normal fault truncates the window. Quaternary and Tertiary sediments are found to the north of this fault.
Published reserve and production figures include:
3.2 Mt @ 9.8 g/t Au = 31 t Au (Cortez production, 1968-73, Radtke, et al., 1987).
3.1 Mt @ 9.6 g/t Au (Cortez initial reserve, 1967, McFarland et al., 1991).
103 Mt @ 1.7 g/t Au = 174 t of contained gold, Proved + Probable Reserve, Cortez operation
31 December, 1999 (Placer Dome Ann. Rep, 1999).
139 Mt @ 1.61 g/t Au = 169 t Proved + Probable Reserve, Cortez/Pipeline operation
31 December, 2000 (Rio Tinto Ann. Rep, 1999). Reserves additional to Resources..
141 Mt @ 1.2 g/t Au = 169 t Measured + Indicated Resource + Inferred, Cortez/Pipeline operation
31 December, 2000 (Rio Tinto Ann. Rep, 1999).
234 Mt @ 1.4 g/t Au (Pipeline+Cortez Hills+Cross Roads total reserves, 2005, Rio Tinto, 2006).
137 Mt @ 1.6 g/t Au (Pipeline+Cortez Hills+Cross Roads total resources, 2005, Rio Tinto, 2006).
Reserves additional to Resources.
293 Mt @ 1.6g/t Au (Pipeline & South Pipeline proven and probable reserves, 2005,
Rio Tinto website, 2007).
Production in 2005 amounted to 28 t Au (Rio Tinto Annual Rept. 2006).
Pipeline Complex (Pipeline, South Pipeline, Crossroads, Gap and Cortez Hills) are estimated to have
originally contained: 715 t of gold (Wikipedia; USGS).
Gold Acres Refractory Ore 31 December 2018 - 8.07 Mt @ 2.88 g/t Au (Miranda et al., 2019,)
Remaining Mineral Resources at 31 December, 2018 were (Miranda et al., 2019, NI 43-101 Technical Report for Barrick Gold Corp):
Cortez Hills Complex - Breccia, Middle, Lower and Deep Zones - Underground
Measured + Indicated Resource - Underground
Mill Ore - 1.003 Mt @ 10.01 g/t Au; Refractory Ore - 2.028 Mt @ 9.74 g/t Au; TOTAL - 3.031 Mt @ 9.81 g/t Au
Inferred Resource - Underground
Mill Ore - 0.329 Mt @ 5.51 g/t Au; Refractory Ore - 1.863 Mt @ 6.82 g/t Au; TOTAL - 2.193 Mt @ 6.75 g/t Au
Inferred Resource - Open Pit
Leach Ore - 0.003 Mt @ 0.27 g/t Au
TOTAL contained gold in Measured + Indicated + Inferred Resource = 44.5 t.
Pipeline Complex - Pipeline and Crossroads - Underground
Measured + Indicated Resource
Mill - 3.646 Mt @ 2.26 g/t Au; Leach - 29.298 Mt @ 0.24 g/t Au; Refractory - 6.182 Mt @ 3.33 g/t Au; TOTAL - 39.126 Mt @ 0.89 g/t Au
Inferred Resource
Mill - 0.046 Mt @ 3.77 g/t Au; Leach - 6.621 Mt @ 0.27 g/t Au; Refractory - 0.066 Mt @ 5.18 g/t Au; TOTAL - 6.734 Mt @ 0.34 g/t Au
TOTAL contained gold - 37.1 t.
Remaining Ore Reserves at 31 December, 2018 were (Miranda et al., 2019, NI 43-101 Technical Report for Barrick Gold Corp):
Open pit Pipeline, Crossroads, Cortez Hills
Proved + Probable Reserves - 126 Mt @ 1.0 g/t Au = 126 t of contained gold;
Underground Middle, Lower and Deep South zones
Probable (only) Reserves - 11.389 Mt @ 10.63 g/t Au = 121 t of contained gold;
Stockpiles Mill + Leach + Refractory
Proved (only) Reserves - 7.665 Mt @ 3.28 g/t Au = 25 t of contained gold;
TOTAL Ore Reserves - 145.054 @ 1.87 g/t Au = 271 t of contained gold.
Mineral Resources are exclusive of Ore Reserves - total contained gold in Ore Reserves and all Mineral Resources at Dec 2019 = 352.8 tonnes
Mining in the Cortez area dates back to 1862 with the discovery of rich silver bearing float on the side of Mt Tenabo, some 7 km to the south-east of the present Cortez Gold Mine. This discovery led to the development of the Cortez silver mines in 1864. Operations persisted at these mines until 1903, producing silver lead and zinc. Most production was from the Garrison (or Cortez Silver) Mine between 1864 and 1895. The mines were intermittently active again from 1919 to 1930 with a cyanide and later a flotation mill recovering silver and gold from ore and dumps. Intermittent small scale operations continued until 1937 when the Cortez Metals Co. was formed to take over the properties. The total metal production of the Cortez district from 1862 to 1958 was $US 14 m in gold, silver, lead and zinc (Radtke, et al., 1987; McFarland and Kirshenbaum, 1991).
In 1959 the American Exploration and Mining Co (Amex), a wholly owned US subsidiary of Placer Development Limited, entered into a joint venture with the Cortez Metals Co. and commenced exploration of the old mine workings and surrounding areas. An agreement was entered into with both Webb Resources and Idaho Mining Corp. which had separately and respectively investigated and acquired ground adjacent to that controlled by Amex. In 1964 the Cortez Joint Venture was formed between Amex, The Bunker Hill Company, V F Taylor and Webb Resources to pursue further exploration (McFarland and Kirshenbaum, 1991).
Regional geological studies of the Cortez district by the US Geological Survey from 1959 and into the early 1960's indicated that anomalous amounts of metal were concentrated in siliceous rocks above the Roberts Mountains Thrust and in carbonates below the same thrust. This work had detected anomalous As, Sb and W in jasperoids in the Wenban and Roberts Mountains Formation limestones. When it was realised that these metals were associated with Au and Hg in jasperoids on the Carlin trend, the anomalous samples were assayed for those elements also. Anomalous amounts of Hg, up to 'a few' ppm, were detected, with corresponding Au maxima of 14 ppm in surface samples and 8 ppm in heavy mineral concentrates from drill cuttings. These samples defined a new area along the mountain front north-west of the old Cortez silver mines. A further 238 follow-up samples were subsequently collected and assayed for Au. Of these, 38 contained >0.3 ppm Au (300 ppb), 5 had between 34 and 102 ppm Au, and 2 exceeded 102 ppm Au (Wells, et al., 1969).
What was to become known as the 'discovery outcrop' over the Cortez orebody returned a rock chip value of 3 ppm Au. It was a red to grey, altered, silty carbonate. The anomalous 3 ppm Au sample was from a fault breccia within that outcrop which had a deep red colour. This colour was quoted as being un-impressive, as it was similar to the colour of many outcrops in the district, most of which are un-mineralised. The gold was all too fine to be panned from surrounding drainages (Wells, et al., 1969).
This anomalous zone was located in a largely gravel covered area on claims controlled by the Cortez Joint Venture. It prompted an extensive surface sampling and drilling program by Placer Amex, on behalf of the Cortez Joint Venture, that led directly to the discovery of the Cortez deposit (Radtke, et al., 1987). Drilling commenced in the anomalous area in September 1966 and by early 1967 sufficient data had been collected to indicate the existence of a significant tonnage of low grade open pit ore. By the end of 1967 the reserve quoted above had been delineated (McFarland and Kirshenbaum, 1991).
Following a feasibility study, a decision to mine was made in March 1968 and pre-production stripping started in the same year. In January 1969 a 1550 tpd standard counter-current decantation cyanide mill went into operation and the first shipment of bullion was despatched in mid February. The Cortez orebody was exhausted in 1973 with production continuing until 1976. The mill ore cutoff in that period had been 1.4 g/t Au (McFarland and Kirshenbaum, 1991).
In 1973 additional reserves were located at Gold Acres, which had originally been discovered in 1922 and mined by open pit and underground mines in the 1930s and 1940s until 1965 (Radtke, et al., 1978), while the Horse Canyon deposits were discovered in 1976 (Foo and Herbert, 1987). Mill grade ore from these deposits was treated at the Cortez Mill. The United States Bureau of Mines built a pilot plant in 1969 which would make heap leaching for gold commercially viable by 1971. Heap leaching commenced at Cortez in 1971, and continued through the 1970's, with a cut-off grade of 0.5 g/t Au, and an extraction rate of 65% from the blocky Cortez ore stacked in both 6 and 9 m lifts (Radtke, et al., 1987; McFarland and Kirshenbaum, 1991). The low grade ore and dump material from the Gold Acres operation were heap leached during this period.
Following the surge in gold prices in 1979-80, heap leaching of the waste dumps at Cortez was commenced during 1980. Feed for the mill was also provided by the nearby Horse Canyon deposit between 1983 and 1987. Recovery was 45% in the first 90 days and a further 10% in the second such period (McFarland and Kirshenbaum, 1991).
Further exploration of the region was carried out by Cortez Joint Venture, which by then involved Placer Dome and Kenecott (at the time Kenecott was a BP Minerals subsidiary). The result of the joint venture's exploration was the 1991 discovery of the Pipeline deposits below alluvial cover 2 to 3 km to the north-east of Cortez in and delineated over the following years. The Pipeline orebody is near the Gold Acres operation, instead of being located in the foothills, it was entirely within the Crescent Valley. The Pipeline orebody was much larger than the original orebodies in the region. The entire Pipeline complex contained 715 tonnes of gold between the original orebodies, the Pipeline orebody and the later discoveries of South Pipeline, Crossroads and Gap. The Cortez Joint Venture built the Pipeline Mine and mill in early 1996 following a series of ownership disputes with junior mining companies. The Pipeline mine had an annual output between 1998 and 2005 of >30 t of gold per annum. The Cortez Hills deposit was discovered in 2002, followed by underground development to define the resource limits which began in 2006. Barrick Gold acquired 60% in Cortez in 2006 when it purchased Placer Dome and acquired the remaining 40% from Rio Tinto in March 2008. Production from the Pipeline Complex mines was from 1994 to 2011. The Cortez mines are now (2021) operated by Nevada Gold Mines LLC, a joint venture between Barrick Gold Corporation (61.5%) and Newmont Corporation (38.5%).
Geology
The orebodies of the main Cortez deposit were hosted within the upper sections of the Siluro-Devonian Roberts Mountains Formation carbonates, below the shallow dipping, folded and undulose, Devono-Carboniferous Roberts Mountains Thrust sheet. These carbonates are exposed where the capping Roberts Mountains Thrust has been up-domed and up-faulted, and the underlying rocks are exposed by erosion to form the Cortez Window. Also exposed within this window are the underlying Cambrian Hamburg Dolomite, and the Eureka Quartzite and Hanson Creek Formation, both of Ordovician age. Overlying the Roberts Mountains Formation within the Cortez Window is the Devonian Wenban Limestone. All of these are intruded by the Jurassic Mill Canyon Quartz Monzonite/adamellite (150 ±3 Ma) and by Oligocene biotite-quartz-sanidine porphyry (38 Ma) dykes and sills. The Roberts Mountains Thrust is overlain by the Ordovician Valmy and Vinini Formations, the Silurian Fourmile Canyon Formation and the Devonian Slaven Chert, which have been thrust eastward onto the time equivalent autochthonous, carbonate assemblage. These are all overlain in turn by the Oligocene Caetano Tuff, which is apparently comparable in age to the intrusive biotite-quartz-sanidine porphyry. Younger basaltic-andesite flows, rhyolitic flows and a related plug are found to the east, while extensive unconsolidated Tertiary and Quaternary sediments mask exposure in low lying areas (Radtke, et al., 1987; S Foo, Pers. comm., 1993). For detail of the regional geological setting, see the 'Battle Mountain - Eureka Gold Trend - Geology' record.
The main stratigraphic units within the Cortez mine area are as follow:
• Siluro-Devonian, Roberts Mountains Formation, ~300 m thick - laminated, black, silty, graptolite-bearing limestone. The upper part is a thinly laminated, dark grey to light grey, dolomitic siltstone, calcareous siltstone and silty limestone that contain some carbon and locally show scour, graded bedding and cross-bedding. Pyrite cubes and aggregates of cubes less than 6 mm across occur throughout the unit and show no genetic relationship to igneous bodies or mineralised areas. Generally the most abundant pyrite is within the coarser grained silty layers. The pyrite grains are euhedral and much coarser than the silt grains, indicating that they are diagenetic (Wells, et al., 1969).
• Devonian, Wenban Limestone, 880 m thick - a massive, thin bedded to argillaceous and bioclastic, grey limestone. The lower part, found in the mine area, is around 30 m thick and comprises a thin bedded limestone, similar to the Roberts Mountains Formation limestone, interbedded with medium bedded and bioclastic limestone. The contact between the Wenban Limestone and the Roberts Mountains Formation is placed by Gilluly and Masursky (1965) at the base of the lowest bioclastic limestone above the thin bedded, grey, pyritic, graptolite-bearing Roberts Mountains Formation (Wells, et al., 1969). The Wenban Limestone is equivalent to the Popovich Formation in the Lynn Window on the Carlin Trend.
• Devonian, Horse Canyon Formation,, ~60 m thick - laminated calcareous siltstone, mudstones with interbedded chert and silicified siltstones.
Four periods of igneous activity are recognised in the Cortez Window by Gilluly and Masursky (1965), as follows;
• Jurassic, Mill Canyon Stock, 150±Ma - of a quartz-monzonite (ie. adamellite) composition which occurs to the east of and possibly below the orebody. Low grade contact metamorphism surrounds the intrusive (Wells, et al., 1969; Radtke, et al., 1978). The dominant rock of the stock is a biotite-quartz monzonite, although it ranges in composition from quartz-diorite to alaskite. Fresh exposures of the dominant rock type are light grey with a pepper and salt texture. Some are porphyritic, with phenocrysts of plagioclase up to 3 mm in diameter set in an equigranular groundmass whose grain size is perhaps 0.5 mm or smaller. Biotite is conspicuous, although the K-feldspar (perthitic-microcline in thin section) is only suggested by a flesh-pink hue to the fine grained matrix of white plagioclase. Quartz is abundant, but not in phenocrysts. Accessories are magnetite, zircon and minor apatite. Much of the rock is mildly altered so that the biotite is partly replaced by muscovite and chlorite and the plagioclase by sericite and minor zoisite (Gilluly and Masursky, 1965). This stock is essentially of the same age as the older sections of the Goldstrike Stock on the Carlin Trend, between the Post/Goldstrike and Blue Star/Genesis orebodies.
• Oligocene, Caetano Tuff and Biotite-Quartz-Sanidine Porphyry Dykes - which have variously reported dates. S Foo (Pers. comm., 1993) stated that the 'dykes' are 38 Ma, while Wells, et al., (1969) reported 34 Ma and Foo and Herbert (1987) suggest 35.7 to 33.7 Ma. Radtke et al., (1978) quote dates ranging from 31.0 to 33.6 Ma for the Caetano Tuff, while Foo and Herbert (1987) gave 32.7 to 32.3 Ma. The Caetano Tuffs are composed of water laid rhyolitic tuffs, together with lesser amounts of andesitic tuff, sandstone and conglomerate (Radtke, et al., 1987). The biotite-quartz-sanidine porphyry is present as both dykes and sills, and is apparently post ore in age as it is un-mineralised, has a remobilised selvage of mineralisation up to 5 m wide and includes clasts of mineralised sediment (D Bernosky, Pers. comm., 1993). Where seen in the pit it was very variable in colour, texture and composition (Pers. observ., 1993). Radtke, et al., (1978) however reported that the margins of the porphyry dykes have undergone argillic alteration and are weakly mineralised. According to Rytuba (1985), the tuffs and dykes are both part of a caldera related volcanic event. The dykes and sills at Cortez are the same age as the main stock at Battle Mountain (H Bonham, Pers. comm., 1993). They are also of a similar age to the biotite-feldspar porphyry dykes found within the Carlin Trend, in particular the younger phase of the Goldstrike Stock at Post/Goldstrike.
• Basaltic Andesite Flows and Associated Dolerite Dykes, of Pliocene age, in the range 16.7 to 15.9 Ma (Wells, et al., 1969; Radtke, et al., 1978; Foo and Herbert, 1987).
• Rhyolite Plugs and Flows, of Pliocene to Pleistocene age, occurring to the east of the Cortez Window, dated at 15.0 to 14.0 Ma (Wells, et al., 1969; Radtke, et al., 1978; Foo and Herbert, 1987).
Deposit Geology
The geology of the other individual deposits and complexes may be summarised as follows (after Miranda et al., 2019, NI 43-101 Technical Report for Barrick Gold Corp):
Pipeline Complex - Pipeline, Pipeline South, Crossroads
The three deposits are separate zones of a single gold-mineralised system, that collectively strike at ~340° for >2500 m and up to 1500 m in width, east-west. Economic gold grades are not found across the full extent of this system, although variable degrees of hydrothermal alteration are evident throughout, including oxidation, decalcification, weak contact metamorphism and argillic, silica and carbonate alteration. Mineralisation within the complex occurs just outside of the metamorphic aureole that is associated with the Gold Acres intrusive.
Alluvial cover varies across the deposit, from being absent in the NW, but thickens up to 140 m in the eastern Pipeline Pit area, and ranges from 95 to 235 m over the Crossroads deposit in the south.
The geology of these deposits is characterised by folded and low-angle faulted Palaeozoic carbonate rocks. The primary hosts are variably altered, thin- to thick-bedded, carbonate turbidites, debris flows, micrites and silty limestones of the Devonian Wenban Limestone and thin-bedded, planar-laminated calcareous siltstones, mudstones, inter-bedded chert and silicified siltstones of the overlying Devonian Horse Canyon Formation. At depth, planar laminated, silty limestones of the Silurian Roberts Mountains Formation also host mineralisation.
The pre-existing porosity of the turbidites, siltstones and silty limestones were enhanced through argillic alteration and carbonate dissolution via structural and stratigraphic controls. Thrust and normal faulting shattered the more brittle cherty and silicified beds, also creating a secondary porosity. The strongest and most continuous gold grades occur in the inter-bedded cherts and silicified turbidites of the Horse Canyon Formation and in the Wenban Formation, either where capped by the Horse Canyon Formation or in areas of more intense carbonate dissolution. Host formations have also been thickened and repeated by low-angle thrusting largely associated with the Late Devonian Antler Orogeny.
Gold Acres
This pit is centred along the axis of a low-amplitude, NNW trending antiform, where the primary hosts to mineralisation are sheared Upper Plate siliciclastics and greenstones of the Ordovician Valmy Formation and cherts and quartz siltstone of the Devonian Slaven Formation, sheared Lower Plate silty limestone with discontinuous thin phosphatic black lenses of the Silurian Roberts Mountains Formation and micrite to silty micrite of the Devonian Wenban Limestone.
The intensity of thermal metamorphism related with the Gold Acres Stock is variable, dependent upon the original lithology, ranging from hornfels to calc-silicate marble to skarn. The pluton is apparently 120 to 180 m below the current (2019) Gold Acres pits.
Two discrete bands, known as 'Upper Skarn' and 'Lower Skarn', have been delineated at Gold Acres. The former is a bleached felsic sill-like body with endoskarn development, presumed to be associated with the Jurassic to Cretaceous (104 to 150 Ma) granodioritic Gold Acres Stock. The 'Lower Skarn' comprises a garnet–diopside assemblage believed to have be after the lower Wenban Limestone. The two skarn bands are separated by a 25 to 60 m thick zone comprising slices of Upper and Lower Plate rocks known as the Imbricate Thrust Zone.
NOTE: This brief summary is from Miranda et al. (2019) while the more extensive summary at the main Gold Acres record comes from a more extensive review in 1996 and may reflect the evolution of ides and observations.
Cortez Hills Complex - Cortez Hills Breccia and the Middle and Lower Zones, and Cortez Pits and NW Deeps
In the Cortez Hills Breccia and the Middle and Lower Zones of the Cortez Hills deposit, the upper mineralised levels are hosted within the Horse Canyon Formation, while the bulk of the remainder lies within the Devonian Wenban Limestone. Mineralisation is also hosted at depth by the Roberts Mountains Formation as well as Hanson Creek Dolomite. The major Cortez Fault, which bounds the Cortez Range, is located just to the east of the deposit.
Breccia gold mineralisation appears to have been emplaced in hydrothermally brecciated and fractured rocks centred on a NW striking and moderately SW dipping fault, known as the Voodoo Fault, and its associated structures. The stratigraphy at depth has been deformed by thrust faulting resulting to both folding and fracturing, with gold mineralisation occurring as tabular, sub-horizontal to shallow dipping zones associated with calcareous rocks subject to preparation by alteration and deformation. This deeper mineralisation forms the Middle and Lower Zones at Cortez Hills. Post-mineral quartz porphyry dykes and sills intrude the Cortez Hills deposits. A NW trending swarm of steeply dipping dykes defines the limits between the Middle and Lower Zones.
In the Cortez Pits and NW Deeps sections of the Cortez Hills deposit, mineralisation is hosted by strongly altered, thin- to medium-bedded silty limestone of the Roberts Mountains Formation and by sheared and altered interbedded dolostone and limestone of the underlying Ordovician Hanson Creek Formation. The Devonian Wenban Limestone caps most ridges and hills around the NW Deeps deposit and locally appears to have acted as a cap rock over alteration systems in the underlying Roberts Mountains Formation. The deposit was originally covered by a thin veneer of Quaternary alluvium. This alluvium thickens abruptly to the north across the Cortez range-front normal fault.
The Jurassic Mill Canyon Stock quartz monzonites occur to the east of the deposit on the uplifted side of a NNW trending normal fault, whilst numerous, apparently post-mineral Oligocene quartz porphyry dykes and sills intrude the mineralisation. A series of NNW and NE trending faults cut the Roberts Mountains Formation in the deposit area. Mineralisation is found where these faults intersect shallow east dipping thrust breccia zones (thrust duplexes) within the Roberts Mountains Formation.
The southern, deep extension of the Lower Zone of Cortez Hills Complex is known as the Deep South section of the deposit.
Structure
Thrust faulting during the Antler Orogeny moved an allochthonous slice of clastic, siliceous sedimentary and volcanic rocks eastward over time correlative rocks during the Devono-Carboniferous. The main Thrust package over which this movement took place was the Roberts Mountains Thrust. A projection of this thrust would place it well above the Cortez gold deposit (Radtke, et al., 1987; Wells, et al., 1969). H Bonham (Pers. comm., 1993) believes that the fracturing associated with the movement on the Roberts Mountains Thrust has been important in 'ground preparation' and the localising of mineralisation in that part of the sequence above and below its trace.
Folding in the area has formed many anticlines and synclines, although no regional pattern had been discerned in 1969. Many complex minor folds are known within the vicinity of the Cortez deposit, although the beds dip generally to the east or north (Wells, et al., 1969). Foo and Herbert (1987) however advise that northerly trending folds are common in the area and are thought to be contemporaneous with thrusting. These folds are evident throughout the upper and lower plates. Large scale folding was accentuated by doming associated with the intrusion of the Jurassic Mill Canyon Stock. The Cortez Widow represents a broad NNW trending and SSE plunging antiform within the Roberts Mountains Thrust surface (Madrid and Roberts, 1991).
Several periods of faulting are evident at Cortez. Pre-Oligocene north-west trending high angle faults have controlled the emplacement of subsequent Oligocene dykes and sills. Post-Oligocene high angle faulting and subsequent erosion have strongly affected the location of the Cortez Window. The major structural features responsible for the main relief are the Basin and Range faults. The major Cortez Fault is one of these. It trends NNW along the core of the antiform forming the Cortez Window, and is reflected by a steep scarp along the south-west end of the Cortez Mountains. Normal movement of approximately 900 m down-dropped the west side, containing the Cortez deposit, relative to the eastern block. The other major fault, the north-east trending Crescent Fault, which has a down-drop to the north-west of approximately 3000 m, bounds the Cortez Window in that direction. This latter fault cuts off both the orebody and the Cortez Fault. Tilting of the 14 to 10 Ma basaltic andesite flows and very limited outwash fans show that movement on the Crescent Valley Fault was as recent as Pliocene or Pleistocene (Radtke, et al., 1987; Wells, et al., 1969).
Mineralisation and Alteration
The immediate Roberts Mountains Formation host to ore at Cortez is a characteristic thin to medium bedded, dark-grey, argillaceous, siliceous, calcareous dolomite, as distinct from the more calcareous un-mineralised Wenban Limestone. In addition to dolomite and calcite, the rocks contain large amounts of fine grained quartz and illite, as well as minor quantities of kaolin, chlorite, K-feldspar, sphene, pyrite and carbonaceous material. Upon weathering and oxidation the rocks take on a shaly character and a distinct light grey to tan colour. Oxidation at the deposit and in adjacent wall rocks is extensive, and only a few small zones of apparently fresh, dark-grey carbonate rocks remain. On the basis of chemical analyses the host is made up of 30 to 40% dolomite, 20 to 30% calcite, 15 to 20% illite and 15 to 30% quartz (Radtke, et al., 1987). The diagenetic pyrite content of the upper Roberts Mountains Formation has a mean value of around 1.2% (Wells, et al., 1969).
The Cortez gold deposit is located where altered Roberts Mountains Formation hosts were faulted, brecciated and folded along the margins of the biotite-quartz-sanidine porphyry intrusive. The gold deposit cuts across the bedding along the intruded front of the thick sill like mass of porphyry. Considerable crenulation of bedding, strong drag-folding, jointing and crackling of the siltstone took place in this zone. The orebodies have long dimensions that correspond to the strike of faults, dyke filled faults and sills. The presence of pre-ore high angle normal faults and the wide-spread brecciation of the host rocks adjacent to the intrusive, which marks a strong competence contrast, are regarded as important pre-cursors to mineralisation. Further away from the intrusive the mineralisation only occupies certain altered beds. Post mineralisation faults disrupt the ore zone while the north-east trending Crescent Valley Fault offsets it (Wells, et al., 1969; Radtke, et al., 1987; D Bernosky, Pers. comm., 1993).
The normally dark-grey laminated carbonatic siltstone beds of the Roberts Mountains Formation have been variably bleached and leached over a large area surrounding and embracing the orebody. The more carbonatic Wenban Limestone was only mildly altered. The biotite-quartz-sanidine porphyry has also been bleached and altered to clay on its margins (Wells, et al., 1969).
The argillised fringes of the porphyry dykes were generally 0.2 to 1 m thick where observed in the Cortez pit. These fringes had a strong white clay development, but retained the texture of the un-altered porphyry. Mineralisation is slightly higher grade on the fringes of these dykes occurring as an interpreted remobilised halo. The dykes apparently contain clasts of mineralised sediments, indicating that they predate the introduction of the ore (D Bernosky, Pers. comm., 1993; Pers. observ., 1993).
Alteration, in the form of bleaching, leaching and oxidation has been most intense in the highly fractured limestones. This alteration has changed pyrite to iron oxide and mobilised minor amounts of Fe. The interface between the fresh, dark grey, un-altered rock and the altered zones is generally sharp. The altered rocks show a decrease in carbonaceous material and carbonate and a corresponding increase in porosity. This increased porosity associated with the decalcification (or de-carbonatisation) is accompanied by late chalcedonic silicification to form a jasperoid. The silicification cements and coats rock fragments within the decalcified sediment. The jasperoid is a semi-brittle rock ranging in colour from predominantly light grey with some greyish-brown, to red-brown. The greyish-brown variety breaks with a characteristic conchoidal fracture (Wells, et al., 1969).
The decalcified Roberts Mountains Formation, where observed in the pit, is a porous, silty to fine sandy, tan coloured rock which still 'fizzes' with acid. It is patchily silicified to a jasperoid which is grey in colour with a reddish tinge and does not 'fizz'. Within the ore zone there are zones of brecciated calcite veins which enclose angular fragments of country rock, some of which are fragment supported, while others are calcite supported. The oxidised decalcified host has a mottled, porous, grey-green-red colour, different in character from the un-altered well laminated (1 to 2 mm), grey, silty-limestone. Elsewhere on the margins of the pit grey limestone has white quartz veins from 1 mm to 1 cm thick which follow particular beds or 'wriggle' through the rock. The rocks in the pit appear to have undergone multiple stages of brecciation, veining, decalcification and silicification. As stated previously, the host rock in the ore zone appears to have been chaotically folded with irregular fold axes and folds with wavelengths down to a few metres (Pers. observ., 1993).
Studies indicate that gold, quartz, pyrite and other sulphides were deposited in carbonate rocks from low salinity solutions at temperatures of more than 175°. Chemical analyses indicate that, relative to the unaltered hosts, the ore has a decreased content of CaO, MgO and CO2, and an increase in SiO2. Although Al2O3 shows an increase, the argillic alteration is weak and no definitive data is available on mineral assemblages (Radtke, et al., 1987).
The silicified siltstone beds of the Roberts Mountains Formation contains the bulk of the gold mineralisation. However not all silicified rocks are gold bearing, while relatively un-silicified lithologies may contain significant gold values. The altered intrusive contains only trace amounts of gold, except along the contact with altered siltstone where values are higher. Fingers and lenses of silicified rock in the Wenban Limestone may contain minor amounts of gold (Wells, et al., 1969).
There is an apparent close relationship between Au and SiO2 within the orebody, except where multiple generations of silica are indicated. In a particular study, data indicated that a value of 126 ppm Au corresponded to 59% added silica, 20 ppm Au at 10% added silica and 0.1 ppm Au at 1% silica. A linear relationship is indicated between the carbonate removed and silica added, with up to 11% carbonate depletion before silica is added. The relationship of gold to faulting was also indicated as important by the same study, with jasperoids within a few metres of a fault usually containing large amounts of Au, whereas those located 15 m or more from a structure commonly contain <0.5 ppm Au (Radtke, et al., 1987).
The gold in the oxidised ore is present as native metal in micron to sub-micron sized particles. Particles as large as 10 µm and as small as 0.5 µm have been observed in polished section. Rarely grains as large as 100 µm (0.1 mm) have been encountered. The gold occurs as 1). clusters of particles between silt grains in siltstones (decalcified silty limestones), although none occur within original silt grains; 2). scattered grains in quartz veinlets; and 3). individual grains in hematite-goethite pseudomorphs after pyrite (Wells, et al., 1969).
Gold from deep un-oxidised refractory ores occurs as fine grained particles in pyrite, as coatings on pyrite grains and as sparse <1 micro;m grains locked in hydrothermal quartz. Near surface un-oxidised or weakly oxidised ores contain gold in the same forms. In the latter case however, it generally occurs in coarser grained particles (0.5 to 20 µm ) disseminated throughout partially silicified carbonate beds and intergrown with pyrite, and as grains of metallic gold up to 300 &mocro;m (0.3 mm) in quartz veinlets cutting partially replaced carbonate beds (Radtke, et al., 1987).
Analytical data indicate that the gold mineralisation was accompanied by an increase in As, Sb, Hg, W, Ba, Ag, B, Cu, Mo, Pb, Zn, Co and Ti. Gold in slightly altered rocks is of the order of 0.04 ppm (40 ppb), but increases to a maximum of 46 ppm in high grade ore (a more than 1000x upgrading). Rocks in the ore zone are also particularly enriched in As (up to 45x), Hg (up to 35x) and to a lesser extent in Sb (up to 7x) and W (up to 4x), relative to un-mineralised equivalents. Ba and Sr are depleted, and may have been removed during the decalcification. Calcite veins, some of which contain admixed barite, occur in fractures that cut the mineralised rocks. These may be related to late stage meteoric waters (Radtke, et al., 1987; Wells. et al., 1969).
Hydrothermal sulphide minerals which were introduced and occur as ubiquitous, fine grained and dispersed disseminations, include pyrite, arsenopyrite, pyrrhotite (and other FexSy minerals) and a very minor amount of realgar and stibnite which are found in deep, un-weathered zones. The organic carbon content varies from an average of 0.4% in the fresh un-mineralised host, although values of up to 3% are recorded. In the oxidised mineralised rock the average is 0.04%. In general the carbon appears to have been removed with the introduction of mineralisation (Radtke, et al., 1987; Wells. et al., 1969).
The orebodies at Cortez have been oxidised to a depth of at least 60 m. Hydrothermal sulphide minerals are partially or totally altered in situ to iron oxides and the small amounts of original carbonate that survived the decalcification were removed. In this oxidised ore, both the disseminated and veinlet oxidised ores contain gold surrounded by, and intergrown with quartz and iron oxides that resulted from late stage oxidation of hydrothermal sulphides. Many of the iron oxides contain significant amounts of As, with the gold commonly being concentrated in the zones of highest As (Radtke, et al., 1987).
The Pipeline, South Pipeline and Crossroads deposits are located approximately 8 km NW of Cortez. Submicroscopic gold particles are evenly distributed throughout carbonatic sedimentary host rocks, commonly in association with secondary silica, iron oxides or pyrite. The principal host units are the Silurian Roberts Mountains Formation and Devonian Wenban Limestone, which are overlain by Quaternary alluvium. Both deposits are principally hosted in sheared and variably altered thinly bedded calcareous siltstone (silty carbonate unit) of the Roberts Mountains Formation, which is >600 m thick in the deposit area.
Mineralisation at Pipeline occurs where an east dipping thrust duplex crosses a deep seated 305° striking fracture system. The bulk of the mineralisation is tabular and stratabound with a shallow easterly dip.
The main Pipeline deposit is a 15 to 90 m thick, tabular zone at 150 to 180 ft below the surface, dipping at a low angle to the east, covering an area of 230 m north-south x 450 m east-west.
The South Pipeline comprises two zones i). a shallow zone starting at ~20 to 45 m depth and ii). a deeper zone that begins ~300 m below the pre- mining surface. The shallow zone occupies an area of ~550 x 600 m, north-south and east-west respectively, and has both low-angle and high-angle structural controls on gold distribution. The deep zone covers an area of ~60 x >180 north-south and east-west respectively, is up to 75 m thick, and is more closely associated with high-angle structures. The combined mineralisation thickness ranges from 120 to >300 m in thickness.
Crossroads occurs at the southern end of the Pipeline trend and is deeper, varying from <3 to >90 m in thickness, with ore primary control being low-angle structures sub-parallel to bedding with an overall 20°E dip. The ore zone is intensely sheared, shattered, and/or brecciated, with minor offsets along high-angle faults. Oxidation extends to depths of >400 m. Crossroads comprises two mineralised zones: an upper stratabound zone following the Horse Canyon-Wenban Formation contact, and a deeper zone controlled by an ENE striking, 20 to 25°W dipping structural zone that cuts across stratigraphy (Miranda et al., 2019).
The Gold Acres deposit has a mineralised area of ~180 x 750 m with an average thickness ranging from 25 to 60 m. Mineralisation is mainly refractory with high gold grades (i.e., >3.4 g/t Au) associated with secondary carbon and/or fine-grained sooty sulphide minerals. Minor oxide gold mineralisation is hosted within the Upper Plate rocks overlying the Imbricate Thrust Zone (described in the geology section above). The 'Lower Skarn' is largely barren of gold, although it does host minor polymetallic Zn-Mo-Cu mineralisation presumed to be coeval with intrusive emplacement and skarn formation.
The Gold Acres deposit occurs as two lobes, i). the north (London Extension Pit) and ii). the south (Old Gold Acres Pit). The London Extension Pit is bounded on the north by the NE striking, 50 to 60°W dipping Gold Acres Fault. The Gold Acres Fault down drops the Imbricate Thrust Zone and Gold Acres Stock by ~60 m to the NW. The Island Fault separates the London Extension and Old Gold Acres pits and strikes ~NNE, dipping at 50°NW. The Island Fault apparently down-drops mineralisation in the London Extension Pit relative to the Old Gold Acres Pit. Multiple NE trending faults between the Gold Acres and Island Faults incrementally down-drop mineralised stratigraphy to the north in a stair-step pattern. Both pits have been inactive since 1995 except for a small program in the London Extension Pit in 2000-2001 when refractory ore was mined to supply a third-party for processing (Miranda et al., 2019).
NOTE: This brief summary is from Miranda et al. (2019) while the more extensive summary at the main Gold Acres record comes from a more extensive review in 1996 and may reflect the evolution of ides and observations.
The Cortez Hills deposit and the pediment accumulation on its southern margin, Cortez Pediment are around 1.5 km SE of the main Cortez pit. Cortez Pediment has one primary mineralised zone, the top of which from the south where it is 45 m below the surface, dips northward to a depth of 170 m on its other extremity. This gently dipping tabular body is about 75 m thick and covers an area of approximately 1000x180 m, elongated north-south.
Gold mineralisation in the Breccia Zone is hosted by hydrothermally brecciated and fractured rocks that are spatially associated with the Voodoo Fault and its associated structures. The highest gold grades are within altered, matrix supported breccias, whilst strongly fractured rock with moderate gold grades continuing outwards to poorly fractured rocks with lower grades. The bulk of the 'breccia mineralisation' dips moderately SW, enveloping the Voodoo Fault. The upper part of this mineralisation has a NE dip, possibly reflecting control by an antithetic structure. Breccia Zone mineralisation extends from near surface at an elevation of 1784 to 1240 m, terminating just east of the Middle Zone. It is approximately 300 m wide with a NW trend, and varies in width from 76 to 580 m (Miranda et al., 2019).
The mineralised Middle and Lower zones occur at depth to the west and SW of the Breccia Zone. They are sub-horizontal and tabular, associated with alteration localised along a complex zone of thrust faulting and back thrusts in the Roberts Mountain Formation that has also incorporated slices of Devonian Wenban Limestone. A swarm of NW trending post mineral quartz porphyry dikes separate the Middle from the Lower Zone. The Lower Zone has a distinct NW-SE trend in the Roberts Mountain and Hanson Creek formations, interpreted to reflect the crest of a plunging antiform. The Middle Zone occurs is found between RL 1290 and 1166 m above sea level. It is ~550 m wide NW-SE x 395 m long NE-SW, and varies from 3 to 80 M in thickness. The Lower Zone occurs at an RL of 1300 m in the NW and at 935 m to the SE, extends 1300 m NW-SE, varies in width from 440 m in the north to 150 m in the south, and ranges from 18 to 80 m in thickness. Both the Middle and Lower zones wee open to both the NW and SE in 2019 (Miranda et al., 2019).
Post-mineral dykes and sills are estimated to constitute up to 10 vol.% of the waste within sections the Cortez Hills deposits.
Gold mineralisation is frequently spatially associated with decalcification (carbonate dissolution), and to a lesser degree with silicification. The deep oxidation evident at Cortez Hills is inferred to be the product of deep, convection-driven circulation of mixed meteoric and spent hydrothermal fluids during the waning stages of the mineralising event. This resulted in significant carbonate dissolution and clay generation, as well as extremely deep oxidation of gold-bearing iron sulphide minerals. Copper and zinc arsenates are found within, and adjacent to, oxidised mineralisation (Miranda et al., 2019).
The Cortez NW Deep deposit is a continuation of the mined-out main Cortez deposit and comprises remnants of oxide mineralisation exposed in the east wall of the Bass Pond pit, as well as deeper, sulphide and carbonaceous mineralisation. The Roberts Mountains Formation is cut by a series of NNW and NE trending faults in the deposit area. Gold mineralisation is localised at the intersection of these faults and shallow east dipping thrust breccia zones (thrust duplexes). The bulk of the higher-grade (i.e., >3 g/t Au) Cortez NW Deep gold mineralisation occurs in two zones lying between RL 1280 and 1460 m elevations, beneath the old Cortez open pit floor, where the current surface is between RL 1460 and 1600. The first of these two zones comprises of an oxidised and strongly altered thrust zone within the Roberts Mountains Formation, whilst the second an unoxidised, sulphide-bearing thrust zone at the top of the Hanson Creek Formation. Post mineral quartz porphyry dykes have been emplaced along high-angle faults. Silica locally overprints all lithologies, but does not have a strong correlation with gold at a local scale. It occurs as massive fault fill in both NNW and NE trending faults, as bedding replacement after decalcification, and as micro-veinlets. Bedding replacement by silica follows beds that were originally carbonate-rich. Oxidation is pervasive at the RL 1430 m elevation. Mineralisation becomes dominantly refractory at between RL 1280 and 1325 m elevation (Miranda et al., 2019).
The most recent source geological information used to prepare this decription was dated: 2019.
Record last updated: 16/3/2021
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.
Pipeline Cortez Cortez Hills
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Arehart G B and Donelick R A, 2006 - Thermal and isotopic profiling of the Pipeline hydrothermal system: Application to exploration for Carlin-type gold deposits: in J. of Geochemical Exploration v91 pp 27-40
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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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Henry, C.D., John, D.A., Leonardson, R.W., McIntosh, W.C., Heizler, M.T., Colgan, J.P. and Watts, K.E., 2023 - Timing of Rhyolite Intrusion and Carlin-Type Gold Mineralization at the Cortez Hills Carlin-Type Deposit, Nevada, USA: in Econ. Geol. v.118, pp. 57-92.
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Maroun, L.R.C., Cline, J.S., Simon, A., Anderson, P. and Muntean, J., 2017 - High-Grade Gold Deposition and Collapse Breccia Formation, Cortez Hills Carlin-Type Gold Deposit, Nevada, USA: in Econ. Geol. v.112, pp. 707-740.
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Mercer, C.N., 2021 - Eocene Magma Plumbing System Beneath Cortez Hills Carlin-Type Gold Deposit, Nevada: Is There A Deep-Seated Pluton?: in Econ. Geol. v.116, pp. 501-513.
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Muntean J and Taufen P, 2011 - Geochemical Exploration for Gold Through Transported Alluvial Cover in Nevada: Examples from the Cortez Mine: in Econ. Geol. v.106 pp. 809-833
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Radke A S, Foo S T and Percival T J, 1987 - Geological and chemical features of the Cortez gold deposit, Lander County, Nevada: in Johnson J L (Ed.), 1987 Bulk Mineable Precious Metal Deposits of the Western United States - Guidebook for Field Trips Geol. Soc. Nevada pp 319-325
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Wells J D, Stoiser L R and Elliott J E, 1969 - Geology and geochemistry of the Cortez gold deposit, Nevada : in Econ. Geol. v.64 pp. 526-537
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