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Cortez, Pipeline, South Pipeline, Crossroads, Cortez Hills, Cortez Pediment

Nevada, USA

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The Cortez, Pipeline, South Pipeline, Crossroads, Cortez Hills 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).

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 are:

      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).
    293 Mt @ 1.6g/t Au (Pipeline & South Pipeline proven and probable reserves, 2005,
                Rio Tinto website, 2007).
    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).
    Production in 2005 amounted to 28 t Au (Rio Tinto Annual Rept. 2006).

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 & 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 & 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, etal., 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 & 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. The mill ore cutoff in that period had been 1.4 g/t Au (McFarland & Kirshenbaum, 1991).

In 1973 additional reserves were located at Gold Acres, which had originally been discovered in 1922 and mined until 1965 (Radtke, et al., 1978), while the Horse Canyon deposits were discovered in 1976 (Foo & Herbert, 1987). Mill grade ore from these deposits was treated at the Cortez Mill. 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 & Kirshenbaum, 1991).

Following the surge in gold prices in 1979-80, heap leaching of the waste dumps at Cortez was commenced during 1980. Recovery was 45% in the first 90 days and a further 10% in the second such period (McFarland & Kirshenbaum, 1991).

The Pipeline deposits were discovered under alluvial cover 2 to 3 km to the north-east of Cortez in the early 1990's and delineated over the following years. The operation is now (2006) owned 60% by Barrick Gold and 40% by Rio Tinto.

Geology

The orebodies of the Cortez deposit are 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, etal., 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, about 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, etal., 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 & Masursky (1965) at the base of the lowest bioclastic limestone above the thin bedded, grey, pyritic, graptolite-bearing Roberts Mountains Formation (Wells, etal., 1969). The Wenban Limestone is equivalent to the Popovich Formation in the Lynn Window on the Carlin Trend.

Four periods of igneous activity are recognised in the Cortez Window by Gilluly & 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, etal., 1969; Radtke, etal., 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 & 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, etal., (1969) reported 34 Ma and Foo & Herbert (1987) suggest 35.7 to 33.7 Ma. Radtke etal., (1978) quote dates ranging from 31.0 to 33.6 Ma for the Caetano Tuff, while Foo & 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, etal., 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, etal., (1978) however reported that the margins of the porphyry dykes have undergone argillic alteration and are weakly mineralised. According to Rytuba (1985a), 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, etal., 1969; Radtke, et al., 1978; Foo & 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 & Herbert, 1987)

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 & 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 & 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, etal., 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, etal., 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 SiO
2 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, etal., 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 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.

Both Pipeline and South Pipeline were masked by Quaternary alluvium ranging in thickness from around 10 m to >200 m. The base of the alluvium dips at <10° to the east, marginally shallower than the sub-parallel bedding of the underlying Palaeozic host sediments. The Wenban Limestone forms the western highwall of the Pipeline/South Pipeline open pit.

The Pipeline/South Pipeline deposits comprise two zones of mineralisation:
i). a shallow zone, the top of which is from 10 to 180 m below the pre-mining surface, and
ii). a deeper zone that begins approximately 300 m below the pre- mining surface.

Both zones are low angle and tabular and range from 15 to 110 m in thickness and dip to the east. The deposits are both part of the same mineralised system and cover a north-south elongated areal extent of approximately 3000x1000 m.

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.

Conventional open-pit mining methods are used for the Pipeline and South Pipeline deposits, scheduled in nine stages. Between 2001 and 2005, production was expected to average 70 Mt pa. Pit wall slopes vary from 34 to 50°.

The most recent source geological information used to prepare this summary 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.


  References & Additional Information
   Selected References:
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
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. 451484
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.
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
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
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


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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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