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The Horse Canyon gold Mine was located within Eureka County in north-central Nevada, some 54 km to the south-east of the town of Battle Mountain and 5.8 km to the south-east of the Cortez mine and mill and falls on the Battle Mountain - Eureka Trend.
Due to the extreme elevation difference between the two sites, the Horse Canyon orebody was accessed from the Cortez mill by a 22 km haul road. The open pits are on the eastern flank of Center Peak, in the Cortez Range and are within an area of 1070 x 250 m (Foo & Herbert, 1987).
The orebody was on the eastern margin of the Cortez Window which exposes autochthonous Eastern, Carbonate Assemblage rocks below the Devono-Carboniferous Roberts Mountains Thrust, framed on three sides by the overlying allochthonous Western, Siliceous Assemblage succession. The ore is hosted by both upper plate siliceous and by lower plate carbonate lithologies.
Published reserve and production figures include:
3.2 Mt @ 3.8 g/t Au =12 t Au (Production, 1983-87, S Foo, Pers. comm., 1993)
2.99 Mt @ 4.45 g/t Au (Production+Reserves, 1986, Foo & Herbert, 1987)
2.8 Mt @ 4.2 g/t Au (Initial Reserve, 1983, Foo & Herbert, 1987)
4.5 Mt @ 3.4 g/t Au (Reserve, 1983, Bagby & Berger, 1985)
The Horse Canyon deposits was discovered in early 1976 as the result of an outcrop sampling program by Placer Amex on behalf of the Cortez Joint Venture. This program was conducted in the vicinity of a previously discovered gold occurrence, some 1 km to the south, which had resulted from work by a Homestake-Getty joint venture program. Placer Amex sampled silicified Palaeozoic units. The assay values obtained ranged up to a maximum of 19 ppm Au. Geological mapping and follow-up sampling, both from outcrop and trenching, were sufficiently encouraging to warrant a drilling program. A total of 294 holes were drilled between 1976 and 1981, totalling 24 045 m. The maximum depth of drilling was 100 m. Vertical and angled holes were drilled, comprising both rotary percussion and reverse circulation drilling. This program outlined the initial reserve listed above, namely 2.81 mt @ 4.16 g/t Au. Production commenced in May 1983 after modification to the Cortez mill to accept the more carbonaceous ore, and continued until 1987 when the orebody was exhausted (Foo & Herbert, 1987).
The metallurgical characteristics of the ore were classified as follows, based on the ratio of AAS:Fire Assay values; 1). >0.7 = oxide ore; 2). 0.5 to 0.7 = direct shipping ore; 3). 0.2 to 0.5 = carbon ore; and 4). <0.2 = low ratio ore to be stockpiled.
In plan, the orebody had a dumb-bell shape and was initially mined in two main pits, the South Pit which covered a 150x100 m area and was 100 to 150 m deep, and the North Pit, some 150 m to the north, which is roughly 300x100 m in plan area. A third pit, the South Extension was worked last. Mining commenced in the North Pit which was completed by late 1985. The North Pit orebody was originally found by step-out drilling along the narrow connecting neck, as there was no outcropping silicification similar to that at the South Pit (Foo & Herbert, 1987).
The Horse Canyon orebody lies on the eastern margin of the Cortez Window, and is hosted by autochthonous carbonates of the Devonian Wenban Limestone and by allochthonous silici-clastics of the Ordovician Vinini Formation separated by the Devono-Carboniferous Roberts Mountains Thrust (Foo & Herbert, 1987). For detail of the regional geological setting, see the 'Battle Mountain - Eureka Gold Trend - Geology' record.
The stratigraphy of the host succession within the mine area is as follows, from the structurally lower unit, upwards:
Devonian, Wenban Limestone - thin to medium bedded, generally laminated, crystalline limestone to silty limestone. When un-altered it varies in colour from medium to dark grey and is weakly to moderately carbonaceous. Calcite veins are common throughout the unit, as are euhedral pyrite crystals. Pyrite is occasionally concentrated along bedding planes. The unit is composed principally of calcite with small amounts of illite, dolomite, quartz and traces of carbonaceous material. It is a primary host to both oxidised and refractory ores, and where mineralised is weakly to intensely altered, including decalcification, silicification, argillisation, oxidation and is enriched in carbon (Foo & Herbert, 1987).
Devono-Carboniferous, Roberts Mountains Thrust - which separates the autochthonous Wenban Limestone from the overlying, eastward thrust allochthonous Vinini Formation. The thrust surface has been recumbently folded in the mine area (Foo & Herbert, 1987). For more detail see the 'Structure' segment below.
Ordovician, Vinini Formation, which has been separated into two mappable units within the mine area. These are:
- Ovi 1 - a thin bedded, siliceous to cherty siltstone, with light grey to tan shaly interbeds. It is not generally carbonaceous but is commonly highly contorted and fractured (Foo & Herbert, 1987).
- Ovi 2 - a thin to medium bedded, laminated, argillaceous siltstone to silty mudstone. It varies from being weakly to non-calcareous, and from tan to buff when oxidised, to very dark grey and carbonaceous. Pyrite is often disseminated throughout the unit and concentrated along laminations and bedding planes. In physical appearance the Ovi 2 is similar to the decalcified Wenban Limestone (Foo & Herbert, 1987).
Undated, Felsic Dykes - which are strongly altered and thought to be of Oligocene age. The dykes are both mineralised and un-mineralised, and they are therefor though to be pre-ore. They are however, controlled by faults which have had movement after the emplacement of the ore. The dykes become sills at depth. They are a pale grey to buff in colour, with a light grey to tan outer margin. The altered dykes are composed of quartz, kaolinite, sericite and celadonite, with pyrite in the outer margins (Foo & Herbert, 1987).
Within the North Pit, 90% of the ore is hosted by the Wenban Limestone, while in the South Pit the great majority of the ore is within the Vinini Formation.
The principal structural features at Horse Canyon include the Devono-Carboniferous Roberts Mountains Thrust, several sets of high angle faults and large and small scale overturned to recumbent folds of both upper and lower plate rocks. The thrust plane has itself been recumbently folded (Foo & Herbert, 1987).
Three principle high angle normal fault sets that off-set the orebody and alteration are recognised. These are, from oldest to youngest, 1). NNW (varying from north to NW), 2). east to ENE and 3). NW sets. The NNW set, which is the most strongly developed in both the mine area and in the district as a whole, tends to control lateral upper to lower plate contacts and is interpreted to be the main conduit for the introduction of mineralisation. At Horse Canyon the ore is localised along and between two main NNW trending faults, occurring where these intersect favourable, permeable lithologies or structurally prepared lithologies adjacent to the Roberts Mountains Thrust. ENE faults control higher grades and more oxidised ore locally within the deposit (Foo & Herbert, 1987).
The Roberts Mountains Thrust plane has been recumbently folded within the orebody. The axis of the fold has a northerly trend, with its axial plane overturned to the east. Although the hinge of the folded thrust plane is truncated by normal faults in the North Pit, large scale parallel folding is obvious in the Vinini Formation above the thrust, while small scale parallel folding is mappable below the thrust in the Wenban Limestone. Permeability appears to have been enhanced, particularly in the more brittle Vinini Formation, near the hinges of the larger scale folds, and is reflected in higher grade mineralisation (Foo & Herbert, 1987).
The controls of ore are interpreted to be both structural and lithological. Pre-mineralisation NNW faults broadly bound the orebody, while higher grades are locally concentrated along ENE faults within the deposit. The Roberts Mountains Thrust between the bounding normal faults also localises mineralisation. In the North Pit the majority of the mineralisation is hosted by Wenban Limestone between a high angle portion of the recumbent folded thrust to the east and a high angle normal fault on the west. Only a small portion of the ore in this pit is hosted by the Vinini Formation silicic-clastics. In contrast, in the South Pit, where the thrust contact has a relatively low angle, Vinini Formation rocks above the sole of the thrust are the dominant hosts, although significant quantities of ore are also within the Wenban Limestone. Stratabound lenses of ore in the Wenban Limestone tend to parallel the thrust plane (Foo & Herbert, 1987).
Within the South Extension Pit, the ore is sharply confined to stratabound jasperoid lenses within the Wenban Limestone. Gold and silica are interpreted to have migrated into the hangingwall of a NNW high angle fault. Such lithological control, within the Horse Canyon deposit as a whole, is reflected by high grade in relatively undisrupted lithologies away from major structures and by ore to waste boundaries which are defined locally by lithological contacts. This is also seen on a small scale by wildly fluctuating gold values in samples collected normal to bedding (Foo & Herbert, 1987).
The orebody is, overall, a sub-horizontal tabular mass, with local high angle tabular extensions parallel to high angle faults. In plan it trends NNW and has a dumb-bell shape (Foo & Herbert, 1987). Gold is thought to occur as sub-micron particles, although none had been identified to 1987 ( Foo & Herbert, 1987), nor were we told had any been seen subsequently (C Gillette, Pers. comm., 1993). Selected sampling suggests a fracture control as distinct from disseminations. The age of mineralisation is also un-certain (Foo & Herbert, 1987). At Cortez ore is believed to be pre-porphyry dyke, while it appears to be post-dyke at Horse Canyon. This implies either multiple ore, or multiple dyke emplacement pulses within the Cortez Window.
Gangue minerals include calcite, quartz, crystalline jarosite, hematite and rare barite. Very rare and restricted stibnite, fluorite and cinnabar have also been identified. Alunite has been recognised macroscopically within jasperoid breccias associated with fault zones, while As, Sb, Hg and Tl are anomalous in samples in the mine area. Tl, however is the only element with a positive correlation with Au (Foo & Herbert, 1987).
Soil sampling over Horse Canyon would apparently give a spotty anomalous zone with peaks of around 1 ppm Au which would outline the orebody. The fine nature of the gold, which has apparently never been seen at the deposit, was such that it had no expression in panned samples (C Gillette, Pers. comm., 1993).
The following alteration styles are recognised at Horse Canyon, all of which are associated with both ore and waste zones:
Decalcification - is most evident in the Wenban Limestone, and is preferentially developed in the more silty members. This alteration results in a relatively porous, poorly indurated siltstone. Decalcified Wenban Limestone is sometimes difficult to differentiate from the Ovi 2 facies of the Ordovician Vinini Formation. Decalcified rock is typically oxidised and is commonly argillaceous. Relict bedding features are generally evident (Foo & Herbert, 1987). Large dissolution cavities have been found within the orebody (Pers. observ., 1993; C Gillette, Pers. comm., 1993).
Silicification - where structurally controlled, occurs as massive breccias along high angle fault zones and as localised lenses along the Roberts Mountains Thrust surface. Stratabound silicification is also observed, occurring within the Vinini Formation siltstones, but more frequently within Wenban Limestone. Jasperoids within the Wenban Limestone may either be passively replaced with preserved bedding, or occur as zones of multiple brecciation and re-silicification. Silicified rock is present as both ore and waste, although both stratabound and fault controlled silicification within the deposit may host high grade values of more than 8 g/t Au. Jasperoid and silicified Vinini Formation in the hangingwall of the west bounding NNW fault is typically only weakly mineralised. The wide variation in gold values of jasperoids suggests multiple silicification events which may be pre-, syn- and post-ore (Foo & Herbert, 1987). Outcropping jasperoid is a red-brown siliceous, silty rock with conchoidal fractures (Pers. observ., 1993).
Argillisation - is best developed along fault zones, with discontinuous bodies of clay occurring along high angle faults and along the Roberts Mountains Thrust surface. Interbedded clayey horizons are also found within oxidised and to a lesser extent, carbonaceous Wenban Limestone and Vinini Formation. The clays are indicated to be mixed layer illitic types, although kaolinite has been recognised along structures (Foo & Herbert, 1987).
Carbonisation - refers to the introduction and concentration of sooty organic carbon in less carbonaceous or oxidised rock. Carbon is strongly localised by high angle faults, on the Roberts Mountains Thrust surface and along large scale fold hinges. Isolated carbon pods whose margins are discordant to bedding are recorded (Foo & Herbert, 1987). Within the pit carbon is very obvious on faces, outlining both bedding and faults. It readily rubs off onto hands when the outcrop is handled. The carbon rich rocks may contain abundant sulphides, mainly pyrite, with contents of up to several percent. Zones of carbon rich host rock in places are cut by abundant calcite veins (Pers. observ., 1993).
Oxidation - is considered to have occurred late in the paragenetic sequence. There appears to be no clear relationship between grade and the degree of oxidation. High values in carbonaceous limestones and Vinini Formation shales suggest the host rocks were carbonaceous at the time of ore deposition. Transitional or decreasing oxidation does seem to correlate locally to transitional boundaries of ore to waste, but this may be a function of permeability enhancing first the introduction and deposition of gold and subsequently oxidation along the same channels. Significant quantities of carbonaceous rock, both ore and waste, are found above oxidised ores. No distinction has been made between supergene and hypogene oxidation. Rock textures and the preservation of pyrite within silicified rocks suggest that silicification preceded oxidation (Foo & Herbert, 1987).
The most recent source geological information used to prepare this summary 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.
Colgan J P, Henry C D and John D A, 2014 - Evidence for Large-Magnitude, Post-Eocene Extension in the Northern Shoshone Range, Nevada, and Its Implications for the Structural Setting of Carlin-Type Gold Deposits in the Lower Plate of the Roberts Mountains Allochthon: in Econ. Geol. v.109 pp. 1843-1862|
Foo S T and Herbert J P, 1987 - Geology of the Horse Canyon deposit, Eureka 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 326-332|
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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