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Pinson

Nevada, USA

Main commodities: Au
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Deposit Description

The Pinson gold deposit is located 43 km to the north-east of the town of Winnemucca, Humboldt County, in north-central Nevada, USA. The ore deposit occurs on the eastern margin of the Osgood Mountains, within the Potosi Mining District and is on the Getchell Trend. It is about 8 km to the south-west of the Getchell mine and 19 km north-east of the Preble orebody.

The Pinson Mines exploit six separate orebodies aligned in a north-easterly direction. These are the A and B orebodies which were mined out prior to 1990, two smaller satellite bodies, the C orebody and Felix Canyon which were being mined in 1990, the larger Mag orebody which was also in production in 1990 and the CX deposit which was due to start-up during the 1990's (Foster & Kretschmer, 1990).

Gold was first discovered in the Potosi Mining District in 1934, with mining commencing at the Getchell Mine in 1936. In 1945 two local prospectors, Clovis Getchell and Charles Ogee found small siliceous outcrops that resembled the early Getchell ore and staked claims over them. Getchell Mines Inc. leased the claims and mined approximately 50 000 t of ore in 1949-50, apparently exhausting the mine. The leases were dormant until the late 1960's when some exploration was carried out in the area (Kretschmer, 1986).

In 1970 the Cordex I Syndicate was formed by several small Canadian companies and individuals. Their aim was to explore for deposits that would require a low capital outlay to bring into production. In 1970 the syndicate leased the old Ogee and Pinson Gold Mine which they recognised as lying on the Getchell structural zone. Geological mapping, geochemical sampling and geophysical programs in 1970 led to a rotary drilling program in 1970-71. The first 17 rotary holes which were drilled mainly around the old Ogee and Pinson Pit, only encountered low grade values. The 18th hole, drilled to test a potential north-eastern extension, encountered 27 m of 'good mineralisation'. This was the discoveryhole for the Pinson A orebody which, after additional drilling was found to be 300 m long and 18 m thick. It was obscured by 3 to 15 m of alluvium and contained 1.4 mt @ 6.2 g/t Au. Further drilling outlined the B orebody which comprised a further 1.5 mt @ 5.1 g/t Au, plus an additional tonnage of low grade ore (Kretschmer, 1986).

Grid soil sampling in 1971 detected anomalous Hg values above the Mag deposit, while geological observation of samples indicated the presence of a weathered shear zone. Ground magnetics and VLF-EM produced unclear results over the same area. Following the discovery of the A orebody, two holes were drilled to test the Mag area in the summer of 1972. One encountered gold mineralisation. Additional drilling continued on the perceived north-east trend of the deposit through the 1970's, mainly to fulfil claim assessment obligations. A second hole encountered significant gold in 1978. However due to drilling difficulties in penetrating the alluvial cover (Mag lies under 12 to 60 m of alluvium), and the relatively low grades in metallurgically unfavourable black argillite, the results were not regarded as being attractive (Foster & Kretschmer, 1990).

Various evaluations were made of the Pinson A and B orebodies during the early 1970's. In 1975 a 100 m adit was driven for bulk sampling and metallurgical testing. In 1979, when the gold price had reached $US 250/oz, and over $ US 1 m had been expended on the property, a new feasibility study was undertaken. On this occasion it was indicated as being profitable. The Cordex Syndicate was re-organised into a partnership and the Pinson Mining Co was formed (Kretschmer, 1986). The first production was in 1982 (Foster & Kretschmer, 1990). By 1982 Cordex was a partnership between Dome Exploration (US) Ltd, Lacana Gold Inc and Rayrock Mines Inc, each with 29.33% interest, and J S Livermore with 12% (Ellis, 1987).

In 1983 funding was provided to prove up the indicated CX deposit which lies between the Pinson A and Mag orebodies. During development drilling at CX, mapping to the north indicated a hydrothermally altered limestone that appeared to project into the Mag area. Step-out drilling in May 1984, along the Pinson-CX trend, intersected values of 0.4 to 4.8 ppm between 23 and 84 m within the Mag area. A two stage 25 hole program followed, and in January 1985 a reserve of 1 mt was announced. Continued drilling on 30 m centres, with 15 m offsets where greater detail was required, outlined 6 mt of ore as listed below. Gold mineralisation continues below this reserve, which is to a depth of 150 m, but was not minable on 1990 economics. Mining commenced in the Mag Pit in 1987 (Foster & Kretschmer, 1990).

Published reserve and production figures include:

    3.6 Mt @ 4.1 g/t Au = 15 t Au (Mill Production to 1988, Foster, et al., 1990).
    2.7 Mt @ 0.68 g/t Au = 1.8 t Au (Leach Production to 1988, Foster, et al., 1990).
    1.4 Mt @ 6.2 g/t Au (Initial Reserve, A orebody, 1971, Kretschmer, 1986).
    1.5 Mt @ 5.1 g/t Au (Initial Reserve, B orebody, 1971, Kretschmer, 1986).
    3.2 Mt @ 4.1 g/t Au (Production+Reserve, 1984, Bagby & Berger, 1985).
    3.9 Mt @ 2.7 g/t Au = 10 t Au (Mill Reserve, Mag O/b, 1987, Foster & Kretschmer, 1990).
    2.1 Mt @ 1 g/t Au = 2 t Au (Leach Reserve, Mag, 1987, Foster & Kretschmer, 1990).
    4.7 Mt @ 2.3 g/t Au (Proven+Probable Reserve, 1994, AME, 1995).

In 1990, mining at the Mag Pit was on 6 m benches with a 38° slope. The mill ore was treated using a carbon-in-pulp process at a rate of around 1500 tpd. The mill cut-off grade was 1.6 g/t Au, while material with grades from 0.69 to 1.54 g/t was treated in run-of-mine dump leach pads. Mill recoveries in 1990 averaged 88%, although these are expected to decline with depth due to elevated levels of organic carbon and sulphide sulphur in the deeper, less oxidised parts of the orebody. Average organic carbon and sulphide-sulphur are 0.21% and 0.22% respectively at less than 60 m below the base of cover. These increase to 0.62% and 1.03% respectively below that depth. The Mag pit has planned final dimensions of 975 x 365 m and a depth of 150 m (Foster & Kretschmer, 1990). Production in 1993 was 1.54 t Au from 0.51 mt of ore at an average grade of 2.7 g/t Au (AME, 1995).

Geology

The rocks in the vicinity of the Pinson Mines comprise a lower Palaeozoic sequence made up of the Cambrian Preble Formation comprising calcareous, sandy and carbonaceous shales, sandstones and limestones, conformably overlain by the late Cambrian to Ordovician Comus Formation which is composed of thin bedded carbonates and shales. The Comus Formation is the host to ore. These sediments are intruded by a 10 km long, dumb-bell shaped mass of Cretaceous granodiorite, the Osgood Mountain Stock. The margin of this stock is within 500 m to the north-west of, and parallel to, the north-east trending string of six orebodies at Pinson. Both the Preble and Comus Formations belong to the Transition Assemblage (Kretschmer, 1986; Foster & Kretschmer, 1990). The six orebodies are distributed along a north-east trending fault set which may be a continuation of the Getchell Fault Zone that controls the Getchell deposits 8 km to the north.

The regional geology and setting of these units is described in the Getchell Trend Geology record.

The stratigraphy in the Pinson area is as follows, from the base:

Middle to Late Cambrian, Preble Formation - which may be sub-divided into three distinct members as follows (Kretschmer, 1986):
- Lower Member - sandy shale, quartzitic sandstone and phyllitic shale.
- Middle Member - limestone, carbonaceous shale and calcareous shale with subordinate phyllitic shale and quartzitic sandstone.
- Upper Member - phyllitic shale, sandy shale and a few carbonaceous beds. According to Foster & Kretschmer (1990), the Upper Member is represented by maroon-brown and black phyllitic hornfels and argillites in the Pinson Mine area, which pass conformably upward into the overlying Comus Formation.
Upper Cambrian to Lower Ordovician, Comus Formation - which at Pinson is predominantly a thin bedded carbonate and shale unit and is the host to ore. Rhythmic interbedding of carbonates and shaly beds is often seen, sometimes occurring as fine laminations (Kretschmer, 1986). Intra-formational conglomerate occurs in the section near the intercalated limestone and cherty beds. It is entirely calcareous, containing fragments or plates of medium dark grey to greyish-black limestone in a limestone of sugary textured dolomite (Kretschmer, 1987). The Comus Formation may be divided into:
- Lower Comus Limestones, 245 m thick - composed of bedded calc-siltite and micrite that are commonly silty, shaly, cherty or conglomeratic;
- Upper Comus, 810 m thick - which is of late Ordovician age. It contains black, carbonaceous shales and argillites, with locally interbedded limestone.

These sediments are intruded by the granodiorite of the Osgood Mountain Stock and in the mine area by dacite dykes which are possibly related to that stock. All of the dykes are highly altered making their origin uncertain (Foster & Kretschmer, 1990).

The Pinson pits lie within a conspicuous contact metamorphic aureole that extends irregularly for up to 3000 m outwards from the margins of the Osgood Mountain Stock. Within the aureole shale is metamorphosed to chiastolite/andalusite-biotite or cordierite-muscovite hornfels and carbonate rocks are converted to marble, light coloured calc-silicates containing diopside, actinolite and quartz, and to dark garnet tactites. The tactite/skarns host the scheelite mineralisation of the district (Kretschmer, 1986). An inner zone of more intense metamorphism is found within 600 m of the contact. Beyond this the metamorphism is weaker and more spotty. At a distance of 1200 m or more the Upper Comus shales are typically fused, hard argillites with graphitic carbon (Foster & Kretschmer, 1990).

Within the immediate Mag open pit the host sediments of the Upper Comus Formation are sub-divided as follows, from the base (Foster & Kretschmer, 1990):

Carbonaceous shales and argillites that are commonly weakly metamorphosed to biotite and cordierite hornfels. This band is 150 m thick.
Massive, carbonaceous, micritic limestone interbedded with shale. This unit reaches 40 m in thickness. The limestone fraction is locally conglomeratic and commonly contains calc-silicate minerals.
Thin to very thinly bedded carbonaceous shale and argillite with interbedded limestone and calcareous shale. Micritic, carbonaceous lenses occur locally. The unit is between 60 and 90 m thick, although the actual thickness is uncertain due to faulting. The lowest 30 m is composed mainly of thinly bedded, blocky argillite. These shales host most of the ore. They are locally weakly metamorphosed to calc-silicates or hornfels.
Shale, and limestone, reaching 30 m in thickness. Carbonates in this unit are typically metamorphosed to calc-silicate minerals.
Shale and argillite, which is a thicker unit not exposed in the open pit.

The Ordovician bedrock in the Mag Pit is covered by 12 to 60 m of Quaternary alluvium (Foster & Kretschmer, 1990).

Structure

The major structure in the mine area is a prominent north-east trending normal fault which has controlled the gold mineralisation. This structure has been traced by drilling for 2500 m along strike and shows a uniform dip of 49 to 50° to the south-east. For most of its length it forms the contact between the Preble and Comus Formations. It is a 15 to 20 m wide zone of shearing and brecciation. For the remainder of its length it cuts through and offsets limestones and intercalated beds of the Comus Formation (Kretschmer, 1986).

Within the Mag Pit, bedding is locally contorted, although mappable faults are rare due to abrupt changes in fold orientation and to later, closely spaced faulting. The Mag deposit contains an abundance of fault 'rubble' and gouge. There are two orientations of extensional faulting at Mag. The dominant trend, controlling the strike of the ore is from 330 to 360°, dipping at 40 to 65° SE. This is approximately normal to the trend of the string of deposits at Pinson. The second fault set, which parallels the dominant fault direction at Pinson, trends at 30 to 60° and dips at 50 to 65° to the south-east. Faults are characterised by rubble zones up to 6 m thick that contain a central core of fault gouge and sheared surfaces. These faults are repeated at intervals of 15 to 30 m across the Mag deposit. The alluvium to bedrock contact in the hangingwall of the deposit is a 345° trending fault contact. Alluvium has also been offset on north-east trending faults, indicating the most recent movement on these structures. A less common fault trend is east-west, with sub-vertical dips. Two such faults are indicated at each end of the NW-SE trending Mag deposit (Foster & Kretschmer, 1990).

Mineralisation and Alteration

The six main orebodies at Pinson are distribute in a generally north-easterly direction, from Felix Canyon in the south-west, through the B, A and C orebodies, the CX body and finally Mag. The A, B, C and Mag orebodies are briefly described below.

A Orebody - The A orebody is localised along the 20 m wide north-east trending shear zone, bounded by definite footwall and hangingwall strands. The orebody is characterised by intense silicification, in which intercalated limestone and siltstone beds of the Comus Formation are completely replaced by a dense jasperoid that locally carries gold values of more than 6.8 g/t. The jasperoid is confined to an 18 to 20 m wide zone along the fault. Polished sections of the jasperoid reveal a relatively simple mineralogy of silica, goethite, lepidocrocite and hematite, with sparse remnants of pyrite, marcasite and gold (Kretschmer, 1986).

Gold particles are less than 30 µm in diameter, and most commonly <5 µm. It occurs as a free phase and has been seen as micron sized inclusions in As bearing pyrite. The gold:silver ratio often approaches 100:1. Highly altered dacite or andesite porphyry sills and dykes lace the A orebody in an irregular fashion. These dykes are strongly argillised, sericitised and contain low gold values near the ore zone (Kretschmer, 1986).

B Orebody - The B orebody starts in the vicinity of the old Ogee & Pinson gold mine and trends southward for about 300 m. The host rocks are a less silicified, leached, silty, carbonate horizon of the Comus Formation. It occurs in a fold axis that has been sheared and brecciated along a north trending shear zone, with influence from a north-east trending fracture system that is sympathetic with that of the A orebody structure. Drilling indicates that carbonaceous shales of the Comus Formation surround the ore bearing carbonates, above, below and to the west (Kretschmer, 1986).

Gold mineralisation occurs with limonite and kaolinite along numerous fractures in a zone of subtle, pervasive silicification. This fracture system has allowed good dissemination and uniform grade. Although of lower grade than the A orebody, it is more extensive. In addition the fracturing has allowed more thorough oxidation, allowing the ore to be more efficiently treated (Kretschmer, 1986).

C Orebody - Only a modest tonnage was outlined in the C orebody which occurred along a cross-cutting normal fault at the eastern end of the A orebody. It had a similar grade and mineralogy to that of the A orebody (Kretschmer, 1986).

Mag Orebody - The location of gold mineralisation in the Mag orebody is controlled by faulting and host rock lithology. Within the overall north-west configuration of the deposit, the higher grade areas typically follow north-west or north-east trends. Several faults have been observed with higher degrees of silicification and higher gold content in the hangingwall than in the footwall. Most faults however also exhibit post-mineralisation movement, thus obscuring such relationships (Foster & Kretschmer, 1990).

Gold occurs mainly in the carbonate bearing argillite and shale of the 'thin to very thinly bedded carbonaceous shale and argillite' unit in the middle of the upper member of the Comus Formation. The underlying massive carbonaceous micritic limestones at the northern end of the pit also host mineable gold. Unfavourable lithologies include the pure shale/argillite sub-units and the rocks which have been metamorphosed to calc-silicates and hornfels. In general gold grades decline in areas of increased contact metamorphism. The massive carbonaceous micritic limestones, which are mineable at the northern end of the pit, for instance, are only weakly mineralised on the western section of the pit where they are moderately metamorphosed (Foster & Kretschmer, 1990).

The Mag deposit ores contain minor to abundant brown, yellowish and locally red, hydrated iron oxide and minor scorodite. Partially oxidised ores at depth contain very fine disseminations and micro-veins of pyrite and rare occurrences of chalcopyrite and sphalerite. Quartz veinlets, druse and micro-veins occur throughout the deposit (Foster & Kretschmer, 1990).

Gold occurs as grains that are <5 µm across, with very rare occurrences of larger particles up to 15 µm. Gold is enclosed in pyrite and supergene hydrated iron oxides, and less commonly within quartz or silicified rock. Metallic gangue minerals, other than pyrite and iron oxides have extremely fine grain sizes, are widely isolated and typically show no intergrowth with other metallic minerals. The most common are cinnabar ( which may in some cases be intergrown with pyrite and hydrated iron oxides), arsenopyrite, chalcopyrite, sphalerite, galena, stibnite and a Pb-Sb sulphide (Foster & Kretschmer, 1990).

Decalcification, leaching, silicification and kaolinisation are spatially associated with the ore at Mag. The carbonate content of the host rock, with the exception of local massive limestone, was completely removed during leaching, leaving a porous, silty-textured rock. Weak to strong jasperoid silicification is pervasive, occurring in previously leached rock and undisturbed rock, as well as healing fault gouge and breccia. White kaolinite fills fractures in the central portion of the deposit. Outside of this core, kaolinite is found as argillic alteration of the host rock. Montmorillonite accompanies kaolinite in argillised rocks formed from calc-silicates (Foster & Kretschmer, 1990).

With the exception of silicification, all of the alteration outlined above occurred during both supergene and hypogene mineralisation. Decalcification, leaching, argillisation and white kaolinite fracture fillings are common in drill cuttings from deep un-oxidised sections of the orebody. The intensity of alteration increases in the shallow oxidised sections of the orebody. The amount of silicification however, is comparable in both the oxidised and deep un-oxidised parts of the orebody, suggesting a hypogene origin. Supergene oxidation produced hydrated iron oxides and pseudomorphs after pyrite. Iron oxides do not occur at depth in the un-oxidised ore (Foster & Kretschmer, 1990).

Calcite veining is only common in massive, un-mineralised limestone (Foster & Kretschmer, 1990).

Massive, banded and botryoidal chalcedonic silica with dehydration cracks locally fill voids in fault rubble. Unlike the jasperoid however, this silica does not correlate with higher gold grades, nor is it associated with As or Hg, suggesting a second, post-ore period of silicification (Foster & Kretschmer, 1990).

Compared to background values of un-altered host rocks, the Mag deposit is strongly enriched in Au, Hg, As, Tl, Sb and W, and weakly enriched in Ag (Foster & Kretschmer, 1990).

The Mag orebody is strongly oxidised to a depth of 60 m below the alluvium/bedrock interface. Below this zone the deposit is partially oxidised, and gradually becomes un-oxidised at depths of 90 to 120 m below the base of the alluvium. The only element that correlates well with Au is Hg, except in the oxidised zone where Hg is depleted. The Au-As correlation is positive, but much weaker than the Au-Hg association, while As correlates with Tl throughout the orebody

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.


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