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Rain, Emigrant Springs, Gnome, SMZ (Southern Mineralized Zone), Tess

Nevada, USA

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The Rain mine is located in south-western Elko County, north-eastern Nevada, USA. It is some 14 km to the south-east of the township of Carlin and 25 km to the south-east of Gold Quarry, the next significant deposit on the Carlin Trend. The orebody lies within the Rain Window, and is the south-eastern most mined occurrence of the Carlin Trend. Three other satellite deposits lie within the same window, Emigrant Springs, Gnome and the Southern Mineralised Zone (SMZ). The Gnome deposit is some 600 m to the south of the Rain Deposit, while the SMZ is a further 350 m to the south-east of Gnome and 800 m south of Rain. The Emigrant Springs mineralisation is just over 2.5 km to the east of the Rain Mine,. The Rain orebody covers a plan area of some 750 x 150 m (Thoreson, 1978 & 1990).

The Rain Claims were originally staked by a local prospector, over occurrences of thick barite veins and jasperoid which were to constitute the discovery outcrop. Perceived similarities with the other gold bearing outcrops of the Carlin Trend resulted in an approach to Newmont Exploration Ltd in 1979. Outcrop sampling of the jasperoid in the discovery outcrop returned anomalous gold levels, including a peak value of 17 g/t Au (Thoreson, 1987 & 1990).

Detailed soil geochemistry identified an arsenic anomaly sub-parallel to the Rain jasperoid, with some peak values of up to, and more than, 1000 ppm As over the jasperoid. Minor erratic Au-Ag values occurred over the deposit, but at a 50 ppb Au detection limit gold from soils was in-adequate as a pathfinder. Subsequent drilling in 1982 and 1983 outlined the 7.5 Mt @ 2.85 g/t Au reserve. Continued drilling defined in the main pre-mining reserve of 14 Mt @ 2.26 g/t Au. The three satellite deposits were discovered during this period, resulting in a total reserve in the sub-district of 35 Mt @ 1.44 g/t Au (Thoreson, 1990).

Construction of the access road commenced in July 1987, with the first Rain Mine blast in October of the same year. The first gold bar was poured in June 1988. Planned production was 36 000 tpd mined, 2100 tpd milled and 2700 tpd stacked on the leach pads for an annual 3 t of Au (Thoreson, 1990).

Open pit mining between 1987 and 1994 concentrated on the main Rain and the SMZ deposits as two separate pits.   In 1993 underground mining of the deeper extension of the Rain deposit commenced and its continuation, Tess (Williams et al., 2000).

Published reserve figures for the deposits of the Rain Window are as follows:

Rain 36.4 Mt @ 1.8 g/t Au (Open pit production from Rain & SMZ to 2000, Williams et al., 2000)
    4.9 Mt @ 7.7 g/t Au (Underground production+reserve at Rain & Tess, 2000, Williams et al., 2000)
    Total contained gold = 103 tonnes from open pit+underground (from Williams, et al., 2000)
   14 Mt @ 2.26 g/t Au = 31.6 t Au (Pre-mining Reserve, 1986, Thoreson, 1990)
    7.5 Mt @ 2.85 g/t Au (Resource, 1983, Thoreson, 1990)
    12.8 Mt @ 2.6 g/t Au (Initial reserve, 1985, Ryneer, 1991)
    4.1 Mt @ 5.6 g/t Au, plus
    2.9 Mt @ 1.17 g/t Au (Initial reserve, 1985, Thoreson, 1987)
    4 Mt @ 1.7 g/t Au  (Proven+probable reserve, 31 Dec. 1992, Christensen, 1993)
    22.9 Mt @ 1.64 g/t Au (Total geological reserve, 27 Dec. 1989, McFarlane, 1991)
    8.7 Mt @ 1.1 g/t Au (Proven+probable leach reserve, 1994, AME, 1995)
    7.9 Mt @ 1.2 g/t Au (Proven+probable milling reserve, 1994, AME, 1995)
Gnome
    2.5 Mt @ 1.65 g/t Au = 4t Au (Total geological reserve, 1989, Coope, 1991)
SMZ
  1.6 Mt @ 0.58 g/t Au =0.93 t Au (Total geological reserve, 1989, Thoreson, 1990)
Emigrant Springs
      4.5 Mt @ 1.2 g/t Au =5.2t Au (Proven+probable reserve, 31/12/92, Christensen, 1993)
    11.7 Mt @ 0.76 g/t Au (Total geological reserve, 27/12/89, McFarlane, 1991)
    27.5 Mt @ 0.72 g/t Au (Total geological reserve, 1989, Coope, 1991)
    46.6 t Au (Total mineable reserve, 2005, Newmont, 2005)
    4.18 Mt @ 0.24 g/t Au (Indicated resource, 2005, Newmont, 2005)
    1.22 Mt @ 0.49 g/t Au (Inferred resource, 2005, Newmont, 2005)

Gold recoveries within the siliceous and baritic ore are 85 to 95%, while in the carbonaceous mineralisation these are reduced to 70%. The siliceous and siliceous/baritic ores however, require extensive milling and grinding to reduce the ore to the required 80% -200 mesh. The increased grinding time for these hard ores lowers the throughput of the mill (Thoreson, 1990).

Geology

The deposits of the Rain District are hosted by the lower sections of the lower Carboniferous (Mississippian) Overlap Assemblage of the Rain Window. These sediments unconformably overlie the Eastern Carbonate Assemblage and post date the Antler Orogeny and the eastward movement during that orogeny of the Western Siliceous Assemblage above the Roberts Mountains Thrust.

The sequence within the Rain District is as follows, from the structurally lowest:

Devonian, Nevada Formation - dolomite, limestone, and minor amounts of sandstone and quartzite (Stewart, 1980). This is the lowest unit exposed in the immediate Rain District.
Devonian, Devils Gate Limestone >198 m thick locally, 260 m thick regionally - a medium to thick bedded, fine grained, medium to dark grey, micritic limestone. It outcrops on the south-eastern edge of the Rain deposit, weathering to a medium to light grey colour and forms massive, well rounded outcrops with locally abundant stromatoporoid colonies, horn corals and fossil hash (Thoreson, 1990). This unit is equivalent to the Popovich Formation of the Lynn Window and the Devonian Limestones of the Carlin Window.
Unconformity - representing the Antler Orogeny. The sequence below belongs to the Eastern, or Carbonate Assemblage, and the Western, or Siliceous Assemblage. The latter is not represented in the immediate vicinity of the Rain deposit. The unconformable contact between the Devils Gate Limestone and the Webb Formation is represented by a 1.5 to 6 m thick zone of brick red clay composed of quartz rich illite-sericite clays with increasing amounts of alunite proximal to and within the Rain deposit. This clay layer also contains fragments of silicified and baritised Webb Formation sediments and calcite veining. (Thoreson, 1990).
Lower Carboniferous (Mississippian), Webb Formation up to 240 m thick - commencing with a lower sequence of sediments comprising interbedded mudstones and siltstones with minor quartz and chert sandstones and quartzites. Basal fine grained mudstones and siltstones of the Webb Formation at Rain grade upwards to coarser grained siltstone, sandstone, conglomerate and quartzite, which are in transitional contact with the sandstones and conglomerates of the overlying Chainman Shale (Thoreson, 1987 & 1990).
  At the surface, the Webb Formation sediments have been oxidised. All pyrite has been converted to hematite and other limonites, with leisegang banding being locally abundant. Below the oxidised cap these same rocks contain abundant carbonaceous material and pyrite. The Rain deposit occurs below these carbonaceous rocks in oxidised and hematite stained mudstones and siltstones. Arenaceous interbeds occur within the formation and are composed of strongly hematite stained, fine to medium grained (0.25 to 0.5 mm), well rounded and well sorted quartz and chert sandstone and quartzite. These arenaceous beds range in thickness from less than 2.5 cm to more than 1.5 m. Away from the ore system these arenites are carbonaceous and contain up to 1% disseminated pyrite (Thoreson, 1990).
  A unit of pebbly siltstone some 46 m above the base of the Webb Formation is composed of well rounded and poorly sorted quartzite, chert and sandstone pebbles and cobbles randomly set within a matrix of mud and silt. On the west side of the Rain pit the pebbly siltstone unit contains thin interbeds of fossil plant material. This marker has been described by other authors as being near the base of the overlying Chainman Shale. Drilling indicates that the Webb Formation is >153 m thick at Rain and Gnome and <30 m at Emigrant Springs. The transitional nature of the contact and the lack of a reliable marker makes the definition of the contact between the Webb Formation and Chainman Shale difficult to locate. If the marker above does represent the contact, the Webb Formation is only 46 m thick at Rain (Thoreson, 1990).
Lower Carboniferous (Mississippian), Chainman Shale - a clastic sequence of fine to coarse grained sandstones, quartzites and conglomerates with thin interbeds of mudstones and siltstones. These are coarser than those of the Webb Formation and are commonly carbonaceous with abundant (0 to 5%) primary pyrite. These sediments may be distinguished from the underlying Webb Formation by the increase in coarse clastics relative to the abundance of siltstone and mudstone in the Webb Formation. At Emigrant Springs however, the Chainman Shale is composed of black, pyritic shales with few interbedded sands. The Chainman Shale coarsens upwards and is overlain by the even coarser sandstones and conglomerates of the Diamond Peak Formation (Thoreson, 1990).

Structure

The deposits of the Rain district are all found near the faulted cores of antiformal structures. The most prominent structure at the Rain Mine is the 290° to 310° striking and 70°SW dipping Rain Fault. This fault apparently has a reverse sense of movement. It is interpreted as the pathway of mineralising media and is expressed at the surface as a jasperoid ridge. Although the surface exposure of the jasperoid has been excavated, it maintains its character along the north-eastern wall of the Rain Pit as a brecciated and silicified zone with abundant barite and minor alunite, jarosite, dussertite, hematite, other limonites and cinnabar (Thoreson, 1990).

All of the mineralisation occurs within a hydrothemal breccia mass that extends for some 5 km in the hangingwall of the Rain fault (Williams et al., 2000), penetrating from 45 to 180 m into the Webb Formation above. To the north-east and south-east the gold mineralisation is restricted by high angle reverse faults. To the south-west and north-west the limits are determined by grade and depth respectively. Away from the main orebody at Rain the gold is restricted to the high angle reverse Rain Fault and to zones a metre or so thick above the contact between the Devils Gate and Webb Formations (Thoreson, 1987).

There are four texturally and genetically distinct breccias types at Rain as follows:
 1). Hydrothermal breccia, which occured as multistage pre- and syn-ore phases during at least three episodes of oc convective fluidisation, followed by quartz-sulphide-barite cementation - matrix supported, heterolithic hydrothermal breccias at Rain comprise sediments including sandstone, siltstone, mudstone, limestone and conglomerate,
 2). Crackle breccias which formed a capping over the multistage hydrothermal breccias,
 3). Tuffisite with accretionary lapilli, that occur as tabular pipe shaped dykes which cut the hydrothermal breccias and
 4). Collapse breccias which occur along the floor of the composite breccia mass and have irregular upper and lower contacts - the lower contact is on a dissolution boundary with the Devils Gate Limestone (Williams, et al., 2000).

Three joint orientations are prominent, two are generally NE trending with 10° and 45° trends and steep north-west dips, while the third parallels the Rain Fault. All three are veined with silica, barite, alunite, kaolinite, cinnabar and jarosite within the Rain Jasperoid and in un-altered sediments above the ore deposit. At the intersection of the NE and NW striking fractures there are hydrothermal breccias with both silicified and un-silicified Webb Formation fragments in a matrix of silica and/or barite. Hydrothermal breccias increase in abundance as mining penetrates into the main ore zone (Thoreson, 1990).

Well defined sedimentary beds in the Rain pit dip gently at 10° to 30° south-west and are accentuated by increased hematite staining of silty and sandy interbeds. Bedding surfaces are well defined and often display a thin clay gouge which sometimes displays striations indicating bedding plane slip. Low angle faults (<45°) may emanate from a bedding plane, then cross-cut overlying bedding and re-merge along another bedding plane at a different level within the sequence. These low angle structures can have either a normal or a reverse sense of movement. Tight, high amplitude drag folds, the axes of which parallel the Rain fault, occur in the sedimentary rocks in the hanging wall of the Rain Fault. These folds have wavelengths of greater than 9 m and amplitude of more than 18 m. Un-silicified and barren footwall sediments associated with the Rain Fault are highly contorted, folded, fractured and oxidised Webb Formation siltstones and mudstones (Thoreson, 1990).

At Emigrant Springs the structural control of ore was apparently provided by the high angle, north to north-west striking Emigrant Springs Fault. This fault dips steeply west and juxtaposes Devonian Devils Gate Limestone in the footwall against Chainman Shale in the hangingwall. A vertical displacement of at least 550 m is inferred from drilling. All gold at the prospect lies within the footwall of this structure (Thoreson, 1990).

Alteration and Mineralisation

Alteration and mineralisation at each of the four deposits is as follows:

Rain - Gold mineralisation at Rain is hosted by a hydrothermal zone developed immediately in the hangingwall of the Rain Fault which is 60 m wide and traceable over a length of 5 km (Williams, et al., 2000). The fault zone is occupied by a jasperoid zone which forms a topographic high up to 40 m above the neighbouring terrain. Outcrops along the jasperoid were highly brecciated, silicified and barite veined with scattered gold mineralisation. Locally abundant concentrations of alunite, hematite, jarosite, other limonites and dussertite were also observed along the jasperoid outcrop. Six ore types are recognised at Rain, namely,  1). Siliceous - which contain in excess of 40% quartz, with less than 30% barite;  2). Siliceous/baritic - siliceous ores containing between 30 and 40% barite;  3). Baritic - having in excess of 40% barite;  4). Carbonaceous - those ores with a high total carbon content and generally accompanied by more than 0.5% pyrite;  5). Argillaceous - which have a total clay content of more than 40%; and  6). Calcareous - containing predominantly carbonate, principally calcite (Thoreson, 1990). These ores reflect varying combinations of the main alteration modes, namely  1). silicification;  2). argillisation;  3). baritisation;  4). carbonatisation; and  5). oxidation, which are superimposed upon the oxidised and carbonaceous host sequence (Thoreson, 1987).

The main orebody at Rain, as known in 1990 had plan dimensions of approximately 750 x 150 m, with a maximum vertical extent of around 250 m (Thoreson, 1989 & 1990). However Williams et al., (2000) show the Rain pit having a length of 800 m, while the underground high grade reserve extends down plunge for at least another 750 m.

Carbonaceous, pyritic ores only account for approximately 2% of the total orebody at Rain, and are concentrated in the south-western portion, away from the Rain Fault. The carbon rich zone forms a wedge that thickens to the south-west and pinches out to the north-east near the Rain Fault. This carbonaceous wedge is developed in the lower sections of the Webb Formation, with thicknesses in the mine area of up to 50 m (Thoreson, 1987 & 1990).

The bulk of the gold mineralisation occurs proximal to, and within, the Rain Fault, confined to oxidised Webb Formation sediments, and immediately above the unconformity with the underlying Devonian Devils Gate Limestone. This unconformity is characterised in drill holes by an up to 6 m thick, brick-red, clay layer containing fragments of silicified and baritised Webb Formation sediments and calcite veining. The main ore zone forms a flat lying blanket that is thickest near the Rain Fault and thins to the south-west. The intensity of north-east and north-west fractures increases near the orebody, associated with an increase in hydrothermal breccias which are veined with quartz, barite, alunite, jarosite and kaolinite, both within the Rain Fault and peripheral to the gold mineralisation (Thoreson, 1990).

The main mass of hydrothermal silicification is concentrated in the immediate hangingwall of the Rain Fault and occurs as quartz veinlets and as cherty to massive, crypto-crystalline replacement of clastics and limestones. Within the orebody area the zone of silicification defines an envelope that is slightly smaller than the mineralised volume. Hydrothermal barite forms an envelope contained within the silicified volume, occurring within the jasperoid breccia as breccia fragments, as replacement of breccia fragments and as massive white interstitial barite. Later barite occurs as fracture filling of crypto-crystalline and crystalline barite. Baritisation both precedes and post-dates argillisation and was contemporaneous with silicification. Silicification and barite veined ores are abundant along the Rain Jasperoid and in the main ore zone. Only a small portion of the ore outside of the carbonaceous ores are not silicified or barite veined (Thoreson, 1987, 1989 & 1990).

Argillisation is structurally controlled and followed the earlier silicification and baritisation (Thoreson, 1987). While much of the ore bearing host rock surrounding the orebody is oxidised and argillised, the silicification and baritisation is confined to the immediate orebody vicinity (Thoreson, 1989).

In the primary ore zone the total sulphide content of unoxidised ore is less than 5% by volume (Williams, et al., 2000).

The Rain orebody is interpreted to have resulted from five stages of development (according to Williams, et al., 2000), namely:
 1). Structural preparation along the right lateral oblique Rain Fault system and conjugate lataeral oblique NE striking faults
 2). Multiple episodes of hydrothermal breccia formation during at least three stages of convective fluidisation, with high grade gold deposition immediately following the last brecciation event associated with quartz-sulphide-barite cementation. High grade gold was deposited as a late phase along the upper portion of the hydrothermal breccia mass and extended into the crackel breccia zone.
 3). Late channelisation and fluidised rock fragments and fine clays forming tuffusute bodies with accretionary lapilli,
 4). Post-mineral extension reactivation structures, and
 5). Collapse brecciation resulting from postore supergene acidic fluid ponding on and dissolving the upper Devils Gate Limestone.

Gnome - The Gnome deposit is some 600 m to the south of Rain and has plan dimensions of approximately 200 x 150 m. Mineralisation is confined to Webb Formation sediments directly above the unconformable contact with the Devonian Devils Gate Limestone. Gold mineralised rock is predominantly located within carbonaceous and pyritic sediments. Minor mineralisation is found within silicified and baritic veined and oxidised sediments below the carbonaceous and pyritic ore (Thoreson, 1990).

Southern Mineralised Zone (SMZ) - is a small 200 x 120 m deposit containing around 0.95 t of Au, some 800 m to the south of the Rain open pit and 350 m to the south-east of Gnome. Gold mineralisation again occurs within clastic sediments of the Carboniferous Webb Formation above the unconformity with the underlying Devonian Devils Gate Limestone. The ore is located at the intersection of north-east, north-west and east-west striking high angle structures, although these are not evident in the plans in the literature. The Devils Gate Limestone is folded into a steeply south-west plunging antiform and is exposed near the centre of the deposit. Gold values drape off on all sides of the antiform. The hosts are oxidised with silicified breccias exhibiting minor hydrothermal barite (Thoreson, 1990).

Emigrant Springs - This orebody has maximum plan dimensions of 1100 x 600 m and occurs as a flat lying, faulted sheet up to 240 m, but generally <50 m thick. The maximum thickness is adjacent to the Emigrant Springs Fault. The deposit is located some 2.4 km to the east of the Rain mine and is immediately adjacent to, and in the footwall of the NNE trending and west dipping, normal Emigrant Springs Fault. The Emigrant Springs Fault is occupied by a 15 m wide and 675 m long jasperoid breccia which is highly silicified with minor barite, alunite, jarosite and kaolinite. The ore is contained within the lower Carboniferous Webb Formation clastic sediments, closely overlying the unconformity with the underlying Devonian Devils Gate Limestone. Thin jasperoid outcrops to the east of the Emigrant Springs Fault occur at the surface and form a bedding parallel dip-slope. Gold grades are consistently lower than at Rain, as illustrated by the reserve figures above. Approximately 1 Mt of the total 11.7 Mt resource is classified as mill grade reserve with a grade of +1.7 g/t Au. The remainder of the ore constitutes a near surface, leach grade deposit (Thoreson, 1990).

The ore types are siliceous and oxidised. Although barite is present, concentrations are significantly less than at Rain. All of the ore is oxidised and no carbon/sulphide mineralisation has been identified (Thoreson, 1990).

Arsenic soil geochemistry proved to be the best pathfinder for the Rain deposit with values reported from 10 to 950 ppm, with some peaks of more than 1000 ppm As, while geochemically detectable gold was a poor pathfinder (Thoreson, 1990).

Note: this summary was prepared in two stages, the first in the mid 1990s, which was updated in 2003 from additional information then available. As such it may mix different interpretations.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 2000.    
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:
Thoreson R F, Newmont Exploration Ltd  1987 - The Rain gold deposit: in Johnson J L (Ed.), 1987 Bulk Mineable Precious Metal Deposits of the Western United States - Guidebook for Field Trips Geol. Soc. Nevada    pp 269-270
Williams C L, Thompson T B, Powell J L, Dunbar W W  2000 - Gold-bearing breccias of the Rain Mine, Carlin Trend, Nevada: in    Econ. Geol.   v95 pp 391-404


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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