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Sleeper

Nevada, USA

Main commodities: Au Ag
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The Sleeper gold deposit is within the Awakening mining district and is located 45km to the north-west of the town of Winnemucca, in Humboldt County, north-western Nevada, USA. It lies beneath 20to50m of alluvium in Desert Valley, approximately 750m west of the western flanks of the Slumbering Hills. Sleeper is 65km to the WNW of the Getchell orebodies and 115km to the north-west of the Battle Mountain mine, along the projected trend of the Cortez-Battle Mountain Trend.

Geology

The major veins and disseminated mineralisation of the Sleeper Au-Ag orebody are primarily hosted by Miocene rhyolite porphyry, although deeper vein segments occur in propylitically altered tuffaceous sediments and andesite lavas, and in the unconformably underlying folded Triassic to Jurassic slate, phyllite and quartzite. The orebody is concealed by 20to50m of alluvium and lacustrine sediments. The main lithologies recognised in drilling and in the open pit are exposed in the Slumbering hills on the eastern up-thrown side of a major range front fault. Lower grade mineralisation has also been found within the exposed sequences of the range. A Cretaceous granodiorite stock with a diameter of approximately 10km intrudes the sediments and underlies much of the Slumbering Hills to the south (Saunders, 1994).

Most of the Miocene volcanics of the northern Slumbering Hills are outflow facies of the McDermitt Volcanic field and its nested calderas, 55km to the north. Large volumes of per-alkaline ash-flow tuffs were erupted from the McDermitt calderas between 16and15Ma. A younger, local volcanic complex, represented by the rhyolite porphyry dome, was also present, and was important in the localisation of the Sleeper deposit (Nash, etal., 1990).

The geological succession in the northern Slumbering Hills is as follows (Nash, etal., 1990; Conrad, etal., 1993), from the base:

Upper Triassic to Lower Jurassic, Auld Lang Syne Group - dark, fine grained slate, phyllite, quartzite and calcareous phyllite. These sediments were folded, faulted and regionally metamorphosed to greenschist facies during the Mesozoic. The hills to the immediate east of the orebody are underlain by slate, phyllite and calcareous phyllite. Much of this sequence has a subdued expression and is overlain by Quaternary aeolian sand. Wood (1987) notes that the calcareous strata are separated from the overlying non-calcareous beds by a west dipping, low angle thrust.

Cretaceous Granitoid - principally of granodiorite to monzonite composition.
Disconformity to Unconformity.
Tertiary, Succession, composed of (mainly from Conrad, etal., 1993):
  Basal Unit, 200m thick, but locally 40m in the mine, thinning to the west - This unit rests disconformably on the Mesozoic sequence and is composed of varied volcaniclastic rocks, including conglomerate and greywacke beds which are as much as 15m thick, but are typically 1to3m thick. These lithologies grade upward into thick, massive, clay-rich siltstone or water laid tuff. Shard textures and laminated bedding are seen locally in the latter rocks. They may represent deposition in a fluvial or lacustrine environment.
  Intermediate Flows, 150m thick - intermediate lavas flows and breccias, with minor volcaniclastic interbeds. The flow rocks are variably altered and appear to be mainly andesite, with lesser basalt.
  Lapilli Tuffs, >40m thick - which have felsic pumice fragments. It appears to be thickest north-east of the Sleeper pit, but is not recognised in the foothills to the east of the pit.
  Rhyolite Porphyry, 19to16Ma, >300m thick - composed of plagioclase, sanidine, quartz and sparse mafic phenocrysts. This unit is host to more than 90% of the ore at Sleeper. Glassy and partly devitrified zones in the rhyolite porphyry are found in drill holes 250m to the west of the orebody. Texturally and compositionally similar dykes and sills are cut by drill holes below the orebody. Where not destroyed by alteration, the flow banding and flow brecciation are sub-horizontal. Compositionally similar flows are found in the foothills of the Slumbering Hills as much as 2.4km to the east and south-east of the mine.
  Per-alkaline Ash Flow Tuffs, up to 80m thick - overlie the intermediate flows 2km to the north-east of the mine, but have not been found in contact with the rhyolite porphyry.
  Basalt and Andesite Flows, approximately 300m thick - These outcrop 2to4km to the south-east of the mine, but are not in contact with any of the other Tertiary units described above. These may be as young as Pliocene in age.
  Pliocene, Sediments, 10to50m thick - overlie the rhyolite porphyry dome complex in the mine area. These are un-consolidated fluvial to lacustrine sediments, including poorly sorted conglomerates at the base which locally carry alluvial gold. The conglomerate beds are overlain by a sequence of lacustrine beds with two interbeds of air-fall ash. The lower bed, which is 0.2m thick, has been dated at 2.1±0.1Ma.

In the mine area two highly fractured and altered Tertiary volcanic units are disconformably overlain by Pliocene gravel and sand. The major Tertiary unit is the Miocene Rhyolite Porphyry, dated at around 16.5Ma. Plagioclase, sanidine and quartz phenocryst abundances and textures within this lithology are very similar over much of the pit, despite variable hydrothermal alteration. It generally has a massive fabric and uniform primary texture. Interpreted flow foliation is visible at a few localities. Plagioclase phenocrysts comprise about 20to25%, sanidine 5%, and quartz 3to5% of the rock. Silicification in the ore zone preserves textures, although peripheral argillisation produces crumbly rocks that appear to comprise a different unit, although phenocryst assemblages remain the same (Nash, etal., 1990).

The rhyolite porphyry in the pit is very similar to several exposures 1to2km to the east, but rarely displays the same degree of flow banding and flow breccias as the latter. Dyke like bodies of this porphyry intrude the underlying andesite in the Sleeper Pit. Rhyolite porphyry occupies much of the surface below the Pliocene alluvium to the east, west and south of the mine. Away from the main portion of the orebody, generally >300m west of the pit, the rhyolite porphyry has less altered bands which are characterised by the same recognisable phenocrysts, but have a black vitreous matrix (Nash, etal., 1990).

Below the rhyolite porphyry, the other Tertiary unit exposed in the pit comprises a variety of layered volcanic rocks of intermediate composition. These probably include flows, flow breccias and air-fall tuffs, but are mainly lapilli tuffs. The lapilli tuff contains 20to50%, 1to3cm size, angular to rounded fragments of light coloured pumice or 'dark rock'. The lapilli tuff is about 60m thick in the south-eastern section of the Sleeper Pit. The composition of these rocks has been considerably changed by alteration, but was probably andesitic or basaltic. Micro-phenocrysts of tabular plagioclase 10to200m long by 30to50m wide are the only phenocrysts present and are found in both the lapilli tuffs and flows. Some soft, clay-rich layers of possible air fall tuff are interbedded with the lapilli tuff. Dykes of rhyolite porphyry intrude these intermediate rocks. Some beds and compositional layering can be recognised through the alteration. The rhyolite is generally in faulted contact with the underlying intermediate unit (Nash, etal., 1990).

Structure

The 'basement' Triassic strata in the Northern Slumbering Hills form a 20° trending monocline which dips at 45° to the east. South-east of the Sleeper deposit the Triassic beds are dislocated by faulting, are moderately folded, and generally dip to the north-west. A low angle, west dipping, thrust fault separates calcareous beds from the overlying non-calcareous strata in the same area (Wood, 1987).

The Sleeper gold deposit occurs in an extensive 10° to20° trending fault zone that is more than 1000m wide and has been traced for 4000 m along strike. Large normal faults bound the east side of this zone and form the scarps of the range front on the western margin of the Slumbering Hills (Wood, 1987). This fault zone is represented by west side down faulting of the rocks in the mine area along a series of step-like faults which belong to the regional, generally north-striking, Basin and Range set. These faults are sub-parallel to the gold veins, and dip at 50° to 70° to the west. The offset on individual structures is hard to determine, but is believed to be of the order of up to 100m. A few splays of these Basin and Range faults dip at about 45°E. Several of the Basin and Range faults cut the orebody and are well exposed in the pit, occurring as broken and sheared zones as much as 20m wide. A few post ore faults that strike north-west are filled by barren quartz which are as much as 1m thick. Un-healed faults and fractures have steep dips and diverse strikes, most commonly 30to60°, 300 to 330° and due north to 10°. Displacement on these late faults is believed to be generally <3m, although some may be up to 6 m (Nash, etal., 1990; Conrad, etal., 1993).

A large 310° trending normal fault which is 3 to 30 m wide cuts the sequence within the Slumbering Hills to the east and intersects the main north-trending major fault zone in the vicinity of the Sleeper ore deposit. The Sleeper gold deposit formed in a complex breccia zone which was known to be 180 m wide and more than 550 m long in 1986, localised at the intersection of the two fault systems (Wood, 1987).

The high grade gold bearing veins occupy fractures that consistently strike at 30° and dip at 60° to 65°NW. Ore is clearly fracture controlled, although there is little evidence of pre-ore displacement along these fractures. All faults with a measurable off-set, and the majority of fractures are post-ore. Displacement on the Basin and Range faults is complex and probably multi-stage, including offset of Pliocene and Quaternary lake bed sediments. Faults with local west-up displacement are also seen at various points throughout the pit. Although the complex faulting pattern makes displacement hard to determine, it appears that the Sleeper deposit has been down faulted about 300 to 600 m relative to the foothills of the adjacent Slumbering Hills to the east. The major phase of faulting appears to be bracketed by the age range of 15to2Ma, with only two faults displacing the Pliocene sediments (Nash, etal., 1990; Conrad, etal., 1993).

Wall rock and ore are broken into centimetre sized fragments in zones of intersecting fractures. Most of the late fracture surfaces are coated with red to brown earthy iron oxides that give the prominent red colouration to the upper parts of the pit. These fracture zones appear to narrow downward and are tight about 30 m above the unconformity with the Mesozoic sediments. Some of the north and north-west striking faults were the sites of acid leaching and subsequent infilling by opaline quartz and white to brown alunite-jarosite (Nash, etal., 1990).

Mineralisation and Alteration

The Sleeper gold-silver deposit consists of 1). high grade bonanza veins, 2). medium grade breccias and 3). low grade stockworks. The high grade veins contain approximately 60% of the gold, while the breccia and stockwork zones account for the remaining 40%, which is present as easily extracted bulk mineable heap leach ore. Stockwork ore contributes about 20% of the total contained gold. The Au:Ag ratio of between 0.1 and 0.01 in the marcasite rich breccia and stockwork zones is much lower than that of the bonanza veins which is generally >1 (Saunders, 1994; Conrad, etal., 1993; Nash, etal., 1990).

The bonanza veins assay as high as 6700 g/t Au over a 6.1 m vertical blast hole, or 5500 g/t Au over a 1.5 m true thickness of reverse circulation drill intersection. Bonanza veins are as much as 4 or 5m thick and average 50 to 100 g/t Au. Breccia ore is generally within 5m of high grade veins, but also occurs as discrete zones several metres wide in both footwall and hangingwall rocks. Approximately half of the breccia ore, chiefly that near the veins, is processed by the mill with the vein ore. The stockwork mineralisation has grades represented in the leach reserve above, namely around 0.7 to 0.8 g/t Au and 8 g/t Ag (Nash, etal., 1990; Saunders, 1994; Conrad, etal., 1993).

The bonanza veins - are banded quartz-adularia-gold/electrum veins with minor carbonate, barite and late stibnite. They are situated near the centre of the extensive, lower grade stockwork zones and breccias. Although disrupted by post-ore faulting, the bonanza veins are quite consistent along strike for distances of more than 200m, and down-dip also for more than 200 m. Veins occur in a zone that is at least 450 m wide and more than 1200 m long. In most pit exposures a series of parallel sheeted veins occur in a zone 10 to 25 m wide. Intervening wall rocks are medium grade breccias and stockworks. Scattered veins that are less than 0.5m thick in the hangingwall are possibly branching splays of the main, west dipping vein system. Significant high grade intersections of the main veins have been encountered as deep as 500m below the surface (Nash, etal., 1990; Saunders, 1994; Conrad, etal., 1993).

Hand specimens of the bonanza veins may have up to 30 separate bands of silica and electrum, indicating multiple mineralising pulses. Individual bands are generally less than 1mm thick and tend to be mono- or bi-metallic. Some bands may contain as much as 50% electrum by volume. The electrum bands are separated by one or more layers of silica that contain little or no electrum and rarely any sulphides. The electrum is typically 62 to 71% Au. The trace sulphides, generally <1%, within the quartz-electrum veins include miargyrite1, naumannite, acanthite, tetrahedrite and marcasite. Fine grained adularia and minor barite are locally present in either silica or electrum bands. No significant pyrite or marcasite has been observed in the gold-silica bands. The early stage silica is generally milky or grey, while the late stage is dark grey or black. Carbonate minerals may have been part of the early stage, although these have been subsequently replaced by silica leaving only tabular forms within the veins (Nash, etal., 1990).

Three major veins have been recognised at Sleeper, the East and West Veins in the southern half of the deposit, and the Sleeper Main Vein in the northern. All strike approximately north-south and dip to the west (Saunders, 1994).

Gold in the veins appears, on textural and other criteria, to have been deposited from solutions carrying supersaturated amorphous silica and colloidal gold. It has been postulated that precipitation was triggered by a significant pressure drop, possibly caused by hydro-fracturing of the enclosing rock (Saunders, 1994).

Dating of high grade banded vein material has yielded ages of from 16.6±0.5to 15.0±0.8Ma. Similar dating of a late stage mineralised adularia-quartz vein cutting the breccia zone gave an age of 14.1±0.6Ma. The host rhyolite has been dated at around 16.5Ma (Conrad, etal., 1993).

The breccias and stockworks - are cemented by silica-pyrite-marcasite and contain variable amounts of silver and gold. Stockwork ore extends outwards from bonanza veins for up to 200m, especially into the structurally preserved western, hangingwall side of the veins. The stockworks are fractured rocks cut by numerous thin (<1cm) veinlets of random orientation. Some breccias may relate to faults, although most appear to be erratic local structures with no displacement. The breccias are typically clast supported, 0.5to5m wide and extend gradationally outwards into stockwork ore. The matrix makes up around 15to 30% of the breccia and comprises fine rock particles and hydrothermal cement, mainly pyrite-marcasite. Breccias invariably only contain fragments of the enclosing rock type and are almost invariably mono-lithologic. Clasts are angular and grade downwards in size from 30cm to the <0.1mm rock flour of the matrix. Banded quartz vein clasts in some breccias indicate that those breccias are younger than the veins, whereas other older breccias are cut by banded quartz veins. Breccias and stockworks are flooded by silica and pyrite, although there is no apparent reaction selvage around fragments or on veinlet walls (Saunders, 1994; Conrad, etal., 1993; Nash, etal., 1990).

Ore bearing breccias are invariably siliceous and are much richer in sulphides than are the veins. The breccias typically contain 5to10% sulphides as matrix cement, giving the matrix its characteristic dark colour. One variety of black matrix contains abundant stibnite. Microscopically the matrix iron sulphides are very fine euhedral crystals that are 10to30m across. Local concentrations of adularia have been found in matrix cement along with predominant micro-quartz. Opal is not common in breccias, but may have been a precursor of the micro-quartz with jigsaw textures (Nash, etal., 1990).

Large volumes of rock are cut by quartz-sulphide veinlets containing small amounts of Au and Ag that contribute about 20% of the reserve. The veinlets are generally only a few millimetres thick, spaced at several centimetres apart, and are most widespread in silicified rhyolite porphyry. Most stockwork veins are associated in time and space with pervasive early silicification, although there are also later cross-cutting veinlets. The stockwork mineralisation is only found within argillic alteration where that alteration has overprinted earlier silicification. As such the stockwork appears to be controlled by the physical properties of the host (Nash, etal., 1990).

One of the key interpretive features of the Sleeper deposit is the superposition of multiple structurally controlled stages of mineralisation. It has been inferred that an ancestral fracture zone established the location of fracturing and pre-vein silicification. Once established the zone of early silicification became the locus of repeated brittle fracturing, stockwork veinlets and breccias. Silicified rhyolite apparently was most favoured for brittle deformation (Nash, etal., 1990).

Supergene ore - is found in the upper parts of the Sleeper orebody. Gold has been redistributed into un-cemented fractures and voids, while silver is leached from electrum and re-deposited as halide minerals. Naumannite was replaced by gold and cerargyrite. Electrum in samples from the upper 30m of the vein typically has rims that are gold rich (up to 86% Au, 14%Ag) and is intergrown with cerargyrite. Cerargyrite also fills fractures and vugs. Extremely rich samples of gold with moss textures formed in voids and fractures, indicating that gold was mobile. Kaolinite, halite and films of FeCl, FeSO4 and Co-oxide minerals also fill fractures and vugs. Despite the evidence of gold and silver mobilisation, bulk Au:Ag ratios do not change appreciably in the upper sections of blast holes, nor is there evidence of any enrichment blanket (Nash, etal., 1990).

All of the Tertiary rocks within the open pit were moderately to intensely altered during multiple stages of hydrothermal and supergene alteration. Drilling indicates that argillic alteration extends for 500to1500 m beyond known ore. Silica-pyrite alteration dominates a broad area from the veins outwards for more than 700m. Silicification and argillic alteration zones are diffuse, rather than localised along individual centimetre scale veins. Four generic alteration types have been distinguished, 1). silicification, 2). argillisation, 3). acid leaching and 4). supergene.

These styles of alteration can be summarised as follows:

Silicification - involves replacement and open space filling by silica minerals, the most prominent and symptomatic alteration at Sleeper. Early silicification, accompanied by pyrite, resulted in the replacement of plagioclase phenocrysts and groundmass by silica. Rock textures remained intact. Broad zones of rhyolite porphyry were silicified at this stage. This early silicification also filled vesicles with microcrystalline opaline silica and peppered pyrite through the groundmass of tuffs and flows. Analyses of early silicified rock generally show <0.1ppmAu (Nash, etal., 1990).

Silicification of several types accompanied the ore stages. Massive white opal as much as a metre thick commonly occurs parallel to high grade veins, generally on the footwall side. This opal fills re-opened parts of these veins. Silicification is intense in breccia and stockwork ore zones, although most, if not all, of this ore stage silicification seems to be superimposed on previously silicified rocks (Nash, etal., 1990).

Late stage silicification, in the form of opal is controlled by post-ore faults and fractures. This late multi-stage opaline silicification is barren and typically contains abundant light coloured alunite, kaolinite or jarosite. Small amounts of native sulphur are enclosed in the late silica. Some of the opaline silica fills sponge like voids created by acid leaching and some occurs as fine powdery white opal superimposed on hard grey silica-pyrite alteration. Late white powdery fracture coatings are widespread comprising very fine alunite, kaolinite and opal. Opal has recrystallised to cristobalite in many places (Nash, etal., 1990).

Argillic Alteration - feldspar phenocrysts and aphanitic groundmass in barren and low grade peripheral zones are commonly altered to clay minerals. X-ray diffraction indicates that these are most commonly composed of kaolinite, smectite-montmorillonite and illite-sericite. Plagioclase phenocrysts are altered to very fine clay minerals and alunite. Sanidine survives most argillic alteration, but is locally altered to kaolinite or sericite. Groundmass is generally altered to a very fine clay-silica mass. The least altered lithologies contain chiefly smectite, while more intensely modified rocks are composed of a mixture of kaolinite, smectite and sericite (Nash, etal., 1990).

Hypogene argillic alteration occurs peripheral to zones of early silicification, commonly with a transition zone of 3to50m wide of clay altered plagioclase in a silicified groundmass. Most argillised rocks are friable, containing a hard siliceous groundmass with a few percent clay along fine fractures that weakens the rock. The argillic alteration zone also includes sericite-pyrite-microquartz assemblages, as the sericite cannot be discerned in hand specimen. Sericite appears to be more abundant than smectite and kaolinite adjacent to silicified zones (Nash, etal., 1990).

Acid Leaching - Thoroughly leached rocks with a sponge like texture occur in local zones along post ore faults. These oxidised, light coloured rocks contain more than 90% silica with most of the other constituents having been leached. Subsequently opal, alunite and jarosite were deposited in the porous rocks. Late powdery silica seems to be a variety of acid leach alteration. This leaching appears to be post ore and has been dated at 5.4Ma (Nash, etal., 1990).

Supergene Alteration - Oxidation is widespread in the top of the orebody. The prominent red colouration in the upper parts of the pit reflect the destruction of sulphides and local redistribution of iron onto fractures. The interior, protected parts of rocks, away from fracture surfaces, are partially oxidised to green or pink hues, while pyrite can persist within silicified rocks. Pyritic tuffs on the eastern side of the pit have been oxidised to tan alunite-jarosite-limonite bearing mixtures. Broad areas of friable altered rhyolite porphyry in the south and east side of the southern section of the pit and the western side of the northern half of the mine have been influenced by supergene alteration of earlier argillic rocks. Available information implies that most of the alteration was probably produced during the hydrothermal stages (Nash, etal., 1990).

Published reserve and production figures include:

Mill Production to 1989 - 0.78 Mt @ 19.17 g/tAu = 13.75 t Au (Wood & Hamilton, 1991).
Leach Production to 1989 - 5.55 Mt @ 0.45 g/t Au = 2.43 t Au (Wood & Hamilton, 1991).
Mill Reserve, 1989 - 3.10 Mt @ 10.87 g/t Au = 33.62 t Au (Wood & Hamilton, 1991).
Leach Reserve, 1989 - 40.71 Mt @ 0.72 g/t Au = 29.30 t Au (Wood & Hamilton, 1991).
Geol. Resource, 1986 - 3.4 Mt @ 7.5 g/t Au, 37.4 g/t Ag (Wood, 1987).
Feasibility Mineable Reserve, 1986 - 1.3 Mt @ 11.0 g/t Au, 30.9 g/t Ag (Wood & Hamilton, 1991).

For detail see the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1994.    
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:
Conrad J E, McKee E H, Rytuba J J, Nash J T, Utterback W C  1993 - Geochronology of the Sleeper Deposit, Humboldt County, Nevada: Epithermal gold-silver mineralization following emplacement of a silica flow-dome complex: in    Econ. Geol.   v88 pp 317-327
Saunders J A  1994 - Silica and gold textures in bonanza ores of the Sleeper deposit, Humboldt County, Nevada: Evidence for colloids and implications for epithermal ore-forming processes: in    Econ. Geol.   v 89 pp 628-638
Wood J D,  1988 - Geology of the Sleeper gold deposit, Humboldt County, Nevada: in Schafer R W, Cooper J J, Vikre P G (Eds), 1988 Bulk Mineable Precious Metal Deposits of the Western United States Geol Soc of Nevada, Reno,    pp 293-302
Wood J D, Amax Exploration  1987 - General geology of the Sleeper gold deposit, Humboldt 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 173-174


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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