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Gold Quarry - Geology |
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Nevada, USA |
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Au
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Super Porphyry Cu and Au
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The Gold Quarry mine is located some 11 km to the north-west of the town of Carlin in Eureka County, north-eastern Nevada, USA. It lies on the Carlin Trend of deposits and is part of the Carlin operations of Newmont.
Published reserve and production figures include the following:
348 Mt @ 1.15 g/t Au = 400 t Au (Proven+probable reserve+production, 1993, Rota, 1993)
163 Mt @ 1.39 g/t Au (Proven+probable reserve, 31 Dec. 1992, Christensen, 1993)
543 Mt @ 1.17 g/t Au = 635 t Au (Total Geological reserve, 27 Dec. 1989, McFarlane, 1991)
130 Mt @ 1.68 g/t Au (Initial pre-Mining reserve after 550 holes, Ryneer, 1991)
4.35 Mt @ 3.2 g/t Au (Reserve Maggie Creek Mine 1980, Christensen, 1993)
0.308 Mt @ 4.1 g/t Au (Initial reserve, Gold Quarry, 1970, Rota, 1993)
Total contained gold in production + reserve/resource in 2002 = 746 t Au (Harlan et al., 2002)
373 t Au (Total production to end 2002, Newmont 2005)
Rota (1991 & 1993) notes that initial prospecting in the area surrounding Gold Quarry began in around 1870. By 1935 a number of small underground and open cut mines along the Good Hope and Gold Quarry Faults had produced several hundred tonnes of Cu, Pb and Ba. Intermittent small scale mining continued into the 1960's. The total Au and Ag production from 1932 to 1958 was 26 kg and 136 kg respectively. Between 1960 and 1962 Newmont geologists Coope and Livermoore mapped and sampled jasperoid outcrops near the intersection of the Good Hope and Gold Quarry Faults. They obtained anomalous Au values of 1 ppm, with up to a 2 ppm maximum, at the surface. This was the Gold Quarry discovery outcrop. Elsewhere in the Gold Quarry area jasperoids assayed around 100 ppb Au. In 1962 Newmont acquired the leases over the Maggie Creek/Gold Quarry Claims. Drilling of 130 shallow (around 120 m) rotary drill holes between 1962 and 1970 outlined 0.308 Mt @ 4.1 g/t Au. Metallurgical testing however, indicated poor recoveries caused by the silicification in the ore zone. As a consequence of the discouraging metallurgical tests, the low grade at the prevailing gold price and the fact that the nearby Carlin prospect showed more promise, title prioritisation led to the Gold Quarry/Maggie Creek leases being sold in 1970 for $US 8000 to the owners of the nearby T Lazy S Ranch (Ryneer, 1987; Rota, 1991 & 1993).
The same leases were re-acquired in 1972 at the price of a 16% production royalty, and further testing commenced. This resulted from the re-appraisal of the prospect suggesting it was open-ended. Between 1972 and 1977 drilling continued on the Maggie Creek/Gold Quarry Claims and surrounds. In 1976-77 testing of anomalous areas south-west of the main claims delineated a resource of 4.35 Mt @ 3.2 g/t Au at what was to become the Maggie Creek Mine. This is now known to represent the upper-most portion of the Deep West ore zone of the Gold Quarry deposit. Mining commenced at Maggie Creek in July 1980 based on this reserve, although at that stage the existence of the main Gold Quarry deposit had been indicated. The Maggie Creek orebody was located approximately 900m south-west of the Gold Quarry discovery outcrop jasperoids and occurred along splays of the Gold Quarry fault systems. There was apparently no direct mineralisation connection between the Gold Quarry discovery outcrop and the Maggie Creek orebody. High grade ore (>2.4 g/t Au) from Maggie Creek was trucked to Carlin while the bulk of the ore was heap leached on site (Rota 1991 & 1993; MacFarlane, 1987; Rota, pers. comm., 1993).
Exploration drilling 600m south of the original Maggie Creek/Gold Quarry Claims and 300m east of Maggie Creek Mine intersected further significant Au in 1979, below 75 m of cover. After a program of 550 holes some 130 Mt @ 1.68 g/t Au was delineated, establishing the main Gold Quarry deposit (Ryneer, 1991; Rota 1991). Production at Gold Quarry commenced in 1985, following a feasibility study, stripping, and construction of both Mill #2 and the dump-leach facilities between 1982 and 1984. Exploration drilling continued between 1986 and 1990, with Mill #5 and the South Area Leach facilities being constructed. During this period the Deep West ore zone was outlined beneath the Main Gold Quarry orebody. In 1989-90 deep exploration discovered the 'structural feeder zone' in the south-western corner of the deposit. In the same year the Maggie Creek heap leach facility was de-commissioned to allow for expansion, while significant 'refractory' resources were discovered below the oxidised portions of the orebody. A refractory ore process plant was designed in 1992, constructed in 1993 and began production in 1994. At this stage all remnants of the Maggie Creek Mine had been removed. By 1991 the total exploration and development drilling at Gold Quarry had reached 450 000 m (Rota, 1991 & 1993).
Geology
Within the Gold Quarry Mine area three successions are represented. These are the lower plate autochthonous Eastern or Carbonate Assemblage, separated from the allochthonous Western or Siliceous Assemblage by the regional package of thrust surfaces known as the Roberts Mountains Thrust. Possible Transition Assemblage rocks which constitute the main host, the Rodeo Creek Unit, separate the two assemblages, sandwiched by splays within the Roberts Mountains Thrust. These assemblages are unconformably overlain by Tertiary and Quaternary cover. For regional detail see the 'Carlin Trend Ð Geology' and 'Carlin Trend Ð Mineralisation' records.
The stratigraphy in the mine area is as follows, from the base, commencing with the Eastern or Carbonate Assemblage:
Ordovician, Hanson Creek Formation, 180 to 275 m thick - this unit is the structurally lowest known within the mine area. It is characterised by black, massive dolomite to dolomitic limestone with white quartz veins. The middle portion has been dated as upper Ordovician. It contains black knobby to lensoidal chert developments which increase in frequency up-section. The top of the formation is commonly marked by a tan-brown sandy dolomite or sandstone. This unit locally contains scattered geochemically anomalous Au and Ag values (Rota, 1991 & 1993).
Siluro-Devonian, Roberts Mountains Formation, 365 to 460 m thick - composed overall of medium to thinly bedded, grey, silty dolomitic limestone. The bedding thins and the silt content increases up-section. It grades upward from a medium to thick bedded grey dolomitic limestone with black chert, similar to the Hanson Creek Formation at the base, to a medium to thick bedded, very fine grained calcarenite and limy mudstone. Higher still, with the detrital sand and silt content increasing, it grades into a laminated to thin bedded silty to sandy limestone. Sedimentary features include repeated minor turbidites (graded bedding), soft sediment deformation and cross-bedding. The uppermost portions host locally economic concentrations of disseminated gold, including the Tusc and Deep West mineralisation (Rota, 1991 & 1993).
Devonian, Un-named Limestone (probable Popovich Formation equivalent), 60 to 120 m thick - which comprises massive, grey limestone and calcarenite which grade upwards into thin-bedded, silty limestone. It contains sedimentary breccias, conglomerates and possible debris flows. It has been interpreted to represent locally derived lime debris deposited in a discrete basin. This unit is a probable equivalent of the Popovich and Bootstrap Limestone units and hosts part of the Deep West mineralisation (Rota, 1991 & 1993).
Roberts Mountains Thrust (Chukar Gulch Fault) Splay, up to 45 m thick - which is represented by a variably argillised mylonite breccia locally separating the Quarry Member siltstone of the Rodeo Creek Unit (described below), from the Devonian Limestone (described above). A low angle structure within this zone which strikes at 20° and dips at 40°SE, is coincident with the Deep West ore zone. The thrust splay is often masked by alteration associated with the Deep West mineralisation (Rota, 1991).
Devonian, Rodeo Creek Unit, up to 300 m thick - this unit may be part of the transition facies representing deposition between the Eastern autochthonous carbonate and Western allochthonous silici-clastic Assemblages. It appears to be sandwiched above and below by splays of the Chukar Gulch Fault (or Roberts Mountains Thrust) zone. The sequence is made up predominantly of a thin to medium bedded, grey succession of siltstone, mudstone, chert and argillite - principally representing a 'ribbon argillite' unit. It may represent a structurally repeated and thickened section. Locally it has been divided into two members, namely the:
- James Creek Member (60-120 m thick) - the lower member which may locally be in stratigraphic contact with the Devonian Limestone. There is a stratigraphic transition from carbonate to the lowest siliceous member of the Rodeo Creek Unit. It comprises a light grey, thinly laminated to thinly bedded silty limestone, limy siltstone and siltstone. The carbonate content and bedding thickness decrease upwards. It has been de-calcified in mineralised zones and hosts significant gold in the Maggie Creek and Deep West orebodies (Rota, 1991 & 1993).
- Quarry Member (approx. 200 m thick) - normally a thin to thick bedded light grey siltstone and mudstone with minor black chert lenses or boudins. The siltstones thin up-section. The lower section contains thin to medium bedded, alternating bands of argillaceous black chert, siliceous mudstone and light grey siltstone, with the chert content decreasing upwards from the bottom of the sequence. Soft sediment deformation features indicate rapid loading. A 25 m thick coarse siltstone unit occurs near the top. Upper Devonian to lower Carboniferous ages are indicated from palynology. Chaotic folding and possible structural repetition occur near thrusts and reverse faults. Cross and rare graded bedding are found in the coarse siltstone beds, while any original carbonate will have been removed by the mineralising event. This member is the host to the main Gold Quarry ore zone (Rota, 1991 & 1993).
Roberts Mountains Thrust (Chukar Gulch Fault), generally up to 12 m thick - this structure represents a regional break. It varies from a small thin, red silicified mylonitic breccia to a large, argillised mylonitic zone locally up to 40 m thick. It has abundant low angle slickensides in variable orientations and emplaces the Ordovician Vinini Formation over the Devonian Limestone (Rota, 1991). The major east directed movement on the Roberts Mountains Thrust was in association with the Devono-Carboniferous Antler Orogeny (Rota, 1991 & 1993).
Ordovician, Vinini Formation, 90 to 180 m thick - representing the allochthonous upper plate of Western or Siliceous Assemblage above the Roberts Mountains Thrust, the main splay of which defines its base locally. It is composed of interbedded, thin to medium bedded, black argillaceous chert or siliceous mudstone and light grey argillite. Significant thicknesses of siltstone and mudstone locally host lower grade ore. Isoclinal recumbent, chaotic folding is common. This unit hosts low grade ores in the north-eastern parts of the Main Gold Quarry ore (Rota, 1991 & 1993).
Unconformity - which cuts both the Eastern and Western Assemblage units.
Pliocene, Carlin Formation, which varies from <30 to >600 m thick - basin fill lacustrine, tuffaceous sediments and fluvial conglomerates derived from the surrounding Palaeozoic ranges. It includes erratic, un-economic channel deposits of gold mineralised clasts up to 3 km to the south-east of Gold Quarry. It may be sub-divided into a:
- Lower Member - composed of a basal conglomerate of grey Quarry Member clasts and greenish clays, often above a red palaeosol at the regional unconformity. Only the clasts are mineralised (Rota, 1991).
- Middle Member - white to greyish blue, thin bedded montmorillonitic lacustrine tuffs. Fluvial channel conglomerates are also present, with well rounded, poorly sorted chert and siltstone. The clasts are seldom mineralised (Rota, 1991).
- Upper Member - a brown, calcareous colluvium or debris flow which is found above an erosional unconformity with the lacustrine sediments. This unit is not mineralised (Rota, 1991).
Local Unconformity
Quaternary, Alluvium - mainly occurring as fanglomerates made up of rounded andesite, chert and siliceous mudstone clasts which form the ridge caps above the Maggie Creek Mine. No mineralisation is known from these lithologies (Rota, 1991).
The thickness of the Tertiary and Quaternary cover thickens quickly to more than 2000 m to the north-west of the deposit. The main ore deposit, which was 98% concealed, is under 75 m of cover in a pediment below the level of the more resistant un-mineralised rocks (Rota, pers. comm., 1993).
Structure
The major geological feature of the Maggie Creek Sub-district is the Carlin Window, a roughly circular feature which is approximately 3 km in diameter. It is composed predominantly of carbonates of Ordovician to lower Carboniferous age, overlain above the Devonian Roberts Mountains Thrust by the Ordovician Vinini Formation silici-clastics. These Palaeozoic rocks define an exposure which is some 7 x 3 km, elongated parallel to the Humboldt Lineament, and surrounded by Cenozoic cover of varying thickness.
Antiformal doming of the Carlin Window is indicated by the exposure of the Ordovician Hanson Creek Formation within its interior. Window bounding faults include the shallow east dipping Roberts Mountains Thrust on the north-east, and the Gold Quarry oblique/normal and Good Hope reverse fault systems on the south-east and south-west respectively. Cenozoic sediments have buried the north-western boundary which is inferred to be the NE-SW trending Soap Creek Fault (Rota, 1991 & 1993).
Some 1.5 to 3 km to the south-west of the Carlin Window, the Tuscarora Fault, which is sub-parallel to the Good Hope Fault, marks the north-eastern margin of another window which is in turn bounded to the south-west by the shallow west dipping Roberts Mountains Thrust. These two windows appear to represent an earlier larger window bounded on two sides by the anti-formally folded Roberts Mountains Thrust. This older structure appears to have been dissected by a down-faulted corridor of allochthonous upper plate silici-clastics between the Good Hope and Tuscarora Faults (Rota, 1991).
Three major structural stages are indicated. These are:
i). Compressional - comprising eastward transport during the Devono-Carboniferous Antler Orogeny, reflected by the Roberts Mountains Thrust system and associated minor to significant individual splays (Rota, 1993). The shallowly south-east dipping Chukar Gulch Fault is interpreted to be a splay of the Roberts Mountains Thrust. It is an important locus of mineralisation in the Deep West ore zone, forming the contact between the Devonian carbonates and the Devonian Rodeo Creek Unit;
ii). Translational - which followed the emplacement of the allochthonous plate and appears to have been driven by a NNE principal compressive stress direction. Significant dextral shearing along the Carlin Trend is inferred, with shear zones having interpreted widths of 30 m to 3 km, parallel to the Good Hope and Tuscarora Fault systems. The development of the Carlin Window is described as the response to reverse fault termination by the Good Hope Fault system, of the Gold Quarry sinistral shear system. These were internal to a wider zone of dextral movement. Detailed structural studies have indicated that the Good Hope and Gold Quarry systems are contemporaneous conjugate sets with a 60° separation. These structures pre-date mineralisation, although additional disruption is indicated during the emplacement of ore. Some workers have suggested that the faulting and fracturing involved in this block rotation may be the result of accommodation of volume loss within the carbonates structurally below. Volume losses of up to 50% by de-calcification are indicated in such carbonates at Carlin. De-calcification is also indicated at Gold Quarry. Collapse of host beds due to de-calcification of the Deep West ore zone below the Main Gold Quarry orebody may have led to upward stoping or propagation of normal faults, which would then enhance permeability within the overlying silici-clastics (Rota, 1991 & 1993).
iii). Extensional - following the deposition of mineralisation, extensional reactivation of all of the faults and eastward rotation of the deposit occurred during Basin and Range tectonism. Episodic rejuvenation of nearly all structures is indicated by well developed argillic slickensides that display both strike-slip and dip-slip movement, in particular along the Chukar Gulch Fault.
Both the Good Hope and Gold Quarry Fault systems appear to exert a strong control on the distribution of gold mineralisation in the sub-district, with the Gold Quarry system being locally dominant (Rota, 1993).
The Gold Quarry Fault System a is developed parallel to the major Humboldt Lineament which crosses the Carlin Trend immediately to the south. The Gold Quarry Fault System is a set of high angle structures which strike at about 40° and dip at 75°SE. They include the Les, Footwall Carbon and Alunite Faults. The fault system is marked by prominent jasperoid outcrops along a 2000 m strike length, with silicification decreasing towards the south from its intersection with the Good Hope Fault. Three major fault planes aggregate approximately 180 m of vertical displacement with down-throw to the east. Anomalous gold values within the fault decrease rapidly to the north-east of the intersection with the Good Hope Fault, while barite is concentrated in open fractures in the southern half of the system. An early phase of sinistral strike slip, followed by later dip-slip reactivation is interpreted (Rota, 1991). Note however that on the most recent mine planthe Gold Quarry system Les, Barstow and Noble faults are parallel to and in part coincident with the Chukar Gulch Fault, dipping shallowly to the south-east. These are taken to be listric splays of the Gold Quarry Fault which have followed the older Chukar Gulch Thrust.
The Good Hope Fault is the major connection between the Mike, Tusc, MAC and Gold Quarry deposits. It strikes at roughly 140° over a 2.5 km interval and dips at between 45 and 60°NE. Some field evidence indicates that this is a simple, moderate angle reverse fault, while other observations suggest that it may be related to a regional dextral strike-slip fault zone. Footwall jasperoid outcrops trace the northern two thirds of the system, with the northern end being concealed beneath Tertiary valley fill. Several base metal and barite occurrences within the district are associated with this system.
Steeply dipping normal faults and associated fracturing are the primary controlling features of gold mineralisation, with permeability interpreted as having been enhanced by fracturing. Structural destruction of host sediments is complex enough to give the upper levels of the deposit the appearance of a 'mega-breccia' (Rota, 1991).
The overall strike trend of the mineralised area at Gold Quarry is around 45°, sub-parallel to and bounded on the west by the Gold Quarry System, and to the east by the parallel Challenger Fault System. This latter fault system, which is composed of two main strands the 'Challenger' and 'Grey', constrain all Gold Quarry mineralisation to the east. They comprise a NNE trending, east dipping system of faults separating the underlying host Devonian Rodeo Creek Unit from the overlying Ordovician Vinini Formation silici-clastics. Pliocene lacustrine sediments and overlying colluvium have been vertically offset along the Grey Fault by at least 45 m in the north-west wall of the mine. East of the Challenger system, Devonian bedrock seems to have been steeply offset and down-dropped by about 120 m. The Challenger and Grey Faults converge to the south and were reactivated during the Basin and Range episode (Rota, 1991).
Tabular, higher or mill grade (ie. >1.1 g/t Au) ore zones cross-cut the north-easterly trending envelope of lower grade material bounded by the Gold Quarry and Challenger System Faults. These higher grade shoots have a northerly to north-west strike, tightly controlled by 345 to 350° faults. These are known as the 'Kristalle System' of faults. Silicified faults within all ore zones appear to have been enhanced by 'hydrothermal' dissolution and brecciation. Evidence of faulting becomes less distinct and fault density decreases away from the centre of alteration. The ore deposits are said to be focused in the zones of concentrated deformation. Discrete blocks of siliceous host rocks appear to be interlocked and rotated and are usually bounded by faults and shears of variable orientation. Exposures of low angle structures have been found to increase with depth (Rota, 1991; Rota, pers. comm., 1993).
A major ore-bounding structure, the 'Hangfire Fault' cross-cuts the 'Kristalle System' with a 315 to 320° trend and terminates in a shatter zone at the intersection with the north-east trending 'Sulphide Feeder Zone' which is interpreted as being the main structural conduit of the Gold Quarry hydrothermal system.
Very few of the structures within the silici-clastics continue into the carbonates with the exception of those of the 'Sulphide Feeder Zone' (Rota, pers. comm., 1993).
The other fault which appears to exert a strong control on the localisation of ore is the Chukar Gulch Fault which is interpreted to be a splay of the Devono-Carboniferous Roberts Mountains thrust. The Deep West orebody mineralisation is concentrated on the contact between the underlying Devonian carbonates and the Rodeo Creek Unit which this fault in part defines. This relationship appears implicit in the mine sections, but is not emphasised in the literature.
Continued as the Gold Quarry Ð Mineralisation record
The most recent source geological information used to prepare this decription 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.
Gold Quarry
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Selected References:
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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. 451-484.
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McFarlane D N 1987 - Maggie Creek 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 274-275
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Rota C R, 1988 - The Gold Quarry Mine: History and general geology: 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 49-56
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Rota J C 1987 - The Gold Quarry 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 271-273
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Rota J C and Hausen D M, 1991 - Geology of the Gold Quarry mine: in Ore Geology Reviews v6 pp 83-105
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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, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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