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Gold Acres |
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Nevada, USA |
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Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Gold Acres gold orebody, the principal deposit of the Bullion Mining District, is located some 45 km to the south-east of the town of Battle Mountain, 13 km north-west of the Cortez mine and mill, and lies within Lander County, north-central Nevada, USA.
The deposit is principally hosted by the Siluro-Devonian Roberts Mountains Formation silty limestones with lesser ore in the overlying strongly deformed Devonian Slaven Chert and Ordovician Valmy Formation. Mineralisation is found within an imbricate shear zone belonging to the Devono-Carboniferous Roberts Mountains Thrust, at the western margin of the Gold Acres Window. The 9 sq. km Gold Acres Window comprises autochthonous Eastern or Carbonate Assemblage Palaeozoic rocks surrounded above the folded and faulted Roberts Mountains Thrust by allochthonous Western or Siliceous Assemblage Palaeozoic rocks (Hays and Foo, 1990).
Prospecting and mining in the district dates back to 1862. The Gold Acres deposit was originally discovered in 1922 (Radtke, et al., 1978) although gold mining in the North Pit area only commenced in 1935. Underground methods were employed initially, with open pit mining introduced at a later stage. It was one of the few gold mines to continue production through World War II, and was one of the largest gold producers in Nevada in the 1950's. The main North Pit was mined from 1942 until 1962 (Hays and Foo, 1990). Carbon in the clayey ore, which made it refractory, was the main reason for the cessation of operations (McFarland and Kirshenbaum, 1991).
Placer Amex, signed an agreement to assess the Kelly-Moloney property at Gold Acres in 1969, followed in 1970 by a further agreement over the adjacent London Extension property. These were both acquired in 1972 and drilled through 1973. In 1973 drill indicated ore was defined to the south of the existing North Pit, with an initial minable reserve of 1.45 Mt @ 3.29 g/t Au. Cortez Gold Mines began mining the less oxidised, high clay ores of the South Pit in 1973 and continued until 1976. Confirmation and exploration drilling continued through that period (Hays and Foo, 1990). The higher grade milling ore was hauled to the Cortez Mill and treated there (McFarland and Kirshenbaum, 1991).
Production from the Bullion District to the end of 1977 included 9.3 t of Au, around 90% of which had come from Gold Acres, although approximately 0.3 t was from placer accumulations. Gold Acres also produced significant amounts of silver and a small quantity of tungsten (McFarland and Kirshenbaum, 1991). Limited heap leaching and milling of dumps and stockpiles from Gold Acres continued until 1983 (Hays and Foo, 1990). Heap leaching facilities had been constructed at Gold Acres in 1973, with a carbon extraction plant, to treat the lower grade ore on site (McFarland and Kirshenbaum, 1991).
Between 1984 and 1986 rotary and diamond drilling added further to the reserves and produced samples for bulk metallurgical testing. In 1987 mining re-commenced, centred around the North Pit where a predominantly refractory minable reserve of 1.99 Mt @ 3.67 g/t Au had been outlined. Ongoing exploration had also delineated additional geological reserves around the South Pit (Hays and Foo, 1990). Autoclave roasting of the refractory ore oxidises it and allows the gold to be retrieved (S Foo, Pers. comm., 1993).
Published reserve figures are as follows:
1.45 Mt @ 3.6 g/t Au = 4.8 t Au (Initial Minable Reserve, 1973, Hays and Foo, 1987).
2.5 Mt @ 2.2 g/t Au = 5.5 t Au (Resource, 1983, Bagby and Berger, 1985).
1.99 Mt @ 4 g/t Au = 8 t Au (Additional Refractory Reserve, 1987, Hays and Foo, 1987).
8.07 Mt @ 2.88 g/t Au = 23.25 t Au (Gold Acres Refractory Ore, December 2018, Miranda et al., 2019,)
NOTE: The Gold Acres deposit has been mined as part of the Cortez Complex operation. This summary is the result of a literature review conducted in 1996. A less detailed description of the understanding of the deposit in 2019 taken from Miranda et al., 2019 (an NI 43-101 Technical Report for Barrick Gold Corp) is included in the Cortez summary, and may reflect the evolution of ides and observations since 1996.
Geology
As outlined above the Gold Acres orebody is principally hosted by the autochthonous Eastern or Carbonate Assemblage within the Gold Acres Window, below the Devono-Carboniferous Roberts Mountain Thrust. The allochthonous upper plate rocks have been displaced up to 80 km towards the east over this thrust during the Antler Orogeny, and are composed of Western, or Siliceous Assemblage Palaeozoic rocks (Hays and Foo, 1990).
The exposed Gold Acres Window is bounded to the south-west by the upper plate allochthonous rocks above the Roberts Mountains Thrust and to the north-west by similar rocks across a north-east trending steep normal fault. To the east the window is concealed by the alluvial cover of a north-east trending Basin and Range basin which also forms the north-western margin of the Cortez Window 10 km to the south-east (Hays and Foo, 1990).
Regionally around Gold Acres the allochthonous upper plate sequence includes the Cambrian Harmony Formation; Ordovician Valmy and Vinini Formations; the Silurian Elder Sandstone and Fourmile Canyon Formations; and the Devonian Slaven Chert. The lower plate autochthonous carbonate rich sequence is made up of the Cambrian Prospect Mountain Quartzite, Pioche Shale, Eldorado Dolomite and Hamburg Dolomite; the Ordovician Eureka Quartzite and Hanson Creek Formation; the Siluro-Devonian Roberts Mountains Formation; the Devonian Wenban Limestone; and the lower Carboniferous (early Mississippian) Pilot Shale (Hays and Foo, 1990).
Overlying both assemblages are the Overlap Assemblage rocks which comprise the upper Carboniferous (mid-Pennsylvanian) Battle Formation conglomerates and the upper Carboniferous (late Pennsylvanian) to lower Permian Antler Peak Limestone. These are overlain in turn above the Golconda Thrust by the lower Permian Havallah Formation which was thrust eastward over the assemblages described above during the Permo-Triassic Sonoma Orogeny. These rocks are in turn unconformably overlain by the early Triassic China Mountain Formation. Further post-early Triassic thrusting is also implied by regional rock relationships (Hays and Foo, 1990).
Later emplacement of granitic stocks during the Mesozoic resulted in localised doming throughout the region and accentuated regional large scale folding, as well as causing upwarping of the Roberts Mountains Thrust. Subsequent erosion of these updomed sections of the thrust led to the formation of windows such as the Gold Acres Window. Regional contact metamorphism produced hornfels and skarns with associated base metal mineralisation and possibly some gold. All of these rocks were overprinted by Tertiary igneous events which included quartz porphyry dykes, rhyodacite and rhyolite tuffs and basaltic-andesite flows (Hays and Foo, 1990).
For more detail of the setting and composition of the individual stratigraphic units listed above see the Battle Mountain - Eureka Trend Ð Geology record.
High angle fault systems with northerly, north-east and north-west strikes are also evident. They have both pre- and post-Antler Orogeny movement recorded, with the most recent displacement being associated with the Tertiary Basin and Range activity (Hays and Foo, 1990).
Within the Gold Acres deposit the following sequence has been mapped, from the structurally lowest unit upwards (Hays and Foo, 1990):
Siluro-Devonian, Roberts Mountains Formation - This is the principal host to the mineralisation. It comprises thin to medium bedded and thinly laminated, calcareous to dolomitic, silty limestone. In the ore zone it is a dark grey in colour, thin bedded and carbonaceous, with discontinuous lenses of black chert.
Devonian, Wenban Limestone - occurs as a medium to thick bedded, silty to crystalline limestone, which in the ore deposit area may be altered into a skarn with a calc-silicate and sulphide assemblage. No fossil evidence has been located in the mine area to distinguish it from the Roberts Mountains Formation, other than on the basis of physical appearance.
Roberts Mountains Thrust - which is present as an updomed regional thrust separating the allochthonous lower plate, reasonably un-deformed carbonates, from the strongly contorted and fractured upper plate silici-clastics.
Ordovician, Valmy Formation - This unit contains a variety of rocks, including quartzite, sandstone, chert, shale, siltstone, greenstone and minor amounts of limestone. Overall it is dominated by quartzites and siltstones, with thin bedded chert that hosts ore at the north end of the deposit. The chert ranges in colour from dark to red and green. Shale, sandstone and greenstone also occur in large quantities.
Devonian, Slaven Chert - This is a reasonably resistant unit composed mainly of thin bedded cherts. Nodular, black chert beds 2 to 10 cm thick typify the formation. These chert are made up of very fine to fine silt-sized quartz grains with small amounts of iron oxides, organic matter and sericite.
In addition the following igneous rocks are mapped:
Early Cretaceous, Granite - which is present as a pluton beneath the deposit, although it is not exposed within the open pits. The pluton is believed to be centred approximately 1.6 km to the south-west of the deposit, based on a magnetic high. Drilling indicates a depth of 120 to 180 m to the top of the pluton in the mine area. In core it is a light grey, clay altered adamellite (quartz-monzonite) with approximately 1% biotite and pyrite, while thin section examination identified a medium greenish-grey, sericitised granitic rock. Biotite yielded an age of 98.8andplusmn;2 Ma.
Tertiary, Quartz Porphyry Dykes - have been mapped in the Gold Acres area. Related dykes may be associated with gouge zones along high angle normal faults.
Gold Acres, like the Cortez and Horse Canyon deposits, lies within 4 to 6 km of the margins of the Oligocene Cortez Caldera, to the south of the Gold Acres Window. This Caldera is filled with 1500 m of tuff and interstratified collapse mega-breccia of the Oligocene Caetano Tuff. These tuffs have been interpreted to as being the co-magmatic equivalent of the quartz porphyry dykes found within the orebody area (Rytuba, 1985).
Structure
The most significant structure at Gold Acres is the Roberts Mountains Thrust which forms the south-western margin of the Gold Acres Window and strikes at approximately 330°, with a dip of 20 to 30° to the south-west. An imbricate thrust zone that is sub-parallel to and beneath the Roberts Mountains Thrust hosts most of the mineralisation. The orebody is bounded to the north-west by the north-east striking, high angle, normal Gold Acres fault which has a vertical displacement of 30 m. This latter fault cuts off the North Pit at its northern end and down-drops the stratigraphy to the north. The Island Fault which strikes at 20° and dips at 50° to the north-west is sub-parallel to the Gold Acres Fault and displaces the North Pit strata downwards relative to the South Pit. Four prominent, north-east striking, high angle, normal faults, with displacements ranging from 1 to 6 m lie between, and parallel to the Island and Gold Acres Faults. This block of faults down-drops both the Robert Mountains Thrust and the imbricate thrust zone to the north in a progressive step-wise fashion. Two of these structures bound ore within the Valmy Formation on its east and west margins at the north end of the North Pit. The eastern of these faults juxtaposes the upper plate silici-clastics against the Roberts Mountains Formation carbonates. In addition, a north-striking high angle fault with a westerly dip, which juxtaposes Valmy Formation in the hangingwall against the Roberts Mountains Formation in the footwall, exhibits local controls on the ore along the eastern margin of the deposit (Hays and Foo, 1990).
Alteration
The most significant alteration styles in the mine area are:
Carbon Enrichment - This is the dominant alteration type and is evident in both the upper siliceous and lower carbonate plates. While carbon concentrations occur along fractures and bedding planes suggesting small scale migration within the deposit, it is predominantly pervasively distributed throughout the rock matrix. The pronounced sooty appearance is the result of mature hydrocarbons which are anomalous within the rocks of the ore deposit, relative to the surrounding un-mineralised equivalents. The carbonaceous alteration also generally includes a significant amount of disseminated pyrite and may contain up to 3% by weight organic carbon (Hays and Foo, 1990).
Silicification - also exists in both the upper and lower plate hosts, occurring as a weak to a very intense replacement feature. It is strongest on the northern margins of the ore deposit, waning both to the south and with depth. Silicified upper plate rocks are found peripheral to the open pits (Hays and Foo, 1990).
Argillic Alteration - is found primarily in lower plate carbonates, commonly associated with carbon addition, and is largely confined to fractures. These fractures often form an extensive network throughout the host rock units. The clays are predominantly illitic (Hays and Foo, 1990).
Oxidation - is evident throughout the deposit area, primarily as limonitic and hematitic staining along fractures. Oxidation of sulphides is considered to be a late stage supergene effect (Hays and Foo, 1990).
Skarn - The skarn alteration is apparently most extensive in two separate zones, both of which are barren of gold. These zones are found above and below the imbricate thrust zone that hosts the mineralisation. On the basis of the preserved bedding, the upper skarn is believed to have replaced the Wenban Limestone, while the lower skarn, which is seen only in drill holes, is interpreted to be a replacement of Roberts Mountains Formation. A retrograde skarn assemblage has been identified overprinting the earlier prograde skarn. This retrograde assemblage includes chlorite, tremolite, epidote, calcite, quartz, illite, nontronite, smithsonite, pyrite, pyrrhotite, chalcopyrite, sphalerite, arsenopyrite and galena. In addition, molybdenite and garnet are found in the lower skarn. The sulphides occur as pervasive disseminations, large blebs, veinlets and massive replacements. No gold is associated with the prograde or retrograde skarn alteration at Gold Acres (Hays and Foo, 1990). Note: the source paper does not list the mineralogy of the prograde skarn.
Mineralisation
Two phases of mineralisation are recognised at Gold Acres. The first is the skarn calc-silicates and sulphide assemblage, while the second is the gold mineralisation within the imbricate thrust zone. During both stages the north-east trending normal faults appear to have influenced the distribution of alteration and mineralisation. The majority of the gold mineralisation was deposited within the Roberts Mountains Formation of the imbricate thrust zone, with a lesser amount in juxtaposed upper plate silici-clastics. Some gold was also deposited in the high angle structures and along the Roberts Mountains Thrust above (Hays and Foo, 1990).
Within the ore zone pyrite is disseminated throughout the host rock matrix and is also concentrated along fractures and bedding planes. At least two stages of pyrite have been identified. The first is an euhedral, possibly diagenetic development, while there is also at least one over printing replacement phase. Quartz occurs as micro-fracture fillings and as pervasive replacements of silt grains in the host rocks. Calcite veining is a late stage feature that cross-cuts quartz veinlets and pyrite crystals. Carbon addition, where present, is accompanied by fine grained disseminated pyrite and is pervasive throughout the rock matrix, as well as along fractures and bedding planes in the ore zone (Hays and Foo, 1990).
The gangue minerals include calcite, quartz, melanterite, azurite, jarosite, realgar and various base metal sulphides and calc-silicate minerals related to the skarn (Hays and Foo, 1990).
Two geochemical associations are observed in the orebody area. The oldest, which coincides with the skarns, involves Ag, Cu, Pb, Zn and Mo. Ag values average 1 g/t. The upper skarn has elevated values of Pb, Zn and Cu while the lower has higher Cu, with Mo increasing with depth. The second association is of Au, As, Sb, Hg and Tl. This suite displays anomalous values along the north-east fault sets within the ore zone. Where the faults pass into the skarns the values drop to below detection limits. Positive correlations with Au exist for As, Ag, Hg, Sb, Tl, SO4, S2- and organic C. Tl displays the best, while Cu and Zn have the worst correlation with gold (Hays and Foo, 1990).
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.
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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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