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Summitville |
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Colorado, USA |
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Au
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Super Porphyry Cu and Au
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The Summitville gold deposit is located in south-western Colorado, USA, approximately 290 km south-west of Denver. It lies within the Summitville Caldera which is one of a group of nested structures on the SE margin of the San Juan Caldera Complex. The San Juan Complex is on the north-south Rio Grande Rift, at the intersection with the major north-east Colorado Mineral Belt lineament. The deposit lies within a 21 to 20 Ma silicic dacite dome, which has a monzonite core. The dome cuts the 29 to 27 Ma San Juan Volcanics.
The ore occurs as irregular tabular bodies, structurally controlled by a vertical fracture system, which is discordant to the enclosing rocks. The orebody lies on the major post-mineralisation South Mountain Fault, occurring in the footwall of that structure. Fine sulphides dominate, occurring as disseminations in vuggy silica, to a lesser extent in minor micro veinlets, and as a matrix to micro-breccias. The gold mineralisation is present as native gold in vugs as drusy material, and associated with iron oxides and barite. The Au:Ag ratio averages around 10:1, ranging from <2:1 in the centre to 20:1 in the peripheries.
The ore minerals are native gold, covellite, enargite, electrum, argentiferous covellite, chalcopyrite, tennantite and argentiferous jarosite, with minor galena, sphalerite and barite. The Au:Cu and Au:Ag ratios decrease with depth, due to the decrease in Au and increase in Cu content, prompting the suggestion that the deposit may pass down into a porphyry Cu orebody. The upper auriferous zone contains covellite, enargite, luzonite and minor pyrite, with local galena, sphalerite, hinsdalite, marcasite, native sulphur and chalcopyrite. The deeper gold poor interval is characterised by a tennantite-chalcopyrite-pyrite assemblage. The gangue mineralogy of the vein system includes quartz, alunite, kaolinite, barite, hematite and vuggy silica. The alteration is characterised by acid sulphate activity producing a siliceous core, surrounded by advanced argillic and montmorillonite rich argillic peripheries, and an outer propylitic zoning. At a depth of >600 m the system passes into a phyllic zone. In detail the individual veins have acid sulphate siliceous fillings along fractures, with a quartz-alunite envelope. This is enclosed progressively by quartz-illite and illite layers. The surrounding argillic and propylitic alteration however is pervasive. Superimposed oxidation is believed to be supergene.
The orebody has plan dimensions of approximately 500 x 100 m, to a depth of 175 m. It occurs within a mineralised zone that is of the order of 1500 x 600 m at the surface and extended over a vertical interval of 1500 m.
Following discovery of gold in 1870, placer workings were undertaken in the Summitville district before hard rock mining was commenced in 1876 with production peaking in the early 1880s with 250 individual claims in operation by 1885. The known payable ore was mined out soon after. The site was re-opened on a number of occasions for gold or other metals but with little success. The main periods of production were between 1873 and 1894, and from 1926 to 1942. Grades mined in 1944 to 1946 averaged 21 to 28 g/t Au. From 1950 onwards the area was tested by a number of companies, including ASARCO and Anaconda. The total amount of gold produced between 1873 and 1959 was estimated at ~8 t. In 1984 most of the leases were consolidated by the Canadian-based Galactic Resources Ltd. subsidiary Summitville Consolidated Mining Company, Inc.. A total of 800 angled holes were drilled for 7000 m of drilling. The orebody gives anomalies in soil and rock for Au, Hg, As, Ag and Ba. Summitville Consolidated Mining Company, Inc. commenced mining in 1986 (Bethke and Lipman, 1989) from a new large-scale open pit operation covering an area of 2.2 km2.
In 1988 some 12 Mt of waste+ore was mined by open-cut and 4 Mt treated to produce 6.2 t of Au (i.e., at a grade of 1.55 g/t Au). The overall striping ratio is to be 1.67:1, waste:ore. The ore is crushed to -30 mm and treated by cyanide heap leaching. The gold is extracted by a carbon-in-pulp plant with an overall recovery of 70 to 80% for oxide ore and 40% for sulphide mineralisation. Approximately 90% of the orebody is oxidised.
The mine was closed in 1992 and rehabilitation work undertaken to remediate the acid drainage into the regional river system and impact on agriculture.
The total production from Summitville amounted to 17.4 t of gold (Bethke et al., 2005).
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription 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.
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Selected References:
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Bethke, P.M., Rye, R.O., Stoffregen, R.E. and Vikre, P.G., 2005 - Evolution of the magmatic-hydrothermal acid-sulfate system at Summitville, Colorado: integration of geological, stable-isotope, and fluid-inclusion evidence: in Chemical Geology, v.215, pp. 281-315.
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Gray J E, Coolbaugh M F 1994 - Geology and geochemistry of Summitville, Colorado: an epithermal acid Sulfate deposit in a volcanic dome: in Econ. Geol. v89 pp 1906-1923
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Gray J E, Coolbaugh M F, Plumlee G S, Atkinson W W 1994 - Environmental geology of the Summitville mine, Colorado: in Econ. Geol. v89 pp 2006-2014
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Rye, R.O., 2005 - A review of the stable-isotope geochemistry of sulfate minerals in selected igneous environments and related hydrothermal systems: in Chemical Geology v.215 pp. 5-36
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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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