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Cripple Creek |
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Colorado, 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 Cripple Creek mining district, which is located approximately 120 km WSW of Denver in Colorado, USA, has produced over 650 tonnes (21 Moz) of gold since its discovery in 1891 (#Location: 38° 43' 37"N, 105° 8' 34"W).
The orebodies that have yielded this gold have historically been narrow veins within both Precambrian and Tertiary rocks, with increasing contributions from bulk tonnage deposits in tectonic and hydrothermal breccias.
The Cripple Creek district is part of a belt of alkaline-related epithermal vein, breccia, disseminated, skarn, and porphyry gold deposits in the southern Rocky Mountains along the eastern edge of the North American Cordillera. It lies at the intersection of the north-east trending Colorado Mineral Belt and the north-south Rio Grande Rift, associated with a remnant of a roughly 400 x 500 km composite volcanic field that covered much of the southern Rocky Mountains in middle Tertiary time and consists mainly of intermediate composition lavas and breccias. Alkaline igneous rocks and associated hydrothermal deposits formed during two periods. The first erupted between 70 and 40 Ma from scattered central volcanoes, with deposits restricted spatially to the Colorado mineral belt. They are overlain by 15 widespread voluminous ash flow sheets aged between 35 to 27 Ma, which originating from as many calderas. The younger alkaline rocks and associated ore deposits are the more widespread, distributed along the eastern margin of the Cordillera, following the Rocky Mountain front southward from Cripple Creek in Colorado to New Mexico, while voluminous calc-alkaline rocks were deposited to the west. The largest deposits in the belt, are in the Colorado Mineral Belt and include Cripple Creek and Summitville. After about 26 Ma volcanism became bi-modal, dominated by alkalic basalt and silicic rhyolite, corresponding to the development of extension on the Rio Grande Rift.
The Cripple Creek mineralisation is localised within and adjacent to a nested set of 29.3 to 27.9 Ma diatreme-intrusive clusters, approximately circular in shape covering 18.4 sq. km, surrounded by Proterozoic rocks. Two magmas, one of phonolitic and the other of alkalic basaltic composition are interpreted to have mixed and generated volcanic flows, sub-volcanic intrusions and phreatomagmatic breccias.
The diatreme-intrusive clusters are developed within a 1030 Ma late Mesoproterozoic granite and older meta-sedimentary and igneous rocks. The older Proterozoic Yavapai province sedimentary and igneous rocks were metamorphosed from around 1790 Ma and fall into two groups, namely: i). interlayered quartzo-feldspathic gneiss and amphibolite; and ii). interlayered biotite schist, gneiss and migmatite. The biotitic rocks contain layers and lenses of marble, quartzite and conglomerate. Locally they are seen to grade laterally into greywacke, and are regarded as being largely of sedimentary origin. Proterozoic, calc-alkaline granitoids of granodiorite to quartz monzonite composition vary from strongly foliated, concordant bodies to discordant varieties with only weak foliation. They range in age from 1780 to 1650 Ma, although a later 1400 Ma phase of dykes, stocks and irregular, discordant plutons of non-foliated two mica granite is also mapped in the district. The younger 1030 Ma granite mass is largely composed of biotite and hornblende-biotite potassic granite (Bryant & Beatty, 1989). These three Mesoproterozoic sites of intrusions were important precursors to Tertiary alkaline-related gold deposits. Many of the larger gold deposits are located at sites of Proterozoic intrusions, localised at the intersection of northeast-trending ductile shear zones formed during Mesoproterozoic deformation, and an important north-trending fault formed during 1.1 Ga rifting (Kelley and Ludington, 2002). The intersection of these structures and the dsitribution of Proterozoic intrusives and metamorphics formed an area of regional dilation which subsequently facilitated the formation of the Tertiary Volcanic complex, the majority of which was then infilled with the eruptive phase Cripple Creek Breccia host rock (AngloGold Ashanti website, 2006).
The Tertiary volcanic complex in the Cripple Creek district occurs in three coalescing basins and comprise flows, sub-volcanic intrusives and phreatomagmatic breccias. The Cripple Creek Breccia was intruded by a series of Tertiary dykes and sills ranging in composition from syenite to phonolite/ phonotephrite, trachyte and alkali basalt to lamprophyre. These intrusives occupy all of the dominant district structural orientations, which are generally near vertical and strike NNW to NE, as well as occuring as laccoliths, cryptodomes and surficial flows.
Subsidence related to the diatreme cluster is indicated by a thick overlying fluviatile to lacustrine sequence, with carbonaceous and red ripple marked sediments more than 300 m below the present surface, and by fracture systems near the margins of the complex (Thompson, 1989).
There are indications that fluidisation was important in the emplacement of the diatreme breccias. These include: i). accretionary lapilli within the breccia masses; ii). the sorting of the diatreme breccia matrix resulting in the removal of almost all clay sized material; iii). the fact that the breccia is bi-modal, with the matrix predominating; and iv). the presence of discordant hydrothermal breccias occurring as steep or flat dipping bodies with fragments of veins within them (Thompson, 1989).
Mineralisation at Cripple Creek is apparently independent of host lithology, but is related to the occurrence of breccias, although, un-fractured tonalites also locally host significant gold grades. The main Tertiary diatreme-intrusive complex has an irregular outline and covers an area of approximately 4.5 x 5.5 km. It is surrounded by Proterozoic rocks, with one larger island of the same country-rock within the complex. There are a large number of individual veins, and lesser bulk-mineable shoots, within the complex. The bulk mineable blocks can be tabular, pipe-like, irregular or massive but have order of magnitude dimensions of around 60 x 20 x 600 m, occurring as elongated, plunging, tabular to pipe-like bodies which are discordant, with disseminated and fracture controlled mineralisation.
Typically within the Precambrian crystalline rocks adjacent to the Tertiary volcanic complexes the veins are radial to the diatreme system margins and are sheeted zones with rock dissolution and open space fillings. Where the veins cut the Cripple Creek breccias they tend to occur as an irregular anastomosing fracture zone.
The veins at Cripple Creek formed in sheeted zones with only minor quartz and base metal sulphides, accompanied by fluorite, adularia, carbonate, hematite, gold and tellurides. The ores are vuggy, resulting in many places from country rock dissolution. These veins occur as radiating and concentric bodies near the outer margins of the diatreme complex, cutting both Tertiary breccias and Proterozoic granitoids. They also occur in fracture zones overlying basement ridges beneath the diatreme complex. The major veins may be very continuous vertically, extending for more than 1000 m below the present surface (Thompson, 1989).
Five stages of minerals are recognised in some vein deposits (eg. at the Ajax mine), namely:
i). quartz-fluorite-adularia-pyrite-(dolomite-marcaste),
ii). base metals-quartz-pyrite,
iii). quartz-fluorite-pyrite-hematite-rutile,
iv). quartz-pyrite-rutile-calaverite-acanthite,
v). quartz-pyrite-fluorite-dolomite. Gold values varied from 15 to 30 g/t in the better veins.
The bulk tonnage deposits consist of mineralised tectonic and hydrothermal breccias cutting Tertiary alkali volcanic rocks. Higher grade pods of mineralisation occur at structural intersections and/or as sheeted vein zones along zones of strike deflection. Broader zones of disseminated mineralisation occur primarily as micro-fracture halos around the stronger alteration zones in the more permeable Cripple Creek Breccia wall rocks. At an example of this style at Globe Hill, there have been four structural events, namely:
i). emplacement of hydrothermal breccias along a NW trending 1800 x 700 m zone,
ii). intersecting tectonic adjustment along steep zones with variable strike on the western margin of the preceding stage,
iii). intrusive breccia emplacement at the major stage 2 fault intersections, and
iv). hydrothermal brecciation with a matrix of anhydrite, carbonate, fluorite, pyrite and base metal sulphides. Two hydrothermal events generated Au-Ag mineralisation and associated alteration, resulting in base and precious metals being found in both clasts and the matrix of the hydrothermal and tectonic breccias.
There is no recognised zonation in either the vein or bulk tonnage deposits, and no expansion of the alteration zones in the upper levels of the mine. Vein related hydrothermal alteration occurs as narrow selvages that are no thicker than 5 times the width of the associated vein. Secondary K-feldspar, roscoelite, pyrite and dolomite form an inner zone adjacent to the vein walls, flanked by an outer zone of sericite, montmorillonite and magnetite with minor secondary K-feldspar and pyrite. A similar alteration pattern is exhibited in the bulk-mineable deposits, with more overprinting by argillic assemblages (Thompson, 1989).
The hypogene ore minerals in the bulk-mineable deposits are chiefly tellurides with only minor native gold. Gangue minerals are pyrite, fluorite, celestite and quartz. Alteration styles include carbonatisation which is apparently pre-, syn- and post-mineralisation, and is pervasively associated with the brecciation, producing both calcite and dolomite. Potassic alteration is also pervasive, but zoned, with a suite comprising adularia, orthoclase, sericite, alunite, kaolin, silica, pyrite and chlorite. Proximal to ore there is intense adularia and orthoclase, passing outwards into sericite and then smectite clays, argillic alteration and possibly an outer propylitic zone.
The ore has been oxidised to an average depth of 120 m, although it occurs to greater depths along major structural zones. In the oxide zone, gold occurs within hydrous iron and manganese oxides and as gold-silver tellurides, accompanied by a gangue of hematite and quartz. Silver is present but is economically unimportant. Gold mineralisation can be encapsulated by iron and manganese oxides, pyrite, K-feldspar alteration and quartz. Individual gold particles are generally less than 20 µm in size and occur as native gold with pyrite or native gold after gold-silver tellurides
The recovered grade at the Cripple Creek mine in 2005 was 0.62 g/t Au.
Proven + probable reserves (end 2005) at the Cripple Creek and the associated Victor mines was 119.1 Mt @ 0.86 g/t Au.
Measured + indicated + inferred resources (end 2005) for the same mines was: 227.2 Mt @ 0.93 g/t Au
The source of the above was the AngloGold Ashanti Annual Report, 2006.
For detail see the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2002.
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.
Cripple Creek
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Selected References:
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Kelley K D and Ludington S 2002 - Cripple Creek and other alkaline-related gold deposits in the southern Rocky Mountains, USA: influence of regional tectonics: in Mineralium Deposita v37 pp 38-60
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Richards, J.P., 2009 - Postsubduction porphyry Cu-Au and epithermal Au deposits: Products of remelting of subduction-modified lithosphere: in Geology v.37, pp. 247-250.
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Thompson T B, Trippel A D, Dwelley P C 1985 - Mineralized veins and breccias of the Cripple Creek district, Colorado: in Econ. Geol. v80 pp 1669-1688
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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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