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Creede |
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Colorado, USA |
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Ag Au Pb
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Super Porphyry Cu and Au
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The Creede mining district, which lies within the Colorado Mineral Belt, is located in the San Juan Mountains of south-western Colorado, USA, approximately 260 km south-west of Denver. It is a low sulphidation silver-gold-base metal district.
Bonanza vein mineralisation was first discovered in the Creede district in the mid 1870's, although little mining was undertaken until oxidised silver ore was located along the Solomon-Holy Moses fault by Nicholas Creede in the late 1880's. Mining boomed in the period 1890 to 1893, especially after the discovery of the Amethyst vein system in 1891, with the peak production being realised in 1892 and 1893. In 1893, the repeal of 'silver-support' by the US government caused the silver market to collapse. However, mining continued sporadically until 1955. Initial mining was from seven veins, the Amethyst, OH, Park Regent, Solomon-Holy Moses, hangingwall Amethyst, Alpha-Corsair and Monon Hill. These were mined primarily for Ag, although Pb, Zn and Cu were also extracted. Gold production was minimal (Plumlee et al., 1989). Most of the Zn was discarded during the early mining. Consequently, production statistics indicate a misleading Pb:Zn ratio. Galena and sphalerite actually occur within the ore in sub-equal amounts (Bethke & Lipman, 1989).
Historic production from the 1890's to 1988 was approximately 2500 t of Ag and 5 t Au, plus 0.15 Mt of base metals.
An exploration program undertaken by the Homestake Mining Co. in 1963 resulted in the delineation of economic mineralisation in the Bulldog Mountain Fault zone from a depth of 150 m below barren outcrop. Production commenced in 1969 and continued until 1985, with 708.75 t of Ag and 22 000 t of Pb being extracted (Plumlee et al., 1989).
Production and reserve figures include:
4.33 Mt @ 540 g/t Ag, 15.5% Pb, 4.5 Zn, 0.3% Cu, 1 g/t Au (Historic Production, to 1988, Plumlee et al., 1989).
3 Mt @ 181 g/t Ag (Res., Caldera moat seds., 1988, Plumlee et al., 1989).
1.36 Mt @ 564 g/t Ag, 1.9% Pb (Production, Bulldog Mt. Mine, 1969-85, Plumlee et al., 1989).
Geology
The Creede district lies within the centre of a 25 000 km2 remnant of a roughly 400 x 500 km composite volcanic field that covered much of the southern Rocky Mountains in middle Tertiary time. This field comprises a lower, approximately 2 km thick volcanic pile, composed mainly of intermediate composition lavas and breccias, erupted between 35 and 30 Ma from scattered central volcanoes. These are overlain in turn by 15 widespread, voluminous, rhyodacite to rhyolite ash flow sheets aged between 30 and 26 Ma, originating from as many calderas. The calderas are spatially related to mineralisation at a number of centres, including Silverton, Summitville, Creede, etc.. After about 26 Ma, volcanism became bi-modal, dominated by alkalic basalt and silicic rhyolite, corresponding with the development of extension on the Rio Grande Rift (Plumlee et al., 1989).
The Creede mineralised system lies in the central section of the remnant San Juan volcanic field, in a complex set of nested calderas formed by the eruption of 6 major ash-flow sheets between 28.4 and 26.1 Ma (Plumlee et al., 1989). The calderas and associated ash flows during this interval are dated as follows: Mt Hope Caldera - 28.4 Ma; La Garita Caldera - 27.8 Ma; Bachelor Caldera - 27.4 Ma; South River Caldera - 27.2 Ma; Creede Caldera - 26.6 Ma; and San Luis Caldera - four units between 26.4 and 26.1 Ma. The six over printing calderas formed in the locus of pre-caldera volcanoes. The largest of the calderas is the 30x40 km La Garita, with the younger Creede and San Luis structures being 20x16 km and 22x20 km respectively. The four younger calderas are developed in a north-south trending alignment along the western margin of the larger, older, La Garita Caldera. The ash flow tuffs vary from silicic rhyolite to silicic dacite and mafic dacite (Bethke & Lipman, 1989).
The ore veins of the Creede district occupy reactivated older faults that formed the Creede graben cutting volcaniclastics of the 27.5 Ma Bachelor caldera on the northern margin of the Creede Caldera. The Bachelor caldera was formed by the eruption of voluminous rhyolitic ash flow tuffs, collapse and resurgence, which formed the host rocks and structures. The younger Creede caldera erupted voluminous dacitic ash flow tuffs, collapsed, resurged and created a deep moat lake (surrounding the resurgent central dome) that was filled by >1000 m of volcaniclastic sediments and lava flows just south of the Creede district at 26.9 Ma. Erosion cut a deep canyon through the Creede caldera crater wall, and incised the fill of the adjacent Bachelor caldera. Conglomerates were deposited within the canyon, forming a channel way for the discharge of hydrothermal fluids and waters from the the Creede caldera moat (Campbell & Barton, 2005).
At around 25.3 Ma a probable cryptic pluton emplaced below the northern part of the district is interpreted to have heated, acidified, circulated and mixed magmatic, meteoric and connate waters in the Creede caldera moat. This event produced the ores (Campbell & Barton, 2005).
Pre-Tertiary rocks are not evident in the immediate Creede district.
Mineralisation
The base and precious metal ores in the Bachelor Caldera to the north of the town of Creede, represent the principal style of mineralisation within the central caldera cluster of the San Juan Mountains. In addition, fluorite, occurring with barite, was mined on the eastern margin of the Creede Caldera, about 13 km to the SSE of Creede, while barite and base metals have also been exploited in the Spar City district on the southern margin of the Creede Caldera, 16 km to the south of Creede (Bethke & Lipman, 1989).
Most of the production in the Creede district has come from open space fill veins occupying fractures in the Creede Graben. This graben extends from the 26.6 Ma Creede Caldera in the south-east, to the 26.4 to 26.1 Ma San Luis Caldera in the north-west. The major host to mineralisation is the 27.4 Ma Bachelor Mountain Member of the Carpenter Ridge Tuff which fills the older Bachelor Caldera between the two mentioned above. In the northern part of the district, mineralisation is hosted in part by the 26.1 Ma Nelson Mountain Tuff, one of the ash-flows which fill the San Luis Caldera. Vein mineralisation in both the northern and southern sections of the field has been dated at 25.1 Ma, approximately 1 m.y. younger than the last volcanism in the area. The majority of the production has come from the southern third of the graben (Plumlee et al., 1989).
The Creede Graben trends at roughly 345°. It is 13 km long and 6 km wide. Four major faults form the framework of the graben. These are, from east to west, the west dipping i). Solomon-Holy Moses and ii). Amethyst faults; and the iii). Bulldog Mountain and iv). Alpha-Corsair faults which both dip to the east.
Numerous intervening faults form a complex network of conjugate shears and extensional fractures. Structural relationships indicate that the Creede Graben underwent five periods of deformation, mainly sinistral wrench and oblique-slip movement. Branching conjugate structures had dextral strike slip displacement. The fourth phase of deformation was apparently contemporaneous with mineralisation (Plumlee et al., 1989). More than 97% of the district's production has come from the Amethyst, Bulldog Mountain, OH and P vein systems. The Alpha-Corsair and Solomon-Holy Moses fault systems, although mineralised, together only account for around 2% of the total output (Bethke & Lipman, 1989).
Differences in the competency of the various volcanic units, due to differential welding, resulted in refraction of faults, with steep and shallow dipping segments being produced. A dip-slip component, dominant in the southern part of the district during the period of vein formation, led to the opening of the steeper section of the faults and the closing of the shallower dipping fault segments. Steeper dips were prominent in the competent Campbell Mountain welding zone of the Bachelor Mountain Member. Ore shoots tend to be concentrated in the dilation zone where the faulting steepens crossing the Campbell Mountain welded zone. Dilation zones were also formed at strike deflections to the left, indicating normal/sinistral oblique-slip deformation during vein development. Some veins carry base metal veins, while others tend to be silver rich. This is taken to reflect opening during different stages of the mineralising event (Plumlee et al., 1989).
The ore zone occupies a narrow, approximately 250 to 400 m, vertical interval, although ore has been mined from a near continuous horizontal strike length of 3 km along the Amethyst-OH vein system, and 2 km along the Bulldog Mountain Fault. It has been indicated that sub-economic mineralisation may extend for over 8 km along the Amethyst Fault (Bethke & Lipman, 1989).
The ore deposits are mineralogically complex and spatially zoned. The southernmost veins are silver rich and gold poor, with native silver, acanthite, sulphosalts, sphalerite, galena and copper sulphides in a gangue of abundant barite, rhodochrosite, quartz, pyrite, hematite and adularia. Ag:Au ratios are >1000:1. This is the Bulldog Assemblage (Plumlee et al., 1989).
Further north along the OH, P, Central Amethyst and Bulldog Mountain vein systems, the ores become base metal rich. This is the OH Assemblage, comprising sphalerite, galena, chalcopyrite and lesser tetrahedrite, in a gangue of quartz, pyrite, chlorite, hematite and lesser adularia and fluorite. Gold is present in slightly greater abundance, but is still extremely rare (Plumlee et al., 1989).
To the north the vein systems take on the Northern Assemblage, marked by high precious metal values, and Ag:Au ratios below 100:1. Ore minerals include electrum, acanthite, pyrargyrite, jalpaite, sphalerite and galena, in a gangue of quartz, adularia, calcite, kutnahorite, rhodonite, rhodochrosite, pyrite, fluorite, hematite, magnetite and chlorite. Higher precious metal levels are marked by adularia-rhodonite-kutnahorite gangue mineralogies. Barite is rare, but increases to the south (Plumlee et al., 1989).
Alteration
Three periods of wall-rock alteration are recognised, all younger than the San Luis Caldera. These are:
i). an early, pervasive, K-feldspar stable potassium metasomatism which affected intra-caldera ash flows of both the Bachelor and San Luis calderas. It is characterised by K2O enrichment and depletion of Na2O and CaO.
ii). a second stage is recognised as erratic bleaching of vein selvages, also K-feldspar stable, but in addition sometimes containing albite. Both the K-feldspar and bleaching stages are crosscut by the earliest mineralised veins.
iii). a third stage is intense and comprises a mixed layer illite/smectite capping to the vein systems and is part of the mineralising event, yielding the same dates as the vein adularia (Bethke & Lipman, 1989).
Plumlee et al., (1989) record that in the southern parts of the district, wall rock alteration in lower and middle levels of the vein systems is generally minor. There is some silicification and sericitisation of feldspar phenocrysts, and pyritisation of biotite phenocrysts is noted locally. The alteration increases to the north, with the volcanics locally being extensively altered to sericite, pyrite and chlorite. The upper sections of the veins are capped by clay alteration, as described above. These caps bottom in narrow zones along the veins and blossom upwards into masses at least several metres wide along most of the vein lengths (Plumlee et al., 1989).
The most recent source geological information used to prepare this decription was dated: 1983.
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 and Rye R O, 1979 - Environment of ore deposition in the Creede Mining District, San Juan Mountains, Colorado: Part IV. Source of fluids from oxygen, hydrogen and carbon isotope studies: in Econ. Geol. v74 pp 1832-1851
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Campbell W R and Barton P B, 2005 - Environment of Ore Deposition in the Creede Mining District, San Juan Mountains, Colorado: Part VI. Maximum Duration for Mineralization of the OH Vein : in Econ. Geol. v100 pp 1313-1324
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Foley N K, Ayuso R A 1994 - Lead isotope compositions as guides to early Gold mineralization: the north Amethyst vein system, Creede district, Colorado: in Econ. Geol. v89 pp 1842-1859
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Hayba D O 1997 - Environment of ore deposition in the Creede Mining district, San Juan Mountains, Colorado: Part V. Epithermal mineralization from fluid mixing in the OH vein: in Econ. Geol. v92 pp 29-44
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Plumlee G S 1994 - Fluid chemistry evolution and mineral deposition in the main-stage Creede Epithermal system: in Econ. Geol. v89 pp 1860-1882
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Plumlee G S and Whitehouse-Veaux P H, 1994 - Mineralogy, paragenesis and mineral zoning of the Bulldog Mountain vein system, Creede District, Colorado: in Econ. Geol. v89 pp 1883-1905
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Robinson R W, Norman D I 1984 - Mineralogy and fluid inclusion study of the Southern Amethyst Vein System, Creede Mining District, Colorado: in Econ. Geol. v79 pp 439-447
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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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