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Central City District |
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Colorado, USA |
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Au Ag Cu Zn Pb
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Super Porphyry Cu and Au
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The Central City lead-zinc-silver-gold deposits are located in Gilpin County, Colorado, United States, on the east flank of the Front Range, ~50 km west of Denver.
The Central City - Idaho Springs area of Colorado represents the earliest mining activity in the state of Colorado. Placer gold was encountered in Clear Creek in January of 1859 and shortly afterwards lode gold was discovered at a site halfway between the present towns of Central City and Black Hawk. The initial workings exploited placer gold and oxidised lode gold ores. The oxidised gold ores gave way to sulphides at depths of 30 to 40 m. These sulphides could not be processed until an improvement in extraction methods in the late 1860’s allowed their treatment. The rich silver veins were not discovered until 1877. Gold has accounted for 80% of the districts production and silver around 10%. Until the mid 1880’s metal output from this field exceeded all other districts in Colorado. Production declined sharply however, from 1914, and has been negligible since 1930. The lode deposits of the district have also yielded copper, zinc, lead and some uranium (Sims, 1989).
Ore was produced from about 500 mines in the Central City district, although a substantial proportion came from a relatively few large mines, which included the Hidee, Glory Hole and Argo. Recorded production from 1859 to 1914 was around:
70 000 t of Pb; 4300 t of Cu; 8700 t of Zn; 28.35 t of Au and 155.63 t of Ag.
The production of the adjacent Idaho Springs district is about two thirds of that at Central City, with silver being more important than gold (Sims, 1989).
The Central City district is principally a vein field within Proterozoic metamorphics. However it is also a relatively significant Ag producer in the same province as the Ag-Zn-Pb deposits of Mexico and the western United States. In addition, it is possibly related to the same processes that formed the manto, chimney and vein deposits of the Leadville-Aspen-Gilman district some 85 km to the south-west, but is hosted by a different lithological suite. Both districts are within the NE-SW trending Colorado mineral Belt, which also embraces the Climax, Henderson-Urad and Mt Emmons porphyry Mo deposits, and the Creede and Cripple Creek caldera related Au/Ag deposits of the San Juan Mountains volcanic field.
Geology
The country rock in the Central City - Idaho Springs district is composed predominantly of gneiss, migmatite and intrusives of Proterozoic age. The lithologic succession is dominated by two principal rock types, namely, a biotite-quartz gneiss and a microcline-quartz gneiss. The maximum exposed thickness of the original sequence is believed to have been less than 5000 m. Older, approximately 1700 Ma, granodiorite and quartz-diorite masses, and younger bodies of two mica granite, intrude the gneisses (Sims, 1989).
Deformation of the Proterozoic rocks produced a series of mainly open synforms and antiforms whose axes are spaced at around 1 to 2 km and trend NNE. In the south-east a younger ductile phase was superimposed on the regional deformation pattern. This zone is represented by a 2 km wide belt of folds, mylonites and cataclasite which trends NE and makes up section of the Colorado Mineral Belt. This trend has apparently localised the Laramide intrusive phase, which in the district is composed of dykes and small plutons of calc-alkalic and alkalic intrusives. The country rock is cut by abundant faults, some of which were Proterozoic structures which were reactivated during the Laramide Orogeny. Most however appear to be largely Laramide in age (Sims, 1989).
Mineralisation and Alteration
The bulk of the ore in the area occurs in veins that follow zones of minor faulting, although a few important deposits are present as chimney-like zones of brecciation that are better described as stockworks. The veins are composed predominantly of quartz-sulphide containing precious metals, base metals and sparse uranium. They are believed to have formed at around 60 Ma, or younger, and are interpreted to be related to the associated porphyries (Sims, 1989).
The abundance of veins in different localities appears to be related to the amount and character of fracturing, which is largely determined by the character of the country rocks. In the Central City area, to the north of Idaho Springs, there appears to be a close relationship to areas of granite gneiss, pegmatite and porphyry dykes. The absence of veins in certain areas seems to be due to the presence of the Proterozoic Idaho Springs Formation which is composed principally of quartz-biotite schist and quartz-biotite-sillimanite schist, with lesser, but common quartzite, quartz-schist or quartz-gneiss (Lovering & Goddard, 1950).
The strike of nearly all veins lies between east-west and 45° , although in some it is north-westerly, while a small number are in other directions. When examined in detail the veins are found to be part of a complicated network composed of master veins connected by oblique cross-veins, each fracture having its smaller branches and spur veins. In cross-section the pattern is similar. Few veins can be definitely followed as individual fractures for more than 1 km. The longest north-easterly vein is traceable continuously at the surface for nearly 3.5 km. One WNW vein, just to the north of Idaho Springs is more extensive, but is less continuously mineralised. Many of the smaller veins pinch out at moderate depths, although several of the large veins have been mined to depths of 300 to 450 m, while at least one is to 660 m. Most veins have a steep dip, generally greater than 60° . Changes in dip and strike along a vein by as much as 20° are common. The width of workable veins varies from around 1 cm or less in the telluride veins, up to about 12 m in a few broad mineralised shears. The common widths of commercial ore have been 0.3 to 1.5 m (Lovering & Goddard, 1950).
Most veins are the result of mineralisation along a fracture zone, the walls of which underwent repeated movement before and during mineralisation. The stockworks are pipes or chimneys of irregularly fractured and brecciated rock that have been cemented by ore minerals. The most noteworthy of these is The Patch, about 1.5 km south-west of Central City. In outcrop it has an ovoid shape of approximately 230 x 120 m, elongated in a north-westerly direction. It can be traced to a depth of 180 m without decrease in size, but with a marked decrease in grade. The breccia varies from a passive, to a rotated breccia with transported clasts (Lovering & Goddard, 1950).
There are several types of gold-silver veins, namely:
i). Pyritic veins - composed of pyrite and gangue, with subordinate chalcopyrite, tennantite, gold, and in places enargite1, in a gangue of predominantly quartz, with locally abundant siderite and/or fluorite. Grades range from 30 to 100 g/t Au, 120 to 250 g/t Ag, but generally <1.5% Cu, although locally they contain 15 to 16% Cu;
ii). Galena-sphalerite veins - composed dominantly of galena, sphalerite and pyrite, subordinate chalcopyrite, a little tennantite and bornite. Small amounts of enargite, native bismuth and molybdenite are found locally. The gangue is chiefly quartz and either calcite or siderite, although rhodochrosite is present in some veins. In general these are poorer in Au and Ag than the pyritic ores. Grades are of the order of 5 to 100 g/t Au, 170 to 1350 g/t Ag, 1 to 17% Cu, up to 54% Pb and up to 32% Zn;
iii). Composite ores - composed of a combination of pyritic and galena-sphalerite veining. Although pyritic veins are generally older than galena-sphalerite veins, these composite veins appears to be a combination of both and may represent an overlap of the two stages;
iv). Telluride ores - composed of gold and silver tellurides in a gangue of blue-grey quartz and small amounts of fluorite, ferruginous calcite and fine pyrite. The Au:Ag ratio in some mines varied from 1:1 to 3:1, with grades of up to 550 g/t Au; and 5) Uranium ores - in which pitchblende is found associated with sulphides in several veins in the district (Lovering & Goddard, 1950).
In summary, the principal ore minerals of the district are pyrite, sphalerite, galena, chalcopyrite and tennantite. Lesser enargite, tellurides and molybdenite are also represented. The gangue minerals include quartz, barite, fluorite and peripheral Ca-Mg carbonates (Sims, 1989). Sericite is the dominant alteration associated with gold-silver ores of both the pyritic and galena-sphalerite types, although it is usually also accompanied by pyrite. Alteration has been most severe in the vicinity of veins, with in many places, all of the original rock being replaced by sulphides, sericite and in some places calcite and siderite (Lovering & Goddard, 1950).
Four stages of mineralisation are inferred, namely: i). uranium; ii). pyrite; iii). base metals; and iv). tellurides. Most of the precious metals are associated with stages ii and iii. Two molybdenite stages are indicated, an initial phase which was early in the paragenesis, and a late stage with coeval fluorite which post-dates the base metals. The pyrite-base metal ores have a well defined, concentric, hypogene mineral zoning. There is a large central core containing pyrite-quartz veins, surrounded by an intermediate zone of pyrite veins that also contain copper, lead and zinc minerals. Still farther out in the periphery of the system there are predominantly galena-sphalerite-quartz-carbonate veins. An elliptical area of molybdenite and fluorite bearing veins are centred on the NNE trending Dory Hill Fault on the eastern margin of the Central City District. This zone is spatially independent of the concentric pyrite-base metal zoning pattern, although apparently of the same age. The wall rocks of the fissure veins are altered to successive zones of sericitised and argillised rocks (Sims, 1989).
Fluid inclusion studies suggest a homogenisation temperature in the range 380 to 220°C for the precious metal bearing pyrite veins and the base metal veins. The early molybdenite veins yield common CO2 rich inclusions with temperatures of 420 to 340°C, and rare halite bearing inclusions yielding temperatures of 340 to 240°C. Homogenisation temperatures from the late phase molybdenum inclusions have been measured at 280 to 200°C. Salinities, as wt % NaCl equivalents have been measured at 42 to 32% for halite bearing solutions and 2 to 12 for all other types (Sims, 1989).
The most recent source geological information used to prepare this decription was dated: 1989.
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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Sims, P.K., 1989 - Central City and Idaho Springs Districts, Front range, Colorado: in Bryant, B. and Beaty, W., (Eds), 1989 Mineral Deposits and Geology of Central Colorado, Mineral Deposits of North America, 28th International Geological Congress, American Geophysical Union, Washington, Field Trip Guidebook T129, pp. T129:15-27.
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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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