Gilman District - Mineralisation

Colorado, USA

Main commodities: Zn Pb Au Ag Cu
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See the record entitled Leadville, Aspen and Gilman Districts for geological setting, structure and mineralisation overview.

The Gilman group of silver, lead and zinc mines are part of a larger district spread over an area of approximately 240 x 100 km within central Colorado which embraces a series of Pb-Zn-Ag deposits. All of these deposits lie within the broad, north-east trending Colorado Mineral Belt. The Aspen mines are located some 40 km to the NNW of the larger Leadville District of similar mineralisation in the state of Colorado, USA.

Published production and reserve figures for the district are as follows (Bryant & Beaty, 1989):

Gilman - 11.7 Mt @ 8.5% Zn, 1.5% Pb, 0.7% Cu, 228 g/t Ag, 1.7 g/t Au (Production to 1989),

The ore deposits at Gilman were discovered outcropping on the side of Battle Mountain, Colorado, in 1879 by prospectors from Leadville. The town of Gilman was founded in 1886. The New Jersey Zinc Company began purchasing mines in the district in 1912 and by 1915 had acquired the town and most of the large mines, which they consolidated into the Eagle Mine. Early production had mainly comprised rich oxidised Ag-Pb-Zn and gold ores. By 1910 to 1915 however, most of the rich oxide ore had been exhausted and marmatite-bearing sulphide ores had been reached. In 1929 an underground flotation mill was built. The mill closed in 1977 due to falling zinc prices and depletion of Pb-Zn reserves. Production of high grade direct shipping Cu-Ag-Au ore continued until 1984 when the mine and town were abandoned. Cumulative production from all ore types at Gilman has been 11.7 Mt of ore with an average grade of 8.5% Zn, 1.5% Pb, 0.7% Cu, 228 g/t Ag and 1.7 g/t Au (Beaty, et al., 1989).

Mineralisation and Alteration

The mineralisation at Gilman is hosted by the 200 m thickness of lower to middle Palaeozoic sediments, predominantly carbonates, that overlie the Proterozoic granitoid basement, and are in turn overlain by the 3000 m thick coarse clastics of the upper Carboniferous Minturn Formation. The sediments at Gilman are intruded by only one porphyry body, that of the Pando Porphyry which forms a 12 to 27 m thick sill within the Upper Carboniferous (Pennsylvanian) Belden Shale (Beaty, et al., 1989).

The Eagle mine at Aspen was developed on a large interconnected sulphide deposit which was composed of two principal ore types, namely  i). Zn-Pb mantos and  ii). Cu-Ag-Au chimneys (Beaty, et al., 1989).

Mantos - The mantos comprise long, stratabound, sub-cylindrical, Zn-rich deposits within the upper sections of the Leadville Dolomite. Four main mantos, each around 600 m to 1200 m long, converge down dip where they are connected by a north-west trending manto and the chimney deposit (Beaty, et al., 1989).

The most abundant minerals in the manto are pyrite, marmatite and siderite. Galena and chalcopyrite are always present in small amounts. Typically the mantos are surrounded by an irregular shell of siderite which is in turn enveloped by a shell of sanded dolomite. The up-dip sections of these mantos exposed at the surface have been oxidised to a depth of as much as 400 m down dip (Beaty, et al., 1989).

Chimneys - These are funnel shaped, copper, silver and gold rich massive sulphide bodies which cross-cut bedding and extend from the upper Leadville Dolomite, through the Dyer Dolomite, into the Parting Formation or Harding Sandstone. The chimneys are typically zoned with a pyritic core, surrounded by irregular and incomplete shells of Zn-rich ore, siderite and sanded dolomite. The principal sulphides are chalcopyrite, tetrahedrite, freibergite and other silver minerals, as well as galena and gold. There are eleven chimneys known at Gilman, ranging in width from 12 to 120 m at their tops, and vertical extents of as much as 80 m (Beaty, et al., 1989).

Exploration of the lateral and down dip equivalents of the manto ore, in conjunction with the knowledge of the ore mined, has indicated a concentric metal zonation centred on the chimney zone. A number of smaller mantos were discovered although these decrease in size and grade to the north-east. The zonation, from the chimney outwards was found to progress from Cu-Ag-Au → Zn → Pb-Zn → Mn. In addition however, distinctive Ag-Pb-Zn orebodies which are paragenetically older are distributed across the concentric paragenetic zones (Beaty, et al., 1989).

Above the manto orebodies the Pando Porphyry sill is typically altered along its base, although it is essentially unaltered everywhere at its top. The altered sections are mineralogically zoned away from ore from muscovite rich -> kaolinite rich -> chlorite rich. The Belden and Minturn Formation rocks above the porphyry are everywhere unaltered and unmineralised. Drilling below the manto/chimney ore systems has indicated local stockwork mineralisation and alteration, but no post-Proterozoic intrusives. The deep veins are essentially mineralogically identical to the manto-chimney complex. Zonal alteration around the veins, from quartz-sericite-pyrite-sphalerite -> chlorite-clay-sericite-pyrite -> to chlorite rich is similar to that in the Pando porphyry above the ore (Beaty, et al., 1989).

Fluid inclusion studies indicate temperatures of formation as follows: manto sphalerite - 413 to 391°C; deep vein sphalerite - 306 to 174°C, averaging 223°C; and deep vein quartz - 306 to 174°C, averaging 250°C. Freezing measurements indicate fluid inclusion salinities of 2.7 to 7.7 weight % equivalent NaCl for manto sphalerite and about 2.8 weight % for the deep vein sphalerite. Age dating of apatite in vugs of the orebody has indicated that the mineralisation was emplaced at around 34 Ma (Beaty, et al., 1989).

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary 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.

  References & Additional Information

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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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