Aspen 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 Aspen 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 45 km to the west 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):

Aspen - 2.3 Mt @ 2.1% Zn, 5.9% Pb, 16.4% Ba, 1500 g/t Ag (Production+Reserve, 1989).

Outcropping manto deposits were discovered at Aspen in 1879 by prospectors from Leadville. Many claims were pegged and after the first smelter commenced operation in 1882 and the railway reached the town in 1887, Aspen grew quickly. Peak production was reached in 1892 when around 260 t Ag was mined. The devaluation of silver in 1893 and the exhaustion of the large, rich mantos resulted in a decrease in production, although many of the larger nuggets were found in 1894, the largest of which weighed 770 kg. Mining continued into the 1920s, although production declined as the mines became deeper and more water had to be pumped. By 1952 production had ceased. The total historic production at Aspen has been 2.3 Mt of ore at an average grade of 2.1% Zn, 5.9% Pb, 1500 g/t Ag, 16.4% Ba (Stegen, et al., 1989).

Mineralisation and Alteration

The Aspen mineralisation is hosted by 200 to 320 m of lower to middle Palaeozoic sediments, mainly carbonates, which unconformably overlie Proterozoic gneisses schist and granitoids. It is disconformably overlain in turn by more than 5000 m of upper Carboniferous (Pennsylvanian) to Tertiary sediments. The ore deposit occurs at a point that lies on the Proterozoic NE-SW trending Homestake Shear zone, but also coincides with the change in structural fabric from a 350° trend in the south to 30° in the north (Stegen, et al., 1989).

Aspen lies on the western margin of a block faulted uplift, in a zone of numerous faults which apparently localise in part, the Ag-Pb-Zn-Ba manto deposits. In the south-western part of the field four types of Laramide age igneous intrusion are identified. Two have been dated at 69 and 74 Ma. Within the immediate Aspen area however, only one Laramide intrusion is exposed. This occurs as an aplite porphyry forming a sill as much as 100 m thick, near the base of the Pennsylvanian Belden Formation. This sill thins and pinches out to the north (Stegen, et al., 1989).

Most of the Aspen mineralisation is contained within the Redcliff and Castle Butte Members of the Leadville Dolomite. Both have abrupt thickness variations which are attributed to two separate dissolution events. As a consequence two laterally continuous breccia beds were formed. A lower bed, the 'Contact Breccia', is found at the contact between the Castle Butte and the underlying Redcliff Members, while the upper 'Silver Breccia' is found at the top of the Castle Butte Limestone. The 'Contact Breccia' contained the bulk of the ore accumulations. A younger breccia, mineralised with Pb & Zn sulphides in the Smuggler mine, cuts both the Contact Breccia and the large baritic manto orebodies (Stegen, et al., 1989).

Three principal ore types are recognised at Aspen. These are:

Main Ag-Pb-Zn-Ba Mantos - which are found within the Contact Breccia of the Leadville Dolomite. The principal production has been from these mantos which are relatively large, laterally continuous and stratabound. Throughout the district around 50% of the breccia was mineralised. The main mantos comprise galena, sphalerite and chalcopyrite with tennantite, pearceite, acanthite and native silver in a barite rich (30 to 70%) gangue (Stegen, et al., 1989).

Younger Pb-Zn Mantos - in the younger breccia detailed above. Ore and gangue minerals are in two settings, namely:  i). as open space fillings between breccia fragments and  ii). as replacements of a basal bed of stratified dolomite sand. There was virtually no replacement of the fragments in the breccia. This ore is composed of varying proportions of galena, sphalerite and pyrite that replaced the breccia matrix. Mineralised rock ranges from sparse, fine grained disseminations to massive ore. The massive ore was mined along intersections of the brecciated strata and Laramide faults (Stegen, et al., 1989).

Pb-Zn Mantos - in the Pennsylvanian Belden Formation. These replaced carbonaceous shale and limestone, with the principal ore minerals being galena and sphalerite. Seven separate, tabular manto orebodies have been mined. These extend laterally for 70 to 140 m and are up to 1.5 m thick. They are found stratigraphically above the Main mantos of the Contact Breccia. However these two different manto types have not been geologically connected (Stegen, et al., 1989).

Hydrothermal Alteration - At Aspen there is no alteration noted in the carbonates below the ore of the Main mantos, although the carbonates are recrystallised within 0.3 m of the ore. Similarly the orebodies within the Younger breccia and the Belden Formation exhibit no alteration. Jasperoid is localised along pre- and post-ore faults, but bears no obvious relationship to ore. Petrographic studies indicate that the jasperoid is post-ore. Hydrothermal (?) dolomite has been reported as enveloping the ore within the Castle Butte Limestone of the Leadville Formation. The aplite porphyry shows strong alteration of feldspar and biotite phenocrysts to form sericite and minor kaolinite, while the groundmass feldspar is extensively replaced by quartz and 1 to 3% disseminated pyrite throughout the rock. Alteration of the aplite porphyry does not have any zoning relative to the sulphide mantos (Stegen, et al., 1989).

Average homogenisation temperatures from fluid inclusions are as follows: barite - 252°C; sphalerite - 252°C; tennantite - 255°C; and pearcite - 257°C (Stegen, 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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