Epithermal vein and carbonate replacement deposits in Cunningham
Gulch are located within the western San Juan Tertiary volcanic field in
southwestern Colorado. The Pride of the West epithermal vein system is
hosted within the intracaldera facies of the Sapinero Mesa Tuff, a voluminous
ash-flow tuff that erupted from and resulted in the formation of the
San Juan Caldera at 28 mybp. The Pride of the West vein system is developed
along a radial fracture formed during resurgence of the San Juan Caldera prior to eruption of the Crystal Lake Tuff (27.5 mybp). This eruption led to the concomitant collapse of the Silverton Caldera, nested within the
larger San Juan Caldera. The Pride of the West, Osceola, and Little Fanny mines are positioned near the intersection of the Pride radial fracture system and the buried structural margin of the San Juan Caldera, suggesting
that ore concentration was controlled by this structural setting.
Large limestone blocks of the Mississippian Leadville Formation are incorporated into the intracaldera fill volcanics in the mine area. These
blocks appear to have been engulfed within mudflow breccias of the Tertiary San Juan Formation (32.1 mybp). They were then emplaced in their present
structural position within a caldera-collapse breccia which caved from the
oversteepened margin of the San Juan Caldera.
Regional propylitic alteration of the hosting volcanics to a chlorite-calcite-pyrite assemblage preceded vein-associated alteration and mineralization.
The veins are enveloped by a narrow phyllic alteration assemblage of quartz, sericite, illite, kaolinite, and pyrite. The veins are comprised
of sphalerite, galena, chalcopyrite, pyrite, hematite, magnetite, quartz,
pyroxmangite, calcite, and minor barite. Substantial bodies of replacement
ore are present where the vein structures intersect the limestone blocks;
the mineral assemblages of the replacement deposits are identical to those
of the feeding vein structures. Commonly, replacement textures are spectacular
concentrations, especially the "zebra ore" which primarily consists
of regularly spaced, alternating bands of sulfides and quartz. These
"zebra" laminations are stratigraphically controlled and appear to represent
replacement of a depositional or diagenetic fabric. Main ore-stage
mineralization began with widespread deposition of quartz with or without
pyrite, followed by sphalerite, chalcopyrite, and galena. Post ore-stage
brecciation and silicification events are evident and were followed by
deposition of calcite and minor barite during the waning stages of the
hydrothermal system.
The distributions of Fe, Mn, Pb, and Ca suggest a lateral component of
fluid flow from northwest the southeast, away from the structural margin of the Silverton Caldera. Fluid inclusion data from both vein and replacement-type sphalerite and quartz indicate that mineral deposition occurred over a range of 200 to 312°C (mean 243°C) from solutions containing 1 to
5% total salts. The high base metal to precious metal content of the ore,
the phyllic alteration assemblage, and the temperature and composition of
the ore-forming fluid indicate that the mine workings are within the lower portion of a fossil geothermal system. / text
| Identifer | oai:union.ndltd.org:UTEXAS/oai:repositories.lib.utexas.edu:2152/6767 |
| Date | 08 December 2009 |
| Creators | Hardwick, James Fredrick, 1955- |
| Source Sets | University of Texas |
| Language | English |
| Detected Language | English |
| Format | electronic |
| Rights | Copyright is held by the author. Presentation of this material on the Libraries' web site by University Libraries, The University of Texas at Austin was made possible under a limited license grant from the author who has retained all copyrights in the works. |
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