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Geology and mineralization of the Uncle Sam vein and surrounding area, San Juan County, Colorado
Waegli, Jerome A.
Waegli, Jerome A.
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1979
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Abstract
The Uncle Sam vein forms a broad, prominent structure cropping out at the surface about three miles north of Silverton in the central portion of the Silverton caldera. Although numerous studies have been done on the regional geology of the San Juan volcanic field and on the geology and economics of specific localities within the field, no previous detailed study has been done on the area covered in this report. The oldest rock unit cropping out in the area is the Burns Formation. It consists predominantly of thick dacite flows but includes a few outcrops of dacitic flow breccia. Rocks of the Burns Formation are almost always highly altered, often to the extent that original textures are partially of wholly destroyed. The Henson Formation overlies the Burns Formation and consists of andesitic tuff breccias, flows, and volcaniclastic sedimentary rocks. Weak propylitization and local phyllic alteration and hematitization have affected rocks of the Henson Formation, but in general, they are not as highly altered as flows of the Burns Formation. Various Quaternary deposits, including glacial till, stream gravels, and landslide deposits are the youngest units in the area. Metal dispersion in wall rocks adjacent to the Uncle Sam vein on the surface indicates that there is an inverse relationship between intensity of alteration and concentration of Cu, Zn, and Mn in the rocks. It appears that altering solutions have "washed" these loosely bound metals out of the rocks. In contrast, the concentration of lead is not significantly lower in rocks that are intensely altered because it can be tightly held in the crystal structure of the feldspars. At depth this trend is reversed; concentration of Cu, Zn, and Mn increase with increasing alteration. Furthermore, there appears to be a vertical metal zoning in the area, with Pb decreasing with increasing depth, and Cu, Zn, and Mn increasing with increasing depth. Limited paragenetic studies of vein material, and gross characteristics of mineralized veins indicate that there were three stages of mineralization in the thesis are athat are generally correlative with several of the mineralizing stages that affected the Sunnyside Mine to the north. These include a pyrite-quartz stage, a sulfide stage, and a huebnerite stage. Regional zoning trends show that most of the tungsten in San Juan and Ouray Counties occurs in an elliptical zone that is centered around the thesis area. This zone may indicate a tungsten source at depth, perhaps a porphyry. Propylitic alteration is by far the most widespread type. It increases with depth, occurs on a regional scale, and is pre-ore. Intense solfataric alteration has affected rocks in the vicinity of the Big Colorado Mine. Phyllic alteration is characteristic of areas of vein mienralization, and is a potential exploration guide. Structural attitudes of volcanic rocks in the area reflect the "trap door" subsidence of the Silverton caldera, and the broad regional doming of the San Juan-Uncompahgre caldera system. Restored attitudes within the Henson Formation indicate that the vent that supplied the lavas is several miles to the northeast, in the vicinity of Lake Emma. The Uncle Sam vein is the dominant structural feature in the are. It strikes northeast and dips steeply to the northwest. The structure varies from a well-defined, narrow zone of mineralization at depth, to a broad zone of quartz veins and intensely altered wall rock at the surface. Continuity is lost both to the south and to the north, so that the vein does not tie in with other major structures in the area. Geometry of the vein indicates that it formed in response to resurgent doming that affected the San Juan-Uncompahgre caldera system. The Henson Formation, with its numerous layers of tuff breccia, is not as conducive to economic mineralization as the Burns Formation. The tuff breccia layers tend to disperse ore fluids in a large network of small fractures, usually resulting in a broad zone of alteration. These layers may also act as dams to mineralizing fluids, so that the zone immediately below the Burns-Henson contact may be especially rich in ore. Geochemical surveys in area of intense alteration would be useful in selected drill sites. Anomalies of Cu, Zn, Pb, Mn, and Ag are often correlative with areas of economic mineralization. Because the Cement Creek area seems to be high in tungsten, this metal might also be useful in such a geochemical study.
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