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Molybdenum Mineralization in Deep Drill Holes at the Cresson Mine and Geology of Grouse Mountain: Cripple Creek District, Teller County, Colorado
AuthorRobinson, Crystal L.
AdvisorThompson, Tommy B.
Geological Sciences and Engineering
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The Cripple Creek district in Teller County, Colorado has a long and distinguished mining history. From sheeted vein systems to large, low–grade disseminated deposits, ore is still being produced from the district, mainly at the largest operating mine in the district, the Cresson Mine. In 2003 the deepest holes drilled in the district were in the diatreme near Globe Hill (in the north) and the Cresson Pipe (in the south). These drill holes encountered molybdenum and base metal minerals in addition to continued but limited gold mineralization. Outboard of the diatreme there are several volcanic outliers. At the time of this study most had been mapped only in reconnaissance, and the question remained as to whether they were intrusive in origin or remnants of volcanic flows. This study is two separate projects and will be addressed as such herein. The first portion deals with whether gold is related to molybdenum. Several methods were attempted to ascertain their relationship. Core and RC chips were logged onsite during the Summer, 2009, and twenty-six samples were collected for petrography to produce polished thin sections. The paragenesis was established from these sections, and fluid inclusions associated with the molybdenite were sought but not found. No gold–bearing minerals were observed in thin section but molybdenite was consistently later than base metals, and previous workers have documented that gold post–dates base metals. The billets for these samples were then stained to identify down–hole changes in potassium feldspar flooding, as potassic flooding is generally associated with gold in the district. Samples were taken at five–foot intervals every twenty feet for geochemical analysis, and correlation matrices of these data were calculated. The data showed that gold has a weak to locally negative association with molybdenum. Finally, sulfur isotope analysis of molybdenite was conducted. Most of the δ<super>34</super> S values lie within a narrow range of 0 ± 5 /, a range identified by Ohmoto and Goldhaber (1997) as characteristic of porphyry systems in the western U.S. The second goal of this project was to examine the outliers to determine if they are intrusive or remnants of flows. Several outliers were explored, and two were mapped but Grouse Mountain was chosen for this report due to its complexity and degree of alteration. A geologic map, including alteration, was developed and samples were collected for petrography and chemical analyses. Two units not previously recognized at Grouse were found in this study. The first unit had previously been mapped as a hornblende phonolite or just a phonolite, but petrography has revealed it is a clinopyroxene–bearing phonolite. The second unit has been mapped as the Tallahassee Creek Conglomerate but is actually a breccia unit, known herein as “Grouse Mountain breccia.” Staining of billets from Grouse Mountain with sodium cobaltinitrite indicates that potassic alteration is abundant at Grouse. The presence of altered Wall Mountain Tuff on the summit of Grouse Mountain, and the degree of alteration suggest that Grouse Mountain is intrusive in nature. Geochemical analysis indicates three distinct fluids based on elemental correlations. These fluids are a Au – Mo fluid, a Ag – Cu – Te fluid, and a Pb – Zn – Cd fluid. While the transition from one fluid to the next is likely gradual, it is not possible to determine the timing relationship between these fluids because correlation analyses only denote a spatial relationship. Future exploration for the molybdenum within the diatreme should include drilling to delineate the extent and grade of the deposit. Since the molybdenum is mostly concentrated in the north and is more accessible there, drilling should begin in the north. Analysis of the sphalerites to determine if the molybdenum is hosted within the sphalerite lattice would be an important consideration for processing. For Grouse Mountain, samples from drilling should be examined to determine the extent and type of breccia body present. Additional drilling will not only define the breccia but should also aid in refining the surface geology.