Mineralogy and Genesis of Miarolitic Cavities in Altered Andesitic Dikes on West Spanish Peak, Colorado, USA

The focus of this thesis is the mineralogy, chemistry, and the understanding of the origin of small (1-5 cm diameter) miarolitic cavities observed in two altered andesitic dikes on West Spanish Peak, Colorado. Twenty rock samples were collected for analysis, and from them 42 thin sections were made for petrographic investigations. Eight of these thin sections were polished and analyzed with the electron microprobe for chemical compositions of the minerals, as well as identification of opaques. Five samples of cavities and one sample from each host dike were analyzed for trace element abundance using ICP-MS. The miarolitic cavities taken from West Spanish Peak contain quartz, epidote, chlorite, calcite, muscovite, barite, pyrite, chalcopyrite, hematite, and small traces of a cobalt sulfide. There are some chemical variations within and between epidotes and chlorites, involving inverse correlations of Fe with Al and/or Mg, respectively, as manifested visually by changes in color of these minerals. The data suggests that Si, Al, Ca, Fe, Mg, S, Cu, Ba, and Co were mobilized within the H2O, CO2, and SO2 bearing fluids that first generated (by exsolution from the magma and expansion) and then mineralized the cavities of both dikes. Potassium mobilization is only observed to occur in one of these dikes, and the absence of any sodium bearing phase in any cavity is noteworthy. Their mineralogy, which is similar to greenschist facies metamorphic assemblages, suggests a process of deuteric alteration occurring within the dikes at >200°C and 500 m depth. The presence of Cu and Co sulfides in these cavities suggests the potential for mineral deposits in the region, consistent with the occurrence of economic metal deposits associated with other Rio Grande Rift related Tertiary plutons to the south of Spanish Peaks.