PETROLOGY, GEOCHEMISTRY, PROVENANCE, AND ALTERATION OF PENNSYLVANIAN-PERMIAN ARKOSE, COLORADO AND UTAH

Late Paleozoic and modern feldspathic clastic sediments of the Colorad o Rockies were derived from dominantly igneous and meta-igneous Precam brian basement rocks. Average composition, expressed as Gazzi-Dickinso n QFL (quartz-feldspar-lithic fragments) is 36:64:0 in modern stream s ands, for the Cutler Formation is 54:46:0, for the Minturn Formation i s 57:43:0, and for the Fountain Formation is 51:49:0. No major mafic c omponents are present in the provenance as indicated by low Co, Cr, an d Ni abundances, although higher Fe and Mg abundances in Cutler Format ion rocks indicate derivation from somewhat more mafic provenance than the Fountain and Minturn Formation rocks. Fountain Formation rocks ar e more altered by predepositional weathering, diagenesis, and Holocene weathering than their equivalents in the Minturn and Cutler Formation s. Thus plagioclase is albitized and clay altered or absent in the Fou ntain Formation, whereas it is abundant and much less altered in the M inturn and Cutler rocks. Fountain Formation sandstone in the Steamboat Mountain section lacks remaining detrital plagioclase but contains au thigenic albite as overgrowths on, and replacement of, detrital K-feld spar and as pore-filling crystals. In the Eldorado Springs section the re is extensive epithermal potassic alteration in the Fountain Formati on from the detrital assemblage quartz + plagioclase + K-feldspar + bi otite + muscovite to the assemblage quartz + K-feldspar + authigenic a dularia + illite/muscovite + kaolinite. K and Rb have been metasomatic ally added to these rocks with concomitant removal of Ca, Na, Co, Cr, Cu, Ni, Ga, Th, U, Zn, Ba, Ce, La, and Y during passage of hydrotherma l fluids, apparently in the Tertiary. These fluids were probably heate d by deep-seated magma adjacent to the Colorado Lineament and/or range front faults that served as geothermal water conduits toward the surf ace.