Late Paleozoic and modern feldspathic clastic sediments of the Colorado Rockies were derived from dominantly igneous and meta-igneous Precambrian basement rocks. Average composition, expressed as Gazzi-Dickinson QFL (quartz-feldspar-lithicfragments) is 36:64:0 in modern stream sands, for the Cutler Formation is 54:46:0, for the Minturn Formation is 57:43:0, and for the Fountain Formation is 51:49:0. No major mafic components are present in the provenance as indicated by low Co, Cr, and Ni abundances, although higher Fe and Mg abundances in Cutler Formation rocks indicate derivation from somewhat more mafic provenance than the Fountain and Minturn Formation rocks. Fountain Formation rocks are more altered by predepositional weathering, diagenesis, and Holocene weathering than their equivalents in the Minturn and Cutler Formations. Thus plagioclase is albitized and clay altered or absent in the Fountain Formation, whereas it is abundant and much less altered in the Minturn and Cutler rocks. Fountain Formation sandstone in the Steamboat Mountain section lacks remaining detrital plagioclase but contains authigenic albite as overgrowths on, and replacement of, detrital K-feldspar and as pore-filling crystals. In the Eldorado Springs section there is extensive epithermal potassic alteration in the Fountain Formation from the detrital assemblage quartz + plagioclase + K-feldspar + biotite + muscovite to the assemblage quartz + K-feldspar + authigenic adularia + illite/muscovite + kaolinite. K and Rb have been metasomatically 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 hydrothermal fluids, apparently in the Tertiary. These fluids were probably heated by deep-seated magma adjacent to the Colorado Lineament and/or range front faults that served as geothermal water conduits toward the surface.

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