CLAY MINERALOGY OF FAULT ROCKS, ZONES OF HYDROTHERMAL AND LOW TEMPERATURE ALTERATION, AND PROTOLITH AS RECORDERS OF THE REGIONAL SCALE PALEO PLUMBING SYSTEM OF THE CRYSTALLINE BASEMENT OF THE CENTRAL COLORADO ROCKY MOUNTAIN FRONT RANGE

Characterization of regional-scale paleofluid flow and mineral deposit formation in the Proterozoic basement rocks of the Central Colorado Front Range involves detailed, outcrop-scale structural geologic mapping integrated with multi-faceted XRD studies. We use clay mineral samples collected in distinct fault zone components to better understand fundamental clay forming processes and fault-related fluid flow. The following distinct groups of clays and associated processes are identified: 1) clay mineral formation by cataclasis and fluid flow localized in fault cores likely at the expense of protolith feldspars; 2) localized hydrothermal alteration variably associated with damage zone quartz veins; 3) low temperature weathering, and 4) crystallization of trace illite by burial and uplift or weak regional hydrothermal alteration in the protolith.

Clay data from six fault zone component traverses indicate total clay content from 20-40%. Illite (1M >>2M1) and kaolinite are most abundant in fault cores, whereas feldspars and biotite are largely altered to absent. Clay content decreases in damage zones where feldspars and biotite are weakly altered. Hematite and goethite, important signatures of Late Cretaceous to Tertiary mineralization, are common in these fault-related sequences.

In contrast, total clay content in weathering profiles in granitic protolith is low (<10%). Profiles are comprised of kaolinite, smectite, and minor to trace illite. Plagioclase in saprolites are weakly altered to kaolinite. Smectites are dioctahedral, Fe-poor, and nearly devoid of illitic interlayers. Biotites are oxidized with possible K removal and development of vermiculite interlayers. Trace to minor illite (3-5%) is present in nearly pristine granite protolith. The abundance of illite (<5%) remains relatively constant through the weathering profiles and is likely unrelated to low temperature weathering. Instead, it may reflect an illite population equivalent to that in fresh granites that remained refractory to low temperature weathering. The near ubiquitous association of hydrothermal versus weathering related clays in faults over an area spanning more than 1100 km² is indicative of the discrete fault-controlled nature of the paleo plumbing system and it regional extent far from possible intrusive sources.