HYDROTHERMAL ALTERATION AND ITS EFFECTS ON SLOPE EVOLUTION IN ALUM CREEK, EASTERN SAN JUAN MOUNTAINS, SOUTH-CENTRAL, COLORADO

Hydrothermal alteration results in a change in mineralogy, and may structurally weaken or strengthen a rock and hence influence the evolution of a slope. Alum Creek, in southern Colorado, is an intensely altered natural drainage unaffected by mining activities and is used to evaluate the effects of hydrothermal alteration on slope evolution.

Alum Creek is part of a high-sulfidation, sub-economic hydrothermal system associated with the Alamosa River stock. Andesitic lava flows were subsequently intruded by equigranular and porphyritic monzonite. The latter brought a hydrothermal fluid responsible for alteration with zones of quartz-sericite-pyrite (QSP), quartz-kaolinite (QK), argillic, quartz-alunite (QA) and propylitic assemblages.

Alum Creek has downcut dominantly in the argillic, QK and QSP facies. The resulting slopes are oversteepened and show evidence of mass movement events. The slopes in the argillic facies are composed of large volumes of fine-grained unconsolidated clay material and blocky talus. The QK and QSP facies are more coherent with large areas of outcrop persisting on the slopes, however acid production during pyrite weathering has weakened outcrops and resulted in minor supergene alteration.

The stability of the slopes is dominated by two main factors, talus armoring and the angle of repose of the physically weakest mineralogic assemblage, which govern the angle at which the slopes are stable. Field mapping the mineralogy of slopes and slope cover, coupled with a detailed GIS analysis of slope gradient has aided in understanding the degree to which each factor influences slope evolution. A PHREEQC geochemical model was constructed to evaluate the degree to which hypogene mineralogies versus supergene mineralogies, resulting from pyrite oxidation and acid weathering, affect the system.

Preliminary results show alteration mineralogies dominated by clays have the greatest effect on slope angles in Alum Creek. Slopes on argillic facies have angles in the range of 30-55°, while those on other facies are less than 30°. A strong correlation exists between the surficial unit comprised of unconsolidated, unvegetated material and the steep slope angles found in the argillic facies. These results may have global implications for the construction of sulfide-bearing mine waste facilities.