PRELIMINARY CHARACTERIZATION OF GEOLOGICAL CONTROLS ON GROUNDWATER FLOW AND SOLUTE TRANSPORT IN AN ALPINE HYDROTHERMAL METAL DEPOSIT: HANDCART GULCH, MONTEZUMA MINING DISTRICT, COLORADO ROCKY MOUNTAIN FRONT RANGE

Handcart Gulch, located along the Continental Divide, contains unmined hydrothermal deposits of disseminated and vein pyrite, Mo and Cu sulfides. Deep mineral exploration boreholes at the head of the watershed and shallow boreholes along the trunk stream provide a unique opportunity to study geologic controls on (1) the initiation of upper-crustal-scale fluid flow driven by extreme topographic head gradients (~0.4) and (2) the introduction of trace metal-rich waters to the crust. A numerical model is being used to integrate a variety of geologic, geophysical, geochemical, and hydraulic data and test hypotheses regarding the flow system and solute transport.

Complex bedrock consists of folded mafic and felsic Precambrian metamorphic rocks. Geologic structures include the km-scale Montezuma ductile shear zone cut by brittle small displacement faults and high intensity joint networks. Tertiary porphyritic stocks and dikes caused fracturing that is spatially associated with pervasive, quartz-sericite-pyrite (up to 10 wt. %) alteration – the major source of trace-metals and acidity. Uplift, glaciation, and hydrologic processes generated high relief and a carapace of weathered bedrock overlain by local alluvium, colluvium, thin soils, an active rock glacier composed of QSP bedrock, and a narrow band of ferricrete beneath the trunk stream.

Geophysical logging indicates heterogeneous and hydraulically conductive fracture networks allow potentially high and discrete ambient flow. Yet conductive fractures do not uniquely correlate with parameters such as fracture intensity, orientation, or depth. Preliminary bulk hydraulic conductivities (K) derived from hydraulic tests indicate higher Ks in QPS felsic rocks versus lower bulk Ks in propylitic mafic rocks suggesting that alteration may be an important control on permeability structure at the watershed scale. Observed, large seasonal head variations (~40m) are likely governed by the permeability and porosity structure of an upper zone of weathered and altered bedrock, and may enhance the introduction of oxygen to groundwater. Generation of acid and metal-rich waters is likely further enhanced by microbiological activity as waters proceed along shallow and deep flow paths that end at the trunk stream or might proceed to the deeper upper crust.