Arizona Geological Society: Geology and Hydrothermal Alteration of Red Mountain Lithocap, Eastern Lake City Caldera, Hinsdale County, Colorado

Exploration for high-sulfidation epithermal deposits requires the development of protocols that allow distinction between mineralized and barren lithocaps. To better define the characteristics of barren lithocaps, Red Mountain, located at the eastern margin of the Lake City caldera in the San Juan Mountains of southwest Colorado, has been studied. This included logging of drill cores, to reconstruction the distribution of lithological units and alteration styles in the subsurface. Core logging was supplemented by petrographic investigations, scanning electron microscopy, and geochemical analyses. Hyperspectral core scanning was conducted to constrain the alteration zoning within the Red Mountain lithocap.

The research demonstrates that the Red Mountain lithocap exhibits mineralogical zoning. Alteration in the shallow subsurface is dominated by quartz-alunite which grades into kaolinite with depth. In the deep part of the lithocap, smectite-white mica alteration overprints secondary biotite ± K-feldspar alteration. The smectite-white mica alteration commonly shows patchy textures. In mineralized systems, patchy pyrophyllite is a texture observed at the transition from the epithermal to the porphyry environment.

Vuggy quartz occurs as narrow ledges within the advanced argillic alteration halo. The vuggy quartz is interpreted to represent residual quartz formed through intense acid leaching of the volcanic rocks by volcanic vapors condensed into groundwater. The vugs contain small euhedral alunite crystals, but drusy quartz has not been observed at Red Mountain. The absence of widespread silicification sets the barren lithocap at Red Mountain apart from high-sulfidation epithermal deposits. In the case of these deposits, early acid-style alteration can be observed that is comparable to Red Mountain. However, the vapor-dominated magmatic-hydrothermal system changed to a liquid-dominated system prior to mineralization as the ore minerals formed from low-temperature aqueous liquids. The absence of drusy quartz may be a key indicator in exploration allowing distinction between barren and mineralized lithocaps.

Rhenium-osmium geochronology performed on two molybdenite samples yielded ages of 23.21 ± 0.14 Ma and 23.11 ± 0.10 Ma. The obtained ages are within error of the volcanic rocks formed during collapse and resurgence of the Lake City caldera suggesting that magmatic-hydrothermal activity followed immediately after the caldera volcanism.

Presented by

Nirio Mendoza Inca

MSc, Colorado School of Mines