STRUCTURAL ANALYSIS OF MINERALIZED VEINS IN THE SWEET HOME MINE, ALMA COLORADO

The Sweet Home mine is located in the Mosquito Range of central Colorado, where normal faults of the Rio Grande rift cross the Colorado mineral belt. The mesothermal-epithermal Tertiary veins were originally mined for silver (1873-1966), but since 1991 the mine has produced world-class rhodochrosite specimens. The porphyry Mo-related veins are hosted dominantly in Precambrian granite gneiss and formed in two stages approximately 30 Ma. The first stage veins contain primarily quartz, sericite, and pyrite, with green fluorite and hübnerite. The second stage veins contain base metal minerals, purple to blue fluorite, and rhodochrosite.

Structural analyses were performed to model development of the veins and their relationship to regional tectonics and the Colorado mineral belt. First-stage veins and unmineralized faults strike NE, N, and WNW, and second-stage veins strike dominantly NE. All veins, faults, and fractures dip steeply between 70° and 80°, primarily to the NW and SE. The host rock foliation strikes WNW, but with a shallower 50° SW dip. Slickenlines on vein surfaces rake between 40° and 90°, though most rake steeply. Using Riedel steps it was determined that early stage veins show mostly reverse slip whereas second stage rhodochrosite veins show normal slip. Dike offsets and oblique slickenlines indicate both dextral and sinistral slip components. Stress modeling shows a near vertical maximum compressive stress (mean Fisher σ₁ plunge/trend=86°/033°) for rhodochrosite veins. The Colorado mineral belt typically shows Paleocene (Laramide) dextral shear, but our data show post-Laramide Oligocene movement switching to Rio Grande rift extension.

Rhodochrosite specimens occur in vugs and are the exploration target in the Sweet Home mine. The rhodochrosite vugs rake uniformly to the SW an average of 25º to 35º (range=0º to 90º). Pocket formation models tested include kinematic openings that occur along neoformed faults, at fault intersections, or near foliation-influenced orientation changes. The analyses indicate that, although all three models can be used to explain some vugs, the main control on the major vug sets is slightly oblique normal movement on neoformed faults.