Joint 118th Annual Cordilleran/72nd Annual Rocky Mountain Section Meeting - 2022

Paper No. 9-13
Presentation Time: 8:30 AM-6:00 PM


WANDA, Matthew and GONZALES, David, Department of Geosciences, Fort Lewis College, 1000 Rim Drive, Durango, CO 81301

In the western San Juan Mountains, skarn deposits are documented in Paleozoic to Mesozoic strata in close proximity to Cenozoic plutons. The Two Kids mine in Ouray was developed in 1914 in a Cu-Fe skarn hosted by the Pennsylvanian Hermosa Group. Although some ore was mined at this site, detailed studies on the formation of the skarn were not conducted. This site, however, offers insight into the genesis and geologic controls that influenced formation of skarn deposits in the region. The goals of our study were to determine the: 1) geologic controls on skarn formation; 2) mineral assemblages and petrogenesis to gain insight into metamorphic conditions and genesis of ore assemblages; and 3) economic potential of the deposit.

Skarn in the Two Kids mine is hosted in a bed of carbonaceous shale that is up to 2 m thick and situated about 150 m south of the ~65 Ma Blowout granodiorite stock. The skarn forms in irregular zones within the shale over an area of several hundred meters and varies from very coarse grained to fine grained. Chalcopyrite + pyrite ± tetrahedrite ± sphalerite are the dominant metallic minerals. The skarn grades from Ca-rich garnet + diopside + calcite + tremolite (~465° C) with a late overprint defined by actinolite + Fe-rich garnet. Hornfels on the outer margins of the deposit are mostly epidote + hematite + calcite + chlorite. The skarn deposit is displaced ~6 m by a north-dipping normal fault; breccia within the fault zone contains fragments of skarn and extensive Fe oxidation. Chemical analyses of skarn samples reveal high concentration of Cu and Fe with very minor Au and Ag.

Our study concludes that the Two Kids skarn formed by release of magmatic fluids and heat by the Blowout stock into a favorable stratigraphic horizon, likely influenced by reducing conditions in the shale. Contrary to previous interpretations, the fault did not play a part in the skarn formation. The mine was a relatively high grade, low-tonnage deposit that is no longer an economic target. However, our research provides a blueprint for possible future exploration of economic skarn deposits at deeper levels in the Hermosa Group in the western San Juan Mountains.