ORE SULFIDE AND OXIDE MINERAL ASSEMBLAGES AND PARAGENESIS IN SKARN AT EXPECTATION MOUNTAIN, RICO MINING DISTRICT, COLORADO

The Rico Mountains are noted for Pb-Zn-Ag vein and skarn deposits that formed in Paleozoic carbonate sequences. Fe-Cu-Au skarns, that formed in the clastic rocks of the Permian Cutler Formation were documented, but not well studied. In this research, the associations and geochemistry of the metallic mineral associations in the skarn-hosted deposits of the Cutler Formation were investigated to assess their economic viability. A paragenetic sequence of mineralization and geochemical signatures of possible ore minerals are also presented.

Skarn deposits in the Cutler Formation are exposed as prospects on the flanks of the Rico dome near Expectation Mountain. These deposits are distinguished by assemblages of magnetite + pyrite + chalcopyrite + hematite. Magnetite dominates the early stages of mineralization, with the Fe-Cu sulfides dominating the later stages; hematite appears to be a late stage metamorphic or deuteric mineral. The paragenesis of the metallic minerals reveals fluctuations in oxidation states. A single bulk-rock chemical analysis reveals high concentrations of Cu (122 oz/ton) and Zn (0.96 oz/ton) along with minor Ag (<0.015 oz/ton) and Au (0.16 oz/ton). Elevated REE in the rocks are attributed to the dominance of garnet in the gangue assemblages. Microprobe analyses show that both pyrite and chalcopyrite contain minor concentrations of Au and Ag. Preliminary SEM analyses reveal that Ag is concentrated in telluride inclusions within chalcopyrite.

The Fe-Cu-Au skarns in the Cutler Formation were previously mined on a limited scale for copper. The small size of these deposits limits their economic viability, but the presence of gold-bearing pyrite and chalcopyrite holds promise for future exploration.