MINERALOGY AND CHEMISTRY OF SILICATE MINERALS IN SKARN DEPOSITS ON EXPECTATION MOUNTAIN NEAR RICO, COLORADO: INSIGHT INTO METAMORPHIC HISTORY AND CONTROLS

Skarn deposits in the Rico Mountains are hosted in Paleozoic carbonate and clastic sedimentary strata. Although previous studies have described the general mineral associations of the ore deposits, the petrologic features of the skarn are not well constrained. In this research, silicate mineral assemblages associated with Fe-Cu skarns from Expectation Mountain were investigated to determine metamorphic grade and paragenetic evolution.

The Cu-Fe skarns hosted by the Permian Cutler Formation on Expectation Mountain are distinguished by assemblages of garnet + quartz + calcite + epidote-clinozoisite ± actinolite-hornblende ± muscovite ± adularia ± retrograde chlorite. These associations are consistent with a broad peak metamorphic temperature range of 400-600°C in a low pressure environment (estimated at 0.02 GPa). Complex zoning in garnet (82.9% Adr, 15.2% Grs) porphyroblasts reveal fluctuations in composition that were probably influenced by rapid shifts in oxygen fugacity or other environmental conditions. Micron-scale oscillatory zoning and anisotropism highlight the chemical variations between andradite-rich and grossular-rich zones.

The silicate-carbonate assemblages of skarn deposits in the Cutler Formation are broadly similar to those of Pb-Zn-Cu-Ag skarns in the Rico district hosted by carbonate and pelitic strata in the Leadville Limestone and Hermosa Formation. These skarns are distinguished by the presence of carbonates + garnets ± epidote-clinozoisite ± tremolite ± diopside-hedenbergite ± actinolite-hornblende ± chlorite ± albite ± potassium feldspar. Our investigation reveals, however, that the mineral assemblages in high-grade Fe-Cu-Au skarns at Expectation Mountain are more compositionally restricted which is indicative of these deposits. These results are consistent with historical accounts of Fe-Cu-Au skarn deposits in the Cutler Formation in the La Plata Mountains, and other locations in the western San Juan Mountains.