Skarn formation adjacent to the Whitehorn Stock, Chaffee County, Colorado

Paleozoic sediments in the Calumet Mining District were metamorphosed by emplacement of the Whltehorn Granodiorite and related sills during Late Cretaceous time producing metamorphic zonations. Carbonates in the Manitou Dolostone were metamorphosed to dolomite, tremolite and calcite assemblages. The Fremont Dolostone and Leadville Limestone were metamorphosed to dolomite, calcite, muscovite, tremolite, chlorite and minor diopside assemblages. The greatest effects of metamorphism and hydrothermal activity occur in the Pennsylvanian Kerber Sandstone that lie between the stock and a sill that intruded subjacent Leadville Limestone. Numerous vein-like sills intruded the Kerber Sandstone, producing mixtures of hypidiomorphic-granular igneous rocks and calc-silicate assemblages of diopside, plagioclase, scapolite and spinel. These rocks are hosts for large veins of massive epidote, garnet, calcite and wollastonite formed as a result of mass transfer by fluid infiltration in the sediments. Original sand-rich and carbonate layers were metamorphosed to quartzites and wollastonitic marbles, respectively. Pelitie rocks form the central part of the Kerber Sandstone. In these rocks, the andalusite zone lies furthest from the stock and is succeeded by the andalusite + sillimanite and sillimanite zones toward the stock. Cordiarite is a major phase in all three zones. Graphite is abundant in the second and third zone and as inclusions in andalusite in the first zone. Fibrolite is a retrograde (?) mineral in all three zones. Corundum-bearing pelites formed in the sillimanite zone.

The pressure of skarn formation was estimated to be 1.4 kbars by determining thickness of overburden at the time of intrusion. Temperature of formation of the carbonates in the Fremont Dolostone and Leadville Limestone was estimated to be 569°C to 583°C with a fluid composition of XC02 = 0.55 to 0.62. A temperature of 670°C for the pelitic rocks was estimated from the aluminum silicate phase diagram. Fluid composition in the pelitic rocks could not be determined but were estimated to be H20~rieh. The position of the P-T equilibria for the cale-silicate rocks are greatly affected by the fluid composition and/or the mineral composition and did not yield meaningful temperatures. Since the cale-silicate rocks and pelitic rocks are equidistant from the stock, a similar temperature of formation would be expected. At a temperature of 670°C, XC02 = 0.22 for the cale-silicate rocks. The magmatic veins that intruded the Kerber Sandstone must have been at temperatures greater than 725"C (granitic solidus).