COMPOSITIONALLY AND OPTICALLY ZONED SKARN GARNET FROM CALAVERAS COUNTY, CALIFORNIA

Garnet Hill (38º28.45'N, 120º15.17'W) is located in the northern part of the Sierra Nevada range where oxidized W-skarn occurs in a roof pendant enclosed by hornblende biotite granodiorite. Roof pendant lithologies were converted to marble and bimetasomatic skarn and then overprinted by Fe-rich infiltration skarn and late hydrous silicate alteration (epidote, actinolite, chlorite) near igneous contacts.

Reddish brown garnet euhedra (2-4 cm) are present throughout the infiltration skarn near the summit of Garnet Hill. Concentric growth zones parallel dodecahedral faces and record two distinct andradite-grossularite solid solutions: 1) colorless birefringent host garnet with oscillatory zoning (And_52.7 to And_68.2) and 2) reddish brown isotropic unzoned replacement garnet (And_70.8) that separates colorless garnet from epidote. Garnet compositions span the usual range for calcic garnets in oxidized W-skarns. Zone composition controlled replacement of host garnet by epidote (Ps_29.4) and quartz. Co-existing epidote-garnet compositions suggest that epidote formed by fluid reaction with garnet (Pf=1 kb, M-H buffer) in the range of 575-590ºC at logf_O2 » -15. This T-range is a minimum for early infiltration skarn.