14 Mineralogy and Petrology (Posters)

Friday, 14 March 2003
8:00 AM - 12:00 PM

COMPOSITIONALLY AND OPTICALLY ZONED SKARN GARNET FROM CALAVERAS COUNTY, CALIFORNIA

Paper number: 14-6
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

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 (And52.7 to And68.2) and 2) reddish brown isotropic unzoned replacement garnet (And70.8) 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 (Ps29.4) 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 logfO2 » -15. This T-range is a minimum for early infiltration skarn.