COMPOSITION OF COLOR-ZONED AXINITE FROM THE GARNET HILL SKARN, CALIFORNIA

Oxidized W-skarn occurs near roof pendant contacts against hornblende biotite granodiorite in the Garnet Hill area (38°28.45’N/ 120°15.17’W) of the Sierra Nevada range. Marble lenses were initially transformed to zoned Ca-rich bimetasomatic skarn by reaction with adjacent lithologies. Near igneous contacts, these lenses are replaced by Fe-rich garnet infiltration skarn overprinted by hydrous silicate alteration: epidote, actinolite, chlorite, scheelite, axinite, apatite, zircon, sphene, talc, ferrostilpnomelane, laumontite, and molybdenite. Axinite occurrence is restricted to small lenses surrounded by felty amphibole at the White Horse claim. It forms optically negative, color-zoned (clove brown interior-pale gray exterior) coarsely-bladed aggregates with a = 1.674, b = 1.681, g = 1.685, d = 0.011, 2V_x = 73.9° (meas.), and r = 3.27 g/cm³. Electron microprobe data indicate M-site compositions (Mg:Mn:Fe²⁺) of 23.6:27.6:48.7 for early brown axinite and 21.1:32.7:46.2 for late gray axinite. Fe²⁺/Mn and Fe³⁺/Fe²⁺ ratios of 1.76 and 0.13 are recorded by brown axinite; for the gray axinite these ratios are 1.41 and 0.12. In both types of axinite the Si-site and M-site are full and small vacancies (<0.10) occur in the A- and C-sites; traces of Ti, Cr, Zn, F, K, and Na were detected in both color zones. Inclusions of zoned actinolite (rim)-ferroactinolite (core) indicate that axinite appeared late in the hydrous silicate alteration stage after Fe-rich garnet + fluid reactions produced epidote (» Ps₃₃); the latter reaction (at P = 1 kb) suggests conditions on the M-H buffer at log f_O2 » -15, X_CO2 » 0, and T in the range of 575-590°. Evidently, axinite formed at somewhat lower temperatures by solutions that were less oxidizing and lower in Fe.