SINGLE CRYSTAL STUDY OF FE-RICH SKARN EPIDOTE FROM GARNET HILL ROOF PENDANT, CALIFORNIA

SINGLE CRYSTAL STUDY OF FE-RICH SKARN EPIDOTE FROM GARNET HILL ROOF PENDANT, CALIFORNIA

Lordo, K. M.¹, Anderson, K. M.¹, Bond, M. R.², and Lowell, G. R.¹, (1) Geoscience Dept, glowell@semovm.semo.edu (2) Chemistry Dept, Southeast Missouri State University, Cape Girardeau, MO 63701.

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, and molybdenite.

Single crystal XRD analysis (MoKa) of a large dark epidote crystal from Garnet Hill infiltration skarn produced the following refined cell data based on 4096 reflections (R=0.033, s.d.=( ): a=8.9097(1)Å, b=5.6439(1)Å, c=10.1734(1)Å, b=115.3946(7)°, V=462.147(11)ų,  SG=P2₁/m.  Refinement of 125 parameters, including anisotropic thermal parameters, M-site occupation factors, H coordinate and thermal parameters, and an extinction parameter [3621 unique reflections with F_o > 4s (F_o)] yield an agreement factor R=0.0275.  Estimation of total Febased on the b parameter is 0.954 apfu (=Ps₃₂) and approximates the maximum of Ps_33.  The M3 site contains 0.839(3) Fe atoms (87.94%) and therefore the M1 site must contain about 0.115 Fe atoms (12.06%).  The total Fe estimate and assumed stoichiometry gives a calculated cell density=3.420 g/cc that compares closely with measured density (3.45 ±0.03 g/cc, n=10).  The b refractive index (1.760 at l=590.5 nm) supports the Fe estimate based on cell data.

Hydrous silicate alteration in proximal Garnet Hill infiltration skarns selectively replaced Fe³⁺-rich (» Adr₇₇) shells of zoned garnets with epidote while Fe³⁺-poor (»Adr₄₁) zones remained stable. Experimental data show that epidote compositions » Ps₃₃ form as reaction products between Fe³⁺-rich garnet (Adr₆₃)and Si-Fe-rich hyrothermal solutions only on, or above, the M-H buffer (magnetite absent from skarn assemblage).  If P_f is taken as 1 kb on the M-H buffer and X_CO2 of the fluid is » 0, the epidote-forming reaction at Garnet Hill occurred in the range of 575-590°C at log f_O2 » -15.