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)Å³, 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_CO2of the fluid is » 0, the epidote-forming reaction at Garnet Hill occurred in the range of 575-590°C at log f_O2» -15.

Paper No. 37-0
*SINGLE CRYSTAL STUDY OF FE-RICH SKARN EPIDOTE FROM GARNET HILL ROOF PENDANT, CALIFORNIA*
LORDO, Kathleen M.¹, ANDERSON, Kevin M.¹, BOND, Marcus R.², and LOWELL, Gary R.³, (1) Geosciences, Southeast Missouri State Univ, Cape Girardeau, MO 63701, glowell@semovm.semo.edu, (2) Chemistry, Southeast Missouri State Univ, (3) Geosciences, Southeast Missouri State Univ, Cape Girardeau, MO 63701 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)Å³, 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_CO2of the fluid is » 0, the epidote-forming reaction at Garnet Hill occurred in the range of 575-590°C at log f_O2» -15.
North-Central Section (36th) and Southeastern Section (51st), GSA Joint Annual Meeting (April 3–5, 2002)
Session No. 37 Igneous and Metamorphic Petrology Heritage Hall: West 8:20 AM-11:00 AM, Friday, April 5, 2002

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