SCHEELITE GEOCHEMISTRY OF THE PINE CREEK PENDANT: EMPA AND STABLE ISOTOPE MICROANALYSIS

The Pine Creek Pendant located 27 km west of Bishop, California, is host to the largest concentration of tungsten skarn deposits in the United States. Primary scheelite (CaWO₄) bearing skarn development has occurred along the pendant contact with Jurassic and Cretaceous age intrusives. The purpose of this study is to provide compositional data on the fluids responsible for tungsten mineralization within the Pine Creek Pendant using microanalytical techniques for elemental and oxygen isotope analysis.

Stable isotopes are powerful tracers for identifying the origin of mineralizing fluids; mixing between magmatic and metamorphic or meteoric reservoirs may be reflected in changes in d18O and cation chemistry of skarn minerals such as garnet and pyroxene. By examining the cation and isotopic zonation in single crystals of scheelite it may also be possible to identify variation and mixing between fluids throughout the development of skarn mineralization.

Scheelite commonly exhibits compositional zoning in the form of a yellow fluorescent, molybdenum (Mo)-rich core surrounded by a blue, Mo-poor rim. Electron microprobe analyses show blue fluorescent scheelite to contain < 5 mole percent powellite (CaMoO₄) while yellow fluorescence indicates between 5 and 24 mole percent powellite. Unlike prior isotopic research at Pine Creek, which has largely relied on bulk analyses of gangue minerals, this study included microanalysis of the ore mineral scheelite. Initial data from scheelite, analyzed by CO₂-laser fluorination, show d18O variation of 0 to 3.1‰ with a mean of 2.0 ± 0.9‰. One anomalous value of -16.4‰ has been reproduced.

Although present data suggest that in zoned scheelite there is no correlation between composition and d18O of the mineralizing fluid, d18O of fluid inferred from scheelite and garnet differ by up to 6.6‰. At 800K fluids calculated from scheelite range from 3.7 to 6.9‰ while preexisting garnet data (Brown et al., 1985) indicate fluids between 7.3 and 10.3‰. Garnet analyses from this study infer fluids of 7.7 ± 0.1‰. The variation cannot be resolved through a reasonable adjustment of temperature demonstrating that fluids responsible for mineralization of garnet and scheelite are not the same. Fluid with a magmatic signature can be inferred from garnet, whereas scheelite appears to contain some component of meteoric water.