Abstract

A complementary paragenetic and fluid inclusion investigation of the carbonate-hosted Gordonsville zinc deposit was undertaken to trace thermal and chemical evolution of the mineralizing fluid. The following paragenetic sequence was established from field and petrographic study: white (early) calcite-sphalerite-galena-fiuorite-white to lavender (main stage) calcite-barite-clear to amber (late stage) calcite. Sphalerite occurs in three textural varieties: disseminated (yellowish brown), massive (reddish brown), and vug fill (dark brown). The disseminated sphalerite, with somewhat lower Fe and Cd contents, may represent an earlier stage of mineralization, but its fluid inclusion characteristics are similar to the other two.Homogenization and freezing temperatures of two-phase (liquid + vapor) inclusions show a pronounced change in the nature of mineralizing fluids between early-stage and late-stage minerals. The early-stage minerals (early calcite through main-stage calcite) precipitated from moderately hot (97 degrees -133 degrees C) and highly saline Na-Ca-Mg-Cl fluids (salinity 21-23 equiv wt % NaCl). Variations in homogenization temperatures of the early-stage minerals probably reflect periodic influx of basinal fluids of similar salinites. However, the lack of correlation between homogenization and freezing temperatures suggests, but does not prove, that fluid mixing was not the cause of precipitation of these minerals. The fluid inclusion data for sphalerite are consistent with its emplacement from a single solution carrying both zinc and sulfur, although the model remains untested without a knowledge of the pH of the Gordonsville ore fluid.The late-stage minerals (barite and late-stage calcite) precipitated from a fluid of some-what lower temperature (96 degrees -122 degrees C) but markedly lower salinity (7-9 equiv wt % NaCl), suggesting an influx of a more dilute fluid, probably heated meteoric water, at the end of main-stage calcite deposition.

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