Zn-Isotopic Evidence for Fluid-Assisted Ore Remobilization at the Balmat Zinc Mine, NY

Abstract Shear zone-hosted ore bodies at the Balmat, NY, zinc deposit were remobilized at the kilometer scale during amphibolite facies metamorphism ca. 1180 Ma. Despite there being little evidence for interaction of hydrous fluids with ores during deformation, such translocation distances are considered unlikely without the assistance of fluids. Measurements of Zn isotopic composition of sphalerite from six Balmat ore bodies that originated from the same stratigraphic level (Upper Marble unit 6) reveal variation between ore bodies, as well as intra-orebody trends of isotopic lightening down plunge. In general, increasing distance of remobilization correlates with decreasing ẟ66Zn. The syn-tectonic isotopic fractionation recorded in Balmat sphalerite is interpreted to have resulted from the interaction between the ore and sulfide melts that were fluxed by H2S localized primarily in Upper Marble unit 7 (fetid dolomite). Lighter isotopes of Zn were enriched in the melt, leaving the residual ore enriched in heavier Zn isotopes. These observations support previous petrographic evidence for the presence of anatectic melts at Balmat and help to explain the unusual scale of translocation at Balmat by means of fluid-assisted remobilization during which intergranular melts decreased rock competency.