ZINC ISOTOPIC CHARACTER OF ORE BODIES AT BALMAT, NY: PRELIMINARY RESULTS

The zinc deposits of the Balmat-Edwards-Pierrepont district, in the New York Adirondack Lowlands, have been an important source for zinc since the early twentieth century. Although the Balmat mine is currently closed, in 2005 its owner estimated its reserves at 1.9 and its resources at 1.4 million tons, respectively. Research described here is part of an effort to determine the physical state of Balmat sulfide ore during its remobilization ca 1180 Ma, coincident with amphibolite facies regional metamorphism. Zinc is known to fractionate during kinetically variable conditions of precipitation of zinc minerals from hydrothermal fluids, as well as during certain igneous processes. Accordingly, to test the feasibility of using Zn isotopes to better understand sphalerite remobilization at Balmat, sphalerite separates from a group of 4 ore samples, located at different distances from a centralized core block in the Mahler-Upper Fowler ore body, were analyzed for zinc isotopic composition. A fifth sphalerite separate from the Horsetail zone of the Fowler ore body, was also analyzed. The analysis was performed on an ISOPROBE multicollector ICP-MS at the University of Arizona. Errors of the IRMM3702 Zn standard for this session were 0.1 per mil. Solutions were measured at 300 ppb Zn. δ⁶⁶Zn values, given in per mil, vary from a high of 0.24 to a low of -0.26 with increasing distance from the core block. The Horsetail ore has the lowest δ⁶⁶Zn, with a value of -0.04. This systematic variation of δ⁶⁶Zn with distance from the core block is consistent with time dependent fractionation of lighter isotopes into the remobilized orebodies, with the lightest ores representing the earliest remobilized fraction. Therefore, the gash fractures that host Horsetail ores may represent structural traps in which the earliest remobilized ore accumulated. Expansion of the data set is required to confirm these preliminary patterns and interpretations. At this point the mechanism of remobilization is still unclear (i.e., anataxis, solid-state flow, hydrothermal dissolution, or a combination of these mechanisms). Further documentation of ore compositions (elements and minerals), and subsequent melting experiments of model ore compositions are planned, and may help to resolve this question..