PETROGRAPHIC AND MINERALOGIC EXAMINATION OF MYLONITES WITHIN THE BALMAT ZN DEPOSIT, NEW YORK

This is the first petrographic and mineralogic study of the 6B and 6FB subunits within the Balmat Zn deposit near Gouverneur, NY. The Balmat deposit is hosted mainly by mid-Proterozoic metamorphic rocks (siliceous dolomitic marbles, Grenville series) formed under near granulite-facies metamorphic conditions (~6.5 kb, 700°C) during the ~1.1Ga Grenville orogeny.

Subunits 6B and 6FB, part of the 167-183 m-thick Unit 6, vary in thickness (≤ 8m) and continuity throughout the deposit although 6FB is generally thinner and laterally less extensive than 6B. Both subunits are fine-grained, light to dark gray laminated and banded quartzites containing quartz, pyroxene, disseminated pyrite, and rare chlorite and phlogopite, and cross-cut by ≤ 5cm veins of pyrite ± pyrrhotite ± sphalerite ± chalcopyrite ± calcite. Previous workers described the laminations within these subunits as relict bedding features, and the exact mineralogy of the different colored bands had not been determined.

Based on transmitted and reflected light microscopy and electron microprobe analysis of nine polished thin sections prepared from four representative samples (MAMD-GQ, MAMD-6FB, MAMD-1, MA-3710) collected in situ from two different locations within the Balmat deposit, our study shows: 1. laminations within the 6B and 6FB subunits are composed of fine-grained quartz + pyrite and are probably metamorphic in origin and not relict bedding features; 2. diopsidic (Ca>Mg) twinned pyroxene (avg. wt %: MgO, 18.2; CaO, 25.7; FeO, 0.2) is ubiquitous (with quartz) in all samples; diopside and fine-grained sulfides (mostly pyrite) are more abundant within the darker gray bands; 3. Ca-rich diopside within 6FB samples occasionally encapsulates relatively Mg-rich diopside grains (avg wt %: MgO, 24.2; CaO, 13.5, FeO, 0.24); 4. All quartz and diopside grains are recrystallized; some grains are shattered and broken.

Petrographic and mineralogic evidence suggests the 6B and 6FB subunits are probably recrystallized (annealed) "stratigraphic" mylonites whose fabric parallel recrystallized bedding planes and major stratigraphic unit contacts. Mylonitization of these subunits predates major sulfide mobilization or remobilization which occurred after an earlier phase of recrystallization and deformation, but still at high metamorphic grade.