A PETROLOGICAL, MINERALOGICAL, AND ISOTOPIC STUDY OF UNIT 2 WITHIN THE BALMAT ZN DEPOSIT, ST. LAWRENCE COUNTY, NEW YORK

This is the first in-depth petrographic, mineralogic, and isotopic, study of Unit 2 within the host rocks of the Balmat Zn orebodies near Gouverneur, NY. The Balmat Zn deposits occur within mid-Proterozoic siliceous dolomitic marbles metamorphosed to upper amphibolite facies metamorphic conditions during the ~1.1Ga Grenville orogeny.

Sixteen rock units (total maximum thickness = 1250 m) comprise the Balmat section; Unit 2 (“pyritic schist”, 30 m maximum thickness) is one of the stratigraphically lowest in an otherwise marble and metaevaporite-dominated column.

This study confirms historical descriptions of Unit 2 as a schist of three different types: 1) garnet + sillimanite ± quartz ± muscovite ± carbonate, 2) pyrite ± muscovite ± carbonate ± quartz, and 3) quartz + graphite ± carbonate ± sericite ± phlogopite. Sericite and phlogopite occur as groundmass material; carbonate frequently occurs as late-stage vein material.

Pyrite is common throughout Unit 2 and occurs in three general forms: 1.) >0.5 cm euhedral crystals and larger clusters of crystals, 2.) ≤0.5 cm subhedral crystals oriented with schist foliation, and 3.) ≤0.5 cm-wide late-stage veins. The sulfur stable isotope signature of Unit 2 pyrite (n=14; δ³⁴S_CDT range from +17.0 to –10.9‰; -3.9‰, average) is generally similar between all Unit 2 pyrite types but is dissimilar from previously reported S isotope values of Balmat orebody pyrite (δ³⁴S_CDT +14.2‰ average, n=16). Rare sphalerite (n=1, δ³⁴S_CDT –5.5‰) occurs only within carbonate + pyrite late-stage veins in Unit 2 rocks and is isotopically dissimilar to previously-recorded S isotope signatures of orebody sphalerite (δ³⁴S_CDT +14.3‰ average, n=16). The variable but overall isotopically light signature of sulfur within Unit 2 pyrite may infer a bacterialogic influence in its formation.

The overall mineral assemblage for the sillimanite garnet schist and pyrite schist within Unit 2 probably supports a calcareous, paraluminous shale as a protolith; if the pyrite existed in the protolith prior to metamorphism its presence suggests that the protolith formed under anoxic/reducing conditions. The overall mineral assemblage of the Unit 2 quartz graphite schist suggests mudstone or pelitic protoliths that contained some carbonaceous (organic?) material.