Northeastern Section (45th Annual) and Southeastern Section (59th Annual) Joint Meeting (13-16 March 2010)

Paper No. 6
Presentation Time: 8:00 AM-12:05 PM


REMINGTON, Kristen M.1, KELSON, Christopher R.1 and DELORRAINE, William F.2, (1)Department of Geology, State University of New York at Potsdam, 44 Pierrepont Avenue, Potsdam, NY 13676, (2)St. Lawrence Zinc Company, 408 Sylvia Lake Road, Gouverneur, NY 13642,

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; δ34SCDT 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 (δ34SCDT +14.2‰ average, n=16). Rare sphalerite (n=1, δ34SCDT –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 (δ34SCDT +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.