GEOCHEMICAL AND MINERALOGICAL CHARACTERIZATION OF ORE FROM THE PYRITES AND STELLA PYRITE DEPOSITS, ST. LAWRENCE COUNTY, NY

Our study is the first detailed geochemical, mineralogic, and stable isotope analysis of pyrite ore from the Pyrites and Stella deposits – two of nineteen pyrite deposits located within a 56 x 6 km-wide belt within the Adirondack Lowlands of St. Lawrence and Jefferson Counties.  The area is underlain by Precambrian quartz-feldspar-mica gneiss, marbles, and amphibolites (Grenville series) and mafic to felsic intrusive igneous rocks, including pegmatite.  At Pyrites and Stella, earlier workers described ore material consisting mostly of pyrite with lesser pyrrhotite within veins, sheets or lenses (² 6m thick, 0.5km wide, 0.5km long) hosted within rusty (pyritic) quartz-feldspar-mica gneiss; gangue minerals include quartz, sericite, chlorite, and graphite.  Pyrite ore consists of variably-sized crystals or crystal aggregates of pyrite intergrown with lesser pyrrhotite, or as late-stage thinly-banded pyrite veins.  The pyrite ore bodies are generally not sharply defined and grade into the surrounding host rock.

Utilizing transmitted and reflected light microscopy and electron microprobe analysis of Pyrites and Stella ore samples, we documented:  1. Ore pyrite is compositionally similar (48.1% Fe, 52.5% S, Pyrites; 48.3% Fe, 52.0% S, Stella) between deposits; 2. Stella ore is relatively pyrrhotite-poor; 3. Previously undescribed occurrences of sphalerite and chalcopyrite within ore from both deposits; sphalerite composition varies between deposits (33.0% S, 0.4% Fe, 68.1% Zn, Pyrites; 32.2% S, 6.5% Fe, 59.3% Zn, Stella); 4. Previously undescribed gangue minerals including rutile, chromite, and molybdenite (Pyrites) and galena, stannite, and V-bearing rutile (Stella).

Relatively depleted S stable isotope compositions of ore pyrite from the Pyrites (d³⁴S_CDT range from -17.1 to -5.9ä; n=5) and Stella (d³⁴S_CDT range from -13.5 to -9.2ä; n=3) deposits support a possible biogenic influence on ore pyrite formation; individual pyrite bands (²0.2cm) characterized by alternating light and heavy S stable isotope compositions (d³⁴S_CDT range from -4.5 to 4.0ä; n=4) within larger veins (²5cm) that cross-cut earlier Stella ore suggest the Stella deposit formed from multiple pulses of isotopically-distinct S-bearing fluid from different (magmatic or metamorphic and/or biogenic) sources.