A GEOCHEMICAL INVESTIGATION OF A LEAD-BEARING CALCITE VEIN FROM MACOMB, ST.LAWRENCE COUNTY, NEW YORK

The goal of this study was to determine the age and origin of a sulfide-bearing calcite vein in Macomb, St. Lawrence County, NY. The vein (N65W, 65SW; ≤ 2m wide, ~16m long) is hosted by mid-late mesoproterozoic metamorphic and igneous rocks of the Adirondack Lowlands. Sulfide minerals include galena > pyrite > sphalerite.

Microthermometry measurements of primary fluid inclusions (n=4) within vein calcite yield Tm of -47°C to -38°C and Th of 108.0°C to 112.0°C. The very low ice melting temperatures indicate the presence of salts in addition to NaCl, most likely CaCl₂, with CaCl₂ >>> NaCl. Analysis of gases within the inclusions yield Ar/He and N₂/Ar ratios similar to basinal MVT-type brines.

Carbon stable isotope analysis of vein calcite (δ¹³C_VPDB = -0.08‰, avg.; n=5) and calculated δ¹⁸O_VSMOWof vein calcite source fluid (+2.2 to +2.7‰) may suggest the vein formed from magmatic (± meteoric?) water interacting with limestone/marble host rocks.

Attempts at determining the age of pyrite and galena via the Re-Os method and temperature of sulfide formation via S stable isotope fractionation proved inconclusive. Although the calcite+sulfide vein is hosted by metamorphic rocks, the low fluid inclusion homogenization temperatures recorded suggest timing of vein emplacement is post-Grenville.