INQUIRY INTO THE SULFUR ISOTOPE COMPOSITION OF THE VMS DEPOSITS IN THE PENOKEAN VOLCANIC BELT

The Penokean Volcanic Belt (PVB) of Northern Wisconsin hosts several volcanogenic massive sulfide (VMS) deposits. VMS deposits are significant sources of metallic resources such as Cu, Au, Pb, and Zn in the form of sulfide ores. The sulfide ores can be tested for δ³⁴S values in relation to the Vienna-Canyon Diablo Troilite (VCDT). The δ³⁴S values can indicate possible sources of sulfur for sulfide formation.

For this study 185 sulfide mineral samples were taken from seven VMS deposits within the PVB. The deposits spread over 250 km from east to west in this order; Back Forty, Horseshoe, Bend, Flambeau, Eisenbrey, and Schoolhouse, with Reef being 50 km south of the center of the spread . The sulfide species were identified through petrographic study and prepared for sulfur isotope identification. A Delta V Isotope Ratio Mass Spectrometer was used to process the samples to attain δ³⁴S values.

To date the resulting data shows average δ³⁴S values for each deposit of +2.35 ppm for Back Forty, -0.36 ppm for Bend, -0.95 ppm for Reef, -1.00 ppm for Schoolhouse. Possible sources of the δ³⁴S variance in the Back Forty deposit may indicate a paleo geographic setting near ocean margin. The proximity to land could allow for sulfur contamination from evaporite beds along the shoreline. The more negative values of the other deposits may indicate a deeper ocean setting with little continental contamination of sulfur.