Sulfur Isotope Ratios from VMS Deposits in the Penokean Volcanic Belt, Great Lakes Region, USA: Constraints on the Source of Sulfur in a Paleoproterozoic Intra-Arc Rift

Abstract

The Paleoproterozoic (~1.8 Ga) Penokean Volcanic Belt (PVB) in the Great Lakes Region of North America hosts several polymetallic volcanogenic massive sulfide (VMS) deposits. These deposits were formed by back-arc extensional volcanism during the accretion of island-arc terranes along the southern margin of the Archean Superior Craton. This study reports δ34S values obtained from sulfide minerals collected from eight VMS deposits in the PVB: Back Forty, Bend, Eisenbrey, Flambeau, Horseshoe, Lynne, Reef, and Schoolhouse. The average δ34S values from most of these deposits lie within the mantle-range between −2 and 2‰, relative to Vienna Canyon Diablo Troilite (V-CDT). Average δ34S values from Back Forty and Lynne deposits are slightly higher, at 2.5 and 2.4‰, respectively. No systematic variation in δ34S was observed based on factors such as the kind of sulfide mineral, ore-texture, type of host rock, or the nature of host-rock alteration. The narrow observed range from the PVB offers a clear indication that sulfur in the mineralizing fluid, originated predominantly from a magmatic source. If there was a significant contribution of sulfur from seawater, the δ34S of seawater sulfur must also have been close to the mantle range. Slightly higher values from Back Forty and Lynne indicate minor involvement of oxidized sulfur at shallow water levels, possibly derived from the continental margin.

Keywords

• Penokean
• VMS
• Paleoproterozoic
• sulfur
• isotope
• arc
• rift
• seawater