SIMS oxygen isotopes indicate Phanerozoic fluids permeated a Precambrian gold deposit

Abstract The Reef Deposit is an anomalous Au Cu occurrence in the Paleoproterozoic terranes of northern Wisconsin, better known as host to significant Cu Zn volcanogenic massive sulfide (VMS) deposits. Previous work using lead isotopes and fluid inclusions has identified a protracted development of the mineralization from initial formation as the root zone veins of a VMS deposit during the Penokean orogeny (~1.9–1.8 Ga), with the most recent mineralization/remobilization activity associated with late Paleozoic Mississippi Valley-type (MVT) fluid flow (Haroldson et al., 2018a, 2018b). Here we use the oxygen isotope history of the Reef Deposit to verify and further examine the deposit's protracted development. Laser fluorination oxygen isotope measurements of primary mineralized quartz veins range in δ18O from 6.8 to 10.0‰ (VSMOW), and a trend is observed of increasing δ18O values of parallel vein zones along a 400-m traverse from northwest to southeast, likely from a temperature gradient during initial formation. Temperature estimates for a VMS deposit setting (230 to 400 °C) are consistent with a hydrothermally shifted formation fluid, using seawater δ18O estimates during the Penokean Orogeny and evolving to higher δ18O by incorporation of magmatic fluids or interaction with local crust. In situ oxygen isotope measurement by Secondary Ion Mass Spectrometer (SIMS) of cross-cutting quartz and carbonate range in δ18O from 19.4 to 28.4‰ for quartz, 25.3 to 28.1‰ for dolomite, and 9.6 to 29.3‰ (VSMOW) for calcite. High δ18O values (>19‰) are measured in a crustiform-textured quartz stockwork microveinlet, in dolomite observed in late carbonate microveinlets, and calcite in settings associated with the late quartz and dolomite and a separate calcite-only setting directly linked with gold mineralization/remobilization.