PHANEROZOIC AU IN A PALEOPROTEROZOIC CU VEIN DEPOSIT OF CENTRAL WISCONSIN; EVIDENCE FROM PB ISOTOPES USING LA-ICP-MS

The Reef deposit, located 15 miles east of Wausau, WI within the Paleoproterozoic Wausau Volcanic complex, is a vein hosted Au-Cu occurrence which has been described in the past as a lode or shear-zone hosted gold deposit or described separately as the root zone of a VMS deposit.

The Reef deposit consists of quartz-sulfide veins largely hosted in gabbroic intrusions. Chalcopyrite, the primary Cu ore mineral, is found in anhedral masses intergrown with pyrrhotite, mantling subhedral pyrite. Au is found in local clusters of intergrown micron scale native Au(Ag) and Bi, Ag, Au, Ni, Fe, Pb telluride minerals which are texturally late along sulfide grain boundaries and microfractures of sulfides and quartz.

Pb isotope compositions were measured by solution nebulization (SN) and MC-ICP-MS analysis of sulfide phases micro-drilled from drill core selections and aliquots of coarse reject (whole rock (WR)) assay material. Additionally, in-situ analyses were conducted by femto second laser ablation (fs-LA) and MC-ICP-MS. SN WR Pb isotope compositions (measured ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb values range from 15.8 to 21.0, and 35.1 to 35.7, respectively) and define U/Pb and Th/Pb errorchrons of approximately 1.8 Ga. The sulfide Pb analyses overlap with WR analyses but extend to higher ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios; up to 22.9 and 41.8, respectively. These radiogenic samples have low U/Pb and Th/Pb ratios and are distinct from the WR samples in having high ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb.

Fs-LA analyses of individual phases reveals the anomalous radiogenic Pb ratios in the sulfides represent mixing between two endmembers. Chalcopyrite Pb isotopes indicate a source region for sulfides at Reef from juvenile crust at approximately 1.8 – 1.9 Ga that broadly match the initial Pb isotope compositions of the WR. Altaite (PbTe) analyses indicate a separate highly radiogenic source region with a ²⁰⁶Pb/²⁰⁴Pb of 25.2 and ²⁰⁸Pb/²⁰⁴Pb of 44.0.

Altaite Pb and by association Au mineralization was emplaced during a separate hydrothermal event which must be significantly younger (Phanerozoic) than the initial vein formation (Paleoproterozoic). Indeed, there is a good match between the Pb isotope composition of Reef altaite and galena in Ordovician carbonates ~200 km to the south which implies Au mineralization is likely associated with MVT fluids.