Effects of paleoweathering and supergene activity on volcanogenic massive sulfide (VMS) mineralization in the Penokean Volcanic Belt, northern Wisconsin, Michigan and east-central Minnesota, USA: Implications for future exploration

Abstract

Three cycles of paleoweathering and supergene activity have been identified within the Penokean Volcanic Belt (PVB) of northern Wisconsin, Michigan and east-central Minnesota, USA. These cycles are responsible for significant subaerial weathering, erosion, oxidation and most importantly local supergene Au, Ag and Cu enrichment that has a major economic impact upon volcanogenic massive sulfide (VMS)-style mineralization within the PVB. The paleoweathering profile (regolith-saprolite) that developed on the exposed PVB surface now marks an angular unconformity with stratigraphically overlying, late Paleoproterozoic Baraboo Interval quartzites and late Cambrian sandstones.Differential uplift of the PVB surface is a direct result of the ∼1850 Ma compressional tectonic event that occurred during the Penokean orogeny. The uplifted PVB paleosurface experienced its first weathering cycle in the mid to late Paleoproterozoic. VMS mineralization near the paleosurface was exhumed and affected during this protracted cycle which ceased upon deposition of Baraboo Interval quartzites (1788–1714 Ma). Further uplift and subsequent unroofing of the quartzites during the Mazatzal orogeny (∼1650–1630 Ma) initiated a second paleoweathering cycle that culminated within the Neoproterozoic – Cambrian transition. During this cycle of accelerated and uninterrupted paleoweathering, new and surviving supergene Au-Ag-Cu-enriched profiles geochemically matured over exhumed mineralized sections in terrains with favorable fault block movements. Supergene systems that developed during both cycles were highly acidic environments. Low protolith reactivity and high hypogene pyrite concentrations promoted acidity of supergene fluids thus facilitating enhanced Cu, Au and Ag mobility. In those systems that lacked sufficient carbonate, Zn and Pb were lost to surrounding aqueous regimes.The second paleoweathering cycle ended with the deposition of overlying late Cambrian sandstone (497–485.4 Ma). Erosional retreat of the overlying Paleozoic sequence exposed many of these supergene deposits to a third, possibly brief paleoweathering cycle before the onset of Quaternary continental glaciation. VMS profiles not protected by sandstone outliers or within down-dropped fault blocks were glacially eroded.Of the 14 or more known VMS deposits and occurrences in the belt, four have preserved or partially preserved paleoweathering profiles. The most significant are at the Flambeau and Back Forty deposits where Au-Ag-enriched indigenous gossans with extensive or local underlying supergene-enriched Cu sulfide zones have developed over hypogene massive sulfide horizons. These profiles demonstrate efficient cumulative or multicyclic Au, Ag and Cu-enrichment to ore grades.Although commonly small (≤2 million tonnes), such supergene-enriched deposits are generally shallow, allowing for development by low cost open pit methods and production of exceptionally high-grade, direct shipping copper and/or gold ores. As such, the buried PVB unconformity is considered a prime exploration target that is highly prospective for new discoveries of commercially viable, supergene-enriched VMS resources.