Abstract

The Early Proterozoic greenstone belt of northern Wisconsin possesses some of the best volcanogenie (volcanic-hosted) massive sulfide potential in North America. A 100-millionton resource of base and precious metal-bearing mineralization, distributed in 13 or more deposits and occurrences and clustered in three districts, has been identified in the belt. Host rocks for the volcanogenie massive sulfide mineralization are part of the 144-mi-long, east-west-trending Ladysmith-Rhinelander metavolcanic complex, which consists of various greenschists, amphibolites, cherty iron-formations, and sericite to quartz-sericite schists. These 1880 to 1860 Ma metamorphic rocks are concealed beneath Pleistocene glacial cover. Development of the Flambeau deposit, initiation of mine permitting for the Lynne deposit, and reactivation of the Crandon project indicate that the belt will receive a higher level of activity than in the past.Geologic and geophysical data compiled since the late 1960s define three depositional environments, each containing volcanogenie massive sulfide mineralization in the 1880 to 1860 Ma Ladysmith-Rhinelander metavolcanic complex: (1) a main volcanic-arc sequence, the structural core of the complex, (2) a laterally equivalent and/or younger(?), back-are basin, volcanic-volcaniclastic succession that includes a series of marie volcanic piles, and (3) major felsic volcanic centers in the back-arc basin and along the flanks of the main volcanic arc.Volcanogenie massive sulfide mineralization in all three depositional environments includes (1) syngenetic and epigenetic strata-bound to stratiform massive sulfide mineralization within, along the flanks of, or near the top of the felsic volcanic centers, (2) syngenetic strata-bound to stratiform massive sulfide mineralization associated with cherty magnetic iron-formation within the main volcanic-arc sequence, and (3) epigenetic stringer sulfide mineralization and syngenetic stratiform massive sulfide mineralization associated with marie volcanic piles developed within the back-arc basin.Identified volcanogenie massive sulfide deposits and occurrences are classified by metal content into three groups, Cu, Zn-Cu, and Zn-Pb-Cu. Each group exhibits various styles of mineralization which include sheets, mounds, stacked lenses, and replacements.Potentially economic deposits are associated with felsic volcanic centers and sulfide-bearing meta-argillite formations that are favorable stratigraphic units deposited before, after, or during the ore-forming event(s).Stratigraphic correlations supported by lead isotope data suggest that most volcanogenie massive sulfide deposits in the greenschist succession formed in a narrow time interval.

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