The volcanic rocks of the Penokean orogen of northern Wisconsin are well known for hosting 1875 Ma Cu-Zn-Pb-Au-Ag volcanogenic massive sulfide (VMS) deposits that formed in deep marine extensional settings. New U/Pb ages obtained from VMS-hosting volcanic strata suggest the extensional tectonic setting was active at 1835 Ma when previous tectonic models state the Penokean Orogen is thought to have closed the ocean. These inconsistencies in dates of samples from the Penokean Orogen may suggest a different geologic timeline than previously recognized. However, younger tectonic or hydrothermal may have altered the zircon U/Pb systematics. This study compares U/Pb ages taken from zircon cores or rims to test if there is a difference in the data. If there is a correlation between core/rim analyses and ages, then it is possible that the 1835 Ma age is an artifact of tectonic or hydrothermal events and not related to timing of VMS mineralization. Preliminary core vs. rim comparisons indicates there is no distinct difference in the ages. This suggests that the younger submarine extensional VMS-forming environment existed and that the timing of major events in the Penokean orogen need to be revisited.
