Mesoproterozoic porphyry copper mineralization at Mamainse Point, Ontario, Canada in the context of Midcontinent rift metallogeny

Abstract

The Jogran copper prospect in southern Ontario, Canada, is located on the northeastern shoulder of the North American Midcontinent rift system where it is centered on two small, closely spaced quartz monzonite porphyry intrusions emplaced into Neoarchean mafic metavolcanic rocks. Notwithstanding an atypical alteration zoning pattern, the distinctive alteration styles and veinlet types confirm its porphyry copper affiliation. Two samples of molybdenite from chalcopyrite-bearing quartz veinlets in the two centers returned Re-Os ages of 1101 ± 5 and 1094 ± 5 Ma, showing that the mineralization took place during the main stage of extension and magmatic activity that gave rise to the Midcontinent rift and associated large igneous province (LIP). The nearby copper-bearing Tribag breccia pipes and Coppercorp veins have been shown previously to also be intrusion related, and likely formed penecontemporaneously with Jogran. The Jogran and nearby copper mineralization overlapped temporally with generation of low-grade magmatic copper-nickel-PGE deposits in the Duluth intrusive complex but predated the strata-bound native copper and chalcocite deposits in the Keweenaw Peninsula, which are believed to have formed from basinal fluids during rift inversion. Although such magmatic and strata-bound copper deposits typify intracontinental rift metallogeny, porphyry copper deposits are normally confined to convergent margin magmatic arcs, thereby making the rift-LIP setting of Jogran exceptional and possibly unique.