Relations between deformation and sediment-hosted copper mineralization: Evidence from the White Pine part of the Midcontinent rift system

Abstract

Detailed studies over the past decade have significantly extended and revised our knowledge of the geologic history of the well-known White Pine mining district of northern Michigan, and indicate that the location of faults exerted a strong control on copper mineralization in this part of the Midcontinent rift system. At White Pine there is evidence for three episodes of faulting: (1) synsedimentary faulting, (2) subsequent high-angle, dominantly normal faulting, and (3) thrusting. Two stages of copper mineralization are present at White Pine and in the nearby Presque Isle syncline. The first, main-stage mineralization, formed a classic sediment-hosted stratiform copper deposit during early diagenesis. Synsedimentary faults may have provided important conduits for cupriferous brines flowing from underlying red beds of the Copper Harbor Conglomerate into the reduced silts and shales of the Nonesuch Formation, where main-stage copper sulfides and native copper were precipitated. The second stage of copper mineralization was synchronous with thrusting and introduced additional copper to the White Pine ore body and the Presque Isle syndine. Thrust faults and cogenetic tear faults provided conduits for second-stage mineralizing fluids. Collectively, these observations indicate strong control by regional deformation on fluid migration and mineralization in the rocks of the Midcontinent rift, similar to proposed relations between deformation and mineralization in other tectonic settings.