A new look at the geology of the White Pine Cu-Ag deposit and the Midcontinent Rift System

The White Pine Cu-Ag deposit in the Midcontinent Rift System (MRS) is one of the archetype sediment-hosted stratiform copper deposits, with an initial resource of ∼10Mt contained Cu metal. A reinterpretation of reflection seismic data shows the basin lacks many features characteristic of rifts such assyn-sedimentary faults and alluvial fans. Mineralisationis hosted in the ∼1.1 Ga Oronto Group, between braided fluvial sandstone and conglomerate of the Copper Harbor Formation, and siltstones and shales of the Nonesuch Formation deposited in an estuarine setting during a marine transgression. New stable isotope and petrographic data reveal the ingress of seawater into the Copper Harbor Formation aquifer that facilitated the leaching of metals from basaltic detritus during diagenesis. The mineral paragenesis suggests that rapid burial compaction of the aquifer, in tandem with incipient inversion, drove fluid flow. Formation waters reutilised earlier petroleum migration pathways, flowing towards the margins of the sag basin where they were focused by faults on the margins of a paleo-topographic high. Metals were deposited in a zoned sequence during thermochemical sulphate reduction across a redox front of liquid petroleum and in situ organic matter.