Alteration and fluid flow associated with sediment-hosted stratiform copper mineralisation in the 1.1 Ga Midcontinent Rift System, USA

Abstract

The White Pine Cu-Ag deposit is one of the World’s largest examples of a sediment- hosted copper (SHC) mineral system, with a pre-mining resource of >10 Mt of Cu metal. The deposit is located on the southern shore of Lake Superior, in Michigan’s Upper Peninsula. Mineralisation is hosted in the late-Mesoproterozoic Keweenawan Supergroup of the Midcontinent Rift System (MRS). This succession comprises 20 km of flood basalts overlain by 10 km of clastic sedimentary rocks. The age, extraordinary volume of basalt and paucity of evaporite minerals distinguishes the MRS from other major sediment-hosted Cu basins. Basin formation commenced at c. 1111 Ma, possibly in response to a mantle plume impacting the base of the lithosphere. The mass of basalts loaded onto the crust initially drove rapid basin subsidence, forming sag-geometry basalt flows in the absence of syn-volcanic faults. Volcanism ceased abruptly

at

c. 1090 Ma and the Oronto Group sediments were deposited at a lower rate of subsidence. The Copper Harbor and Nonesuch Formations were deposited in a braided fluvial-marine embayment-sabkha environment. Seawater formed the basis for the ore-forming fluids. Metals were sourced from the breakdown of basaltic detrital grains in the Copper Harbor Formation. A Re-Os age of chalcocite mineralisation of 1039 ± 4 Ma corresponds to the early Ottawan phase of the Grenvillian orogenesis, which caused incipient basin inversion and deposition of the Freda Formation molasse. This simultaneous burial and mild compression expelled fluids from the Copper Harbor Formation aquifer. Ore precipitated via reduction of fluids by both in situ organic matter and liquid petroleum, and thermochemical sulphate reduction, at temperatures of c. 125˚C. Early Cu-Fe-sulphides nucleated on detrital and authigenic chlorite. The advancement of the peneconcordant ore zones resulted in their eventual replacement by chalcocite and native Cu, reflecting the relatively oxidised conditions and the low sulphur content of late-Mesoproterozoic seawater.