On the timing and metallogenic implications of the sediment-hosted stratiform copper–silver mineralization in the Creston Formation (Belt-Purcell Supergroup), British Columbia, Canada

Abstract

Sediment-hosted stratiform copper–silver mineralization is common in Middle Creston strata of southeastern British Columbia, a correlative lithostratigraphic unit of the Ravalli Group of western Montana. The Ravalli Group hosts the largest sediment-hosted stratiform copper–silver deposits of the Belt-Purcell Supergroup. Strata of the Belt-Purcell Supergroup accumulated between ~1540 and 1300 Ma in a pericratonic rift basin (Belt-Purcell basin) constructed over Archean and Paleoproterozoic blocks of Laurentia. The basin underwent several events of deformation and metamorphism at ~1380–1325 Ma (East Kootenay orogeny), ~1200–1000 Ma (Grenvillian), and 900–800 Ma (Goat River orogeny) prior to Meso-Cenozoic Cordilleran tectonism. U–Pb geochronology of detrital zircons from Middle Creston rocks define a maximum age of deposition of ~1470 Ma, in agreement with the established age range for the Belt-Purcell Supergroup in southeastern British Columbia. Molybdenite is locally part of the sediment-hosted stratiform copper–silver mineralization in the region, either as isolated grains within mineralized strata or in paragenetic sequence with the cupriferous sulfides. Re–Os geochronology on molybdenite reveals repetitive episodes of fluid flow through Middle Creston strata in the study area, with principal events at 1170–1140 and 1043–990 Ma. Paragenetically constrained molybdenite, in growth sequence with bornite and chalcocite, yields an age of 1043 ± 6 Ma. Hence, the fluid responsible for the sediment-hosted stratiform copper–silver mineralization in southeastern British Columbia permeated the host sequence ~440 m.yr. after its deposition, coincident with the Grenville-age metamorphic event at ~1200–1000 Ma. Such tectono-thermal activity extended along the western margin of Laurentia beyond the limits of the Belt-Purcell basin, and was a far-field response to orogen-scale collisions taking place off Laurentia during Rodinia assembly.