THE METALLOGENY OF BASE AND PRECIOUS METALS IN EVOLVING
PLATE MARGIN RIFT SETTINGS, SOUTHERN APPALACHIA

DUKE, N.A., Geology Dept., Univ. of  Western Ontario, London, Canada N6A 5B7

 In the southern Appalachians the Late Precambrian fragmentation of Grenvillia during opening of lapetus is shown by variously evolved marginal rift settings. The initial rift which developed along the eastern flank of the Blue Ridge was a depository for the mafic volcanics and terrigenous sediments comprising the N.C. Ashe and Va. Lynchburg Formations. Continued eastward propagation of extensional subsidence into the earliest Paleozoic is demonstrated by the change to basinal pelites from basin margin turbidite and shelf deposits in the N C Alligator Back and Va. Candler Formations, respectively. Outboard the deep marine Candler basin transitioned into a volcanic rift represented by the bimodal Chopawamsic metavolcanics and overlying Ordovician-aqe Quantico-Arvonia slates. This assemblage is now in fault contact with Grenvillian basement underlying the Outer Va. Piedmont.
Base and precious metal mineralization is concentrated in both initial and advanced volcanically active rift sites, with deposit-types reflecting the stage of evolution of the extensional terrane Primitive. Besshi-type. cupreous pyrrhotite deposits are hosted in the initial mafic volcanic-clastic sediment association and formed where hydrothermal activity accompanied tholeiitic volcanism. Polymetallic massive sulphide, iron formation, and gold deposits occur with the ad-vanced bimodal volcanic-slate association; exhalative sulphide is hosted in volcanics while iron formation and gold is concentrated along the volcanic-slate contact. This metal partitioning indicates a transition from a high to low water/rock hydrothermal system in the advanced rift site. The metallogemc evolution from copper-iron sulphide to polymetanic sulphide affiliated with iron formation and gold is attributed to the change from tholeiitic to bimodal volcanism.