Post-orogenic exhumation of an auriferous terrane: the paleoplacer potential of the Early Carboniferous St. Marys Basin, Canadian Appalachians

Abstract

The St. Marys Basin of mainland Nova Scotia, Canada, consists of Late Devonian–Early Carboniferous clastic rocks of the Horton Group deposited in the waning stages of the ca. 420–360 Ma Acadian orogeny in the Canadian Appalachians. Clast lithologies and lithogeochemical analyses indicate that the detritus was predominantly derived from the Meguma terrane that occurs to the south of the basin. The Meguma terrane contains abundant mesothermal gold deposits that are coeval with peak magmatic activity from ca. 380 to 370 Ma and underwent rapid uplift and erosion between ca. 370 and 360 Ma. Within the St. Marys Basin, the contact between the lacustrine Little Stewiacke River Formation and the fluviatile Barrens Hills Formation is interpreted to represent a shoreline and a potentially favorable environment for depositing paleoplacer gold. Geochemical analyses of lithologies adjacent to this contact indicate that the siltstones are predominantly derived from Meguma terrane metasedimentary rocks, whereas the sandstones and conglomerates are predominantly derived from Meguma terrane granitoids. Geochemical and mineralogical analysis indicate the accumulation of heavy minerals including zircon and gold. Micron-scale (<150 μm) gold identified in the matrix of the conglomerates has a “nuggety” appearance and flakey microtexture indicative of a detrital origin. This observation indicates that the mesothermal deposits were exhumed by the latest Devonian, consistent with regional tectonic syntheses that invoke rapid uplift of the Meguma terrane following peak orogenic activity. This study suggests that favorable depositional environments for paleoplacer deposits may occur in Horton Group rocks that are derived from the Meguma terrane.