The economic significance of the Bell River Complex, Abitibi subprovince, Quebec

Abstract

The exposed part of the Bell River Complex on the northern edge of the Abitibi subprovince represents the upper portion of a layered complex, with a massive gabbroic main zone being overlain by a magnetite-rich gabbroic–anorthositic layered zone and a granophyre. The cumulate rocks display cryptic variation in mineral and whole-rock compositions towards a more evolved signature with stratigraphic height. There is a decrease in An content of plagioclase (from An73 to An51), and in Mg# of augite (from Mg# 75 to Mg# 56), an increase in La/Yb ratio of whole rocks (from 0.5 to 2.9), and a decrease in Cr (from 770 to 5 ppm) and Ni (from 315 to 45 ppm) contents of whole rocks. Concentrations of platinum-group elements (PGE) are mostly below detection limit. Compositional comparison with the South African Bushveld Complex shows that the lowest exposed levels of the complex are equivalent to the critical–main zone boundary, that is, in the immediate hanging wall of the PGE reefs. Based on recent geophysical studies showing that the Bell River Complex is approximately 5 km thick, we consider the presence of a hidden ultramafic zone a distinct possibility and propose a site for an exploration drilling programme. A comparison of compositional data from the Bell River Complex and associated basalts and rhyolites of the Watson Lake and Wabassee groups suggests that the three suites are cogenetic. Based on the fact that the numerous important volcanogenic massive sulphide (VMS) deposits in the region are located around the intrusion, and the similar age of the intrusion and the volcanic rocks, we support earlier workers in suggesting that the intrusion and the VMS deposits are genetically linked, that is, that the Bell River Complex provided the heat source that drove the hydrothermal fluids responsible for forming the VMS deposits.