On the emplacement of the Voisey's Bay intrusion (Labrador, Canada)

Abstract

The Mesoproterozoic Voisey’s Bay intrusion is a small (∼10 km3) mafic intrusive complex forming part of the 20,000 km2 Nain plutonic suite of Labrador (Canada) and hosting a world-class orthomagmatic Ni-Cu sulfide ore deposit. An extensive database of drill-core and down-hole televiewer data provides a wealth of subsurface three-dimensional information on the intrusion. The intrusion consists of the upper Eastern Deeps Chamber and lower Western Deeps Chamber, connected via a complex network of dikes. The Eastern Deeps Chamber can be divided into six units that are distinguished based on geometrical styles, dominant lithologies, and the occurrence of bodies of wall rock partly separating units. Unit I, occurring at the highest levels of the chamber, is tabular and consists dominantly of olivine gabbro with minor normal-textured (orthocumulate) troctolite. Units at midlevels of the Eastern Deeps Chamber (IIA, IIB, IIC) are sill-shaped bodies consisting of normal troctolite (i.e., orthocumulate, exhibiting tiling of plagioclase laths) with local variably textured troctolite (i.e., mesocumulate, locally pegmatitic, locally dynamically recrystallized). Units from the lowermost level (IIIA, IIIB) are irregularly shaped and consist of variable troctolite, breccias containing centimeter-sized xenoliths of host-rock paragneiss, and strongly sulfide-mineralized rocks. At least three different feeder dikes deliver magmas associated with unit IIIB. The overall dike system has an upward-curving geometry such that the dike dips shallow upward and as they approach the Eastern Deeps Chamber. This is analogous to fault geometries produced during caldera collapse or cauldron subsidence. Evolving emplacement styles reflect internal igneous processes and intermittent normal-sense activity along pre- to synemplacement brittle wall-rock structures. High-temperature normal-sense shear zones are prominent within the chamber within 100 m of its basal contact and are interpreted to have formed in response to late-stage normal faulting of the chamber floor. The geometry and internal structure of the Eastern Deeps Chamber are indicative of top-down incremental emplacement associated with floor subsidence. Our findings provide a kinematic and dynamic framework for petrogenetic models of Voisey’s Bay and other intrusion-hosted Ni-Cu sulfide deposits.