Abstract:

Mafic to intermediate magmatism in the Blake River Group is represented in part by a large number of syn- to post-volcanic intrusions. These intrusions are commonly emplaced along pre-existing structures that formed as a result of regional tectonism, synvolcanic subsidence events, or other synvolcanic processes. Some mafic to intermediate intrusions of the Blake River Group are thought to have filled caldera structures which may be related to VMS mineralisation, having served as conduits for mineralizing fluids.<br/>Three gabbroic-dioritic intrusions (Dufault, Seguin, and Horseshoe) previously interpreted as synvolcanic were selected for study as they are representative of the range in intrusive styles and compositions, therefore providing insights into the evolution of the Blake River Group. The results indentify at least two episodes of mafic magmatism in the Blake River Group: an earlier phase of intrusion contemporaneous with volcanism and hydrothermal activity (-2698 Ma; Dufault, Seguin Gabbros) and thus related to synvolcanic structures, and a later, post-folding phase of intrusion (Horseshoe Gabbro) that was emplaced along later structures and is unrelated to Blake River Group volcanism (ending 2695 Ma). All of these intrusions display internal differentiation that had not been previously described. The Dufault Gabbro is a concordant, multiphase intrusion that contains predominantly tholeiitic rocks and displays a progression of composition and texture from an earlier phase to a later phase. The Seguin Gabbro is a discordant intrusion emplaced along a reverse fault containing predominantly tholeiitic gabbro and leucogabbro. The Horseshoe gabbro is a discordant intrusion dominated by tholeiitic gabbroic phases that display layering defined by variation in oxide mineral contents. The dyke has been emplaced by the intrusion of multiple batches of magma.<br/>There was no evidence that any of these intrusions were emplaced along calderarelated structures or are related to VMS mineralisation. Collectively, the different compositions and ages of mafic intrusions in the Blake River Group indicate that they are the product of multiple episodes of mafic magmatism and only some, that have yet to be clearly identified, were emplaced into and therefore define synvolcanic subsidence structures.