Geochemistry and U–Pb geochronology of the Williams Brook area, Tobique–Chaleur zone, New Brunswick: stratigraphic and geotectonic setting of gold mineralization

Abstract

Gold mineralization at Williams Brook in northern New Brunswick is hosted within the Siluro-Devonian, bimodal, volcano-sedimentary rocks of the Tobique–Chaleur Zone (Wapske Formation). Gold mineralization occurs in two styles: (1) as disseminations (refractory gold) in rhyolite, and (2) in cross-cutting quartz veins (free gold). Dating of the felsic volcanic host rocks by in situ LA–ICP–MS zircon U–Pb geochronology returned ages of 422 ± 3, 409 ± 2, 408 ± 3, 405 ± 2, and 401 ± 9 Ma. Zr/Y of subvolcanic felsic intrusion (<8 for syn-mineralization and >8 for post-mineralization) suggests evolution from transitional to more alkalic affinities. Two mineralizing events are recognized; the first is a disseminated mineralization style formed at ∼422–416 Ma and the second consists of quartz-vein-hosted gold emplaced at 410–408 Ma. Felsic rocks from Williams Brook and elsewhere in the Tobique Group (i.e., Wapske, Costigan Mountain, and Benjamin formations), and the Coastal Volcanic Belt have similar Th/Nb ratios of ∼0.1 to 1, reflecting similar levels of crustal contamination, and similar Nb and Y content, suggesting A-type affinities. These data indicate a similar environment of formation. Regionally, mafic rocks show similar within-plate continental signatures and a E-MORB mantle source that formed from partial melts of 10%–30%. Mafic volcanic rocks from Williams Brook have a more alkaline affinity (based on Ti/V) and derivation from lower percentage partial melting (∼5%). The chemical and temporal variations in the Williams Brook rocks suggest that they were erupted in an evolving transpressional tectonic setting during the oblique convergence of Gondwana and Laurentia.