Geochemistry and petrogenesis of the East Branch Brook metagabbroic dykes in the Sawyer Brook fault zone, Clarence Stream gold prospect, southwestern New Brunswick

Abstract

The East Branch Brook (EBB) metagabbroic dykes, host to a portion of the Clarence Stream gold deposit, are situated within the contact metamorphic aureole of the Middle Devonian I-type Magaguadavic Granite on the northwestern margin of the post-orogenic Saint George Batholith. They are highly deformed, light- (type 1), intermediate- (type 2) to dark-coloured (type 3) dykes containing auriferous quartz veins that occupy brittle to ductile northeast-trending shear zones in shallow marine, hornfelsed, volcaniclastic, sedimentary rocks of the Silurian Waweig and Oak Bay formations. The shear zones parallel the regional structure as a result of proximity to the faulted boundary (Sawyer Brook fault) between the Ordovician St. Croix terrane to the northwest and the Silurian to Early Devonian Mascarene Basin to the southeast. Geochemical studies of the EBB dykes indicate that three pulses (Fe-rich, intermediate, and Mg-rich) of subalkaline to slightly alkaline continental tholeiite magmas were generated in a transpressional environment during the Early Silurian to Early Devonian. Positive εNd values indicate their derivation from a partially depleted mantle source during faulting and rift-related events. Although the geochemical data (Fe-and Ti-depletion) indicate calc-alkaline affinity for the nearby Bocabec intrusive complex, εNd values and primitive mantle-normalized spider diagram patterns are similar to those of the EBB dykes. In contrast, the St. Stephen Intrusion appears more primitive with within-plate tholeiitic to slightly alkalic affinity.