Mineralogical and geochemical analysis of the Benjamin River South Cu-Mo-Au porphyry system in northeastern New Brunswick, Canada: A fertile adakitic magmatic system linked to slab failure

The Benjamin River South porphyry system (BRS) lies within the mid-Paleozoic Appalachian realm of Ganderia, representing an area of approximately 80 km 2 in northeastern New Brunswick. This region hosts the hypabyssal Blue Mountain Granodiorite Suite (BMGS) including BRS porphyries, which contains multiple, locally auriferous porphyry Cu-Mo occurrences associated with several cupolas ranging in size from 60 to ~600 m in diameter, situated northeast of the main Blue Mountain Granodiorite. These mineralized cupolas intrude into two Siluro-Devonian-aged volcano-sedimentary rock units (Benjamin and Bryant Point formations;). The adjacent slightly older Benjamin River Intrusive Complex, dated at 418 ± 1 Ma (U-Pb zircon), is cogenetic with the host regional rift-related bimodal volcano-sedimentary suite. The mineralized cupolas of the BRS porphyries (BMGS-BRS) were dated by U-Pb zircon (400.7 ± 0.4 Ma); these Early Devonian intrusive tonalite-granodiorite phases (to andesitic textured), which host the mineralization are light cream to tan colored, plagioclase and hornblende-biotite- porphyries within a variably f.g.- to m.g. hypidiomorphic granular groundmass. In the samples with only minor alteration-mineralization (<200 ppm Cu, <40 ppm Mo), the average Cu concentration is 69 ppm, whereas Mo averages 5 ppm, with an average of 0.35% S. Primary hydrothermal pyrite and chalcopyrite mineralization occur mostly as disseminations, with local pervasive replacement and stockwork vein-style mineralization. This is associated with potassic alteration (± phyllic) in various proportions within the porphyries, and less commonly in the surrounding volcano-sedimentary bimodal sequence. Pervasive biotitization with K-feldspathization reflecting potassic alteration of pre-existing ferromagnesian phases and igneous feldspars are reflected by increases in K/Na, Fe, and S, along with copper and molybdenum in the lithogeochemical data. Later mineralization consists of base-metal-bearing pyritic veins that cut the pervasive disseminated mineralized zones. Molybdenite commonly occurs in quartz-bearing veins that cut across the primary mineralization. Weak, pervasive propylitic alteration overprints much of the primary mineralization-alteration, although some are controlled by veins. The BMGS suite ranges from diorite to granodiorite, plotting within the alkalic-calcic to calc-alkalic fields. Geochemically, they classify as magnesian granites, with FeOt/(FeOt+MgO) increasing with SiO₂ content, reflecting their oxidized nature. The extremely low Zr (100–160 ppm) and Th (2–5 ppm), along with relatively low Ti and V (with Ti/V > 35), also suggest an oxidized calc-alkalic affinity with minimal crustal contamination. This is further supported by very low Zr/Y (average 22) and Th/Y (average 5.0), but higher Nb/Y (average 2.0), as well as high Sr/Y and La/Yb values, are due to very low Y and HREE, like those found in slab-derived adakitic systems related to late Acadian Orogeny slab failure. High T–P partial melting of hydrothermally altered oceanic slabs generates BMGS magmas enriched in H 2 O, SO 2 , HCl, and chalcophile metals that remain incompatible at higher f O 2 . In this region, these processes occur in a post-collisional setting during the initiation of slab failure, where upwelling asthenosphere supplies advective heat to enhance partial melting. Late collisional transpression to transtension in the region due to oblique collision promote rapid ascent of the fertile BMGS melt through subduction-modified lithosphere into the upper crust. Interaction of oxidized slab melts with subduction-modified lithospheric mantle and juvenile volcano-plutonic crust in the region preserves oxidation, establishing conditions favorable for porphyry Cu-Au mineralization.