GRANITIC INTRUSIONS OF THE MOUNT PLEASANT CALDERA, SOUTHWESTERN NEW BRUNSWICK: PETROCHEMICAL CHARACTERISTICS AND IMPLICATIONS FOR GOLD MINERALIZATION

Granitic intrusions within the Late Devonian Mount Pleasant caldera comprise the McDougall Brook Microgranite (MBM) and the slightly younger Mount Pleasant Granite (MPG). Auriferous quartz breccias and veins cut MBM and its volcanic wall-rock, whereas tungsten-molybdenum-bismuth and later polymetallic (tin-copper-zinc-lead-indium) mineralization are genetically associated with the multiphase MPG. The low silica MBM (< 70 wt.%) is similar in aluminum index to the high silica MPG (> 74 wt.%), both being metaluminous to weakly peraluminous. MBM is calc-alkaline, while MPG has crustal A-type affinities. Trace- and rare-earth elements in MBM are distinct from those in MPG in that MBM has lower incompatible elements, enriched REE patterns with a less pronounced negative Eu anomaly, and lower negative Nb, Ti, Sr and Ba anomalies. Trace-element modeling, major-element composition, and petrological evidence indicate that MBM and MPG may have formed by fractional crystallization involving an assemblage of clinopyroxene, amphibole, plagioclase, ± biotite, ± apatite; the magmas are mainly derived from juvenile materials as indicated by previously published oxygen and neodymium isotopic data.

Although gold content in MBM and MPG are typically low, ranging from < 2 to 8 ppb, W, Sn, and Mo contents in MBM are much lower compared to MPG. Gold mineralization appears to be related to the early MBM, which is broadly comparable to intrusion-related gold systems elsewhere. However, MPG that crosscuts MBM, is analogous to topaz-bearing granites commonly associated with Sn-W-Mo-Bi mineralization. Both intrusive units formed in a within-plate tectonic setting based on their regional geological setting in an epicontinental caldera complex. However, geochemical discrimination diagrams suggest that the MBM could have formed in volcanic arc to within-plate tectonic settings, whereas MPG formed entirely in the within-plate field. This discrepancy in tectonic discrimination suggests that mantle upwelling and supracrustal assimilation were more prevalent during the formation of MBM than MPG.