Lithogeochemical and isotopic characterization of Devonian molybdenite mineralization in the Pabineau Falls Granite, northeastern New Brunswick, Canada

Abstract

An unusual molybdenite occurrence in northeastern New Brunswick, referred to herein as the Pabineau Lake Mo (PLM), is situated to the south of Bathurst. This occurrence consists of high-grade molybdenite-bearing granites that occur within the large Pabineau Falls Granite (PFG). To help evaluate the evolution of this occurrence, a large composite sample containing disseminated, very coarse molybdenite (up to 5.02 wt% MoS2) was collected from mineralized granitic blocks excavated from a blasted trench. Although the host rocks are assumed to be PFG, being the only defined intrusive body in the area, major- and trace-element geochemical data together with U-Pb geochronological results suggest that the molybdenite is hosted by an intrusion distinct from the PFG. The radiogenic Sr-Nd-Hf-Pb isotopic data of the host intrusion are consistent with Gander Zone granites of the Appalachians orogenic system.The PLM's granitic host rock is characterized by a high silica content (78.93 wt% SiO2), an enrichment in incompatible elements, and a high molybdenum content. The high initial 87Sr/86Sr of 0.71268, negative εNd of −1.28, and high Pb isotope ratios together with trace- and major-element concentrations indicate a moderately to highly radiogenic Sr source and derivation from a significant amount of supracrustal materials, with a contribution from the upper mantle; a source similar to that proposed for the Gander Zone Siluro-Devonian granitic bodies. A positive εHf (+1.34) also suggests involvement of sedimentary components. This is consistent with elevated Cr (20 ppm) and Ni (20 ppm). The involvement of a pelagic sedimentary component is invoked to explain the enrichment in elements commonly associated with mafic compositions.A new precise, LA ICP-MS zircon crystallization age of 390±1 Ma makes the PLM host granite Middle Devonian, which argues against a direct relationship with the older PFG (397.2±1.9 Ma). The new age opens the possibility of an as yet undelineated, mineralized intrusion within the Gander Zone, which has perhaps been missed during previous investigations due to the extensive glacio-fluvial cover and very limited bedrock exposures. Alternatively, the younger age could be attributed to very slow cooling and/or intra-pluton fractionation, resulting in a younger much more fractionated phase of the PFG. However, there is no geochemical evidence for such a prolonged continuous fractional crystallization process.Notably, recent geophysical surveys on the Bathurst Mining Camp indicate that the PFG is substantively larger than its surface expression as it continues to the east beneath the Carboniferous unconformity.Comparing ages of PLM and PFG with the nearby Nicholas-Denys Granodiorite (381±4 Ma) and Antinouri Lake Granite (372±4 Ma), it appears that magmatic activity in the region occurred every 10 Myr over a span of about 30 Myr.Collectively, this evidence suggests the potential of significant buried intrusions within this tectonic belt of the Gander Zone. At the PLM, this might include exposure of an otherwise hidden intrusion that is younger and much more fractionated than the PFG. More detailed geophysical and geochemical analyses would be required to revise the current geological map and, most importantly, trace unmapped units at depth to possible additional mineralized sequences.