U-Pb and Re-Os geochronology and lithogeochemistry of granitoid rocks from the Burnthill Brook area in central New Brunswick, Canada: Implications for critical mineral exploration

Abstract

The Burnthill Brook area of central New Brunswick contains four Late Devonian intrusions (Burnthill, Buttermilk Brook, Dungarvon, and Sisters Brook plutons) emplaced into greenschist-facies Cambro-Ordovician graphite-bearing meta-sedimentary rocks of the Miramichi Group and/or Middle Ordovician bimodal metavolcanic rocks of the Tetagouche Group. To help resolve the complex nature of molybdenite (± tungsten) mineralization and its relationship with the magmatic evolution of the Burnthill Brook area, this study combines whole-rock geochemistry (major- and trace-elements, and Sr-Nd-Hf-Pb isotopes) with U-Pb zircon and Re-Os molybdenite analyses of samples collected from the Falls Creek occurrence (Mo ± W), the Burnthill Sn-W-Mo deposit, and previously obtained Re-Os ages from the Sisson Brook W-Mo-Cu deposit.

New geochronological data from the Burnthill Brook area, in combination with previous published Ar-Ar and U-Pb data, defines a protracted (25 Ma) period of magmatism from 396 to 371 Ma. These data indicate that peaks of mineralization, at ca. 380 and 378 Ma, correlate with distinct pulses of magmatism. The younger mineralization corresponds with the final magmatic pulse in the area and is likely genetically related to aplite dykes present within the Falls Creek (FC) occurrence. Furthermore, this study has identified a younger generation of Mo-W-REE mineralization that is hosted by FC aplite dykes.

Burnthill Brook plutons are highly evolved, high-silica peraluminous A-type granites that have undergone extensive fractionation, leading to elevated concentrations of incompatible elements (e.g., Rb, Y, Nb, Cs, Th, and U) and fluorine (mean = 0.08 ± 0.05 wt%), as well as economically important elements, such as Sn, Ta, W, Mo, Zn, and Pb. The enrichment of these elements is primarily associated with fractional crystallization, a process that was amplified by repeated magmatic pulses over a protracted period, resulting in Mo + W saturation.

The evolution of granite-related mineral deposits in the Burnthill Brook area is attributed to a regional-scale metallogenic epoch associated with the Neoacadian Orogeny. This same metallogenic epoch is likely responsible for the formation of numerous granite-related mineral deposits across the Canadian Appalachians, and further reinforces the conclusion that pulsed magmatism during the Neoacadian Orogeny (390–350 Ma) played an integral role in the formation of these deposits.

Geochemical and geochronological evidence collectively suggests that the various plutons are individual components of a Middle to Late Devonian batholith that underlies this region, and is provisionally herein referred to as the Peaked Mountain Batholith. In addition to the area encompassing the exposed Burnthill, Buttermilk Brook, Dungarvon and Sisters Brook granites, this presumptive batholith is projected to extend at least 24 km to the south-southwest to include the unidentified, buried pluton(s) responsible for the coeval Sisson Brook area mineralization (W-Mo-Cu).