The Late Devonian (368 ± 1 Ma) post-orogenic peraluminous Mount Douglas Granite, located in southwestern New Brunswick, Canada, forms the eastern part of the Saint George Batholith. The batholith was emplaced following the accretion of the Gander and Avalon zones of the northern Appalachians during the Neoacadian Orogeny. The Mount Douglas Granite is divided into three units, Dmd1, Dmd2, and Dmd3, that formed by progressive high degrees of fractional crystallization. The two most fractionated units, i.e., Dmd2 and Dmd3, are locally associated with greisen/sheeted veins-related endogranitic Sn-W-Mo mineralization. Whole-rock δ18O values of +6.0 to +7.3‰, high initial 87Sr/86Sr ratios of 0.70550 to 0.71665, slightly positive εNd(368 Ma) values (+0.3 to +1.1), and Pb isotopic data indicate that the granite was derived dominantly from partial melting of juvenile Avalonian crust, contaminated by supracrustal rocks. Petrochemical features support the idea of multiple periods of fractional crystallization at lower temperatures producing compositionally more evolved magmas. This is supported by estimated zircon saturation temperatures that decrease from Dmd1 (747–826 °C) → Dmd2 (733–817 °C) → Dmd3 (729–816 °C). The estimated crystallization pressure based on normative quartz and feldspar (albite + orthoclase) contents also decreases with increasing silica content, from 2.3 kbar (~7.7 km) in Dmd1 to 0.3 kbar (~1 km) in Dmd3. Whole-rock geochemical characteristics of this system show that unit Dmd3 is the most highly evolved unit; it has the highest SiO2 (avg. 76.4 wt.%.), LILE contents (e.g., Li, Rb, Cs), Rb/Sr (mean = 42), certain HFSE (Ta, Th, U) including Y (≤138 ppm), and REE, and has the most pronounced negative Eu anomalies (avg. Eu/Eu* = 0.08) and the lowest K/Rb (70–127), Nb/Ta (mean = 4.9), and Zr/Hf (mean = 23.5). Therefore, unit Dmd3 with the highest content of incompatible elements is the most prospective for Sn, W, Zn, Bi, and U mineralization, and then Dmd2.

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Ore Geology Reviews
Department of Earth Sciences

Mohammadi, N. (Nadia), Lentz, D.R. (David R.), McFarlane, C.R.M. (Chris R.M.), & Cousens, B. (2020). Geochemistry of the highly evolved Sn-W-Mo-bearing Mount Douglas Granite, New Brunswick, Canada: Implications for origin and mineralization. Ore Geology Reviews, 117. doi:10.1016/j.oregeorev.2019.103266