Petrological and metallogenic studies of the Nashwaak granite and felsic dykes associated with the Sisson Book W-Mo-(Cu) deposit, west-central New Brunswick, Canada
University of New Brunswick
The Sisson Brook W-Mo-Cu deposit was formed by hydrothermal fluids likely related to the Nashwaak Granites and related felsic dykes. These granites consist of two pluton subfacies: muscovite-biotite granite (Group I) and biotite granite (Group II), and dykes with various textures (aplitic to pegmatoidal dykes, Group III; and a porphyry dyke, Group IV). This deposit formed at 376.45 ± 1.64 Ma to 378.54 ± 1.71 Ma (Re-Os molybdenite) that is older than the porphyry dyke (364.5 ± 1.3 Ma, earlier U-Pb zircon age), but younger than the volumetrically dominant medium- to coarse-grained biotite granitic dykes (405.6 ± 2.5 Ma, U-Pb zircon age), as well as the Nashwaak granitic plutons. The syn-hydrothermal dykes could be the other dykes with different textures in Group III, if they are not contemporary, or possibly related to a deeply buried large granitic pluton has not been intersected by drilling thus far. The Nashwaak Granite and related dykes are highly siliceous (SiO2 > 69 wt. %), peraluminous, calc-alkaline, and magnesian I-type granites. They formed in a volcanic arc type setting and are characterized by depletion of HFSE and enrichment of LILE. Oxygen isotope data (9.3 - 12.3 ‰), (87Sr/86Sr)i (0.702 - 0.710), and ɛNd(t) (-4.51 to -1.42) of the whole rock, and in situ δ18O analyses of magmatic zircons (4.9 - 9.9 ‰) and quartz show the granitic magmas are probably derived from bulk assimilation of Mesoproterozoic Gondwanan basement ± the overlying Gander Zone sedimentary prism, by mantle-derived melts. The magmas of the Nashwaak Granite and related dykes formed at temperatures below 800°C (TZr) with the aid of water-rich fluid infiltration. These magmas with a initial water content of 5-6 wt.% increased upwards until they intersect the watersaturated granite solidus at pressures lower than 2.5 to 3.0 kbar. Assimilation and fractional crystallization is the mechanism that controlled magma evolution. Trace element contents in quartz and biotite are not correlated with that of the whole rock, but the K/Rb of biotite decrease, and Al/Ti and Ge/Ti of quartz increase with differentiation of the magmas and show that the Group III is the most evolved. Oxygen fugacity of these magmas is close to the nickel-nickel oxide buffers, thus they are oxidized I-type magmas, and only the two-mica granite is reduced due to later, strongly supracrustal, contamination (ASI > 1.1, δ18OZr > 8‰). Halogen fugacity study shows that the Group I suite have higher F relative to Cl, with other groups having higher H2O and Cl activity than F, indicative of build-up of chlorine and water in the evolving magma. These high HCl/HF and H2O/HF ratios are suggested as tungsten mineralization vectors and only the dyke phases have similar HCl/HF ratios to that of granites typically associated with Sn-Wo-Bi deposits. Comparing the geochemical characteristics of all the Nashwaak Granites and associated dykes with the granites related to the W-Mo deposits, the biotite granitic dykes are the “best” candidates for the W-Mo deposit mineralization, since they are the most differentiated, oxidized, and relatively ‘wet’ type of magma (compared to magmas of porphyry Cu, < 4 H2O wt.%), and with a similar halogen fugacity. Further geochronology studies are needed in order to better identify the syn-mineralization intrusions.