Biotite composition as a tool for exploration: An example from Sn-W-Mo-bearing Mount Douglas Granite, New Brunswick, Canada

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The Mount Douglas Granite (MDG) in southwestern New Brunswick (Canada) consists of three geochemically different intrusive phases, from earlier to late, changing from barren coarse-grained granite (Dmd1) to more fractionated medium- to fine-grained fertile granites (Dmd2 and Dmd3). This provides a unique geological setting to apply different techniques in order to constrain petrogenesis and metallogeny of the system. This study uses biotite chemistry to test its applicability as a mineral exploration tool to discriminate the least evolved non-mineralized Dmd1 from more highly evolved mineralized Dmd2 and Dmd3. Chemical composition of magmatic biotite was determined by EPMA and in situ laser ablation ICP-MS for different phases within the MDG. Applying the reconstruction of trace element contents accompanied with determined minerals-melt partition coefficients, we were able to further constrain the genetic relationship between the Dmd1, Dmd2, and Dmd3 from the composition of biotite. Biotite trace element contents follow a differentiation trend from the least to most evolved phases, indicative of a continuous magmatic evolutionary series. The fact that high degrees of fractional crystallization produced units Dmd2 and Dmd3 associated with endogranitic Sn, W (Mo), Bi, Zn and U mineralization is reflected in the composition of biotite from mineralized units. Remarkably, the biotite from the Dmd2 and Dmd3 is enriched in Zn (≤ 879 ppm), Pb (≤ 248 ppm), Sn (≤ 164 ppm), Ta (≤ 163 ppm), Ga (≤ 113 ppm), Cu (≤ 44 ppm), W (≤ 14 ppm), and U (≤ 6 ppm); it is also rich in incompatible elements, such as Li, Cs, Rb, Ta, and Nb, as well as substantial contents of F (≤ 2.87 wt%) and Cl (≤ 0.71 wt%). The geochemical compositions of biotite not only demonstrate a continuous fractional crystallization of magmas, but also can be used as a fertility indicator.