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

Abstract

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.