| Abstract | The Mount Douglas intrusive suite consists of an assemblage of peraluminous leucogranites located along the eastern part of the Saint George Batholith in southwestern New Brunswick, Canada. This late
Devonian suite extends from Red Rock Lake to Mount Douglas, and is interpreted to represent the subvolcanic portion of the Mount Pleasant Sn-W-Mo-Bi-In-Zn deposits. The magmatic systems in the Mount Douglas suite have undergone extreme
differentiation producing three distinct phases, including Dmd1, Dmd2, and Dmd3, and are associated with various granophile-element occurrences, such as Sn, W, and Mo. It seems the three phases might originate from a single source that evolves with
increasing fractionation from the earliest unit (Dmd1) to the youngest unit (Dmd3).
Metal behavior during fractional crystallization is one of the most important factors affecting the types of mineral occurrences associated with the Mount Douglas
granites. To establish the metal behavior in the Mount Douglas suite, analytical data from Malcolm McLeod's 1990 report was used in this study. The most incompatible element, Ta, has been selected for defining partition coefficients (D) using the
Allegre method. Based on their D values, as expected, Sn with Rb, Y, Nb, LREE [La, Ce, Nd, and Sm], HREE [Tb, Dy, Tm, Yb, and Lu], Pb, Th, and U behave as incompatible elements; showing the Mount Douglas Granite could be considered for exploration of
Sn deposits as previous studies have mentioned. However, W and Mo accompanied with the other elements, such as Li, P, Sc, Ti, Cu, Zn, Sr, Zr, Cs, Ba, Eu, and Hf seem to act as compatible elements during magmatic evolution, although melt -
supercritical fluid evolution can easily affect W, Mo, Cs, and Li abundances. The apparent compatibility of W and Mo is in contrast with the expected results in which they might act as incompatible elements for developing considerable potential for
W-Mo deposits. However, W and Mo concentrations increased with the degree of fractionation and show a significant enrichment in the Mount Douglas granites relative to other granitoid suites in the region; thus their compatibility may be the result of
leaching or partitioning out during volatile exsolution, or low pressure fractionation. Furthermore, very low K/Rb (average 102.7), Nb/Ta (? 6.83), and Zr/Hf (? 35.58) ratios, and high Ti/ Sc (125.3 ?) ratio in Dmd3 compared to Dmd1 possibly reflect
significant involvement of extreme low T crystal fractionation or even fluid fractionation in the last-stage magmatic differentiation. Consequently, although much more analytical data are needed for exact evaluation of the mineral occurrences in the
Mount Douglas Granites, Dmd3 as the most highly differentiated phases might be considered a superior candidate for hosting of Sn-W-Mo deposits. |