Abstract

A study of granitoid plutons of the western part of the Dorsey terrane in the southern Yukon has shown that two distinct suites exist on the basis of their mineralogy, chemistry, and metallogeny. Hornblende-rich granodiorite and granodiorite with minor gabbro and diorite form the NW and SW Thirtymile stocks, respectively. These are similar to many calc-alkaline Cordilleran intrusions and are not associated with any mineralization. The Thirtymile and Ork stocks are composed of highly evolved granite that contains hornblende only in the least evolved, volumetrically minor, lithofacies. Biotite granite forms the bulk of the Thirtymile stock, and it displays major- and trace-element trends that correspond to those shown by the adjacent Hake and Seagull batholiths. These latter granites are interpreted to be one magmatic suite, which is associated with F–B-rich skarn tin–tungsten mineralization. The most evolved lithofacies of the Ork and Thirtymile stocks is an alkali-feldspar-rich zinnwaldite–topaz–fluorite leucogranite that has an extremely evolved chemistry (differentiation index up to 99.4 and Rb/Sr ratio > 3000), with trace-element contents normally associated with "A-type" granites. This is interpreted to be the result of ultrafractionation of a halogen-rich residual granitic melt.

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