Geochemical trends in the Ackley Granite, southeast Newfoundland: their relevance to magmatic–metallogenic processes in high-silica granitoid systems

Abstract

The Ackley Granite, occupying an area of approximately 2700 km2, was intruded at about 355 Ma across the boundary between the Avalon and Gander tectonic terranes of southeast Newfoundland. It is dominated by K-feldspar-megacristic and equigranular biotite-bearing granite with subordinate granodiorite, but 10 separate phases have been recognized, also including hornblende- and muscovite-bearing varieties. Small aplite–pegmatite type molybdenite deposits occur in a southwest lobe, and cassiterite–wolframite-bearing quartz–topaz greisens are found as steeply dipping veins and pods within a south-central lobe of the pluton. A geochemical study of 357 rock samples, randomly distributed on a grid of 2 km spacing, shows systematic geographic variation in the concentrations of a range of elements.The concentrations of silica and some other elements show a relatively abrupt change approximately 10–15 km west of the projected boundary between the Avalon and Gander terranes, indicating that the different terranes had some influence on magma compositions, presumably as source rocks, that was preserved through subsequent ascent, cooling, and crystallization. Other elements change along smoother trends and support a model for the southeastern part of the Ackley Granite magma chamber (2100 km2) analogous to those inferred for chemically layered, high-silica ash-flow tuffs. This model entails a process of convective fractionation and (or) liquid-state diffusion that was responsible for early enrichment and depletion of certain elements in the magma and was accompanied by volatile exsolution and mineralization, especially in magma with more than 74% SiO2.Overall, the southeastern Ackley Granite has "I-type" granite affinities, evolving to "A-type" affinities in the shallower, more silicic mineralized varieties. This study indicates that neither the current concepts for generation of different types of granite nor the metallogenic concepts linking Mo and Sn–W deposits to specific types of granite are generally applicable. We suggest that high-level magmatic processes are adequate to produce such features and that any high-silica granitoid pluton that exhibits extreme enrichment of the large-ion lithophile elements and depletion of Ba and Sr is a potential host to granophile deposits.