Abstract

Methods to quantify hydrothermal alteration are used to: document alteration halos; to comprehend hydrothermal processes; and to prospect for mineralisation formed by such processes. The chemical methods available are numerous and each has specific advantages, limitations and fields of application. This study focuses on the hydrothermal process at Beattie, a Neoarchean gold deposit hosted by the Beattie Syenite within the Abitibi Greenstone Belt, Superior Province, Canada. To quantify alteration, it was necessary to use the following chemical methods: (1) mass-balance calculations; (2) normative minerals and related alteration indices; and (3) Pearce Element Ratios (PER) diagrams. In the study area, silicification and carbonatisation are satisfactorily quantified by alteration indices, while alkali metasomatism was best estimated by mass-balance calculations and PER diagrams. Combining these methods, the following alteration types have been documented: K-feldspar alteration, silicification, and Na-Ca-leaching are intense and proximal to gold mineralisation; carbonatisation is widespread and intense; and sericitisation and chloritisation are minor to absent. It is proposed that, at the Beattie Syenite, the formation of white mica and chlorite and the mobilisation of alkali and silica are consequences of the predominant process related to carbonatisation.

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