Lucie Mathieu

Ardnamurchan 3D cone-sheet architecture explained by a single elongate magma chamber

The Palaeogene Ardnamurchan central igneous complex, NW Scotland, was a defining place for the development of the classic concepts of cone-sheet and ring-dyke emplacement and has thus fundamentally influenced our thinking on subvolcanic structures. We have used the available structural information on Ardnamurchan to project the underlying three-dimensional (3D) cone-sheet structure. Here we show that a single elongate magma chamber likely acted as the source of the cone-sheet swarm(s) instead of the traditionally accepted model of three successive centres. This proposal is supported by the ridge-like morphology of the Ardnamurchan volcano and is consistent with the depth and elongation of the gravity anomaly underlying the peninsula. Our model challenges the traditional model of cone-sheet emplacement at Ardnamurchan that involves successive but independent centres in favour of a more dynamical one that involves a single, but elongate and progressively evolving magma chamber system.

Quantifying hydrothermal alteration with normative minerals and other chemical tools at the Beattie Syenite, Abitibi greenstone belt, Canada

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.