<i>Métallogénie des éléments chalcophiles durant les processus magmatiques d'arrière-arc: exemple du domaine volcanique de Futuna</i> (<b>Doctoral dissertation</b>, Université de Bretagne occidentale-Brest, 2020)
Creators: name: Jeanvoine, Aurélien
Publisher: name: figshare
Year: 2024
Subjects: classificationCode: 370301
subject: Exploration geochemistry
classificationCode: 370302
subject: Inorganic geochemistry
classificationCode: 370503
subject: Igneous and metamorphic petrology

description: Doctoral dissertation.Abstract: There is a strong relationship between the formation of deposits enriched in precious metals (Au, Ag) and the subduction, in both oceanic and continental settings. However, the recent discovery of seafloor massive sulfide deposits enriched in Au and Ag in a subduction decoupled oceanic setting (Wallis and Futuna, SW Pacific) raises the questions of both the source of these metals and their transfer towards the surface. The study of the lavas located at the mineralized area of Fatu Kapa, away of the Futuna island, brings constraints on the processes of metal extraction and concentration, in particular precious metals, from the magmatic processes in the source rocks towards the hydrothermal mineralizations on the seafloor. The results of the petrogenetic study show that the Fatu Kapa magmas evolve by two major steps. (1) From trachybasalts to trachyandesites, magmas evolve through fractional crystallisation and then by fractional crystallisation coupled to assimilation. At this step, sulfide saturation is early and a sulfide phase enriched in chalcophile elements sink and accumulate at the bottom of the magmatic chamber. These phases are not easily mobilized by hydrothermal circulation. However (2), from trachyandesites to trachydacites, magmas evolve by mixing between trachyandesitic melts and melts generated by partial melting of the hydrothermally altered wallrock. Despite low copper values, these melts generate sulfide drops enriched in copper, which have been identified. A major result is the observation of a direct link between the liquid sulfide drops and the volatil rich magmatic phase which generate the vesicles during the eruptive phases. These bubbles allow the rise and the concentration of the magmatic sulfide blebs at the top of the magmatic chamber. This process, which have been demonstrated in experimental petrology, has not yet - to our knowledge - been identified in oceanic melts. At Fatu Kapa, two steps are considered to be crucial for moblizing these sulfide blebs and to enrich in metals the hydrothermal mineralization on the seafloor. (1) During the volcanic stage, part of the metals contained in the sulfide blebs is transferred to the magmatic fluid phase extracted in the vacuoles. These magmatic fluids, enriched in metals, rise to the surface and are mixed with the hydrothermal fluid which derives of seawater. (2) After the eruptive stage the hydrothermal cells go down towards the top of the crystallized magmatic chamber and alter the sulfide blebs-rich levels, contributing to the direct enrichement of hydrothermal fluids in metals. These processes shed new light on the possibility of generating oceanic sulfide deposits enriched in precious metals away of any subduction.