Pétrologie et métallogénie d'indices de Ni-Cu-éléments du groupe du platine du domaine de Portneuf-Mauricie, Québec (Canada)

Authors: Sappin, Anne-Aurélie
Advisor: Clark, Thomas; Constantin, Marc
Abstract: The Portneuf-Mauricie Domain, located in the south-central part of the Grenville Province, is composed mainly of metasedimentary and metavolcanic rocks of the Montauban group (1.45 Ga), which are crosscut by La Bostonnais complex plutons (1.40–1.37 Ga). This assemblage was formed in a magmatic arc setting. The sequence was intruded by mafic and ultramafic intrusions hosting Ni-Cu±platinum-group element (PGE) prospects and one former mine. The mineralized intrusions were emplaced into a mature island arc between 1.40 and 1.39 Ga, at the volcanic front of the arc. An exception is the Lac à la Vase intrusion, which was formed in a back arc setting. Geochronological results and the geodynamic setting related to their formation indicate that these intrusions belong to the La Bostonnais complex. The lithotectonic evolution of the Portneuf-Mauricie Domain began at 1.45 Ga with the formation of the Montauban island arc, which was associated with a northwest-dipping intraoceanic subduction zone offshore from the Laurentian continent. At the same time, the continental margin was also the site of northwesterly directed Andean-type subduction. Between 1.45 and 1.40 Ga, this Andean-type subduction led to the formation of a back arc basin behind the Montauban arc. At 1.40 Ga, the La Bostonnais complex plutons, some hosting magmatic sulfides, were injected into the Montauban arc. The intrusion of the Ni-Cu±PGE-bearing plutons ceased at 1.39 Ga, when the arc collided with the continent. Finally, intrusion of La Bostonnais complex plutons ceased entirely at 1.37 Ga. The emplacement of the Ni-Cu±PGE-bearing intrusions was facilitated by the presence of a well-developed magmatic network beneath the oceanic arc. The formation of the mineralized intrusions began with the injection of primitive, tholeiitic, hydrous, sulfide-undersaturated magmas into lower magma chambers. These magmas resulted mainly from the partial melting of a metasomatized mantle source composed of spinel-bearing lherzolite (as indicated by the trace element signatures of the intrusive rocks). During their ascent in conduits, the magmas underwent crystal fractionation and became sulfide-saturated as a result of assimilating felsic material and sulfides from the country rocks. Two main sulfide-saturation/segregation events occurred. The first may have occurred in the lower conduits. This early loss of a small amount of sulfides (less than 0.1 wt%, as suggested by Cu and Pd modeling) caused depletion of base and, above all, precious metals in the magmas. The second sulfide-separation event probably occurred in the lower magma chambers. Liquid-sulfide formation in the chambers was followed by magma–sulfide interactions (field evidence for multiple magmatic injections and R factors between 100 and 100,000) that caused enrichment of the sulfides in Ni, Cu, and PGE. Later, injections of primitive magma entered the lower chambers, and partly remobilized and transported sulfide liquid into the upper magma chambers to form the Portneuf-Mauricie Domain sulfide-bearing intrusions. The parent magmas of these intrusions, derived from the later pulses, are separated into two families. As suggested by petrologic modeling and Mg/Fe ratios, most of the intrusions may have formed from a fractionated magma (a slightly evolved magma with a Mg number (Mg#) of 60) resulting from the fractionation of a more primitive parent melt (primary magma, with Mg# = 68). All these parent magmas underwent fractional crystallization accompanied by various degrees of crustal contamination. This explains the large range of compositions observed in the mafic and ultramafic intrusions. This Ph.D. work provides new constraints for the regional evolution models proposed for the eastern Laurentian margin during the Mesoproterozoic. In addition, the petrologic and metallogenic models presented in this study contribute to a better understanding of the magmatic and metallogenic processes associated with the formation of Ni-Cu-PGE-bearing plutons in subduction-related magmatic arcs, a geodynamic setting unusual for magmatic sulfide deposits.
Document Type: Thèse de doctorat
Issue Date: 2012
Open Access Date: 18 April 2018
Permalink: http://hdl.handle.net/20.500.11794/23362
Grantor: Université Laval
Collection:Thèses et mémoires

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