Quelques aspects pétrographiques et géochimiques du volcanisme archéen du Malartic en Abitibi (Province du Québec, Canada)

The Archean metavolcanic formations of the Malartic Group in Abitibi, northwestern Quebec (Canada), are several kilometres thick and made up of lavas and pyroclastic rocks, a large part of which are of submarine origin with pillows and hyaloclastites.The mafic phases, with ultramafic material (komatiites), dominate at the base of the unit, the felsitic flows and other derived material, locally associated with copper and zinc sulphides, are located towards the summit of the unit.The regional metamorphism of greenschist and locally amphibolite–almandine facies, has destroyed the magmatic paragenesis; on the other hand, the original textures and structures are generally well preserved.A geochemical study of major elements was carried out on 67 well chosen fine-grained lavas. These were retained for analysis after thin section examination showed them to be nonporphyritic, homogeneous, with a minimum of alteration, and a lack of secondary fillings of amygdales and microfissures. This study led to the following conclusions:(1) The original composition of the volcanic rocks of the Malartic Group is still identifiable in spite of the modification in the content of Na, K, Ca, S, H2O, CO2 and to a lesser degree of Mg and Fe.(2) The variable percentages, generally high in Na and low in K, give to this volcanic group a spilitic characteristic; this may simply be an accentuation of an original tendency for series poor in K.(3) Apart from the ultramafic flows and certain magnesian basalts for which a komatiitic relationship is considered, the Malartic volcanism has magmatic characteristics comparable to those of volcanic island arcs. It begins with dominantly mafic extrusions with a tholeiitic composition poor in K. It continues in the upper Malartic with eruptions in which felsitic phases, locally associated with copper–zinc mineralization, become more abundant; these products seem to belong to two different magmatic series: one tholeiitic, the other calc-alkaline, both poor in K. The phases of these two magmatic series are spatially tightly imbricated.