Magmatic and hydrothermal processes in rare-element granite-pagmetite systems: The Preissac-Lacorne batholith, Quebec, Canada

Geological and mineralogical investigations of four moderately peraluminous monzogranitic plutons (Preissac, Moly Hill, Lamotte, Lacorne) in the Archean Preissac-Lacorne batholith (northwestern Quebec) indicate that each consists of biotite, two-mica and muscovite monzogranite facies, and that these facies are zonally distributed in this order from margin to core or the larger Lamotte and Lacorne plutons. Rare-element-enriched granitic pegmatites are associated with the Lamotte and Lacorne plutons and are also regionally zoned, in this case, with respect to their respective plutons, from beryl-bearing in the plutons to spodumene-bearing in the country rocks. Molybdenite mineralized albitites and stockworks occur beyond the spodumene pegmatites and in the Preissac and Moly Hill plutons. The monzogranites display systematic mineralogical and major- and trace-element trends from biotite to muscovite monzogranite in the four plutons that are best explained by fractional crystallization of a biotite monzogranite-forming liquid. These petrochemical trends in the Lamotte and Lacorne plutons extend to the rare-element pegmatites, indicating that the latter are comagmatic with the monzogranite. A model is proposed in which the liquids of the Lamotte and Lacorne plutons underwent an initial side-wall crystallization to produce the observed zonation of the monzogranite types, followed by extreme differentiation inside the magma chamber, from where batches of pegmatite-forming liquid were sequentially injected into the overlying rocks. Pegmatites are rare and barren from the smaller Preissac and Moly Hill plutons due to early saturation of the residual liquid with aqueous fluid, from which quartz vein molybdenite precipitate to form quartz vein stockworks adjacent to the muscovite-bearing monzogranite.;The fractionation of the pegmatite-forming liquid is recorded partly in the crystal-chemistry of columbite-tantalite solid-solutions, by progressively higher Ta/(Ta + Nb) and Mn/(Mn + Fe) values with evolution from beryl- to spodumene-bearing pegmatite. These trends correlate to the greater solubility of Mn- relative to Fe- and of Ta- relative to Nb-columbite-tantalite end-members in the magma. In the Lacorne pagmatites, these trends were modified by contemporaneous crystallization of spessartine garnet, which buffered the Mn and Fe activities of the columbite-tantalite.;The pegmatite-forming liquid became saturated with an aqueous fluid at the onset of crystallization as shown by the entrapment of primary fluid inclusions in the paragenetically early beryl and spodumene. The orthomagmatic fluid was NaCl-dominated, had low salinity, contained appreciable dissolved CO;The study represents the first comprehensive reconstruction of the petrological and fluid evolution in a comagmatic suite of monzogranites and rare-element granitic pegmatites.