THE RARE-ELEMENT-ENRICHED MONZOGRANITE-PEGMATITE-QUARTZ VEIN SYSTEMS IN THE PREISSAC-LACORNE BATHOLITH, QUEBEC .1. GEOLOGY AND MINERALOGY

The monzogranitic plutons in the Preissac-Lacorne batholith, Quebec, i n the Abitibi Greenstone Belt of the Superior Province, display geolog ical and mineralogical features that resemble idealized zoned intrusio ns and associated rare-element pegmatites and quartz veins. The zoning comprises biotite, two-mica, and muscovite monzogranites; the mineral ization in pegmatite-poor plutons is dominated by molybdenite-bearing quartz veins, which are spatially related to the more evolved rocks (i .e., muscovite-bearing monzogranite). In contrast, pegmatite-rich plut ons are surrounded by rare-element pegmatites, which vary systematical ly with distance from the plutons from beryl-bearing through spodumene -beryl-bearing to spodumene-bearing. In addition, Mo-bearing albitite dikes and quartz veins occur beyond the spodumene pegmatites. Mineralo gical changes from biotite to muscovite monzogranite are characterized by a decrease in the abundance of oligoclase, biotite, magnetite, mon azite, apatite, and zircon, and an increase in the abundance of albite , muscovite and garnet. The mineralogical trend continues into the peg matites, which are composed of albite, K-feldspar, quartz, muscovite ( biotite is absent), garnet, beryl, spodumene, molybdenite, and columbi te-tantalite. The major-element chemistry of the rock-forming minerals changes progressively from biotite through two-mica to muscovite monz ogranite; plagioclase compositions vary from An(13-17) to An(6), and t he Fe/(Fe + Mg) of biotite and muscovite increases from 0.7 to 0.85 an d from 0.65 to 0.85, respectively. This mineral-chemical evolution ext ends into the pegmatites; the plagioclase in these rocks is almost pur e albite (An(1-5)), and muscovite is lower in Fe/(Fe + Mg) and richer in Al than that in the muscovite monzogranite. Concentrations of Cs, T a, and Rb in muscovite increase, whereas that of Sc decreases, from th e muscovite monzogranite to the rare-metal pegmatites. The systematic mineralogical evolution and mineral-chemical trends, and the field rel ationships of the monzogranites, are intrepreted to indicate that the various subtypes of monzogranite were produced mainly by fractional cr ystallization of biotite monzogranitic magma. Further differentiation of the fractionated monzogranitic melts produced rare-element pegmatit es.