THE RARE-ELEMENT-ENRICHED MONZOGRANITE - PEGMATITE - QUARTZ VEIN SYSTEMS
IN THE PREISSAC-LACORNE BATHOLITH, QUEBEC. I. GEOLOGY AND MINERALOGY
THOMAS MULJA, ANTHONY E. WILLIAMS-JONES, SCOTT A. WOOD* and
MICHEL BOILY**
Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7
* Present address: Department of Geology and Geological Engineering, University of Idaho, Moscow,
Idaho 83843, U.S.A.
** Present address: Géon, 10785, rue St-Urbain, Montréal, Québec H3L 2V4.
Abstract
The monzogranitic plutons in the Preissac-Lacorne batholith, Quebec, in the Abitibi Greenstone
Belt of the Superior Province, display geological and mineralogical features that resemble idealized zoned
intrusions and associated rare-element quartz veins and pegmatites. The zoning comprises biotite, two-mica, and
muscovite monzogranites; the mineralization 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 plutons are surrounded by rare-element pegmatites, which vary systematically
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.
Mineralogical changes from biotite to muscovite monzogranite are characterized by a decrease in
the abundance of oligoclase, biotite, magnetite, monazite, apatite, and zircon, and an increase in
the abundance of albite, muscovite and garnet. The mineralogical trend continues into the pegmatites,
which are composed of albite, K- feldspar, quartz, muscovite (biotite is absent), garnet, beryl, spodumene,
molybdenite, and columbite-tantalite. The major- element chemistry of the rock-forming minerals changes
progressively from biotite through two-mica to muscovite monzogranite; plagioclase compositions vary
from An13<0150>17 to An6, and the Fe/(Fe + Mg) of biotite and muscovite increases from 0.7 to 0.85
and from 0.65 to 0.85, respectively. This mineral-chemical evolution extends into the
pegmatites; the plagioclase in these rocks is almost pure albite (An1-5), and muscovite is
lower in Fe/(Fe + Mg) and richer in Al than that in the muscovite monzogranite. Concentrations of Cs,
Ta, and Rb in muscovite increase, whereas that of Sc decreases, from the muscovite monzogranite to the rare-metal
pegmatites. The systematic mineralogical evolution and mineral-chemical trends, and the field
relationships of the monzogranites, are interpreted to indicate that the various subtypes of monzogranite were
produced mainly by fractional crystallization of biotite monzogranitic magma. Further differentiation of
the fractionated monzogranitic melts produced rare-element pegmatites.
Keywords: rare-element monzogranite, beryl pegmatite, spodumene pegmatite, mineralogy, mineral
chemistry, Preissac- Lacorne batholith, Quebec.