RARE-EARTH MINERALOGY AND GEOCHEMISTRY OF THE MATTAGAMI LAKE VOLCANOGENIC MASSIVE SULFIDE DEPOSIT, QUEBEC

Allanite, zircon, and other exotic rare-earth minerals, including hell andite, monazite, parisite, synchisite, euxenite. and polycrase, have been found at the Mattagami Lake mine Archean Zn-Cu-Ag-Au volcanogenic massive sulfide (VMS) deposit, Quebec. Zircon in the least-altered rh yolite host-rock is interpreted to be relict, and igneous in origin. F resh grains of allanite, hellandite, monazite, parisite, and synchisit e occur in cross-cutting veins and evidently belong to the late greens chist-facies metamorphism. Metamict grains represent an earlier genera tion of allanite, and may have crystallized during the hydrothermal al teration associated with the formation of the ore deposit on the seafl oor. Euxenite and polycrase replace skeletal ilmenite. Whole-rock anal yses and mass-balance calculations reveal that a large proportion of t he REE in all rocks of the Mattagami Lake mine resides in these rare-e arth minerals, particularly in allanite and zircon, which account larg ely for the abundances of LREE and HREE, respectively. This illustrate s the potential of these minerals for controlling the distribution of REE in VMS deposits. In particular, preferential dissolution and recry stallization of allanite dominate the distribution of the LREE. In con trast, the HREE were relatively immobile, apparently because of the ch emical resistance of zircon and (Y,Nb)-oxides.