MINERALOGY OF THE LAKE ZONE, THOR LAKE RARE METALS DEPOSIT, NWT, CANADA

The Proterozoic (ca. 2.1 Ga) Blatchford Lake suite hosts significant c oncentrations of rare metals at the core of a peralkaline granite- sye nite pluton. After emplacement of the Grace Lake Granite and the Thor Lake Syenite within it, bodies of nepheline syenite, ijolite, and urti te were intruded beneath the present-day Lake zone. This is the larges t of five zones of mineralization and lies close to the apex of the Th or Lake Syenite, a region which was then subjected to albitization, mi croclinization, and, finally, rare-metal mineralization. The underlyin g silica-undersaturated rocks contain clots of rare-metal-bearing mine rals, including cerianite-(Ce), britholite-(Ce), thorite, and calcium catapleiite, interstitial to nepheline and aegirine. The Lake zone its elf contains major quantities of Zr (in zircon), Nb (in ferrocolumbite , pyrochlore group minerals, aeschynite group minerals, and fergusonit e-(Y)), and Ce (in allanite-(Ce), monazite-(Ce), and bastnasite group minerals). Lesser amounts of Ta, Y, heavy rare earth elements, U, Th, and Ga are also present, mainly as minor components of rare-metal-bear ing minerals. Electron microprobe analyses of the major rare-metal-bea ring minerals are presented.