ORIENTED MONAZITE INCLUSIONS IN APATITE PORPHYROBLASTS FROM THE HEMLOGOLD DEPOSIT, ONTARIO, CANADA

Oriented inclusions of monazite occur in the dark core of apatite porp hyroblasts in a muscovite schist from the Archaean Hemlo gold deposit, Ontario, Canada. The monazite inclusions are elongated along the b-ax is and parallel to the c-axis of the apatite host; the complete orient ation relationship of the monazite/apatite intergrowth is bMn2//cAp, c Mnz//aAp. From analysis by SIMS and EMP, the dark core of the apatite porphyroblasts is depleted in LREE (La-N/Yb-N = 0.56). The monazite in clusions are correspondingly enriched in LREE, but markedly depleted i n HREE, compared with monazite grains in the rock matrix and cross-cut ting veins. The monazite inclusions precipitated by oriented reaction through rock-fluid interactions during a late hydrothermal alteration. Their unusual REE composition is probably related to both a preferent ial leaching of LREE from the dark core and a selective transfer of HR EE out of the apatite porphyroblasts.