Oriented inclusions of monazite occur in the dark core of apatite porphyroblasts in a muscovite schist from the Archaean Hemlo gold deposit, Ontario, Canada. The monazite inclusions are elongated along the b-axis and parallel to the c-axis of the apatite host; the complete orientation relationship of the monazite/apatite intergrowth is bMnz//cAp, cMnz//aAp. From analysis by SIMS and EMP, the dark core of the apatite porphyroblasts is depleted in LREE (LaN/YbN = 0.56). The monazite inclusions are correspondingly enriched in LREE, but markedly depleted in HREE, compared with monazite grains in the rock matrix and cross-cutting 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 preferential leaching of LREE from the dark core and a selective transfer of HREE out of the apatite porphyroblasts.
