Oriented Monazite Inclusions in Apatite Porphyroblasts from the Hemlo Gold Deposit, Ontario, Canada

Yuanming Pan, Michael E. Fleet and Neil D. Macrae
Department of Geology University of Western Ontario London, Ontario N6A 5B7, Canada

Abstract: 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.

Keywords: apatite porphyroblasts • monazite • oriented intergrowth • rare earth elements

Mineralogical Magazine; December 1993 v. 57; no. 389; p. 697-707; DOI: 10.1180/minmag.1993.057.389.14
© 1993, The Mineralogical Society
Mineralogical Society (www.minersoc.org)