MONAZITE, XENOTIME AND REE MINERALS IN ARCHEAN BANDED IRON-FORMATION FROM THE SHERMAN AND ADAMS MINES, ONTARIO, CANADA

We have used electron-microprobe analyses to establish the presence of a suite of accessory rare-earth-element (REE) minerals in Archean banded iron-formation (BIF, oxide facies) from the former Sherman mine, at Temagami, and the former Adams mine, south of Kirkland Lake, Ontario. The REE minerals identified are monazite and xenotime. Other REE-rich minerals were found, but their REE patterns and major-element compositions were not sufficiently distinctive to afford unambiguous identification. The grains of phosphate are small, 5 to 10 μm, and characterized by an anhedral morphology considered to reflect resorption. The monazite REE patterns are characterized by LREE enrichment similar to those seen in monazite from igneous and metamorphic rocks. However, all the monazite from BIF samples has a significant positive Eu anomaly, comparable to the Eu anomaly characteristic of the BIF host-rocks. These REE minerals contain no U or Th detectable by the electron microprobe. The positive Eu anomaly and the low U and Th contents argue against a volcaniclastic precursor. The accessory REE phosphate minerals probably arise from REE and phosphate that accompanied the deposition of the precursor oxide minerals that crystallized during the early diagenetic history of the iron formation, during the elimination and dehydration of pore fluids. A comparison of whole-rock REE content with that attributable to accessory minerals tentatively suggests that REE phosphates may significantly influence the bulk REE pattern of oxide facies iron-formation. Pore-water reflux from BIF sediment to the water column may represent a source of fluids with higher HREE proportions than original seawater, as possibly seen in REE profiles observed in xenotime.