FORMATION PROCESS OF ZIRCON ASSOCIATED WITH REE FLUOROCARBONATE AND NIOBIUM MINERALS AT THE NECHALACHO REE DEPOSIT IN THE THOR LAKE SYENITE, CANADA

The core samples of two drill holes, which penetrate sub-horizontal mineralized horizons at Nechalacho REE deposit in the Thor Lake syenite, Canada, were studied to distinguish magmatic and hydrothermal processes that enriched HFSE (Zr, Nb, REE). REE mineralized zones occur in the albitization and potassic altered syenite.

The Zr contents in the core samples are positively correlated with Nb, LREE and HREE. Moreover, the most common REE mineral in the REE mineralized zone is zircon. These suggest that crystallization of zircon plays a major role in enrichment of HFSE. In REE-mineralized zone, zircon is divided into four types: Type-1 and -2 zircons are euhedral and subhedral crystals, and are included in phlogophite, fluorite and microcline. Type 1 zircons show core and rim zoning under the polarizing microscope. Type-2 zircons form complex intergrowth with REE fluorocarbonates. Type-3 zircons consist of porous core and rim zone, and are rimmed by fergusonite. Type-4 zircon is characterized by smaller, porous, subhedral crystals. Grain boundary of small zircon is filled by fergusonite. These small zircon and filled fergusonite are often included in ankerite. The average REE and Nb contents in the four types of zircon are as follows: Type-1 (ΣREE₂O₃ 4.21 wt%, Nb₂O₅ 1.47 wt%), type-2 (ΣREE₂O₃ 4.85 wt%, Nb₂O₅ 2.26 wt%), type-3 (ΣREE₂O₃ 1.05 wt%, Nb₂O₅ 0.15 wt%) and type-4 (ΣREE₂O₃ 1.62 wt%, Nb₂O₅ 0.17 wt%). Type-1 and -2 zircons comparatively occur in shallow zone of the two drill holes, while type-3 and -4 in deep zones. REE-mineralized zones including type-3 and -4 type zircons show the highest REE contents.

These results permit a model for evolution of zircon crystallization in Nechalacho REE deposit: zircon high in REE and Nb (type-1 and -2) crystallized before discrete fergusonite. Zircons low in REE and Nb (type-3 and -4) were formed by dissolution-reprecipitation of type-1 and -2 zircon affected by F-CO₂-rich hydrothermal fluids. REE and Nb were no longer stable in the zircon structure and crystallized as fergusonite around the reequilibrated zircon (type-3 and -4). As the conclusion, F-CO₂-rich hydrothermal fluids, cause dissolution of type-1 and -2 zircons, and crystallization of fergusonite as new REE minerals in Nechalacho REE deposit.