EVOLUTION OF ZIRCON CRYSTALLIZATION FROM THE BLACK MOUNTAIN PEGMATITE, MAINE

Zircon is present in 4 of 5 primary zones and the cleavelandite-rich replacement unit of the complex, Li-enriched Black Mountain granitic pegmatite. Textural evidence indicates that zircon underwent a complex growth history that included magmatic crystallization, radiation-induced fracturing, late-stage subsolidus alteration, and recrystallization. Primary zircon is characterized by Hf-enriched, U-poor compositions along with the presence of coarse-grained uraninite inclusions. Zr/Hf ratios of the primary zircon varies from 11-29; average UO₂ concentration is ~0.5 wt. %. Chemical heterogeneity is further expressed by oscillatory zoning or as patchy domains of variable U contents.

Non-stoichiometric, patchy to diffuse regions of hydrated zirconium silicates (HZS) were generated by the alteration of primary zircon by a late stage (U, P, Ca, Mn, Fe, Al)-enriched residual fluid. HZS frequently impart a fragmented texture to the primary zircon as it replaces it along cracks and fractures generated by metamictization. Compared to primary zircon, the hydrated zirconium silicates have lower concentrations of Si, Zr and Hf, similar Zr/Hf ratios, increased U, P, Ca, Fe, Mn and Al contents and typically host a second generation of finely disseminated uraninite inclusions. Wall zone and 1st intermediate zone HZS show higher values of P (up to 11.5 wt. % P₂O₅) and lower rates of U-Zr substitution compared to those from the 2nd intermediate zone and replacement unit. Secondary zircon formed by recrystallization post-dates and partially obscures primary growth zoning and secondary alteration products.