Zirconium-rich Uraninite

Zirconium-rich uraninite from the Central City District, Gilpin County, Colorado

ZHAO, Donggao, and EWING, Rodney C.

Dept. Nuclear Engin. and Rad. Sci. and Dept. Geol. Sci.

Univ. Michigan, Ann Arbor, MI 48109-2104

Abstract

During the corrosion of uraninite, trace elements may become incorporated into secondary uranium phases. We present data on Zr- and Si-rich uraninites (ZrO₂ up to 8.3 wt% and SiO₂ up to 4.9 wt%) that occur in metal-sulfide veins from Gilpin County, Colorado. The uraninites formed hydrothermally from quartz bostonites. Wall rock alteration includes silicification, sericitization and pyritization, indicative of reducing conditions. BSE images and EDS show that the Zr- and Si-rich regions are adjacent to primary uraninite, which contains UO₂ 90.2 to 96.5 wt%, ZrO₂ 1.1 to 1.7 wt%, and SiO₂ 0.3 to 0.6 wt%. After converting Pb back to U, the formula for the average primary uraninite is approximately (U⁴⁺_0.89,Zr_0.03,Si_0.02,Ca_0.03)O_2.00. Compared with the primary uraninite, the secondary Zr- and Si-rich region has low UO₂ (78.3 to 90.4 wt%) and high ZrO₂ (2.6 to 8.3 wt%) and SiO₂ (1.9 to 4.9 wt%). The formula of the Zr- and Si-rich area is approximately (U⁴⁺_0.36,U⁶⁺_0.33,Zr_0.09,Si_0.11,P_0.01Al_0.02Ca_0.03,Fe_0.01)O_2.28. Compositional ranges for ZrO₂ and SiO₂ do not overlap between the primary and secondary regions, although UO₂ changes continually from the primary to the altered regions. All regions have approximately the same PbO content (average 1.8 wt%). However, PbO of the primary uraninite (0.8 to 5.3 wt%) are slightly higher than those for the secondary phase (0.5 to 4.9 wt%). The PbO content suggests that both uraninites are young (55-70 Ma) and that the primary uraninite is only slightly older than the secondary phase. The contents of trace elements are low or below detection limits (b.d.l.) of EMPA (Na₂O b.d.l., K₂O 0.29, MgO 0.02, MnO 0.09, TiO₂ 0.14, SrO b.d.l., SO₃ 0.58, Y₂O₃ 0.08, La₂O₃ 0.03, Ce₂O₃ 0.08, Pr₂O₃ 0.01, Nd₂O₃ 0.02, Sm₂O₃ 0.02, Eu₂O₃ 0.01, Gd₂O₃ 0.02 and ThO₂ 0.04, wt% in average). Textural and compositional relationships suggest that the primary uraninite has partially altered and that Zr and Si were incorporated during alteration.

Key words: uraninite, zirconium, alteration, chemical composition