In the early days of geochronology, attempts were made to date U-Th bearing rocks and minerals using the Pb-U-Th chemical method, but fundamental assumptions that all the lead was of radiogenic origin, and the system was closed, often were invalid. With the advent of the U-Th-Pb isotopic and common-lead methods of dating rocks and minerals, the chemical method was deemed clinically dead. However, several workers, for example, Montel et al., Suzuki and Adachi, have recently demonstrated new life for the chemical method. Using, the same theoretical basis as the chemical method, geologically reasonable dates often can be obtained by electron microprobe analysis of U-Th rich monazite (Ca-REE-U-Th phosphate). Other U- or Thrich minerals sometimes can be used to obtain microprobe U-Th-Pb dates, e.g., zircon, xenotime, uraninite.

Microprobe U-Th-Pb dates of uraninites from the McAllister Ta-Sn pegmatite, Coosa County, Alabama, are reported here. The McAllister pegmatite is a NW-SE dike-like body in the western part of the Rockford granite pluton. Polished grain mounts were made from screened table runs of heavy mineral separates. Wodginite (Mn, Sn, Nb, Ta-oxide), tantalite-columbite, zircon, cassiterite comprise most of the grains in the samples. Uraninite occurs as inclusions, generally <0.01 mm, in these grains (Figure 1).

U, Th, Pb concentrations were measured using a JEOL 8600 Superprobe. Operating conditions were 100 nA, 20 kV, and count times up to 200 seconds for peak plus background. The U Mβ, Th Mα, and Pb Mα peaks were measured. Standards were UO2, ThO2, and PbS.