Nash and Ludwig reported in 1979 that oxidized and reduced uranium minerals that make up the Midnite uranium deposits occur in metapelite and metacarbonate rocks of the Middle Proterozoic Togo Formation adjacent to a Cretaceous porphyritic quartz monzonite pluton. Numerous genetic hypotheses have been proposed, and all suffer from lack of definitive geologic and geochemical evidence. The present distribution of uranium minerals probably reflects destruction and redeposition of earlier formed minerals, and distribution of original uranium minerals is highly conjectural. The authors' new data do not answer many questions, but do place limits on viable genetic mechanisms and their times of operation. We offer the following new information, some of which constrains genesis.
U-Th-Pb ages from zircon in porphyritic quartz monzonite exposed in the Boyd 2 pit are 75+2 m.y. Apatite from this locality yields a fission-track age of 69 ± 6 m.y. U-Th-Pb ages from zircon in porphyritic quartz monzonite collected at Turtle Lake are 90+2 m.y. Fission-track ages on apatite and sphene from the Turtle Lake locality are 80+8 m.y. and 82+3 m.y. respectively.
Isotopic analyses of many rock and ore samples indicate that common Pb is enriched (^645 ppm) in pitchblende and coffinite, and that Pb tends to follow the distribution of U. Analyses of barren pelites, pyrrhotite, and K-feldspar from the quartz monzonite show grossly similar Pb isotopic ratios, which are consistent with Pb evolution from a Cretaceous S-type granitic rock formed from crustal rocks. The amount of radiogenic Pb observed is much smaller than would be generated if there was Proterozoic uranium mineralization in the Togo.
The Eocene age of mineralization and the interpretation that represents redistribution may have important implications for finding deposits similar to those of the Midnite mine. A characteristic of the magmatic-hydrothermal mechanism, if it indeed operated, may be a tendency to form low-grade disseminations that require subsequent upgrading to be economic. In a general sense, several different processes may be possible for upgrading. Structural traps and iron sulfide appear to be important ingredients, but it is not clear if a volcanic-mild hydrothermal environment is required.
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