PYRITE-INDUCED REDOX PRECIPITATION OF URANIUM IN A DOLOMITE BRECCIA, EPLER FM., ALLENTOWN, PA

Brecciated dolostone along a thrust fault juxtaposing Proterozoic granitic gneiss with Ordovician Epler Fm. hosts detectible, subeconomic U mineralization. The breccia is highly variable, characterized by moderately-rounded 1-10 mm dolostone clasts in a fine-grained, calcite-dolomite matrix. Breccia fragment:matrix ratio varies greatly (matrix 3-75%). U occurs as < 3 μm grains of uraniferous calcium phosphate, as well as subordinate monazite and zircon. EDS analyses of U-bearing grains show a range of Ca-U-P stoichiometries, suggesting the phase may be amorphous or too small to obtain accurate EDS analyses of individual grains. F-bearing apatite, albite, and pyrite are spatially associated with U grains. Ca-U-P-phases occur coating trace amounts of irregularly-shaped < 0.1 mm sulfide grains (pyrite>>sphalerite>galena).

Consistent occurrence of Ca-U-P-phase as overgrowths on pyrite suggest U may have been transported downward as highly soluble U⁺⁶ leached from the granitic gneiss thrust sheet, then precipitated by reduction to U⁺⁴ during oxidation of sulfur by a reaction similar to:

7 U⁺⁶ + 22 H₂O + FeS_2(pyrite) = 7 UO₂ + Fe⁺² + 2 SO₄^-2 + 44 H⁺

The presence of phosphate and the pH-buffering nature of carbonate would have promoted deposition of U by this redox/neutralization mechanism. Similar fault contacts juxtaposing granitic gneisses and pyritiferous carbonate formations elsewhere in populated centers may therefore be a priority for home radon testing.