Uranium mineralization of collapse breccia pipes in Northern Arizona, Western United States

The development of caves within the Mississippian Redwall Limestone of the western United States, accompanied by later upward stoping of overlying Palaeozoic and Triassic rock, resulted in the formation of breccia pipes. The Palaeozoic sedimentary rocks on the Colorado Plateau of northern Arizona are host to hundreds of these breccia pipes. The uranium and copper deposits within these pipes transgress formation boundaries from the Mississippian Redwall Limestone to the Triassic Chinle Formation. Although small in size, these pipes occasionally contain first-size samples with up to 55 wt.% U3O8 and can yield 1800 metric tons of U3O8 averaging between 0.30 and 0.60 wt.%, such as in the Orphan Mine. An extensive suite of elements is significantly enriched in the mineralized rock: Ag, As, Ba, Cd, Co, Cr, Cs, Cu, Hg, Mo, Ni, Pb, Sb, Se, Sr, V, Zn, and the rare earth elements. Of these, Pb, Zn, Ag, and particularly As, appear to be the best geochemical indicators of mineralized pipes. The lack of extensive silicification within the breccia, along with the fluid-inclusion filling temperature in calcite of 60 deg. to 110 deg. C, suggest relatively low temperature mineralizing fluids of unknown origin