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The development of caves within the Mississippian Redwall Limestone, accompanied by later upward stoping of overlying Paleozoic and Triassic rock, resulted in the formation of breccia pipes. Despite the depressed uranium market, some of these pipes are presently being mined for uranium. The pipes apparently formed prior to the Jurassic, as no pipes have been observed to penetrate Jurassic strata, and U-Pb determinations on the Hack I and II pipes by K. R. Ludwig suggest the uranium mineralization occurred around 200 Ma. No brecciated rock within pipes has been observed above its normal stratigraphic position, nor is any volcanic rock associated in space or time with these pipes. Mineralized rock transects any strata from the Redwall Limestone to the Triassic Chinle Format on.
Over 400 collapse structures, believed to represent breccia pipes (many with exposed breccia), have been mapped. Those with gamma radiation exceeding 2.5 times background (57 pipes) have been sampled (155 samples). Of these oxidized surface samples collected solely on the basis of radioactivity, 30% have Ag exceeding 10 ppm, some with up to 1,150 ppm. The Copper Mountain Mine, located near Parachant Canyon on the Sanup Plateau has long been known for its "Au adit." Two samples of brecciated, oxidized sandstone with radioactivity exceeding 20 and 40 times background from this adit, and another sample of hematite-, malachite-, and chalcocite-impregnated sandstone from a higher level adit contained high concentrations of Au, Hg, Cd, and W, along with many elements commonly anomalous in m neralized breccia pipes from northern Arizona: Ag, As, Co, Cu, Mo, Ni, and Pb.
Preliminary oxygen isotope ratio data suggest that samples mineralized strongly with Au and Ag have ^dgr18O of 0.9 to 1.5 ^pmil lighter than non-mineralized or less mineralized samples. The strongly developed epithermal suite (Au, Ag, Hg, and As) and 18O depletion of originally detrital quartz is suggestive of at least moderately high temperature hydrothermal fluids involved in the mineralization of the Copper Mountain system.
Co and Ni concentrations in these breccia pipes are also of interest as a by-product to U; 10% of the above samples contain greater than 100 ppm Co (300 times average crustal abundance for sandstones) and 23% of the samples contain greater than 100 ppm Ni (50 times the average crustal abundance). The potential for economic recovery from breccia pipes of elements other than U, such as Ag, Au, Co, and Ni, should not be ignored, as their concentrations are even more enhanced in unoxidized samples.
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