Lithotectonic setting necessary for formation of a uranium-rich, solution-collapse breccia-pipe province, Grand Canyon region, Arizona

Abstract

Thousands of solution-collapse breccia pipes crop out in the canyons and on the plateaus of northwestern Arizona; some are host to high grade, uraniumrich deposits. Mining activity in breccia pipes of the Grand Canyon region of northern Arizona began during the nineteenth century, although at that time production was primarily for Cu. Later, during the period 1956-1969, the Orphan mine yielded 1.6*1 million kg of uranium (4.26 million Ib of UnOg) with an average grade of 0.36% uranium (0.41$ UoOg). During the period of 1982-198? five additional breccia pipes were mined for uranium. Distinct alignments of breccia pipes in N45W and N50E trends occur in northern Arizona. In addition, the pipes within some of these alignments tend to be equally spaced. A study of joints on the Redwall Limestone-capped Hualapai Plateau showed that N45W-and N50E-trending fracture sets were imposed upon the Mississippian Redwall Limestone prior to the deposition of the overlying Pennsylvanian and Permian Supai Group. The ring fracture, encasing each breccia pipe, and the mineralization apparently predated any jointing in the Pennsylvanian or younger host rocks. Thus, the pipe locations were probably controlled by NW-and NE-trending fracture sets, and the pipes appear to have been both formed and mineralized prior to any other significant jointing in the rock. The breccia pipes contain an extensive mineral suite. The paragenetic sequence begins with the deposition of calcite, dolomite, barite, siderite, and kaolinite. The second stage of mineralization is characterized by minerals rich in Ni, Co, As, Fe, and S, such as siegenite, bravoite, pyrite, millerite, gersdorffite, rammelsbergite, niccolite, arsenopyrite, and marcasite. The third stage of mineralization was characterized by the formation of Cu-Fe-Zn-Pb sulfides. Uraninite was later deposited in the coarsely crystalline calcite matrix, in minor vugs, and as rims around detrital quartz grains. In some pipes, supergene alteration has resulted in the formation of bornite, chalcocite, djurleite,-digenite, and covellite, along with an assemblage of nonopaque supergene minerals. Uranium mineralization occurred in most breccia pipes roughly around 200 Ma. Ore-forming fluids that deposited the sphalerite, calcite, and dolomite had minimum ranging between 80°C and 173°C with salinities always >9 wt. % NaCl equivalent, although most commonly >18 wt. % NaCl equivalent. Mineralized breccia pipes are enriched in a large suite of elements. In addition to the metals U, Cu, Pb, Zn, and Ag that have been mined in the past from various breccia pipes, mineralized rock is also enriched in As, Ba, Cd, Co, Mo, Ni, Se, and V. The breccia pipes were excellent conduits for the movement of any fluids present within the Grand Canyon region since the pipes began to form in the Mississippian. Because this is a region of flat lying strata the breccia pipes provided very permeable vertical conduits for fluid movement. So, the opportunity existed for the movement into the breccia pipes of (1) brines dewatered fro...

Geology, geochemistry, and mineralogy of the Ridenour Mine breccia pipe, Arizona

“…These relations further constrain the time of fracture to a period of minor uplift in Late Mississippian time, during which the Redwall was emergent and stream valleys were cut into its surface. The structural control of the NE-and NW-trending cave passages in the Redwall is repeated in the alignments of pipes derived from them (Wenrich and Sutphin, 1989). Further, if fracture of the Redwall were not uniform, but had been more intense adjacent to pre-existing faults reactivated during the Late Mississippian episode of uplift, enhanced cave development and subsequent lengthy chains of pipes along these trends might be expected.…”

“…It almost certainly is one of those faults, such as the Hurricane, of Precambrian ancestry and repeated movement. The possibility that Paleozoic movement on such faults influenced early fracture of the Redwall Limestone is a recurring theme in attempts to explain structural control of aligned breccia pipes Wenrich and Sutphin, 1989).…”

“…The ore horizon appears to be slightly lower south of the Grand Canyon commonly at the level of the Esplanade Sandstone whereas to the north most ore is located at the Hermit Shale horizon. The mineral paragenesis is fairly consistent from pipe to pipe (earliest to latest): (1) quartz, calcite, dolomite, barite, siderite, kaolinite; (2) Ni-Co-As-Fe sulfides and arsenides, such as siegenite, bravoite, pyrite, millerite, gersdorffite, rammelsbergite, niccolite, arsenopyrite, and marcasite; (3) CuFe-Zn-Pb sulfides such as chalcopyrite, enargite, galena, sphalerite, pyrite and uraninite; (4) supergene alteration minerals, such as malachite, azurite, cyanotrichite, and brochantite (Wenrich and Sutphin, 1989). In addition to the above minerals, pipes on the South Rim (the Orphan, Canyon, and Ridenour pipes) contain copper sulfides, such as bornite, chalcocite, djurleite, digenite, and covellite that form under slightly higher Eh conditions than do chalcopyrite and enargite (Garrels and Christ, 1965).…”

“…The more negative 5 -*C values in the mineralized rock probably reflect the presence of organic compounds in the mineralizing fluids; evidence for this can be found in the association of bitumen with uranium ore in a few breccia pipes and the occurrence of organic material within some sphalerite fluid inclusions (Wenrich and Sutphin, 1989).…”

“…Fluid-inclusion studies of sphalerite, calcite, and dolomite yielded temperatures ranging between 80°C and 173°C with salinities consistently >9 wt. % NaCl equivalent, although most commonly >18 wt.5f NaCl equivalent (Wenrich and Sutphin, 1989). The main uraniummineralizing event occurred at roughly 200 Ma, based on a large set of U-Pb isotopic analyses from the Hack 2, Hack 3, Pigeon, Kanab North, EZ-1, EZ-2, and Arizona 1 breccia pipes (Ludwig and Simmons, 1988).…”