Breccia Pipe Anatomy
The Northern Arizona plateau region is host to thousands of breccia pipes. Breccia pipes are vertical or near vertical, circular to elliptical bodies of broken rock. The broken rock is comprised of slabs and rotated angular blocks and fragments of surrounding and stratigraphically higher formations. Hence, many geologists consider the pipes to have been formed by solution collapse of the underlying Redwall Limestone. Surrounding the blocks and slabs making up the breccia is a matrix of fine material comprised of surrounding and overlying rock from various formations. The matrix has mostly been cemented by silicification and calcification.
The roots of a breccia pipe start within the Mississippian Redwall Limestone and extend up to 3000 ft vertically through various rock strata of the Colorado Plateau in northwestern Arizona. Dissolution of the Redwall Limestone about 330 million years ago formed numerous caves in this thick limestone layer.
When the cave roof collapsed, overlying formations subsided into or were deposited in the resulting sinkhole. After later formations were deposited, later periods of cave formation and limestone dissolution renewed the collapse features, such that later formations collapsed into the breccia pipe. This gravitational collapse produced steep-sided, pipe-like bodies that are filled with brecciated rock.
Brecciated rock rarely extends the entire 3000 ft from the Mississippian Redwall Limestone to the plateau surface. Fortunately for exploration, a collapsed cone commonly occurs above a breccia pipe. The depression created by the collapsed cone (usually a structural basin) can be up to 1 mile in diameter. Such enlargement is due to dissolution of Permian gypsum and limestone beds within and immediately adjacent to the breccia pipes. This dissolution is probably related to fluids percolating through the breccia column and moving laterally into the Lower Permian Toroweap and Kaibab gypsiferous units. While the structural depression over a beccia pipe may range in diameter up to 0.5 mi. or more, the breccia pipe diameters range up to about 600 ft.; the normal range being 200 to 300 ft.
The CSMAT 3D graphic of the newly imaged Makapuu Breccia Pipe can be seen above and here.
BRECCIA PIPE MINERALIZATION
The uranium mineralization occurs in the breccia zone within the core of the pipe, as well as in the annular ring faults surrounding the breccia pipe. The economic uranium mineralization occurs typically as uraninite (UO2).
Most of the minerals in a breccia pipe such as uraninite, copper minerals, vanadium-bearing minerals and the more common minerals such as pyrite, galena, barite, and sphalerite are usually megascopic in size. The obvious presence of these minerals at the surface or in drill core, or alteration products from them, indicates the presence of an underlying mineralized breccia pipe.
Mineralization at the surface of the breccia pipes commonly occurs within nodules and concretions associated with pyrite and goethite and along fractures. The best elemental surface indicators of mineralized pipes are Ag, As, Co, Cu, Mo, Ni, Pb, U, and Zn enrichments in the soil and mineralized rocks near to the breccia pipe.
A new geochemical tool called SGH Analysis has shown good results in identifying uranium mineralized breccia pipes.
Surface expression of mineralization is generally located along the ring fracture of the pipe and is characterized by copper minerals, minor increases in gamma radiation, barite, calcite, goethite, and more rarely pyrite or marcasite (Wenrich, 1985). The highest gamma radiation commonly occurs in comminuted rock or in fracture zones. Most economic uranium pipes contain a pyrite cap that commonly oxidizes to goethite in pseudomorphs after pyrite cubes, concretions, botryoidal masses, and boxwork fracture fillings.