Rock brecciation

In the Atacama, various observers have postulated that salt weathering is an active process that leads to rock brecciation (Searl and Rankin, 1993), slope planation (Abele, 1983) and tafoni development (Tricart and... 

Five different vein phases (Types i to V) are recognized at both deposits, aii have variabie amounts of carbonates and quartz gangue. Type i veins contain oniy brecciated quartz and carbonate minerals and at ED are spatially associated with disseminated arsenopyrite, chalcopyrite, pyrrhotite, and pyrite in the mafic host rock. Type II veins in both deposits are partly brecciated and contain 5-80% sulfides of dominantly pyrite, arsenopyrite, and at GB chalcopyrite. Type III veins are quartz-calcite-tetrahedrite-bismuthinite microveins that cut both Types I and II veins. The fine-grained sulfides replace and enclose arsenopyrite and pyrite in Type II veins and are also visible in microfractures within the Type II sulfides. Type IV veins are base-metal rich and characterized by galena, sphalerite, chalcopyrite, pyrite, and stibnite with a maximum width of 20 cm. The Type V veins are late barren-carbonate veins cutting all previous veins and textural features. 

Figure 19 Hand-specimen photograph of the Bustee aubrite showing the brecciated texture of the rock (9 cm in horizontal dimension) (photograph courtesy of the Smithsonian Institution).

Cavities, caverns, or other openings in salt beds are evidence of past or present dissolution. The cavities commonly are filled with low-salinity to saturated brines that may or may not be under artesian pressure. The cavities may also be partly or totally filled with clay or other sediment deposited from water that had passed through the openings. Cavities may also contain brecciated rock from overlying formations that collapsed into the openings. 

Colorado, is hosted in folded and brecciated Mississippian dolomite, shale, sandstone and coal, which is in fault contact with Precambrian gneiss. The Precambrian gneiss is anomalously enriched in uranium, and a Mesozoic or Caenozoic age of mineralization in the Mississippian host rocks, formed by downward percolation of uraniferous groundwater from the adjacent Precambrian terrain, is inferred. Mineralization in the Pryor Mountains region, Montana, is hosted in karsts developed with the Mississippian Madison limestone and consists of uraninite-tyuyamunite grading up to 7% UsOg associated with clay minerals and silicified collapse breccias. A Caenozoic age of mineralization, under conditions similar to those of Pitch mine, is favoured. 

Much of the world s currently minable uranium is found in vein-like deposits of uncertain origin. They consist of three general types of unequal importance. Most important are unconformity-related deposits and vein-like deposits in meta-morphic rocks. Least important and probably of different origin are vein-like deposits in sedimentary rocks. The origin of all three types is a matter of much conjecture. Major unsolved genetic questions are the source of the uranium and its mode of transport, the source of the mineralizing solutions, the nature and role o f reductants and the control exerted on uranium deposition by structural and lithologic features of the host rocks. Unconformity-related deposits and vein-like deposits in metamorphics commonly occur in brecciated and foliated metamorphic rocks in stable Precambrian Shield areas. They contain about 24.4% of the Western world s reasonably assured 30 uranium resources. 

Uranium occurs as concentrations of pitchblende with some coffinite in veinlets along faults, in brecciated zones and in satellite structures associated with the major structure. These minerals may occur also as fine disseminations in selected horizons in the host rocks. Secondary uranium minerals are locally abundant, especially in near-surface portions of deposits. Small amounts of quartz and carbonates are the gangue minerals. Associated minerals vary in type and abundance and determine whether a deposit is classified as monometallic or polymetallic. The latter may contain a wide variety of associated minerals, including sulphides. Hematitization and chloritization are the most common forms of alteration... 

Vein-like deposits in metamorphic rocks also occur in Pre-cambrian Shield areas, but they differ from unconformity-related uranium deposits in that they are not associated with major regional unconformities, the geometries of orebodies are different and they extend to greater depths. Vein-like deposits are closely associated with steeply dipping, brecciated major fault systems. Uranium minerals (pitchblende with some coffinite and brannerite) occur as open fracture fillings and as fine disseminations adjacent to the fractures in Proterozoic meta-igneous andmetasedimentary rocks. Common associated minerals are chlorite, hematite and pyrite. 

Pitchblende and calcite occur in veins and multiple-vein systems in the Tazin, and some pitchblende occurs in the over-lying Martin. Uranium-bearing veins have a strike length of more than 4500 m and they extend to a depth of more than 1645 m. The pitchblende occurs as vein fillings in shears, fractures and brecciated zones within 100 m of the St. Louis Fault. It is also disseminated in rocks adjacent to the veins. Initial pitchblende deposition (1780m.y. ago) was followed by a thermal event that remobilized and redeposited the uranium about I I40m.y. ago. 

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