Vein uranium deposits

Tremblay L. P. Geologic setting of the Beaverlodge-type of vein-uranium deposit and its comparison to that of the unconformity-type. Reference 4, 431-56. 

The bulk of the world s uranium has been produced historically from (1) lower Proterozoic uraninite placer deposits in quartz-pebble conglomerates, (2) epigenetic uranium deposits in sandstones located in many cases at, or near, groundwater oxidation-reduction interfaces and (3) hydrothermal vein uranium deposits. These three distinctly different geologic environments provided most of the uranium that was produced from the 1940s to the early 1970s and they continue to be important exploration targets in the search for new uranium deposits. 

Raisanen E. (1986) Uraniferous granitic veins in the Sveco-fennian schist belt in Nummi Pusula, Southern Finland. International Atomic Energy Agency (IAEA), Technical Committee Meeting (TC-571) on Uranium Deposits in Magmatic and Metamorphic Rocks, Salamanca, 12p. 

The world s largest and highest-grade uranium deposits occur in metamorphic rocks. An extensive review of the classification of uranium deposits was given by Ruzicka. His classification of uranium vein deposits was subdivided into five types, based mainly on mineralogy. 

Vein-like deposits were described as uranium deposits of uncertain genesis by Mathews etal Except for a small number of these deposits in sedimentary rocks, the group is subdivided into unconformity-related deposits and deposits of metamorphic rocks. In a more genetic classification of the major (Proterozoic) vein-like deposits of the world, Dahlkamp and Adams deciphered the relationship of diagcncsis, metamorphism, metasomatism and weathering on uranium concentrations in various host rocks. The result is a logical genetic explanation for the spectrum of vein-like deposits observed today. 

It may come as a surprise that, when groundwater can be sampled, there is a remarkable similarity between the geochemical model described for sandstone deposits and the geochemistry of vein-like deposits. When the mineralization lies near the groundwater-table, water sampled on a scale of kilometres gives similar results for these two diverse cases in uranium, molybdenum, arsenic, radon, helium, sulphate, total dissolved solids, pH and alkalinity. 

Coffinite was first described as a new mineral by Stieff and co-workers from several localities in the sandstones of the Colorado Plateau deposits often intimately associated with asphaltic material. It was also found in vein-type deposits in Spain by Arribas and has since been found in almost all types of deposits. The composition has been reported as U(SiOi4)i AOrganic matter was always present and organometallic complexes of uranium may have accounted for the excess uranium rather than requiring excess (OH) to account for the U Si ratio deviating from unity. USi04 has been prepared by Fuchs and Gebert with no evidence of OH substitution. 

Ferguson J. and Rowntree J. C. Vein-type uranium deposits in Proterozoic rocks. Reference 22, 485-96. 

Soil and rock geochemical surveys may be usefully conducted in detailed exploration programmes where the uranium or other associated trace-metal content of the bedrock or soil of an area may be a useful guide to a buried uranium deposit. For example, uranium-, or copper- or molybdenum-in-soil geochemical surveys may be of value in detailed exploration programmes in the search for uranium vein deposits in terrain where uranium vein accumulations occur in association with copper and molybdenum sulphides. 

Fisher J. C. Remote sensing applied to exploration for vein-type uranium deposits. Front Range, Colorado. Ph.D. thesis, Colorado School of Mines, 1976. 

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. 

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. 

Vein-like deposits of uranium in sedimentary rocks are epi-... 

Pitchblende is the dominant uranium mineral in all these vein-like deposits in sedimentary rocks. It occurs as small veinlets along fractures within and surrounding the structures and as finely disseminated crystals in porous breccia fragments within the structures. Pitchblende concentrations may be distributed zonally within the ore, and in some deposits may be concentrated in the upper levels of the structure. Associated minerals may include sulphides and sulpharsenides. Calcite and quartz are the most common gangue. Types of alteration include bleaching of red sediments, silicification, carbonatization and argillization. 

Vein-like deposits in sedimentary rocks are generally quite small and relatively low-grade, ranging from a few to several hundred tons of UsOs at grades of 0.05-0.25% UaOg. They represent less than ICKX) ton of the Western world s reasonably assured 30 uranium resources. 

An example of veins with complex mineralogy is the mined-out uranium deposit at Shinkolobwe, Zaire, at the northwest end of the African Copperbelt. It occurs in a faulted transported fold in metamorphosed dolomitic shales of the middle Proterozoic Mine Series of the Roan Group.Uraninite and uranophane mineralization (about 620m.y. ago) was followed by several later mineralizations in which pyrite, molybdenite, monazite, selenium, Co-Ni sulphides and selenides and copper minerals were formed. It is believed that this orebody was formed by redistribution of metals originally deposited in marine sedimentary rocks." The uranium, originally weathered from granites and deposited in the marine sediments, was in low... 

The Precambrian European province proper contains limited resources in vein-type deposits in northern Sweden and, again, a huge low-grade uranium natural stock in Cambrian black shales. Grades, under present market conditions, are not economic. 

Uranium deposits in this unit are of numerous geological types, but three are typical of the Hercynian orogen (1) intra-granitic deposits related to leucogranites, (2) deposits bound to the contact-metamorphic haloes of granite intrusives in Lower Palaeozoic shales (the so-called Iberian type ) and (3) deposits bound to Permian cover rocks or Permian acid volcanics. Other types are veins in less differentiated granites, veins in metamorphic environments, sandstone-type deposits in Mesozoic or Caenozoic cover rocks in basin structures of the Hercynian space included in or adjacent to the Moldanubian zone (or its Iberian equivalent). 

The Jachymov district (Fig. 1) is less typical as the country rock had already undergone a general metamorphism. Uranium deposits again occur within the exocontact zone, developed in the metasediments, around an autometamorphic late Carboniferous or early Permian granite (probably submitted to an intense deuteric process). The orebodies are typically vein-like. The northeast veins contain the Ni-Co-Bi-Ag-U association. The northeast fractures present clay and quartz breccia fillings and at the intersections with the north-south fractures the five-metal association, plus Zn, again appears. 

---