Proposed metallogenic and tectonic classification of uranium ore deposits typesP. Goodell (a) , F. Howari (b) , A. Salman (c)
(a)Department of Geological Sciences, University of Texas, El Paso, Texas, United States of America. (b)Bureau of Economic Geology, Jackson School of Geosciences, University of Texas, Austin, Texas, United States of America. (c)Nuclear Materials Authority, Cairo, Egypt. E-mail address of main author: [email protected]
The behavior of uranium in the natural environment is very versatile. It has a crustal abundance of only 4 ppm, about the same as tin or arsenic, however it is found concentrated into uranium deposits in many different types of rock and geologic environments, in concentrations of 20%, or more. Uranium has two different oxidation states in nature. The plus four (+4) oxidation state is present in reduced conditions, meaning all magmas. In the more oxygenated surface and near-surface regions the plus six (+6) oxidation state is present in the form of the uranyl molecule or ligend, O-U-O ‘dumbbell’ shaped molecule, (UO2)+2. This cationic molecule hydrologizes, is very soluble in water, and is transported in this manner. This is partly the reason for the versatility of uranium. The precipitation of uranium from surface or groundwater is necessary for the formation of several types of deposits.First, chemical reduction is a most efficient method of precipitation, reductants being carbon or sulfur in the formsmethane (natural gas), hydrogen sulfide, coal, organic materials, or shale. Independent of oxidation/reduction, the uranyl cationic molecule in water has attraction to a large variety of anionic liginds, including those of As, P, V, Mo, and where the concentrations are large enough, the solubility product is exceeded, and precipitation takes place. The fact that uranium mineralogy consists of over 100 different species which are members of many anionic mineral families attests to the versatility of uranium. As a result of these properties of uranium, there are many different uranium deposit types and subtypes, and many different deposit type classifications. Since countries have different geologic characteristics, classifications tended to reflect national characteristics. The IAEA in 1988 provided its classification based on uranium production by type, and this is of widespread use. Significant problems exist in understanding genetic relationships between certain uranium deposit types, and the magmatic hosted deposits are most problematic. Several deposit types are the name of a known deposit, such as Rossing, Namibia type, or the Bokan Mountain, Alaska, type, with no relationship between them. It is an objective of the present report to provide an integration and synthesis of the various magmatic uranium deposits. A model is presented integrating the following deposit types 1) metasomatic, 2) magmatic, 3) pegmatitic, 4) contact metamorphic, 5) hydrothermal veins., and 6) carbonatite, within a regional tectonic understanding., Another significant problem is demonstrated by a group of the surficial deposits. The origin of uraniferous coal, lignite, phosphate, and black shales are unified with the recent suggestion that the uranium comes from giant caldera ash eruptions from silicic large igneous provinces (SLIP) in adjacent extensional regimes. This implies that all four surficial regimes can be mineralized by the same event. This realization can be a significant global exploration tool. The proposed classification provides a better understanding of the origin of uranium deposits, and consequently can lead to a more effective exploration program.