Multistage evolution of the Pierres-Plantées uranium ore deposit (Margeride, France): evidence from mineralogy and U-Pb systematics
Respaut, Jean-Patrick; Cathelineau, Michel; Lancelot, Joêl R.
published: Feb 21, 1991
manuscript accepted: Jul 19, 1990
manuscript received: Jun 30, 1989
ArtNo. ESP147050301004, Price: 29.00 €
Abstract The Pierres-Plantées uranium mine (S.E. French Massif Central) is located in an alteration pipe which crosscuts the Margeride monzogranite as well as sills of the Grandrieu peraluminous granite. This alteration occurred at temperatures greater than 300°C and mainly involved quartz leaching which significantly increased the permeability of the altered rocks. During pitchblende deposition, within the pipe, fluid/rock interactions induced the crystallization of illite. U-Pb dating of pitchblende from the first uranium ore stage provides an age of mineralization of 188 ± 12 Ma. This age is clearly younger than the emplacement of the surrounding granitoids (324 ± 7 Ma and 305 ± 9 Ma) or the formation of the alteration pipe (269 ± 9 Ma). During Liassic times uranium was mobilized from an unknown source, the most probable being the Grandrieu leucogranite, by hydrothermal oxidizing fluids at temperatures between 150-250°C ; U mineralization resulted from reaction between these fluids and Ca-Mg rich rocks like quartz diorites and to a smaller extent, the Margeride monzogranite. As with other U ore deposits in southern France (Lodève and Bertholène), the formation of Liassic pitchblende is related to a period of regional hydrothermal activity during a distensive phase i.e. the opening of the Tethys Ocean. A strong alteration of the 188-Ma old U ore occurred at a late stage through large-scale transformation of pitchblende into coffinite, associated with the smectitization of silicates. Pitchblende incorporates up to 11 % SiO2 but contains less Ca and Pb than the original U-ore ; it is transformed into intermediate phases between UO2 and USiO4, which induces leakage of 238U intermediate daughter elements. This reworking of.Liassic pitchblendes along oxidation-reduction fronts is related to low-temperature fluid circulation, and the uranium crystallized as pitchblende nodules about 100 Ma ago.