THE ORIGIN OF BRECCIA-HOSTED URANIUM DEPOSITS IN CARBONACEOUS METASEDIMENTS OF THE IBERIAN PENINSULA - U-PB GEOCHRONOLOGY AND STABLE-ISOTOPE STUDIES OF FE DEPOSIT, SALAMANCA PROVINCE, SPAIN

The Upper Proterozoic-Lower Cambrian schist-gray,ak, complex of the Ib erian peninsula hosts several important uranium deposits, which are ch aracterized by their occurrence in fracture and breecia zones and by t he mineral association pitchblende + coffinite + carbonates + adularia + Fe sulfides. The Fe mine, in Salamanca province, is not only the la rgest known deposit of this type but also the most important uranium d eposit in Spain, with current reserves in excess of 16,000 metric tons of U3O8. Primary mineralization occurs as narrow veins occupying frac tures and in cavities as finely laminated sediments showing a variety of geopetal textures. Three stages of primary mineralization, separate d by episodes of fracturing and brecciation, are recognized. The early stage commenced with chloritization of wall rocks in narrow zones adj acent to fractures, followed by deposition of an assemblage dominated by ankerite and iron sulfides.The second (main)-stage mineralization i s the most important for uranium and consists mainly of pitchblende, c offinite, carbonates, iron sulfides, and adularia. The final stage is characterized by repeated episodic deposition of carbonates, iron sulf ides, pitchblende, and coffinite as layered cavity-filling material. C hlorite compositions and fluid inclusion data show that wall-rock alte ration and ore deposition took place over a temperature range of appro ximately 280-degrees to less than 60-degrees-C, probably to subsurfici al temperatures in the final stage of mineralization. The uranium mine ralization has been dated by U/Pb analyses of pitchblende samples at 3 4.8 +/- 1.6 Ma, indicating formation during the Pyrenean phase of the Alpine orogeny. The U/Pb data also suggest the possibility of a two-st age model for the uranium mineralization, with the 34.8 +/- 1.6 Ma eve nt representing remobilization of an earlier pre-Tertiary mineralizati on. Calculated deltaO-18(H2O) values of the ore-forming fluid range fr om approximately 14.5 per mil for early-stage to near zero per mil for late-stage fluid. The deltaC-13 values vary between -7.3 and -9.6 per mil for early- and main-stage carbonates and show a sharp progressive decline to -23.6 per mil at the end of the final stage. The deltaS-34 values of sulfides also decrease from the early stage (around -10 par ts per thousand) to later stages (down to -51.3 parts per thousand). T he combined geologic evidence suggests deposition of the ore from a hy drothermal system that formed in response to the effects of Alpine tec tonics on the Variscan basement. Meteoric water descended via steeply dipping faults, and as well as undergoing extensive isotope exchange w ith, also leached uranium and other components from, the metasediments of the schist-graywacke complex, particularly carbonaceous slates. Th e fluid was probably expelled toward the surface through the fracture and breccia system by seismic pumping. The episodic nature of the mine ralization may have been controlled by fault movements that initiated brecciation and release of fluid pressure, leading to ore deposition i n fractures and cavities.