The origin of breccia-hosted uranium deposits in carbonaceous metasediments of the Iberian Peninsula; U-Pb geochronology and stable isotope studies of the Fe Deposit, Salamanca Province, Spain

Abstract

The Upper Proterozoic-Lower Cambrian schist-graywacke complex of the Iberian peninsula hosts several important uranium deposits, which are characterized by their occurrence in fracture and breccia zones and by the mineral association pitchblende + coffinite + carbonates + adularia + Fe sulfides. The Fe mine, in Salamanca province, is not only the largest known deposit of this type but also the most important uranium deposit in Spain, with current reserves in excess of 16,000 metric tons of U 3 O 8 . Primary mineralization occurs as narrow veins occupying fractures and in cavities as finely laminated sediments showing a variety of geopetal textures. Three stages of primary mineralization, separated by episodes of fracturing and brecciation, are recognized. The early stage commenced with chloritization of wall rocks in narrow zones adjacent to fractures, followed by deposition of an assemblage dominated by ankerite and iron sulfides. The second (main)-stage mineralization is the most important for uranium and consists mainly of pitchblende, coffinite, carbonates, iron sulfides, and adularia. The final stage is characterized by repeated episodic deposition of carbonates, iron sulfides, pitchblende, and coffinite as layered cavity-filling material.Chlorite compositions and fluid inclusion data show that wall-rock alteration and ore deposition took place over a temperature range of approximately 280 degrees to less than 60 degrees C, probably to subsurficial temperatures in the final stage of mineralization. The uranium mineralization has been dated by U/Pb analyses of pitchblende samples at 34.8 + or - 1.6 Ma, indicating formation during the Pyrenean phase of the Alpine orogeny. The U/Pb data also suggest the possibility of a two-stage model for the uranium mineralization, with the 34.8 + or - 1.6 Ma event representing remobilization of an earlier pre-Tertiary mineralization. Calculated delta 18 O (sub H 2 O) values of the ore-forming fluid range from approximately 14.5 per mil for early-stage to near zero per mil for late-stage fluid. The delta 13 C 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 delta 34 S values of sulfides also decrease from the early stage (around -10ppm) to later stages (down to -51.3ppm).The combined geologic evidence suggests deposition of the ore from a hydrothermal system that formed in response to the effects of Alpine tectonics on the Variscan basement. Meteoric water descended via steeply dipping faults, and as well as undergoing extensive isotope exchange with, also leached uranium and other components from, the metasediments of the schist-graywacke complex, particularly carbonaceous slates. The fluid was probably expelled toward the surface through the fracture and breccia system by seismic pumping. The episodic nature of the mineralization may have been controlled by fault movements that initiated brecciation and release of fluid pressure, leading to ore deposition in fractures and cavities.