Preliminary insights on uranium-bearing Jurassic phosphorites of the Ionian zone, Albania, their distribution and genesis

Phosphorites are well-known, worldwide, for accommodating a significant amount of U relative to other sedimentary rocks, due to the crystal structure of apatite where U substitutes Ca. In Albania, uranium-bearing Lower Jurassic phosphorites form a distinctive horizon identified across the entire Ionian zone. Two different types of U-bearing phosphatic deposits are identified within the Lower Jurassic carbonate succession in Albania; one, stratigraphically younger, layered and parallel to carbonate strata and the other due to infiltration in older, brecciated carbonates. The first type consists of lenticular shape deposits of limited lateral extension intercalated in thin-shelled bivalve packstone/rudstone, possibly corresponding to the Toarcian Bositra-rich organic rich “Posidonia” shales of Austria and Germany. The phosphatic horizon is confined by dark gray J2 limestone on the top, while the bottom shows signs of an early erosion. Their extent varies from 5-10 m to 100-160 m, while their thickness varies from 0.3 to 6 m. The radioactivity measurements indicate that the intensity of gamma radiation varies from 0.6 to 1.66 µSv/h, while the uranium content varies from 50 to 230 ppm, the highest values corresponding to the P2O5 peak of 32.66%. The second type of phosphorite mineralization developed in a widespread horizon throughout the Ionian zone. Its extension varies from 1.5 to 8 km, composed of continuous zones up to 0.5-1 km with some small interruptions. Uranium-bearing phosphatic deposits occur as the infiltrated matrix of breccias (caused by hydraulic or tectonic stress, not of sedimentary origin) made of angular clasts of Pliensbachian inner carbonate platform facies of the J1 (peloidal and oncoidal packstone/grainstone with Siphovalvulina foraminifers and Palaeodasycladus mediterraneous dasyclad algae). The intensity of gamma radiation varies from 0.5 to 1.1 µSv/h, uranium content varies from 50 to 150 ppm and the P2O5 highest peak is 15%. In accordance to the first data achieved, the uranium-bearing phosphorites of Early Jurassic age in Albania formed in deep, oxygen-depleted marine conditions as suggested by the abundance of thin-shelled bivalves. The well-documented early Toarcian Oceanic Anoxic Event driven by an increase in nutrients linked to high volcanic emissions of CO2 may have favoured the formation of the phosphate deposits. The intensive tectonic events associated with the Alpine Tethys Ocean opening during the Early and Middle Jurassic should be the main factor causing the brecciation and controlling the formation of later infiltration mineralization.