Concepts, recognition criteria and deposit modelling applied in uranium exploration of Bulgaria

The systematic uranium exploration in Bulgaria started 47 years ago. During the first 10 years it was influenced by the big uranium rush and the main tasks were radioactive anomalies discovery. Within a short period the whole Bulgarian territory was covered by airborne gamma ray survey followed by ground gamma ray study. The evaluation of the identified radioactive anomalies was mainly based on general geological considerations. If uranium minerals were discovered, the anomaly was quickly evaluated by trenching and the drilling of short bore holes. The first recognition criteria and concepts were developed on the basis of geological environment of deposits discovered. Paleozoic black shales soon become the most promising target for uranium exploration, followed by sandstone and acid volcanic environments. Geological study of different uranium type deposits helped deposit modelling and criteria for application in preliminary estimation of uranium potential of regions to be studied. Six main geological processes of importance for uranium deposit formation are considered: (1) Modification of the global uranium source and formation of local one; (2) Host rocks formation; (3) Mobilization of uranium and its transportation; (4) Fixation; (5) Regeneration of the primary uranium accumulations; (6) Deposit conservation. Eight uranium deposit models are discussed. According to the geological environment each of these models could have four modifications applicable to regions built up mainly be sedimentary, metamorphic, igneous and volcanic rocks. Indicators of the main and associated geological processes are presented. (author). 4 refs, 8 figs