The Late Cretaceous Silistar intrusion comprises of gabbros, gabbro-diorites to quartz-diorites and aplites, that were emplaced into a volcano-sedimentary succession of similar age. Structural data suggest that this intrusion is part of a larger, partially exposed body. A dense network of primary and secondary joints, in many places filled with various ore and gangue minerals, is a conspicuous feature of the intrusion. Hydrothermal alteration affecting the intrusion and the wall rocks includes: uralitization (amphibole±epidote), secondary biotitization and propylitization. Propylitic alteration occurred in two stages: high temperature (epidote-actinolite-chlorite) and middle to low temperature (sericite-chlorite-carbonate-epidote and chlorite-sericite-carbonate). Products of later alteration events include quartz-adularia, quartz-carbonate, carbonate, quartz-zeolites and zeolites. Apart from previously recognized contact-metasomatic mineralization, the presence of stockwork-type and disseminated pyrite and chalcopyrite mineralization, hosted by both the intrusion and the host rocks, is documented here. Two types of magnetite and pyrite (magmatic and metasomatic) are recognized. Based upon the alteration products and ore minerals, the presence of two differing zones is suggested. The first zone, which closely coincides with the intrusion, is potassic and hosts py-ma-cpy-hm-(ilm). The second zone is propylitic with py-ma-cpy-hm-(ilm)+(sph+ga)+(bo+hz). Cu is the main ore element; Mo contents are very low or nil. The types of ore mineralization and alteration products, along with structural data, show elements of both porphyry copper and epithermal systems and suggest their occurrence of such (and the first recognition of such) in the incipient rift zone of the Eastern Srednogorie Zone.
NAKOV, RADOSLAV; KERESTEDJIAN, THOMAS; and KUNOV, ANGEL (2002) "The Silistar Intrusive, Eastern Srednogorie Zone, Bulgaria: Structural Data and Potential for Porphyry Copper and Epithermal Systems," Turkish Journal of Earth Sciences: Vol. 11: No. 3, Article 4. Available at: https://journals.tubitak.gov.tr/earth/vol11/iss3/4