Sedimentology of the “ore-bearing dolomite” of the Kraków-Silesia region (Middle Triassic, southern Poland)

Abstract

The depositional history and facies heterogeneity of the epigenetically dolomitized Middle Triassic carbonates of southern Poland are poorly recognized, and existing concepts of fluid circulation entirely overlook the primary lithology as a factor controlling fluid flow. This study reconstructs the consecutive phases of Kraków-Silesia Sub-basin history in the Anisian and highlights their influence on the development of the so-called “ore-bearing dolomite”. Extensive fieldwork and microfacies analyses were carried out in order to decipher the original depositional fabric of the ore-bearing dolomites. As a rule, epigenetic dolomitization affected a horizon of porous strata, 35 m thick and resting directly on impermeable, wavy-nodular clay-rich calcilutites of the Gogolin Formation, which represent the interval of deepest and fully marine (offshore) sedimentation. The sedimentary succession of the porous strata is bipartite. The lower part (Olkusz Beds) is composed of Balanoglossites and Thalassinoides micritic firmgrounds and peloidal packstones-grainstones, representing shoreface-foreshore facies assemblages, whereas the upper part (Diplopora Beds) consists of dolocretes, rhizolites, cryptalgal laminites, peloidal packstones-grainstones and bioturbated fine-grained dolostones, formed in a system of tidal flats and lagoons. These two parts are separated by a subaerial disconformity, which marks a sequence boundary. During emersion, the underlying deposits were subjected to meteoric diagenesis, which led to the development of moldic porosity. This combination of depositional history and diagenetic alteration determined the routes of initial migration of dolomitizing solutions on the one hand, and the location of cavern formation on the other. Owing to progressive dissolution, small caverns were changed into large karstic forms, in which the ore minerals precipitated ultimately. These findings emphasize the importance of sedimentological analysis to the understanding of the evolution of the Kraków-Silesia ore province.