Geochemistry of major elements in Middle Ordovician carbonate rocks: comparative analysis of alteration zones, North Estonia

Altered and/or mineralized bodies formed due to secondary processes occur frequently in the early Palaeozoic carbonate deposits in Estonia. Selectively dissolved and leached rocks, karst cavities, metasomatic dolomite beds, and fracture-related unconformable bodies, calcite, sulphide, sulphate, and oxide minerals filling pore space are the most common phenomena. Four alteration zones in the Middle Ordovician were studied: three in limestones of the Vao Formation from the Vao and Harku outcrops, and one in the Viivikonna Formation from the Narva opencast. Two geochemical types of dolomitization, and correspondingly two groups of rocks, can be defined considering the distribution of CaO, MgO, total iron, Fe2+ and Fe3+, and sulphur.