Petrology, mineralogy, and geochemistry of submarine coals and petrified forest in the Sozopol Bay, Bulgaria

The mineralogy, petrology, and geochemistry of submarine coals and petrified tree trunks from the Sozopol Bay were investigated. The coals are lignites of Lower–Middle Miocene age. The petrified trunks are 40 cm up to 2 m high, some of which are in growth position and others are fallen. The main minerals in the coal are silicates of detrital origin (clay minerals) and authigenic pyrite. Authigenic sulfur and amber are present as well. In the Sozopol coals stored for 30 years, a neoformed mineral assemblage comprising mainly of gypsum, iron sulfates, sulfates of varying Fe, Na, Al, K, and Mg content, and halite has been observed. The formation of the sulfates was induced by bacterial activity as well. Germanium, Li, Rb, Ti, V, Mo, Cr, Mn, Co, and Ni are enriched in the coals as compared with the Clarke values of the world brown coals. In most of the samples studied, the contents of Ga, B, Y, W, Cu, Zn, Pb, P, and Zr are elevated as well. The element distribution is influenced by the lithotype composition. Most elements have a mixed mode of occurrence. Germanium, V, Ti, Cr, B, Na, and Ni (and to a lesser extent Sr and Cu) are organically associated, while K, Li, Rb, and Fe have a distinct inorganic mode of occurrence. Molybdenum follows strictly the Fe distribution. It is suggested that the main sources of the trace elements were the volcanic rocks in the sediment source area, from which the elements were leached in the peat bog.

► The matrix unstratified Sozopol coals are associated with silicified woods. ► The silicification is most likely a result of hydrothermal fluids. ► The high content of inertinite (16.7 vol. %) suggests an oxidizing environment. ► The novel phases are a result of oxidation, microbial activity, and crystallization of water soluble salts. ► Most of the 33 elements tested are of enhanced contents and of mixed organic and inorganic mode of occurrence.