The carbonate and zeolite alterations in bitumen-containing volcanic rocks of the Minusa Basin, eastern Siberia

Intense alterations in the form of secondary carbonates, zeolites, and phyllosilicates characterize the volcanic rocks widespread in the Minusa Basin. A common feature of the studied Early Devonian sheet-like and Triassic dyke volcanics is the amygdules of solid bitumen, indicating a potential water-volcanic rocks-hydrocarbons interaction. This paper presents new data on petrography, mineralogy, and stable isotope geochemistry of carbonates and zeolites in Devonian and Triassic basaltic rocks of the Minusa Basin. Calcite and analcime are the main secondary minerals filling pores and fractures in the studied volcanic rocks. These rocks include basalts, trachybasalts, and trachyandesibasalts of the Early Devonian Byskar series, as well as altered olivine basalts of the Triassic Kopievo complex. Calcite was found in all studied samples, whereas analcime was identified only in altered olivine basalts. The carbon isotopic composition of calcite in the Devonian volcanic rocks indicates that the main carbon source was seawater, which could infiltrate into the basaltic bodies through numerous fractures after flooding the region. Atmospheric water and hydrothermal influence were present, as indicated by differences in the carbon isotopic composition of calcite from different locations. In Triassic altered olivine basalts, calcite is represented by two types differing in textural features, isotopic composition, and manganese content. Very low δ¹⁸O values indicate that calcite formation in Triassic volcanic rocks occurred through precipitation from high-temperature solutions. It is assumed that these solutions were of mixed origin and consisted of both meteoric waters from the surrounding rocks and deep hydrothermal fluids, the generation of which may have occurred due to the magma cooling. Analcime fills numerous primary pores, and its formation appears to result from the interaction between volcanic material and Na-enriched water. Secondary minerals in both Early Devonian and Triassic volcanics were formed in two stages. Filling of primary pores with calcite and analcime occurred during the early postmagmatic stage. In contrast, calcite veins formed later under the conditions of marine (Devonian rocks) or meteoric (Triassic rocks) water dominance, which was controlled by the fracture development.