Origin of bedding-parallel fibrous calcite veins in lacustrine black shale: A case study from Dongying Depression, Bohai Bay Basin

The bedding-parallel fibrous calcite veins in black shales are common in sedimentary basins and retain significant information about shale diagenesis and organic matter evolution. However, the origin of the bedding parallel fibrous calcite veins in black shales is still of great controversy. Carbonaceous black shales of Es4s-Es3x interval are the main source rock in Dongying Depression, Bohai Bay Basin, and fibrous calcite veins parallel to bedding are widely spread in these shales. Petrographic examination, fluid-inclusion microthermometry, and isotopic analyses were conducted to study the timing, diagenetic fluid conditions and formation mechanism of fibrous calcite veins. The bedding-parallel fibrous calcite veins include beef veins and cone-in-cone structures. The beef veins occur as short lenses and are filled with sub-vertical fibrous calcite. They contain a median line defined with brown granular calcite, scattered host rock fragment and pyrite framboids. Several adjacent beef veins may combine and grow together as a cone-in-cone structure and black shale laminas involved in it occur as sinusoidal solid inclusions in slices. Primary two-phase inclusions in fibrous calcite have homogenization temperatures (Th) between 86.4 C and 117.4 C. The 613C composition of the micritic calcite in host shale ranges from +3.7 parts per thousand to +6.3 parts per thousand VPDB and that of granular calcite ranges from- 0.2%o to +1.4%0VPDB, but the fibrous calcite has a moderate 813C values ranging from +1.8 parts per thousand to +5.0 parts per thousand VPDB. The 813C compositions of these calcite suggest an evolving carbon source at different burial stages. The fibrous calcite likely precipitated from modified pore water at elevated temperature, which is supported by its low 8180 values (-13.7% to- 11.4%o VPDB). The veins formed over two stage and the fibrous calcite growth was continuous and at least partly driven by the force of crystallization. The bicarbonate responsible for the fibrous calcite was derived from a mixed source including inorganic carbon from previous carbonate dissolution, and organic carbon from both fermentation and thermal decarboxylation.