MIXED MICROBIAL AND HYDROTHERMAL DOLOMITE RELATED TO MANTLE-DERIVED MATERIALS – A POSSIBLE FORMING MECHANISM FOR PRIMARY DOLOMITE

The origin of dolomite has been a mystery for many years. Here we propose a possible primary origin of dolomite related to microbial and hydrothermal activities. The dolomite occurs in middle-Permian Lucaogou Fm., Santanghu Basin, NW China, which is an intracontinental rift basin. The formation contains lacustrine black fine-grained sedimentary rocks, including mainly tuffaceous mudstones and dolostones, which are the main source rocks in NW China. The dolostones are mm-laminated, and intercalated with tuffaceous shale and composed mainly of dolomite (77% on average), quartz (14%), alkaline feldspars (3%), and little analcimes and pyrites. Dolomite crystals are commonly less than 3 μm in size, mainly anhedral to subhedral with a low degree of stoichiometry. The silicate mineral grains are angular, shard-like, bedding parallel, and show normal grading in dolomite-dominated laminae, indicating a volcanic origin during deposition and precipitation. Unique nanoscale filamentary minerals and build-up structures were observed at intercrystalline pores of dolomite and laminae of dolostones in some samples, respectively. And δ¹³CPDB (5.2‰ to 9.9‰, 6.9‰ on average) of dolomite suggests the formation of dolomites was affected by the generation of bacterial participating methane. δ¹³OPDB (-1‰ to -17.4‰, -7.5‰ on average) values of dolostones composed of subhedral dolomite are more negative than those of anhedral ones, which indicate recrystallization by hydrothermal activities. The whole-rock strontium isotope values (0.70462 to 0.70525, 0.70501 on average) and δ²⁶Mg (-0.89 to -0.24, -0.52 on average) values indicate that the sedimentary fluids may be derived from mantle. The dolostone is a rare type of microbially-influenced dolostone related to hydrothermal and volcanic activities. Repetitive volcanic-hydrothermal activities raised the temperature unevenly on the lake floor, which broke the threshold dynamic barrier temperature (~50℃). Hydrothermal fluids which reacted with mantle-originated magma and wall rocks provided abundant Mg²⁺. Thermophilic microbes near the hydrothermal vents multiplied rapidly, generating a favorable chemical condition for dolomite precipitation.