HYDROTHERMAL AND MICROBIAL ORIGIN OF MICROCRYSTALLINE AND MEGA-QUARTZ AGGREGATE GRAINS IN PERMIAN LACUSTRINE SHALE, SANTANGHU BASIN, NW CHINA

Lake water and environment are commonly affected by physical, chemical, and biological processes. This is demonstrated in shales of mid-Permian Lucaogou Fm. In the sediment-starved intracontinental lacustrine Santanghu rift basin, NW China. Hydrovolcanic and hydrothermal minerals indicate volcanic and related hydrothermal processes controlling precipitation and replacement, among which is a special type of quartz aggregate. 65 samples from Well N122 were subject to microscopic, SEM, microprobe, XRD, LA-ICP-MS, fluid inclusion and isotopic analyses. Five main features were observed: 1) ~200 quartz-bearing laminae are 0.1-2 mm and mainly 1 mm thick and are intercalated within tuffaceous shale, indicating a low energy profundal environment. 2) Framework quartz-aggregate grains are elongate, bedding-parallel, and 140 × 20 micron in size on average. Individual anhedral microcrystalline quartzes (0.1-20 micron) occur as the core and subhedral megaquartzes (20-60 micron) in the outer part, both are in a sutured contact, indicating multiple recrystallization episodes. Angular albite and orthoclase, organic matter, and rare calcite cement comprise the matrix and are < 10 micron in size. 3) Abundance of Al, Ti, Sr and Ba in quartz aggregate may reflect high-temperature replacement. Low total REE is similar to that of typical marine hydrothermal cherts. 4) irregular-shaped fluid inclusions in quartz are in mainly single-liquid and rarely gas-liquid phase with a homogenization temperature ~185 ^oC and contain CO₂, H₂S, and H₂, indicating that many inclusions formed by hydrothermal activities during early, not late, diagenesis. 5) In comparison to adjacent laminae, δ¹³C values of calcite cement in quartz-bearing laminae are 2.57-5.14 per mil and more positive, suggesting a microbial origin; δ¹³O values are -8.88 to -19.06 per mil and smaller, suggesting a hydrothermal origin. In conclusion, the quartz aggregate grains formed by both microbial and hydrothermal processes. An unknown microbe on the lake floor had changed the chemical environment, resulting in silica precipitation followed by recrystallization by hydrothermal fluid related to volcanic activities. This study shows that petrographic and geochemical data are useful proxies in paleoenvironmental reconstruction.