Geochemical Characteristics and Fluid Properties of the Qixia Formation Dolomites of the Middle Permian in the Shuangyushi Block, NW Sichuan Basin, China

On the basis of petrology, in combination with analysis geochemical characteristics major and trace elements, REE (rare earth elements), carbon isotopes, oxygen strontium isotopes fluid inclusions, properties Qixia Formation dolomites Middle Permian Qixiain Shuangyushi block NW Sichuan Basin were systematically studied. The results samples show that are mainly composed medium, coarse, medium-coarse crystalline polymodal crystal size distribution, cloudy centers clear rims. They generally dark purple cathodoluminescence (CL) colors orange red CL relatively highly ordered (0.79–0.95, averaging 0.90). CaO MgO contents a negative correlation. characterized by higher Mg/Ca ratio (0.89–0.92, 0.91), high Fe (94–2,991 ppm, 622 ppm), Mn (26–185 102 Na (210–374 277 ppm) contents, low Sr (53–218 93 content lower ∑REE (1.10–5.56 2.08 concentrations. patterns similar to those calcites (C1), light (LREE) enrichment, heavy (HREE) depletion (LREE/HREE = 5.38–9.58, 7.10), Eu (0.65–4.19, 1.70) Ce (0.99–9.52, 2.61) positive anomalies. have consistent δ 13 C (1.98–4.27‰ VPDB, 3.01‰ VPDB) 18 O (−9.54‰–−4.65‰ −7.09‰ values coeval seawater. salinity index Z is larger than 120, diagenetic temperature normal temperature. average value 87 Sr/ 86 (0.707150–0.712273, 0.708669), while some Their homogenization temperatures inclusions (96.3°C–105.0°C, 101.1°C) exceed 100°C, which equivalent buried paleogeothermal temperature, their (12.3%–14.3%, 13.5%) 3–5 times Based on regional geological setting previous research achievements, dolomitized fluids probably derived from seawater or marine sealed pores under shallow-burial environment. experienced late alternation deep high-salinity hydrothermal fluids, formed before Middle-Late Triassic Period.