Stratigraphic and Structural Control on Hydrothermal Dolomitization in the Middle Permian Carbonates, Southwestern Sichuan Basin (China)

The Qixia Formation and Maokou Formation of Middle Permian in the southwestern Sichuan Basin were pervasively dolomitized during the diagenetic history. Petrographically, four types of dolomites, namely three replacive dolomites (Rd1, Rd2, and Rd3) and one dolomite cement (Cd), were distinguished. Rd1 dolomite occurs as very fine (&lt:50 &micro:m), planar-s to nonplanar crystals: Rd2 dolomite shows planar-e to planar-s crystal shapes with fine crystal sizes (50&ndash:250 &micro:m) and is characterized by center-frog and margin-clear: Rd3 dolomite occurs as medium to coarse (250 &micro:m&ndash:2 mm), nonplanar crystals: and Cd dolomite is characterized by saddle crystals filling dissolution pores and/or fractures, translucent white color in the hand samples, and strong sweeping extinction under cross-polarized light. In areas close to reactivated basement faults (Zhangcun outcrop and well Hanshen1), Rd3 (~65% by abundance) was the dominant type of replacement dolomite and minor amounts of Rd1 and Rd2 (~10%) were found in this area. Cd (~25%) was extensively developed in fractures and dissolution pores, whereas, in areas far away from the fault zones (Xinjigu outcrop), Rd1 (~20%) and Rd2 (~55%) were dominant replacement dolomites, and only a small portion of them were recrystallized to form Rd3 (~20%), with minor Cd (~5%) dolomite occurring in some dissolution pores. The &delta:13CV-PDB (&minus:0.37&permil: to 4.32&permil:) and &delta:18OV-PDB values (&minus:7.41&permil: to &minus:5.19&permil:), 87Sr/86Sr ratios (0.707085 to 0.707795), and rare earth elements (REE) patterns (flat REE patterns with slight light rare earth element (LREE) enrichment and slight negative Ce anomalies) suggest that Rd1 dolomite was formed penecontemporaneously in an evaporitic tidal flat evaporation environment with salinities higher than seawater. The Rd2 dolomite, characterized by &delta:13CV-PDB (&minus:0.18&permil: to 4.89&permil:) and &delta:18OV-PDB values from &minus:6.6&permil: to &minus:5.5&permil:, 87Sr/86Sr ratios from 0.707292 to 0.707951, and LREE enrichment and slight negative Ce anomalies, was interpreted as forming from the recrystallization of Rd1 at shallow burial. The &delta:18OV-PDB values (&minus:12.01&permil: to &minus:8.23&permil:), the prominent positive anomaly of Eu, high 87Sr/86Sr ratios (0.7081&ndash:0.7198) and high fluid inclusion homogenization temperatures (149&ndash:255 &deg:C) suggest that Rd3 and Cd dolomite were formed from hot fluids. Based on regional stratigraphic data, the Rd3 and Cd were likely formed at depths less than 1500 m: thus, the ambient burial temperature would be lower than 85 &deg:C. The high fluid temperatures recorded by fluid inclusions, thus, indicate that the dolomitization was of hydrothermal nature. The &delta:18OV-SMOW values, homogenization temperatures, and salinities of the fluid inclusions of Rd3 and Cd in proximal areas were systematically higher than those in distal areas, suggesting that the hydrothermal fluid ascended along faults in proximal areas and then migrated laterally along the strata to distal areas. The dolomites of the Middle Permian carbonates in the southwestern Sichuan Basin, thus, resulted from different dolomitization phases, and the latter hydrothermal dolomitization was controlled by a combination of strata and structures.