Multi-stage dolomitization controlled by sedimentary facies and basement faults: Insights from the Middle Permian Maokou Formation of the Eastern Sichuan Basin, SW China

Abstract

The Middle Permian Maokou Formation in China's eastern Sichuan Basin has undergone multiple episodes of dolomitization, recrystallization, and cementation. However, a consensus on the controlling factors for dolomite formation has yet to be reached. This study adopted a multidisciplinary approach, encompassing petrological, geochemical (C-O-Sr stable isotopes, trace elements, and rare earth elements), and fluid inclusion thermometry analyses to determine the spatial distribution characteristics and diagenetic influencing factors of various dolomite types. Core, outcrop, and thin-section observations have identified three types of replacive dolomites and one type of cementing dolomite: very fine crystalline, nonplanar-a to planar-s dolomite (RD1); fine to coarse crystalline, nonplanar-a to planar-s dolomite (RD2-a); fine to coarse crystalline, planar-e dolomite (RD2-b); and medium to coarse crystalline, nonplanar saddle dolomite (CD). All dolomites exhibit more negative δ¹³C and δ¹⁸O values than the host limestone, with ⁸⁷Sr/⁸⁶Sr ratios exceeding the range of Permian seawater. RD1 has negative or no anomaly in the Eu element and shows higher Sr content with lower Ba content, indicating that it was not affected by hydrothermal fluids. RD2-a and CD show positive anomalies in Eu, with lower Sr content and higher Ba content, indicating that both were affected by hydrothermal fluids. RD2-b, although showing negative or no anomaly in Eu, has fluid inclusion homogenization temperatures comparable to CD and also has less Sr with more Ba, suggesting that its formation also involved hydrothermal fluids. All dolomites exhibit a clear negative anomaly in the Ce element. By synthesizing petrological and geochemical findings, a multi-stage dolomitization model was proposed, which includes seepage-reflux, thermal convection, and hydrothermal dolomitization, reflecting the formation mechanisms of various dolomite types. Before the activation of the basement faults (SQ1 ∼ SQ2-TST), RD1 was formed by limited seepage-reflux dolomitization in low-energy shoals and small-scale confined environments on the periphery of open seas, with dolomitization fluids being contemporary seawater and a small amount of meteoric water. In the early stage of basement fault activity (SQ2-HST), abnormal geothermal temperatures promoted the thermal convection dolomitization between hydrothermal fluids and external cold contemporary seawater in high-energy and low-energy shoals, leading to the formation of RD2-a, with dolomitization fluids being contemporary seawater and a small number of hydrothermal fluids. In the middle stage of basement fault activity (SQ3), silica-rich hydrothermal fluids rose along the faults and mixed with contemporary seawater, replacing the previously formed dolostones and host limestone to form RD2-b. In the peak stage of basement fault activity (the end of Guadeloupe), deep hydrothermal fluids were fully transported in high-permeability layers and precipitated dolomite cement (CD) in cavities and fractures near the basement faults. The dolomitization fluids for both RD2-b and CD were hydrothermal fluids mixed with a small amount of contemporary seawater. The model delineated within this paper encapsulates the principal controlling factors governing dolomite formation, thereby facilitating a comparative analysis of the genesis of dolomites from various global locales.