Hydrothermal–Microbial Controls on Carbonate Diagenesis and Organic Matter Enrichment in a Lacustrine System: Evidence from the Upper Bayingebi Formation, Yin’e Basin

Microbe–mineral interactions in lacustrine environments play a critical role in controlling carbonate diagenesis and preserving organic matter, particularly under the influence of hydrothermal processes. To improve the understanding of such processes, this study focuses on the diagenesis of different types of carbonates from the upper section of the Bayingebi Formation in the Yin’e Basin, revealing the association between lacustrine sedimentation and hydrothermal activity. According to mineralogical and geochemical evidence, the carbonates in the studied interval can be broadly classified into hydrothermal, hydrothermal-biogenic, and sedimentary types on the basis of their dominant genetic signatures. Hydrothermal carbonates are dominated by crystalline dolomite, with associated hydrothermal minerals rich in Fe, Ba, and Mg, while 87Sr/86Sr values are close to mantle source values, indicating substantial mantle hydrothermal fluid contributions. Hydrothermal-biogenic carbonates are dominated by mud-crystalline and spherical dolomite enriched in Mg, Na, and P. The 87Sr/86Sr values fall between the typical mantle-derived and paleolake water ranges, indicating a mixed hydrothermal signal, with possible microbial involvement indicated by mineralogical and textural features. Sedimentary carbonates are predominantly crystalline calcite, with 87Sr/86Sr values close to crustal source values, indicating the influence of terrestrial inputs. Moreover, there are significant relationships between different types of diagenetic carbonates and organic matter. Intense hydrothermal activity is associated with low TOC values and relatively unfavorable conditions for organic matter preservation. In contrast, sections with mild hydrothermal activity have higher TOC contents, and the nutrient contents and water temperatures are more conducive to enrichment with organic matter. Although based on a single fully cored borehole from the upper section of the Bayingebi Formation, this study provides a framework for understanding the coupling between carbonate diagenesis and organic matter enrichment in hydrothermally influenced lacustrine systems, with implications for future studies involving multi-core investigations across the basin.