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Carbonate minerals (calcite and dolomite) and pyrite are important brittle minerals in organic-rich shales; however, the mechanism of their formation is still uncertain. This study examined the formation of pyrite and carbonate minerals (mainly calcite and dolomite) in the shales of Longmaxi (LMX) Formation through a combination of petrological and geochemical techniques. The results demonstrated that two types of carbonate minerals (calcite and dolomite) and multiple-types of pyrite were identified in the studied shales. The carbonate minerals comprise (1) subhedral to euhedral calcite and dolomite with large diameters (>10 μm), along with low Fe (<2%) and Mn (<0.2%) contents, and (2) calcite and dolomite cement, along with high Fe (>2%) and Mn (>0.2%) contents. The pyrite is primarily composed of framboidal pyrite, infilled framboidal pyrite, pyritic masses, and euhedral pyrite. This study suggests that depositional and diagenetic processes had significant influences on the formation of calcite-dolomite-pyrite in organic-rich shales. In the synsedimentary stage, the expansion of anoxic seawater contributed to the enrichment of organic matter in the shales of lower LMX Formation. In the early diagenesis stage, both the microbial sulfate reduction (MSR) and sulfate-driven anaerobic oxidation of methane (SD-AOM) would generate abundant CO2, which together with the consumption of further contribute to the precipitation of carbonate minerals (calcite and dolomite). In addition, the MSR would also release 32S-enriched H2S and result in the formation of framboidal pyrite/infilled framboids/pyritic masses with low δ34S values (<0‰). However, the SD-AOM would result in the precipitation of euhedral pyrite with high δ34S values (>20‰) and high Co contents (>700 μg/g). In the late diagenesis stage, carbonate cements with high Fe and Mn contents formed due to the diagenetic fluids (high
Mg2+, Fe2+, Mn2+ concentrations). Overall, the sulfate-driven anaerobic oxidation of methane (AOM) during early diagenesis in sediments constitutes a key process influencing methane release, in which case extensive methane release during this period may have contributed to Early Silurian global warming.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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