Objective In recent years, significant breakthroughs have been achieved in the exploration of sedimentary manganese deposits within the Longmenshan tectonic belt. However, problems such as an unclear sedimentary environment and ambiguous manganese enrichment mechanisms persist. To address these issues, this study focuses on the Maowanli manganese deposit in the Longmenshan tectonic belt to further investigate the regional sedimentary environment and enrichment mechanisms.

Methods Based on detailed field geological surveys and indoor thin−section identification, the morphological characteristics of pyrite in the ore−bearing rock series were systematically documented, and statistical analysis of pyrite particle size was conducted.

Results The ore−bearing rock series of the Maowanli manganese deposit belongs to the First Member of the Lower Cambrian Qiujiahe Formation. The lithology primarily includes siliceous rocks, black slate, dolomite, manganese−bearing black siliceous dolomite, pyrite layers, and manganese−bearing rock series. The primary ore minerals in the manganese−bearing series are dolomite and alabandite, with minor amounts of Mn−calcite and rhodochrosite. The gangue minerals consist of quartz, dolomite, and pyrite, with small amounts of collophane, barite, and organic matter. Framboidal pyrite is well−developed in the manganese−bearing series. The statistical results of particle sizes are as follows: in the manganese−bearing series, it ranges from 1.38 μm to 31.05 μm, with an average of 5.18 μm; in the pyrite layers, from 1.60 μm to 28.42 μm, averaging 6.73 μm; in black slate, from 1.31 μm to 31.34 μm, averaging 6.60 μm; in manganese−bearing black siliceous dolomite, from 3.14 μm to 23.20 μm, averaging 6.74 μm; in dolomite, from 0.88 μm to 7.03 μm; and in siliceous rocks, from 1.73 μm to 22.83 μm, averaging 6.03 μm. Overall, the ore−bearing series was deposited under dysoxic−subxic−euxinic bottom water conditions.

Conclusions Given that the transformation from Mn(Ⅱ) to Mn(Ⅳ) occurs at the oxic−dysoxic interface and no manganese oxides are observed in the ore, it is inferred that manganese enrichment and precipitation are closely related to BSR (bacterial sulfate reduction) and EPS (extracellular polymeric substances): ① Under dysoxic−suboxic bottom water conditions, biological organic matter settled and underwent BSR, releasing CO₃²⁻ and H₂S into the pore water and bottom water; ② EPS adsorbed Mg²⁺, Mn²⁺, Ca²⁺, and CO₃²⁻, providing nucleation sites for kutnohorite and inducing its massive precipitation, followed by further epitaxial growth during burial; ③ H₂S released via BSR into the bottom water preferentially combined with Fe²⁺ to form framboidal pyrite, while the remaining H₂S in the pore water combined with Mn²⁺ to form alabandite.