Ping Wang ^1* Jian Wang ¹ Yuansheng Du ² Wenchao Yu ² Qi Zhou ³ Li Tian ⁴ Liangjun Yuan ⁵ Wen Pan ⁵ Wei Wei ² Yongjun Qin ⁶ Zhixin Ma ^3,7

• ¹ Henan Polytechnic University, Jiaozuo, China
• ² China University of Geosciences, Wuhan, China
• ³ Engineering Technology Innovation Center of Mineral Resources Explorations in Bedrock Zones, Guiyang, China
• ⁴ Xiamen University, Xiamen, China
• ⁵ 103 Geological Party, Tongren, China
• ⁶ Guizhou Geological Survey, Guiyang, China
• ⁷ China Geological Survey, Chengdu, China

The Nanhua Basin of South China recorded complete Cryogenian stratigraphic sequence from the Sturtian Glaciation (~ 717–660 Ma) to the Marinoan Glaciation (~ 654–635 Ma). The interglacial Datangpo Fm in the Nanhua Basin is divided into two members, and the 1st Member consists of Mn-carbonate unit and the overlying black shale unit, containing a series of large and superlarge manganese deposits. The metallogenic process of manganese deposits is not clear, the Mn-carbonates formed through the precursor of Mn-oxide/oxyhydroxide reduction or directly precipitated from anoxic water column. Besides, the redox conditions in the deep Nanhua Basin during the precipitation of manganese deposits are also controversial. In this study, the high-resolution nitrogen contents (TN) and isotope compositions, carbon isotope compositions of organic and inorganic matter from the 1st Member of the Datangpo Fm are analyzed. The δ15N values of the Mn-carbonate unit (+1.53 ‰ to +5.26 ‰, mean +3.36 ‰) are higher than those of the overlying black shale unit (-3.74 ‰ to +3.54 ‰, mean +0.89 ‰). The Mn contents show a negative relationship with TN but a positive relationship with δ15N in the Mn-carbonate unit, implying that the formation of Mn-carbonates is related to redox variations. The relatively higher δ15N values in the Mn-carbonate unit indicated oxic conditions, and NH4+can be released and partially oxidized during the mineralization of organic matter, resulting in that the residual 15N-enriched NH4+ were transferred into clay minerals. Meanwhile, the lower δ15N values in the black shale unit indicated anoxic conditions, which recorded primary N isotope signals. The Mn-carbonate unit is characterized by negative δ13Ccarb values (-11.17 ‰ to -5.22 ‰, mean -8.30 ‰), which show a positive relationship with δ13Corg, but a negative relationship with Mn contents, implying that the negative δ13Ccarb excursions were related to the organic matter degradation during Mn-carbonate formation. The findings of this study indicated that the metallogenesis of manganese deposits in the Cryogenian Nanhua Basin was constrained mainly by the oxic interval in the deep basin. The nitrogen and carbon cycling can provide new insights into the geochemical cycling after the Sturtian Glaciation.