扬子西缘"平溪式"锰矿沉积成因及其找矿潜力分析OA

[Objective]Sedimentary Mn-deposits occur in the middle-lower Cambrian Qiujiahe Formation in the Ningqiang–Qingchuan–Pingwu–Beichuan region along the western Yangtze margin.These deposits,commonly referred to as"Pingxi-type"Mn-deposits,were formed through regional metamorphism and supergene oxidation of primary sedimentary Fe–Mn ores.Investigating the genesis of Mn-carbonate minerals is therefore critical for understanding the mineralization processes of these deposits and contributes to regional mineral exploration and prospectivity assessment.[Methods]This study focuses on the Tianliangshang Mn exploration area at Chenjiaba,Beichuan County.Sedimentological,mineralogical,and geochemical data were integrated with previously published data from adjacent Mn-deposits to investigate the precipitation mechanism of Mn carbonate minerals.[Results]Fe/Ti,(Fe+Mn)/Ti,Al/(Al+Fe+Mn),and SiO2/Al2O3 ratios,together with Eu anomalies,indicate that the manganese was mainly derived from seafloor hydrothermal sources.Framboidal pyrite laminae and thin layers developed in the Mn ores,indicating that Fe–Mn separation preceded Mn carbonate precipitation.However,the relatively high Fe/Mn ratios suggest that this separation was incomplete.Mn carbonate minerals coexist with framboidal pyrite grains smaller than 5 μm,indicating they precipitation under persistently anoxic water-column conditions.The Mn carbonate minerals are characterized by microcrystalline textures and commonly contain rhombic,Ca-rich cores with dissolution textures.The above evidence indicate that Mn carbonate precipitation was primarily induced by calcite.[Conclusions]The results indicate that Mn carbonate minerals in the Qiujiahe Mn deposits precipitated directly from anoxic,Mn-rich waters.Authigenic calcite was introduced into the Mn-rich reducing waters and subsequently dissolved.This dissolution released bicarbonate ions and increased water alkalinity.The residual calcite provided nucleation sites for extensive Mn carbonate precipitation.This process resulted in the formation of zoned textures.The Tianliangshang Mn exploration area contains an identified Mn ore resource of 1.42×107 t and is therefore classified as a medium-sized deposit.However,the deeper part of the main orebody has not yet been fully delineated.The equivalent stratigraphic horizon in the surrounding areas also remains prospective,particularly in favorable shelf-slope and depositional-trough settings,with considerable potential for orebody extension and additional resource discovery.