基于古盐度视角的大塘坡锰矿成矿机制研究^*

Metallogenic mechanism of Datangpo manganese ore from the perspective of paleosalinity

摘要

通常认为,黔东南华系大塘坡组锰矿的发育与氧化还原条件、热液输入和生物等因素密切相关。南华盆地作为一个半封闭盆地,在后冰川时期气候、环境动荡的条件下,水体的盐度可能同样会发生变化,从而影响水体的氧化还原状态并调节锰矿的形成过程,然而盐度对于锰矿形成的控制作用却一直被忽视。本研究基于古盐度指标(B/Ga),结合氧化还原等多个古环境指标,对大塘坡锰矿形成时期的水动力条件和锰来源等开展了进一步研究。结果显示,大塘坡组一段锰含量变化趋势与古盐度、氧化还原指标显著相关,揭示出古盐度在大塘坡锰矿成矿过程中具有重要作用。此外,锰含量与Eu/Eu^*、⁸⁷Sr/⁸⁶Sr和εNd(i)之间显著相关,表明南华盆地大塘坡锰矿主要为热液来源。间歇性的热液活动不仅提供了锰的来源,同时引入了营养物质,促进了初级生产力的发展、微生物的繁盛和锰矿的富集。有机碳的氧化与冰川融水的输入造成了南华盆地高碱度的水体环境(水体富含CO₃^2-和HCO₃^-),长期缺氧的环境以及海底热液活动导致水体中锰离子大量富集,两者相结合促进了盖帽碳酸盐岩的形成。因此本研究认为,较多的碳酸根离子输入、活跃的热液活动以及微生物的催化作用,是促进大塘坡锰矿富集的重要因素。

Abstract

It is widely accepted that the development of manganese deposits in the Datangpo Formation of southeastern Guizhou is closely linked to factors such as redox conditions,hydrothermal input,and biological influences. The Nanhua Basin,characterized as a semi-closed basin,may have undergone fluctuations in water salinity due to climatic and environmental disturbances during the post-glacial period. These fluctuations could impact the redox status of the water and regulate the formation processes of manganese deposits. However,the role of salinity in controlling the formation of manganese deposits has long been overlooked. This study utilizes paleosalinity proxies,particularly the B/Ga ratio,in conjunction with redox and other paleoenvironmental indicators,to investigate the paleo-hydrological conditions and sources of manganese during the formation of Datangpo manganese ore. The results indicate a significant correlation between fluctuations in manganese concentration and the evolution of paleosalinity and redox conditions in the First Member of the Datangpo Formation,highlighting the critical role of paleosalinity in controlling the formation of Datangpo manganese ores. Furthermore,pronounced correlations between manganese content and hydrothermal proxies,including the Eu/Eu^*ratio,strontium isotope ratio(⁸⁷Sr/⁸⁶Sr),and neodymium isotope variation(εNd(i)),suggest that the Datangpo manganese ores in the Nanhua Basin predominantly originate from hydrothermal sources. Episodic hydrothermal activities not only supplied manganese,but also delivered essential nutrients,thereby enhancing primary productivity. The oxidation of organic carbon and the input from glacial melting have resulted in a highly alkaline environment,rich in CO₃^2- and HCO₃^-. The oxygen-deficient conditions,combined with seafloor hydrothermal events,have led to significant accumulation of manganese ions in the water, thereby promoting the formation of cap carbonates. Consequently, this research concludes that bicarbonate ions, vigorous hydrothermal activities, and microbial catalysis are critical for the formation of the Datangpo manganese ores.

关键词

古盐度 / 锰矿 / 大塘坡组 / 水体循环 / 微生物成矿

Key words

paleosalinity / manganese ore / Datangpo Formation / water cycle / microbial mineralization

基金资助

*中国地质大学(武汉)“地大学者”科研启动经费(编号: 2022153),博士后创新人才支持计划(编号: BX20200312)共同资助