扬子北缘埃迪卡拉纪锰矿层中的古氧化还原条件对锰矿成矿过程的影响*

王萍; 崔旭; 赵海彤; 杜远生; 栾进华; 张雄; 罗旭; 王健

doi:10.7605/gdlxb.2025.033

古地理学报 ›› 2025, Vol. 27 ›› Issue (2) : 338 -350. DOI: 10.7605/gdlxb.2025.033

沉积矿产资源专题

扬子北缘埃迪卡拉纪锰矿层中的古氧化还原条件对锰矿成矿过程的影响^*

王萍 ¹ ,
崔旭 ¹ ,
赵海彤 ² ,
杜远生 ³ ,
栾进华 ² ,
张雄 ²^,⁴ ,
罗旭 ²^,⁴ ,
王健 ¹

作者信息 +

Redox conditions of the Ediacaran in northern margin of Yangtze Block and their implications for metallogenic processes

Ping WANG ¹ ,
Xu CUI ¹ ,
Haitong ZHAO ² ,
Yuansheng DU ³ ,
Jinhua LUAN ² ,
Xiong ZHANG ²^,⁴ ,
Xu LUO ²^,⁴ ,
Jian WANG ¹

Author information +

文章历史 +

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摘要

扬子板块北缘埃迪卡拉纪锰矿赋存于陡山沱组上部的黑色页岩中,主要分布在城口次级盆地的边缘,目前对于锰矿形成时的氧化还原条件以及成矿模式等问题仍然存在争议,需要进一步研究。对于锰矿的成矿模式,目前主要有2种观点: 一种认为菱锰矿直接从缺氧水体中沉淀,另一种认为水体中的Mn²⁺首先被氧化,然后在沉积物中被还原成菱锰矿。本研究以陡山沱组上部的锰矿层为研究对象,对其进行系统的矿物组成和地球化学分析。结果显示,锰矿层中发现了少量的水锰矿,而水锰矿是一种锰的氧化物矿物,意味着在含锰矿物形成时,存在锰的氧化物中间产物,最终转化为菱锰矿。此外,锰矿层中TOC和TN具有明显的正相关关系,表明地层中的氮主要来源于有机质; TOC与Mn含量具有明显的负相关关系,表明锰矿形成过程中需要消耗有机质; 锰矿层具有很高的 δ¹⁵N 值(+7.22‰~+10.06‰,平均值+9.22‰),可能与有机氮的硝化作用有关,¹⁴N富集的NH₄⁺优先被氧化,导致剩余有机氮具有较高的 δ¹⁵N 值,反映了当时表层水体处于氧化状态。因此,认为陡山沱组锰矿经历了Mn²⁺被氧化然后被还原成菱锰矿的阶段,其成矿过程受到水体氧化还原状态的影响。该研究成果有助于进一步了解埃迪卡拉纪海洋的生物地球化学循环过程。

Abstract

The Ediacaran manganese deposits located in the northern margin of the Yangtze Block are found within the black shales of the upper Doushantuo Formation,predominantly occurring along the margin of the Chengkou Sub-basin. However,several key issues remain contentious,such as including the redox conditions and the metallogenic processes,which quire further investigation. Two primary metallogenetic models have been proposed: (1)rhodochrosite precipitates directly from an anoxic water column;and(2)dissolved Mn²⁺ is initially oxidized to Mn-oxides,which are then reduced to rhodochrosite within the sediments. This study focuses on the Mn-bearing layers of the Doushantuo Formation,analyzing the high-resolution mineral compositions and geochemical characteristics. The results reveal the presence of minor amounts of manganite,a Mn-oxide mineral,within the manganese deposits. This finding indicates that Mn-oxides likely served as a precursors in the formation of rhodochrosite formation. Additionally,a significant positive correlation between total organic carbon(TOC)and total nitrogen(TN)indicates that nitrogen in the sediments primarily originates from organic matter. Conversely,a negative correlation between TOC and manganese(Mn)content implies that organic matter is consumed during the process of manganese deposition. The Mn-bearing layers are characterized by elevated δ¹⁵N values(+7.22‰ to +10.06‰,mean +9.22‰),which can be attributed to the nitrification of organic nitrogen. During this process,¹⁴N-enriched NH₄⁺ is preferentially oxidized,resulting in an increase in the δ¹⁵N of residual organic nitrogen. This implies that oxic conditions prevailed in the surface waters during the formation of rhodochrosite formation. Therefore,it is concluded that the manganese ore of the Doushantuo Formation underwent a stage of Mn²⁺ oxidation followed by reduction to rhombosite,with its mineralization process being influenced by the redox state of the water. This research further contributes to the understanding of the biogeochemical cycles in the Ediacaran oceans.