贵州省松桃县道坨超大型锰矿床的发现及其成因探讨

Geological and geochemical characteristics of Daotuo superlarge manganese ore deposit in Songtao County of Guizhou Province: Constraint on formation mechanism of Mn_carbonate ores

贵州省松桃县道坨锰矿床是新发现的一个超大型全隐伏锰碳酸盐矿床。文章阐述了该矿床的发现概况及基本的地质和地球化学特征,并应用锰矿石和含锰黑色页岩的元素和碳同位素地球化学特征对菱锰矿的形成机制提出了制约。道坨超大型锰矿床的发现是填图及对区域地质资料综合分析的结果。该矿床具有品位高、厚度大、呈层性好及展布广等特点。其锰矿石的主量元素特征为Al2O3、TiO2、Fe2O3含量较低,P2O5中等程度富集,MnO、MgO含量相对较高,Fe/Mn比值低。在微量元素方面,锰矿石显示出较为明显的Co富集,含锰黑色页岩则显示出较为明显的Co、Mo富集;在稀土元素方面,锰矿石具有较高的稀土元素总量,轻微的"帽式"稀土元素PAAS标准化配分模式及明显的Ce正异常,含锰黑色页岩的稀土元素总量与PAAS接近,其稀土元素PAAS标准化配分模式较为平坦,无Ce异常。碳同位素测试结果显示出菱锰矿富集碳的轻同位素,表明在菱锰矿形成过程中存在有机碳的参与。文章表明,道坨锰矿床内的锰是以氧化物或氢氧化物的形式沉淀,菱锰矿是在缺氧且富含有机物质的成岩环境中转化而成。

The Daotuo manganese ore deposit in Songtao County of Guizhou Province is a newly discovered superlarge and completely concealed Mn-carbonate ore deposit. This paper has reported the discovery history and basic geological and geochemical characteristics of this ore deposit with the purpose of understaning the formation mechanism of rhodochrosite ores. The Daotuo ore deposit should owe its discovery to field mapping and comprehensive analysis of existing regional geological data. This deposit is characterized by high grade Mn, large thickness, and good succession and expansion. The major elements show low Al2O3, TiO2, and Fe2O3 content, moderate P2O5 content, relatively high MnO and MgO content, and low Fe/Mn ratios. The trace elements of rhodochrosite ores show enrichment of Co, and the Mn-bearing black shale series show enrichment of Co and Mo. The REE values of rhodochrosite ores are characterized by relatively high total REE content, relatively moderate extent of REE enrichment compared with light REE and heavy REE, and obvious positive Ce anomalies, while the REE values of Mn-bearing black shale series show similar total REE content to PAAS, flat PAAS-normalized REE distribution patterns and no obvious Ce anomalies. The carbon isotopes of rhodochrosite ores display depletion of heavier isotopes, suggesting that organic carbon was involved in the formation of rhodochrosite. By analyzing the elemental and isotopic fingerprints preserved in rhodochrosite ores and Mn-bearing black shales, the authors hold that, although now preserved as Mn-earbonate minerals, these deposits were initially precipitated as oxides or hydroxides, which were reduced and converted to Mn carbonates in a diagenetic environment coupled with oxidation of organic matter.