贵州铜仁坝黄牛蹄塘组黑色岩系有机质富集机理

Enrichment mechanism of organic matter in black rock series of the Niutitang Formation in Bahuang,Tongren,Guizhou

中国扬子板块早寒武世发育一套富含有机质的黑色岩系,黑色岩系不仅是烃源岩,而且与众多金属、非金属矿床的形成有关,但关于影响其有机质富集的主控因素尚存在较大争议。对贵州铜仁坝黄地区牛蹄塘组黑色岩系样品进行了总有机碳(TOC)、主量、微量及稀土元素测试,解析了其有机质的分布规律、影响因素及其富集机理。研究结果表明:黑色岩系有机碳含量从底部→下部-中部→上部呈现出高→低→高的变化趋势;氧化还原指标Ni/Co、V/Cr、V/(V+Ni)和U/Th及判定盆地水体滞留程度的指标Mo/TOC,分别指示了牛蹄塘组底部与上部富含有机质的炭质页岩、富多金属元素页岩形成于贫氧-缺氧、中等-强局限滞留环境,而下部-中部磷矿层则形成于贫氧-富氧、中等滞留的沉积环境;δCe表明研究区在早寒武世经历过一次海侵-海退的过程;Ba bio揭示了牛蹄塘组底部与上部的高古生产力以及下部-中部的中等-高古生产力;Ti/Al指示整体较弱的陆源输入;稀土元素配分模式、δEu异常以及La/Yb-REE图解说明磷矿层和富多金属元素页岩层受到了热液活动的影响,热液活动为富多金属元素页岩层有机质的富集带来了丰富的营养物质和还原的沉积环境,而磷矿层形成于海平面上升期,洋流上涌导致沉积环境充氧,不利于有机质的保存,故下部-中部磷矿层有机质含量较低。总之,中等-高的古生产力是有机质富集的重要基础,热水活动为局部地层(富多金属元素页岩层)有机质的富集创造了有利条件,陆源碎屑输入对有机质富集的影响较小,而最终制约牛蹄塘组黑色岩系有机质富集的主控因素是普遍还原滞留沉积环境,有机质的富集更倾向于“保存模式”。

[Objective]A set of black rock series rich in organic matter developed in the Yangtze Plate of China in the Early Cambrian.[Methods]To explore the main controlling factors affecting the enrichment of organic matter,the samples of the Niutitang Formation in Bahuang,Tongren Guizhou Province were tested for total organic carbon(TOC),major,trace and rare earth elements.[Results]The results show that the organic carbon content of the black rock series show a trend of high→low→high from bottom→lower middle→upper.The redox indexes Ni/Co,V/Cr,V/(V+Ni)and U/Th and the index Mo/TOC for determining the retention degree of water in the basin indicate that the carbonaceous shale and polymetallic element shale rich in organic matter at the bottom and upper part of the Niutitang Formation are formed in oxygen-poor to anoxic,medium to strong limited retention environments.The lower middle phosphate rock layer is formed in oxygen-poor to oxygen-rich,medium retention sedimentary environment.TheδCe shows that the study area experienced a process of transgression regression in the Early Cambrian.Ba bio reveals the high paleoproductivity at the bottom and upper part of the Niutitang Formation and the medium to high paleoproductivity at the lower-middle part.The Ti/Al indicates weak terrigenous input as a whole.Rare earth distribution mode,δEu anomaly and La/Yb-REE diagram show that the phosphorus ore bed and metal-rich shale are affected by hydrothermal activity,which brings rich nutrients and reducing sedimentary environment for the enrichment of organic matter in metal rich shale.The phosphorus ore bed was formed in the period of sea level rise,and the upwelling of ocean currents led to the oxygenation of the sedimentary environment,which was not conducive to the preservation of organic matter.Therefore,the content of organic matter in the lower middle phosphorus-bearing formation is low.[Conclusion]The key factor restricting the enrichment of organic matter in the black rock series of the Niutitang Formation is the generally reducing and retained sedimentary environment,and the medium-high paleoproductivity is an important basis for the enrichment of organic matter.Hydrothermal activity creates favorable conditions for the enrichment of organic matter in local strata(polymetallic enrichment layer),and terrigenous clastic input has little impact on the enrichment of organic matter.The rise in sea level is an unfavorable factor for the enrichment of organic matter.It is the"comprehensive model"under the joint action of multiple factors that control the enrichment of organic matter in the black rock series of the Niutitang Formation.