鄂尔多斯盆地南北缘煤中金属异常富集成因对比

Comparison of genesis of abnormal enrichment of metals in coal betweenthe northern and southern margins of Ordos Basin

鄂尔多斯盆地含煤地层中蕴藏丰富的金属元素,是煤中金属分布规律及成因研究的热点区域,尤以盆地南北缘煤中金属最为富集,且表现出不同的规律和特征。通过对南缘有代表性的渭北、黄陇煤田和北缘准格尔煤田进行采样分析,结合收集已有测试数据,对比南北缘煤中金属富集的种类、丰度、煤岩特征及差异、成煤环境差异、物源特征,分析盆地南北缘煤中金属富集成因差异。结果表明,与世界煤相比,盆地北部准格尔煤田煤中Li,Zr,Hf富集,Be,Ga,Sr,Nb,Pb,Th轻度富集,盆地南部黄陇煤田煤中Sr,Ba轻度富集,渭北煤田煤中Li富集,Ga,Zr,Nb,Cs,Hf,Ta,Pb,Th轻微富集。盆地晚古生代煤显微煤岩组分,南部镜质组含量大于北部,总体上山西组镜质组含量低于太原组,中生代延安组煤镜质组含量最低。煤中矿物以黏土矿物和碳酸盐矿物为主,在盆地南缘煤中发现磷灰石、锐钛矿,还发现有不规则形状的碎屑石英和蠕虫状高岭石,为火山灰输入含煤地层特征;北缘煤中发现丰富的方解石晶体,以裂隙填充及自生形态为主。盆地北部和盆地南部Al_(2)O_(3)/TiO_(2)(含量比)均在8~70,表明源岩为中性-长英质物质,Nb/Y-Zr/TiO_(2)分布表明,中性-长英质火山岩向鄂尔多斯盆地南北部提供了大量的陆源碎屑。盆地北部煤样与阴山古陆钾长花岗岩、本溪组铝土矿配分模式相似,表明其源岩为阴山古陆钾长花岗岩和本溪组铝土矿;盆地南部煤样表现出与秦岭古陆中性-长英质火山岩相似的稀土元素配分模式,均为稀土元素分异不明显,表明其物源来自秦岭古陆。盆地北部煤中Sr/Ba(含量比)为0.07~7.78,均值3.49,表明其主要受海水影响;盆地南部渭北煤中Sr/Ba为0.60~6.25,均值2.77,表明其成煤过程也主要受海水影响。盆地南部黄陇煤田煤中Sr/Ba为0.60~1.17,均值0.78,表明其成煤过程主要受淡水影响。北部准格尔煤田煤的主要物源为阴山古陆中元古代钾长花岗岩和本溪组风化壳铝土矿,由于源岩的碎屑物质供给,煤中元素含量总体偏高,成煤过程受一定程度的海水影响,为阴山古陆陆源碎屑供给-泥炭形成环境复合型成矿模式;南部煤田主要物源为秦岭古陆的中性-长英质火成岩,由于源岩的碎屑物质供给,煤中Ga,Li,Bi,Co等轻度富集,南缘煤的物源主要来自秦岭古陆,基本不受本溪组铝土矿影响,因此该区煤中发现的磷灰石矿物可能与火山成因的物质有关,为秦岭古陆陆源碎屑供给-酸性火山灰复合型成矿模式。

Ordos Basin is rich in metal elements found in its coal-bearing strata.It is a hot spot for the research on the distribution law and genesis of metals in coal,especially in the north and south edges of the basin.The metals in these areas are enriched and show different laws and characteristics.Based on the systematic sampling,testing,and analysis of the representative coal samples from the Weibei and Huanglong coalfields in the southern margin and the Junger coalfield in the northern margin,combined with the collection of existing test data,this paper compares the types,abundances,coal petrographic characteristics and differences,coal-forming environment differences and provenance characteristics of metal enrichment in coals from the northern and southern margins of the basin,and analyzes the genetic differences of metal enrichment in coals in the northern and southern margins of the basin.The results show that,compared with the world coals,the coals in the Junger coalfield in the northern part of the basin are enriched in Li,Zr,Hf,and slightly enriched in Be,Ga,Sr,Nb,Pb and Th.The coals in the Huanglong coalfield in the southern part of the basin are slightly enriched in Sr,Ba,and the coals in the Weibei coalfield in the southern part of the basin are enriched in Li and slightly enriched in Ga,Zr,Nb,Cs,Hf,Ta,Pb,and Th.The vitrinite content of coal macerals in the late Paleozoic coals in the basin is higher in the south than in the north.The vitrinite content in the Shanxi Formation is lower than that in the Taiyuan Formation generally,and the vitrinite content of coals in the Yan’an Formation in the Mesozoic is the lowest.The minerals in the coals are mainly clay minerals and carbonate minerals.Apatite,anatase,irregularly shaped clastic quartz and vermicular kaolinite are found in the coals in the southern margin of the basin,which characterizes volcanic ash input into coal-bearing strata.Abundant calcite crystals were found in the coal in the northern margin,mainly in the form of fissure filling and authigenic morphology.The Al_(2)O_(3)/TiO_(2) content ratio in the north and south of the basin are all between 8-70,indicating that the source rocks are intermediate-felsic materials.The distribution diagram of Nb/Y-Zr/TiO_(2) shows that the intermediate-felsic volcanic rocks provide a large part of terrigenous debris to the south and north of Ordos Basin.The distribution patterns of coal samples in the northern basin are similar to those of K-feldspar granite in the Yinshan ancient land and bauxite in the Benxi Formation,indicating that their source rocks are K-feldspar granite in the Yinshan ancient land and bauxite in the Benxi Formation.The REE distribution pattern of the coal samples in the southern basin is similar to that of the intermediate-felsic volcanic rocks in the Qinling ancient land,which has no obvious REE fractionation,indicating that their provenance comes from the Qinling ancient land.The Sr/Ba content ratio of coals in the northern basin range from 0.07 to 7.78,with an average value of 3.49,indicating that it is mainly affected by seawater.The Sr/Ba contents ratio of coals in Weibei in the southern basin range from 0.60-6.25,with an average contents of 2.77,which indicates that the coal-forming process in this area is mainly affected by seawater.The Sr/Ba content ratio of coal in the Huanglong coalfield in the southern basin range from 0.60 to 1.17,with an average value of 0.78,which indicates that the coal formation process in this area is mainly affected by freshwater.The main provenance of coals in the north of the Junger coalfield is the Mesoproterozoic K-feldspar granite in the Yinshan ancient land and the crust of weathering bauxite in the Benxi Formation.Due to the supply of clastic materials from the source rocks,the content of elements in coals is generally high.Combined with the affection of seawater to a certain extent in the coal-forming process,its complex metallogenic model of the Junger coalfield is terrestrial debris supply and peat-forming environment in the Yinshan ancient land.The main provenance of coals in the southern field is intermediate-felsic igneous rock in the Qinling ancient land.Because of the supply of detrital materials from the source rocks,the coals in this area are slightly enriched in elements such as Ga,Li,Bi,and Co.The provenance of coal in the southern margin is mainly the Qinling ancient land and is basically not affected by bauxite in the Benxi Formation.Therefore,the apatite minerals found in the coal in this area may be related to volcanic origin materials,and it is a complex metallogenic model of terrestrial debris supply and acid volcanic ash in the Qinling ancient land.