本文分析了鄂尔多斯盆地的古水文地质、古气候及地质构造的演化过程,认为现在为开放式盆地的鄂尔多斯盆地在渐新世以前具典型的渗入盆地特征.突破了前人认为该盆地为开放式盆地,不利于铀成矿的评价结论.其古水文地质条件、古气候条件及地质构造条件对发育区域层间氧化带非常有利,层间氧化带的发育期和主成矿期自中侏罗世晚期至始新世末,持续时间达115 Ma.圈定的古层间氧化带前锋线,经钻探验证,具良好的含矿性,未来该地区有望成为我国一个新的铀矿资源基地.

There are many methods of studying and mapping of lithofacies paleogeography. This paper will describe the methodology of lithofacies paleogeography that was advocated by the present author, "Single factor analysis and comprehensive mapping method", with the example from the lithofacies paleogeographic study and mapping of Majiagou Group of Middle Ordovician of Ordos Region. By " single factor" , meant is the factors which can independently reflect certain aspects of the sedimentary environments. Single Factor Analysis and Comprehensive Mapping Method includes three steps. First, earnest petrological and petrographical study of each section should be made in order to obtain various firsthand complete and reliable quantitative and qualitative data, especially quantitative data, and to understand the characteristics of sedimentary environments of each stratigraphical interval in the section. Second, select those factors which can independently reflect some aspects of sedimentary environments ( i. e, single factors) from these quantitative data. And then the percentage content of each single factor in each section of the whole area, followed the requisite mapping unit, should be calculated to draw out various corresponding fundamental single factor maps. They are mainly various isoline maps. These fundamental single factor maps can quantitatively reflect different characteristics of sedimentary environments of the slratigraphical interval of the area. This is Single Factor Analysis. Third, Having synthesized these fundamental single facotr maps and combined with other quantitative and qualitative data and other regional geology data, through all-sided analysis and comprehensive judgement, with the dross discarded and the essential selected, the false eliminated and the true retained, the lithofacies paleogeographical maps of the mapping unit of the area can be compiled finally. This is Comprehensive Mapping. The core of this method is quantification. That is, on the basis of quantitative data, starting with analysis of quantitative fundamental single factor maps, then through comprehensive analysis and judgement, finally the quantitative lithofacies paleogeographical maps can be compiled. In these lithofacies pqleogeographicai maps, the determination of each plaeogeographical unit is based on definite quantitavtive data. This type of lithofacies paleogeographical maps is therefore no loger sketchy. This makes lithofacies pqleogeographicai maps developed into a quantitative stage. The following single factors are used in this paper: thickness, terrigenous materials, shallow-water grains, penecontemporaneous dolostones, gypsum, colors and so on. On the basis of various fundamental single factor maps of each formation of Majiagou Group of Ordos Region, combined with other quantitative and qualitative data, through all- sided analysis and judgement, the lithofacies paleogeographical maps of Majiagou Age I to VI of Middle Ordovician of Ordos Region wrer drawn out. The lithofacies paleogeography of Majiagou Age I may be generalized by following: Outside the lands wre flats (dolomite flats, etc); Inside the flats were lakes (gypsum salty lakes and gypsum lakes); Outside the flats were banks (embryonkc banks); Outside the banks were seas (open seas) and outside the seas were troughs (deep-water troughs). In Majiagou Age Ⅱ, the transgression was enlarged so that the whole area was open sea except three small lands. In the open sea there were some embryonic banks and penebanks scatterred. The lithofacies plaeogeography of Majiagou Age Ⅲ and Ⅴ were similar to that of Majiagou Age I. The lithofacies paleogeography of Majiagou Age n and IV were similar to that of Majiagou AgeⅡ. Generally speaking, form Majiagou Age Ⅰ to Ⅵ, three transgressions and three regressions appearred alternatively. They constructed a series of complete cycle of transgression and regression of triple assemblage and three similar, opposite and complementary, antithetical assemblages of lithofacies paleogeography in Majiagou Age of Middle Ordovic

古地理学及岩相古地理学的研究及编图方法较多,本文以鄂尔多斯地区天然气的主要产层中奥陶统马家沟群的岩相古地理研究及编图为例,阐述笔者倡导的岩相古地理学的方法论,即单因素分析综合作图法

The author initiated and adopted the single factor analysis and multifactor comprehensive mapping method as a new methodology of quantitative lithofacies palaeogeography in order to reconstruct quantitative lithofacies palaeogeography By meaning， single factors are the factors which can independently reflect some characteristics of sedimentary environments of a stratigraphic interval of a section in an area Presence or absence as well as high or low percentage of single factors can all independently and quantitatively reflect some characteristics of sedimentary environments of the interval of the section in the area, such as water depth, water energy, nature of the sedimentary environments, etc The thickness of the interval and its specific rock types, texture constituents, mineral composition, chemical composition, fossils and their ecological assemblages, etc, can all be taken as single factorsThe single factor analysis and multifactor comprehensive mapping method include three steps First, an indepth petrological and lithofacies study of each section, especially fundamental section, should be made in order to obtain various reliable firsthand qualitative and quantitative data, especially quantitative data, and to understand the characteristics of sedimentary environments of each interval of each section Second, according to the mapping units, from the quantitative data of each section, select those factors which can independently reflect the characteristics of sedimentary environments, ie the single factors Then the statistics of the percent contents of various single factors of each mapping unit of each section in the area are made Based on these percentage data, the various quantitative single factor maps, mainly isoline maps， are composed These single factor maps can quantitatively reflect the characteristics of sedimentary environments of the mapping unit of the area from different aspects This is the single factor analysis Third, through superimposition and synthesis of these quantitative single factor maps, combined with other quantitative and qualitative data of this mapping unit of the area, with the dross discarded and the essential selected, with the false eliminated and the true retained, the quantitative lithofacies palaeogeography map of the mapping unit of the area can be composed This is the multifactor comprehensive mappingThe core of this methodology is quantification That is, based on the quantitative single factor data of each section, starting from analysis of various quantitative single factor maps, through superimposition and comprehensive analysis and judgement, finally the quantitative lithofacies palaeogeography map can be composed In such palaeogeography map, the definition of each palaeogeographic unit is based on quantitative data and quantitative single factor maps Therefore, such lithofacies palaeogeography maps are no longer sketchy and qualitative, but quantitative instead Quantification is the most important character of the author's lithofacies palaeogeography maps This makes lithofacies palaeogeography map and lithofacies palaeogeogaphy develop into the quantitative stage This is an important development in palaeogeography

笔者倡导并持续采用单因素分析多因素综合作图法这一定量的岩相古地理学方法论。单因素是能独立地反映某地区、某地质时期、某沉积层段沉积环境某些特征的因素,它的有无或含量的多少均可独立地反映该地区、该层段沉积环境的某些特征,如沉积环境水体的深浅、能量大小、性质等。某沉积层段的厚度、岩石类型、结构组分、矿物成分、化学成分、化石及其生态组合等，均可作为单因素。单因素分析多因素综合作图可分三个步骤：第一，是对各剖面尤其是各基干剖面进行认真的地层学和定量岩石学研究，取得各种第一手的定性和定量资料，尤其是定量资料，了解各剖面各沉积层段的沉积环境特征。第二，在已取得的各剖面的定量资料中，按要求的作图单位，选择出那些能独立地反映其沉积环境特征的因素，即单因素；并把全区各剖面各作图单位的各种单因素的百分含量都统计出来，作出各种相应的单因素图，主要是等值线图。这些单因素图可以从不同的侧面定量地反映该地区该沉积层段的沉积环境。这就是单因素分析。第三，把这些定量的单因素图叠加起来，并结合该地区该沉积层段的其他定量和定性资料，去粗取精，去伪存真，全面分析，综合判断，即可编制出该地区该沉积层段的定量岩相古地理图。这就是多因素综合作图。这一方法论的核心是定量，即以各剖面的定量单因素资料为基础，从各单因素图的分析入手，再通过各单因素图的叠加和综合分析判断，最后作出定量的岩相古地理图。在这种岩相古地理图中，各古地理单元的确定都有确切的定量资料和单因素图为依据。定量是笔者等用单因素分析多因素综合作图法编制出的岩相古地理图的最大特色。这就使岩相古地理图和岩相古地理学发展到了定量的阶段。这在古地理学中是个重大的进展。

在分析前人对鄂尔多斯盆地东北部铀成矿模式研究的基础之上,对该区铀成矿控矿规律进行了详细研究,提出了鄂尔多斯盆地东北部4阶段铀成矿模式,即:①潜水氧化板状矿体形成阶段(中侏罗世直罗晚期);②早期层间氧化作用及主要卷状矿体形成阶段(中侏罗世直罗期末-早白垩世伊金霍洛初期);③油气还原保矿阶段(早白垩世伊金霍洛期末-东胜期初);④晚期层间氧化铀矿体改造叠加阶段(中新世至今).指出研究区存在2期古层间氧化带和1期新层间氧化带砂岩型铀矿床,当各期层间氧化带相互叠合时,矿化明显加大变富,并认为研究区有利远景地段位于直罗组下段古河道两侧及下游迎水面一带,断层发育地段也是铀矿找矿的重点部位.

鄂尔多斯盆地北部东胜地区砂岩型铀矿主要产在侏罗系直罗组地层中.直罗组地层可分为上、下两段:下段又分为上、下两个亚段,其中下亚段主要为容矿的灰色砂岩;上亚段主要为灰绿色砂岩、泥岩;上段为杂色的中细砂岩、泥岩.笔者通过对直罗组砂岩的镜下观察研究,在垂向上确立了岩石的后生蚀变分带:直罗组下段下亚段为灰色-灰白色砂岩(含矿段)的绿化(Ⅰ)(主要为长石的绿帘石化)+碳酸盐化带;上亚段为灰绿色砂岩的绿化(Ⅱ)(主要为黑云母绿泥石化,也有长石绿泥石化和绿帘石化)+粘土化+碳酸盐化带;直罗组上段杂色细砂岩的褐铁矿化+粘土化+碳酸盐化带.主要的后生蚀变作用发生在灰绿色的砂岩中.砂岩呈现灰绿色的原因主要在于含较多绿色黑云母,而非绿泥石,其本质原因在于矿物中Fe2+含量较高.砂岩层中的后生蚀变现象与铀成矿作用关系密切.

东胜铀矿床产于中侏罗统直罗组下段的辫状河道沉积砂体中,铀矿体受灰色砂岩与灰绿色砂岩的接触带控制.灰绿色砂岩的化学成分中硅酸盐矿物二价铁含量高是该类型砂岩呈现绿色的重要原因.在矿物组成上,灰绿色砂岩主要表现为粘土矿物总量高,特别是绿泥石含量高.岩石学、矿物学和地球化学的证据表明,东胜铀矿区存在后生还原作用.灰绿色砂岩是控矿的古氧化岩石遭受还原性流体改造的产物,后生还原作用掩盖了古氧化蚀变带,并在一定程度上起到了保矿作用.东胜地区后生还原作用在时间上和成因上与河套断陷盆地的产生和形成具有密切联系.铀在矿石中主要呈铀矿物和吸附形式存在.通过电子探针和X-射线粉晶分析鉴定,确定东胜铀矿床矿石中的主要铀矿物为铀石,并含有少量钛铀矿和沥青铀矿.富矿石样品中铀石单矿物的铀铅同位素年龄集中分布于20～10 Ma之间.通过研究,建立了鄂尔多斯盆地东胜铀矿床的"多阶段铀成矿模式",即"成岩期预富集-层间渗入成矿-再改造富集-油气还原保护"的多阶段、多成因流体长期富集改造的铀成矿模式.

鄂尔多斯盆地位于华北地台西部.在详细分析地层接触关系、构造界 面特征、地层发育特征和连续性、盆地沉积与周边构造的关系、物源区性质、沉积体系配置与相分布、古水流样式等各种要素的基础上,重建了鄂尔多斯盆地侏罗纪 至白垩纪的原始面貌,并将其演化划分为4个阶段.其中,第一阶段(早侏罗世至中侏罗世早期)和第二阶段(中侏罗世Bathonian-Callovian 期)沉积盆地的范围和沉积体系的配置关系大体类似,两者最为重要的区别是前者有聚煤作用发生,后者没有煤层发育,其原因是古气候变化造成的;第三阶段(晚 侏罗世)盆地收缩至桌子山-石沟驿-六盘山-线东侧;第四阶段(早白垩世)盆地的范围有所扩大,但东界可能被限制于现今的吕梁山以西.

近年来,鄂尔多斯盆地在石油和天然气勘查取得重大进展的同时,在 盆地北部东胜地区也发现了可地浸砂岩型铀矿床,标志着我国地浸砂岩型铀矿勘查取得了重要突破.从微量元素地球化学入手,初步总结了东胜地区砂岩型铀矿的微 量元素地球化学特征,与新疆伊犁和中亚典型的层间氧化带型砂岩铀矿进行了对比,分析探讨了铀成矿作用,认为东胜砂岩型铀矿成矿作用虽与氧化还原过渡带有 关,也具外生和深源双重性质的微量元素组合,不同于单一的外生水成层间氧化带型砂岩铀矿,铀成矿过程中有深部物质和含煤层气或油气流体参与.