Heavy Mineral Characteristics And Provenance Analysis Of The Zhiluo Formation In Northern Ordos Basin

As one of the main energy bases in China,the Ordos Basin is rich in coal,natural gas,petroleum and several other kinds of energy resources.In the recent decade,there have obtained significant breakthroughs on exploration of uranium deposits and large uranium deposits have been discovered in Zaohuohao,Nalinggou and Daying.However,former researchers do not achieve a persuasive consensus on the material provenance and metallogenic uranium provenance in the north of the basin.Thus,based on field survey and laboratory analysis,taking main metallogenic strata,Zhiluo Formation,of sandstone-type uranium deposits in the northern Ordos Basin as research object,applying petrography,mineralogy and petrology,The following observations have been made after studying the sedimentary facies of the Zhiluo Formation in the northern Ordos Basin and the sandstone lithology,heavy minerals,material sources,and ore-forming uranium sources of the Zhiluo Formation.As a result of petrological study on sandstones from the researched Zhiluo formation,the major rock type is feldspathic litharenite and lithic arkose,the compositional maturity of which are not high and indicates the characteristics of near-source deposition.Referring to former researches,the Zhiluo formation in the northern basin mainly developed depositional systems of braided river,braided river delta,meandering river and meandering river delta and testified NW-SE source direction through its evolution and thickness.Composition of detrital sandstones manifests that the primary tectonic setting of the sandstone is recycled orogens.Through observation on the shape of the heavy minerals and analysis on their combination,the crystal shape of the grinding heavy minerals is dominated by angle and hypo-edge angle.The main mineral assemblage is garnet-zircon-epidote-titanite,followed by spinel,ilmenite,apatite and monazite,which suggests that the majority of parent rocks are middle-degree and high-degree metamorphic rocks.Through the characteristic index of heavy minerals,the low ZTR index reveals that the sediment deposits from the near rocks;the high GZi index indicates that granulite facies and amphibolite facies compose part of the provenance;comparatively high ATi index demonstrates that some of the sediments are from magmatic rocks as well;the relatively low MZi index manifests that plutonic masses also works as a small part of their provenance.By U-Pb dating analysis upon the detrital zircons from the Zhiluo formation in the basin,we found that the ages of most zircons date back to the Archean era,the Paleoproterozoic era and the Phanerozoic eon,among which the Paleoproterozoic era accounts for the highest proportion.Compared with the past dating results of adjacent rocks,the sediments may come from the Archaeozoic TTG gneisses and granulites in the Yinshan block,the Paleoproterozoic khondalite series from the area of Jining-Daqing Mountains-Ural Mountains-Qianli mountains-Helan mountains and the Paleoproterozoic tonalitic gneiss granulite from the area of Guyang-Wuchuan Paleozoic igneous body formed in the Yinshan orogenic belt.Intrusive rocks of each period from the area of provenance in the northern basin and ancient metamorphosed basement possessed relatively high uranium background value,where the loss of uranium is prevalent.The loss works as the provenance pf the Zhiluo formatin and the important provenance of uranium.The minerals also contain Ti-containing minerals such as ilmenite,sphene,rutile,etc.,which are connected with the presence of uranium in space.Coffinite and brannerite frequently surrounds and replaces Ti-containing minerals,which demonstrates the Ti-containing minerals possess absorptivity for the uranium and act in the enrichment of uranium.