豫北卫辉地区寒武系馒头组含铁鲕粒的特征及形成机制OA北大核心CSTPCD

Oolitic limestone is an important type of carbonate rocks in the Cambrian period,with cal-cite and dolomitic ooids being the most common.While there have been numerous studies on the formation of these ooids,the occurrence of iron-bearing ooids in the Cambrian is rare and the formation mechanism re-mains unclear.The iron-bearing ooids of the Cambrian Mantou Formation in the Weihui area of Henan Pro-vince were systematically analyzed using sedimentology,sedimentary paleogeography,mineralogy and geo-chemistry.They can be divided into microsparry iron-bearing ooids and sparry iron ooids,which were formed in the transition zone between the oolitic shoal and back-shoal microbial bioherm of a limited sea in Shanxi and Henan Provinces.The geochemical tests and microscopic observations of iron-bearing ooids indicate that the iron minerals are idiomorphic crystals of hematites with irregular morphology and serrated edges,with no other iron minerals present.The contents of major elements such as Ca,Fe,Al and Si in the samples are high,showing a significant positive correlation between Fe and Al.The abundance and variation trend of trace elements resemble those found in the Earth's crust.The total amount of rare earth elements is high,with a deficiency in light rare earth elements,and an enrichment in heavy rare earth elements.There is no obvious Eu anomaly.These characteristics indicate that the iron minerals in iron-bearing ooids originate from terrestrial detrital input.The analysis of mineralogy and sedimentary palaeogeography of iron-bearing ooids reveals that the iron-bearing minerals likely formed in the study area as a result of the precipitation of Fe(OH)3 colloidal solution from the Ordos land with runoff.Three conditions are necessary for the formation of iron-bearing ooids in the Mantou Formation of the study area:continuous supply of Fe(OH)3 colloidal solu-tion from terrestrial source,iron enrichment in a semi-enclosed coastal bay environment,and continuous oxidation of iron minerals under the hydrodynamic conditions of regular agitation.During oolitic formation,the intermittent input of iron elements from terrestrial weathering leads to the development of alternating la-minae of hematite and illite-bearing calcite,which interbed with calcite lamina lacking hematite crystals to form microsparry iron-bearing ooids.If the iron supply is sufficient,sparry iron ooids are formed,charac-terized by hematite lamina interbedded with calcite lamina.These findings are crucial for understanding the paleo-marine environment and atmospheric environment during the geological period,with significant theo-retical and practical implications for the development and utilization of iron deposits.