Geochemical features and environmental significances of deposits in Ordovician karstic fractures and caves, Lunnan area, Tarim Basin

In the Ordovician karstic fractures and caves in Lunnan area of the Tarim Basin,three types of depo-sits have been identified including water mechanical fillings,chemical deposits and gravity collapse debris.The comparisons of chemical and trace element compositions between deposits and bedrocks have indicated that when different deposits were formed,fluid properties and hydrological environment were different,and the migration,enrichment and fractionation of elements were also different.As a result,the chemical and trace element compositions can be used to judge the fluid properties and hydrological environment when the Ordovician karstic fractures and caves were filled.Three kinds of filling environment have been found including oxidizing active freshwater environment in seepage-active undercurrent zone,freshwater environment in stagnant undercurrent zone as well as pressure-released and hot water environment in shallow-buried zone.The study is helpful for karstic re-servoir prediction and evaluation.