CHLORINE, SULFUR AND OXYGEN ISOTOPIC CONSTRAINTS ON ANCIENT EVAPORITE DEPOSIT IN THE WESTERN TARIM BASIN, CHINA

Abstract

From the late Cretaceous to the early Tertiary period, the Tethys seawater repeatedly invaded the Shaqa sub-basin and entered the Kuqa sub-basin through the Awati straight. At the same time, as a result of tectonic and paleoenvironmental conditions, many sequences of thick strata of evaporites, especially in its two largest sub-basins of Kuqa in the north and Shaqa in the southwest have been deposited. Therefore, the western Tarim Basin has been considered to be a favorable location to prospect for ancient potash deposits. In order to clearly distinguish between a marine or a continental origin, and also to study the evaporite facies (i.e., late vs. early stage), we analyzed the late stages of evaporites of gypsum and salt minerals and measured chlorine isotope values in halite, sulfur isotopes in gypsum and oxygen in crystalline water of gypsum. The results showed that the sulfur isotope composition of gypsum can be used to distinguish between a marine and continental origin and chlorine isotopes of salt and oxygen isotopes of crystalline water in gypsum can be used to judge the sedimentary stage. Based on these results, we conclude that the late Cretaceous halite deposited in the Kashi Depression of the southwestern Tarim Basin have a marine origin and represent a later stage of marine evaporite sequence compared to other evaporites in the regions. Hence, the Kashi Depression should be considered as a promising area for finding new sylvinite deposits. Copyright © 2006 by The Geochemical Society of Japan.