Degree of Brine Evaporation and Origin of the Mengyejing Potash Deposit：Evidence from Fluid Inclusions in Halite

The Mengyejing potash deposit is located in the southern port of the Simao Basin, Yunnan Province, and is hosted in mid-Cretaceous strata. The chemical compositions of fluid inclusions in halite crystals, collected from the level-610 adit in the deposit, were analysed by laser ablation inductively coupled plasma mass spectrometry（LA-ICP-MS）. The results show that the brine is of the Na-K-Mg-Ca-Cl type and has K concentrations that are distinctly higher than those of Mg and Ca, unlike normal brines associated with Cretaceous halite. The high K concentrations indicate that the degree of evaporation of the ancient Mengyejing saline lake was very high, reaching the sylvite deposition stage but rarely reaching the carnallite deposition stage. The trajectory of the H and O isotopic compositions of the brines in the halite-hosted fluid inclusions corresponds to intense evaporation, indicating that the net evaporation exceeded the net inflow of brines. These brine compositions in halite-hosted fluid inclusions were likely formed by the dissolution of previously deposited K-bearing minerals by fresh continental and/or seawater, forming a type of modified seawater, with deep hydrothermal fluids potentially supplying additional potassium. The basin likely experienced multiple seawater incursion, dissolution and redeposition events in a high-temperature environment with high evaporation rates.

Provenance and Paleogeography of the Late Cretaceous Mengyejing Formation,Simao Basin,Southeastern Tibetan Plateau

The Mengyejing potash salt deposit(MPSD)is the only pre-Quaternary potash salt deposit in China.The MPSD is located in the southern Simao Basin,southeastern Tibetan Plateau.The MPSD,along with rock salts and clastic rocks,

New Sr Isotope Evidence to Support the Material Source of the Mengyejing Potash Deposit in the Simao Basin from Ancient Marine Halite or Residual Sea

Objective The Mengyejing potash deposit in the Simao Basin is the only producing area of solid potash at present in China. There is still controversy about the material source and distribution of the potash in this deposit （Shen Lijian et al., 2017）, which has influenced not only the prospecting direction and efficiency but also the understanding of the control of Tethys tectonic evolution on the formation and distribution of the mineral resources. This work analyzed the Sr isotope geochemical characteristics of evaporites from core samples in the well MZK-3 in order to further clarify the material source and to explore the potash distribution in the Simao Basin.

New insights on the age of the Mengyejing Formation in the Simao Basin,SE Tethyan domain and its geological implications

Knowledge on the origin and mechanism of the large potash formation in the southeast Tethyan tectonic domain are still under debate,one of which mainly due to the controversial ages of the potash-bearing strata.Here we present a detailed detrital zircon U-Pb geochronologic-magnetostratigraphic study of the potash-bearing Mengyejing(MYJ)Formation(Fm)from the Simao Basin.948 paleomagnetic cores and 4 detrital zircon U-Pb samples were collected from a 932-m thick Jiangcheng section.Thermal demagnetization isolated 857 primary characteristic remnant magnetization(ChRM)directions,indicated by positive reversals and fold tests and petrologic examinations.Seven normal and seven reversed polarity zones were identified and correlated well with chrons C27r to C34n of the geomagnetic polarity timescale(GPTS2012),yielding the first known age sequence of>112 to~63 Ma for the MYJ Fm.The ages for the potash bearing strata in the Simao commenced~20 Ma earlier than those in the adjacent Khorat basins,opposite to the traditional viewpoint that the former would rather correspond to the middle and lower parts of the latter.Given the paleogeographic reconstruction of likely a pan-Simao-Khorat basin during the Cretaceous,~85 Ma would be another important potential period for future potash exploration in the Simao Basin.