Geology of Evaporite Sulfate Deposits in Late Paleozoic Rocks of Wang Saphung Area, Loei Province, Thailand

The evaporite sulfate deposits in Loei-Wang Saphung (LWS) area of northeastern Thailand is a small evaporite sediment deposit with up to 50 m thick gypsum-anhydrite beds. The evaporite deposits are overlain by cross-laminated and fine-grained siliciclastic and carbonate rocks of the Carboniferous to Permian ages. This study documents some characteristics of the deposits, including lithologies, textures, structures of gypsum-anhydrite and associated rocks, and S, Sr, C, and O Isotope of the deposits, based on the stratigraphic core logging of boreholes, lithofacies analysis, and isotope analysis of selected samples. Morphological and textural mineralogical relationships reveal 10 textures of the evaporite formation viz. alabastrine gypsum, satin spar gypsum, selenite gypsum, gypsarenite, porphyroblastic gypsum, fine lenticular gypsum, crystalloblastic or blocky anhydrite, prismatic anhydrite, epigenetic anhydrite, and felty epigenetic anhydrite. The gypsum-anhydrite beds with associated carbonate and mud rocks suggest that these sedimentary sequences form in a subaqueous, probably shallow marine marginal setting during Late Carboniferous. Sulfate samples yield average values of δ34S of 14.61‰ while the 87Sr/86Sr ratio of gypsum is 0.708282 and anhydrite is 0.708288. Carbonate layers yield average δ18OPDB, and δ13C values of -12.52‰, and -0.1‰, respectively. Our results reveal that the LWS evaporite deposits were originally formed from seawater and the relatively negative value of δ18O is a result of meteoric alteration during subaerial exposure of the sections. The results also indicate that the LWS sulfate deposit has passed through at least 4 evolutionary alterations; (1) original precipitation as gypsum deposit, (2) gypsum-to-anhydrite transformation resulting from burial diagenesis in response to basinal subsidence, (3) rehydration of anhydrite-to-gypsum, indicated by distorted gypsum rocks, resulting from the increase of volume due to the rehydration from anhydrite to gypsum, and recrystallization of anhydrite and/or primary gypsum to secondary gypsum (4) uplift and re-expose of gypsum, indicated by the karstification and dissolution cavities and gypsarenite veins.