Characterization of sulfate mineral deposits in central Thailand

Abstract

In this paper we present petrographic and geochemical data of sulfate mineral deposits in northeast Nakhon Sawan, central Thailand, and provide new constraints on their age. The deposits are made up mainly of strongly deformed nodular and massive gypsum in the upper part, and less deformed layered anhydrite in the lower part. They are intruded by andesitic dikes that contain Middle Triassic zircons (ca 240 Ma). These dikes are probably part of the regional magmatic activity of the Sukhothai Arc during the Early to Middle Triassic. Sulfur (δ34S) and strontium (87Sr/86Sr) isotopic compositions of the sulfates range from 15.86‰ to 16.26‰ and from 0.70810 to 0.70817, respectively. Comparisons with the Phanerozoic seawater isotopic evolution curve indicate that those values are best explained by precipitation of the sulfates from Carboniferous seawater, in particular seawater of late Mississippian age (ca 326 Ma), and this would be consistent with previous studies of calcareous fossils in the limestones that crop out around this site. Our interpretation is that evaporitic gypsum was originally precipitated from hypersaline seawater on a shallow lagoon or shelf on the Khao Khwang Platform during the Serpukhovian, and that this gypsum changed to anhydrite during early burial. The anhydrite was then cut by andesitic dikes during the Middle Triassic, and more recently the upper part of which was rehydrated during exhumation to form secondary gypsum near the surface

Sulfur, Strontium, Carbon, and Oxygen Isotopes of Calcium Sulfate Deposits in Late Carboniferous Rocks of the Loei-Wang Saphung (LWS) Area, Loei Province, Thailand

“…The different lithofacies could have been affected by the burial diagenesis of sulfate rocks in such a way that the gypsum transformed to anhydrite by the dehydration process at depth, and anhydrite rehydrated to gypsum during uplift. The sulfur isotopic compositions of the LWS area are slightly lower than the Nakon Sawan sulfate deposits, as described by Kuroda et al (2016) [21]. The δ 34 S values of our gypsum samples fit well with the ocean water sulfate δ 34 S curves between the Early to Middle Carboniferous and between the Late Triassic and younger ages (Figure 2).…”

“…The latter case also fit well with the stratigraphic data, where the gypsum beds were apparently overlain by limestone with late Moscovian marine fossils from previous works [5,8]. We compared our results to the Nakon Sawan sulfate deposits [21], which are also associated with Carboniferous rock similar to the LWS deposits. Our results were slightly higher than the values of the Nakon Sawan deposits.…”

“…Since the age of carbonate rocks have been herein assigned as Middle Carboniferous by fossil assemblages [5,8], we interpreted that both evaporite and carbonate/clastic sequences were partly coeval and had formed in the same basin during the Middle Carboniferous. The lithological and mineralogical features of the LWS sulfate deposits were similar to those of the Nakhon Sawan area, as described by Kuroda et al (2016) [21], who interpreted that the sulfate deposits of Nakhon Sawan were originally precipitated in a shallow marine environment on the floor of a lagoon or shelf. The δ 34 S value variations were largely resistant to isotopic fractionation during burial alteration and transformation [25].…”

“…[22]), and the δ 34 S values of carbonate-associated sulfates (blue curve) and sulfate evaporites (red) were compiled by Kampschulte and Strauss (2004) [23]. Horizontal lines indicate the data for the LWS sulfate samples when compared with the Nakon Sawan gypsum samples analyzed by Kuroda et al (2016) [21].…”

“…The cross-plots of carbon with oxygen isotopes are useful in determining the origin of evaporitic carbonate [2]. When we compared our cross-plots with the Nammal Formation along the different carbonate rocks proposed by Hudson (1977) [21] (Figure 3), the LWS carbonate rocks fell into the area of common marine limestone. …”