Characterization of sulfate mineral deposits in central Thailand

Abstract

en

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.

Abstract

ja

タイ中央部のナコンサワン北東に分布する硫酸塩鉱物岩体の岩石学的・地球化学的研究を行い，その年代や成因を検討した．この岩体は層状硬石膏と，その上位の変形した石膏からなり，安山岩質ダイクに貫入される．ジルコンU‐Pb年代は中期三畳紀の貫入年代を示し，三畳紀のスコタイ弧の広域的マグマ活動に関連したものと思われる．硫酸塩鉱物の硫黄同位体比とストロンチウム同位体比を，顕生代の海水の同位体進化曲線と比較した結果，石炭紀(サープコビアン)の海水の値に一致した．この年代は，周辺の石灰岩に産する石灰質化石の年代とも整合的である．この硫酸塩岩体の履歴は以下のようになる．まず，石炭紀サープコビアンに浅海域で高塩海水から初生的石膏が晶出し，その後，埋没続成で硬石膏に再結晶した(層状硬石膏)．この硬石膏は中期三畳紀に安山岩質ダイクの貫入を受け，最近になって再び地表近くにもたらされ，上部が水和して石膏が再晶出した．石膏の変形構造は，水和の際の膨張によるものと考えられる.