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Research Article

ScienceAsia 30 (2004): 43-58 |doi: 10.2306/scienceasia1513-1874.2004.30.043

Sulfur Isotopic Implication of
Middle Miocene Marine Incursion in Northern Thailand

Thanuchai Silaratana,a,* Benjavun Ratanasthien,a Katsumi Takayasu,b William S. Fyfe,c Pongpor Asnachinda,a Wittaya Kandharosaa and Minoru Kusakabed

ABSTRACT: A sulfur isotopic study was undertaken to identify environmental changes of the middle Miocene Mae Moh and Chiang Muan coal fields in northern Thailand. Samples of coal, gypsum, and pyrite were collected from the mine fronts and processed for sulfur isotopic analysis. In the Mae Moh coal field, the δ 34S values of pyrite from the Q and K coal zones are between +4.2 and +10.9 per mil and indicate that the source of sulfur came from organic sulfur. The δ 34S values of +11.2 to +12.1 per mil of gypsum from the K zone coal indicate non-marine sulfate. The δ 34S values of J zone coal are highly variable from -18.8 to +17.2 per mil. The δ 34S value of +17.2 per mil in the lower part of the J zone indicates a freshwater source. In the middle part of the J zone, the δ 34S values of -5.8 to +8.8 per mil may indicate a volcanic source, and are confirmed by volcanic debris. In the upper part of the J zone, the very low δ 34S values of –18.8 to –15.5 per mil may indicate a marine incursion during coal deposition in middle Miocene. This is confirmed by the δ 34S values of gypsum, which are +16.1 to +18.6 per mil, from red beds lying above the J zone. In a minor I coal zone, where the pyrite is replaced in gastropods, the δ 34S values of pyrite are between +30.6 to +34.2 per mil, suggesting that the sulfur was reworked from red beds by bacterial reduction. In the Chiang Muan coal field, the high positive δ 34sulfur values of total sulfur from LS and LM zones (+12.1 and +10.3 per mil, respectively) indicate that the LS and LM were deposited in fresh water environment. The δ 34sulfur values of pyrite from U2 coal zone are between –15.6 and –11.2 per mil, which show the associated with marine sulfate. The similar δ 34S values of black gypsum from the U1 coal zone show that the source of sulfur could have come from the oxidation of sulfide in coal. The different δ 34S values between the organic sulfur, total sulfur, and pyritic sulfur in the U2 zone suggest that marine incursion occurred during the deposition of the U2 coal. This is indicated by the similar δ 34S values of pyritic and organic sulfur.

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a Department of Geological Sciences, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand.
b Research Center for Coastal Lagoon Environments, Shimane University, Matsue, Shimane 690-8504, Japan.
c Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada, N6A 5B8.
d Institute for Study of the Earth’s Interior, Okayama University, Misasa, Tottori-ken 682-0193, Japan.

* Corresponding author, E-mail: g4265401@yahoo.com, thanuchai@hotmail.com

Received 17 Jan 2003, Accepted 12 Nov 2003