MINERALOGICAL AND STRUCTURAL GENESIS OF THE UDON THANI SYLVINITE (POTASH) DEPOSITS, UDON THANI PROVINCE, NORTHEASTERN THAILAND

An exploration program conducted by Asia Pacific Potash Corporation, Ltd. during the 1990s delineated two adjacent deposits of sylvinite-type potash in the province of Udon Thani, Thailand. An extensive exploration program that included drilling and 2D-seismic provided a database of sufficient quality and distribution to develop Canadian National Instrument 43-101 and Australian JORC compliant resource estimates for input to feasibility-level studies.

A mining study conducted for the Udon South potash deposit in 2003 confirmed total sylvinite resources of 302 million tones with average grade of 23.5% K2O and average thickness of 3.8 meters. A mine design based on square room and pillar layout resulted in mineable reserves of 118 million tones with average grade of 23.4% K2O over a 25-square kilometer area. The deposit is unique for the Khorat Plateau as it presents a mineable seam of sylvinite mineralization that is comparable in grade and purity of any global sylvinite deposit.

A geological model and resource estimation study conducted for the Udon North potash deposit has outlined a resource of 665 million tones sylvinite with average grade 17.1% K2O and average thickness of 12.0 meters over a 83-square kilometer area. A continuous sylvinite seam represented by two distinct sylvinite intervals of primary and secondary origin defines the resource boundaries of the Udon sylvinite deposits.

Typical of the Khorat Plateau, the potash deposits near Udon Thani occur within the Cretaceous Maha Sarakham Formation. The presence of thick sylvinite within the Lower Salt member is result of primary precipitation of sylvite and a period of intense incongruent solution of carnallite to secondary sylvite. Preservation of the sylvinite appears to be related to an area of tectonic stability and relatively low degree of late-stage dissolution within the southwestern corner of the Sakon Nakhon evaporite basin. The area is characterized by the absence of deep penetrating tectonics and grand-scale salt kinetics.

This paper focuses on the genesis of a mineralogical and structural setting that has preserved economic deposits of sylvinite-type potash in northeast Thailand.