Rare Earth Elements in Hydrothermally Altered Granitic Rocks in theRanong andTakuaPa Tin‐Field, SouthernThailand

Geochemical studies were conducted on the hydrothermally altered granitic rocks in the R anong and T akua P a tin‐fields in southern T hailand in order to investigate the mode of occurrence of REE (rare earth elements), with emphasis placed on a potential REE resource associated with granitic rocks in the S outheast A sian T in B elt. The total REE (Σ REE ) content of altered granitic rocks ranges from 130 to 350 ppm at H aad S on P aen (which is presently mined for kaolin clay) in the R anong tin‐field, and that of altered granitic rocks and kaolinite veinlets reaches up to 424 ppm and 872 ppm, respectively, at N ok H ook in the T akua P a tin‐field. Rare earth elements in the altered granitic rocks and kaolinite veinlets show a relatively flat chondrite‐normalized pattern, thus enriched in heavy REE compared with the original granitic rocks and their weathered crusts. At N ok H ook ( T akua P a), the Σ REE content of kaolinite separated from an altered granitic rock by elutriation is 1313 ppm, a Σ REE amount about four times higher than that of whole‐rock composition of the altered granitic rock. Chondrite‐normalized REE patterns of the elutriated kaolinite and of the altered granite are relatively flat. Sequential extraction experiments suggest that 41 and 85 percent of REE are present as ion exchangeable‐form in the altered granitic rock, and in the kaolinite veinlets, respectively. In addition, more than 90% of REE in the kaolinite veinlets are present as the acid‐soluble state. On the other hand, the Σ REE content of kaolinite veinlets and of the kaolinite concentrated by elutriation from an altered granitic rock at H aad S om P aen ( R anong) is 70 ppm and 75 ppm, respectively, thus enrichment of REE in kaolinite was not confirmed. In addition, by the sequential extraction experiments, 23% and 4% of REE were extracted from the altered granitic rock and the kaolinite veinlets at H aad S om P aen. In the altered granitic rocks at H aad S om P aen, REE are present as refractory phases, and REE in the acid‐soluble states had been leached by hydrothermal fluid.