Abstract

The granitic complexes in the Mae Chedi area, Wing Pa Pao district, Chiang Rai province, northern Thailand, compriSe two granitic suites. The first, the OM-series, is associated with the known primary tin-tungsten mineralization and composed of fine-grained biotite (GM-1), fine-grained muscovite-bearing biotite (OM-2), and fine- to medium-grained leucocratic (GM-3) granites. The second, the OR-series, is generally referred to as tin-tungsten barren granite. It is composed of porphyritic biotite (OR-1), medium- to coarse-grained biotite (OR-2), and fine- to medium-grained leucocratic (OR-3) granites . . Both granitic series are geochemically calc-alkaline, peraluminous, and S-type granites. However, the OM-series appears to be less silicic than that of the OR-series and is characterized by higherTi02 , ~03 , F~03 , FeO, MnO, MgO, CaO, P20 5, ~o+, Li, F, Sr, Ba, Zr, Sn, W, Cu, Zn, Ni, Ce and lower Rb contents. Relative differences in N~O and KzO contents between the OM-1, OM-2 and the OR-1, OR-2 are not remarkably sharp. However, the OM-3 is notably lower in N~O but higher KzO contents than that of the GR-3. This and petrographic evidences have led to the belief that the OM-3 is the late metasomatic alteration product of the OM-1, wheq:as the OR-3 is the late magmatic differentiation product of the OR-1. The primary tin-tungsten mineralizations occur in the area where small granitic plutons of the OM- series intrude metabasite. The tin and tungsten ores are commonly found· in/or adjacent to quartz veins and veinlets cutting through the OM-granitic series, and less commonly, in the metabasite. Wall-rock alterations are characterized by K-feldspathization, toumtalinization, chloritization; sericitization, muscovitization and albitization.

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