Petrochemical features of granites associated with tin-tungsten mineralization at mae chedi, Wiang Pa Pao, Chiang Rai

Abstract

The granitic complexes in the Mae Chedi area, Wiang Pa Pao district, Chiang Rai province, northern Thailand, comprise two granitic suites. These two groups are here referred to as the GM-and the GR-series. The first one is associated with the known primary tin-tungsten mineralization and composed of fine-grained biotite (GM-1), fine-grained muscovite-bearing biotite (GM-2), and fine-to medium-grained leucocratic (GM-3) granites. The other is generally referred to as tin-tungsten-barren and composed of porphyritic biotite (GR-1), medium-to coarse-grained biotite (GR-2), and fine-to medium-grained leucocratic (GR-3) granites. Major element data displayed on Q-Ab-Or plots indicates these two granitic systems have been developed in epizonal plutons that meet Tuttle and Bowen’s (1958) definition of granite. The chemistry of an individual magma series is graphically represented by Harker variation diagrams and classified by means of Wright’s (1969) alkalinity. Based on the chemistry of the source plutons, both granitic series are calc-alkaline, peraluminous and classified as the so-called S-type granites. However, the GM-series appear to be less silicic than those of the GR-series and is characterized by higher TiO₂, Al₂O₃, Fe₂O₃, FeO, MnO, MgO, CaO, P₂O₅, H₂O⁺, Li, F, Sr, Ba, Zr, Sn, W, Cu, Zn, Ni, Ce and lower Rb contents. Relative differences in Na₂O and K₂O contents between the GM-1, GM-2 and the GR-1, GR-2 are not uncommon but insignificant. Nevertheless, the GM-3 is remarkably lower in Na₂O but higher in K₂O contents than that of the GR-3. As a result, it is believed that the GM-3 is the late metasomatic alteration product of the GM-1, whereas the GR-3 is the late magmatic differentiation product of the GR-1. To the author’s knowledge, the primary tin-tungsten mineralizations occur in the study area where small granitic plutons of the GM-series intruded metabasites. The tin and tungsten ores are commonly found in/or adjacent to quartz veins and veinlets cutting through the GM-granitic series and, less commonly, in metabasites. Wall-rock alterations are characterized by K-feldspathization, tourmalinization, chloritization, sericitization, muscoivitization, and albitization.