CHAPTER 1 Introduction 1.1 Purpose and Scope of the Study Igneous rocks at the Doi Kio Lom (DKL) area, Pai district, Mae Hong Son province northward extents from Samoeng granite through Chaing Dao and Pai granite. These granitic rocks are associated with mineral deposits in northern Thailand especially tin- tungsten and fluorite deposit (Department of Mineral Resources, 1971; Cobbing and Pitfield, 1985; Cobbing et al., 1986; 1992; Schwartz et al., 1995). The aim of this study is to discuss the evolution of igneous intrusion, mineralization and alteration of the Doi Kio Lom area. The granitic rocks at the DKL and adjacent areas will be collected for petrographic and ore microscopic study, use for systematic classify igneous rock type, their texture and alteration. Whole rock geochemistry (major-, minor-, trace-, and rare earth elements) of least altered rocks will be analyzed to clarify rocks type and magmatic evolution. Zircon U-Pb age dating will be analyzed to determine the ages of crystallization. 1.2 Granitic Rocks in northern Thailand The Granitic Rocks in northern Thailand (Fig. 1.1) distribute from Myanmar border continuing southward through Mae Hong Son province to Inthanon metarmorphic core complex in Chiang Mai province. They mainly compose of the Central granitic rocks that associated with migmatitic core complex or Doi Inthanon metamorphic core complex (Maldonald et al., 1991; Dunning et al., 1995; Macdonald et al., 2011) that flanked by east and west margin of plutons and small batholiths (Cobbing and Pitfield, 1986; Cobbing et al., 1992) and minor Eastern Belts and Western Belts granitic rocks. 1 Fig. 1.1 Geological map of northern Thailand showing distribution of granitic belts and the study area (after Cobbing, 2010 and Macdonald et al., 2011). 2 1.2.1 Eastern Granitic Belt The Eastern Granitic Belt generally emplaced in Upper Paleozoic sedimentary and volcaniclastic sequences. These granite rocks mainly I-type affinity (Cobbing, 2011), comprised mainly two feldspars (alkali-and calcium-rich) with greenish brown to green color of mafic minerals and biotite (Charusiri et al., 1993). Geochemistry of this belt comprises calc-alkaline I-type or magnetite series (Ishihara et al., 1980) to I-S granites (Mahawat, 1982; Charusiri, 1989; Charusiri et al., 1993; Cobbing, 2011). Mahawat (1982) and Charusiri (1989) studied geochemistry of the Eastern Granitic Belt indicated that these granitiod rocks originate form differential crystallization or partial melt from true magma. Khositanont et al. (2007) the study of Lampang-Phrae granites suggest these granites were formed from lower crustal source with less contamination from upper crust when gradually cooling crystallization process in convergent plate margin. From 40Ar/39Ar technique has been found that ages of granites range from 210 – 245 Ma. (Charusiri et al., 1993). The U-Pb zircon age dating yielded ages 224±4 Ma and 228±3 Ma at Lampang-Phrae granites (Khositanont et al., 2007) The largest body is Tak batholith that has been studied by Teggin (1975), Mahawat (1982) and Yokart et al. (2003) reported that Tak batholith is mainly I- type affinity comprised of hornblede-bearing diorite to granodiorite with inner monzogranite in the east of pluton, the west is biotite monzogranite and an inner is hornblende-biotite quartz monzonite. From Rb-Sr whole-rock isochron yielded 213±10 and 219±12 Ma for white and pink granite respectively (Beckinsale et al., 1979). Khositanont (2007) reported that Tak batholith yielded a zircon age of 228±3 Ma. Small plutons are also scattered in Phrae, Nan, Lampang and Loei provinces. Doi Ngom pluton in Phrea province is small isolated body, with pinkish-grey, biotite-hornblende granodiorite associated with Sb-W-Cu-F ore deposit. 40Ar/39Ar dated the age of tungsten at Doi Ngom is 35 Ma that much younger than the age of emplacement 1.2.2 Central Granitic Belt The Central Granitic Belt occurs as a large batholith covers almost north of 3 Thailand, distributed from Myanmar border continuing south through Chiang Rai, Mae Hong Son, Chiang Mai to Lamphun and Lampang province. These granitic rocks mainly emplaced in Late Triassic to Early Jurassic (Beckinsale, 1979; Cobbing et al., 1986, 1992; Dunning et al., 1995; Darbyshire, 1988; Charusiri et al., 1993). Geochemistry of the Central granitic belts are S-type affinity, peralumiouns potassic (Cobbing et al., 1992; Charusiri et al., 1992, 1993; Imai et al., 2008; Ishihara et al., 2008) and shows highly evolved with high silica content and the high 87Sr/86Sr ratio of the central granite indicated a crustal derivation. This granitic belt is occurred associated with migmatitic core complex (Doi Inthanon metamorphic core complex) (Maldonald et al., 1991; Dunning et al., 1995; Macdonald et al., 2011) that flanked by isolate plutons and small batholith (Cobbing and Pitfield, 1985; Cobbing et al., 1992). The Doi Inthanon metamorphic core complex comprised of highly deformed and high grade metamorphic rocks. These rocks are orthogneiss, paragneiss, granitic mylonite and calc-silicate gniess (Maldonald et al., 1991; Dunning et al., 1995; Rhodes et al., 1997, 2000; Macdonald et al., 2011) which first led to inferred to Precambrian age (Baum et al., 1970; Pongsapitch et al., 1983). However, recent studies reported that the granites and associated migmatite have been assigned to Permo- Triassic age (Cobbing et al., 1986, 1992; Darbyshire, 1988; Pitfield, 1988; Dunning et al., 1995). Moreover, small undeformed plutons/batholiths have been studied by many authors (e.g. Baum et al., 1970; Cobbing and Pitfield, 1986; Cobbing et al., 1992). Eastern Marginal pluton/ batholiths distributed east side of the metamorphic core complex, in Chiang Rai province, east of Chiang Mai province to Li district, Lamphun province. These plutons emplaced into Silurian to Triassic rocks, mainly comprised of medium to coarse-grained K-feldspar megacrystic granite, megacrystic biotite granite, mainly S-type affinity with initial ratio 87Sr/86Sr 0.7280-0.0066 (Fang batholith), 0.7248±0.0014 (Khuntan batholith) (Cobbing et al., 1992) suggested form in syn-collision setting (e.g. Bunopas, 1981; Bunopas and Vella, 1983; Barr and Macdonald, 1991), with Rb-Sr whole- rock yielded age 240±64 Ma. at Fang batholith and 202±3 Ma at Khuntan batholith (Beckinsale, 1979). The western marginal granites formed a discontinuous chain of pluton in the west of the Inthanon metamorphic core 4 complex, distribute in Mae Cham district, Chiang Mai province and Mae Sariang district, Mae Hong Son province. The main component of the western margin is K-feldspar megacrystic biotite-hornblende granite with cognate microdiorite enclaves and small mafic clots. Geochemically, K/Ar biotite age dating yielded 186-207 Ma (Teggin, 1975) and 255 Ma from K/Ar amphibole technique. Braun et al. (1976) suggest 205 Ma from K/Ar biotite age dating or Triassic age. Granitic rocks associated with the Doi Inthanon metamorphic core complex are composed of highly deformed rocks with leucosomes and mafic palaeosomes and formed migmatitic character with some undeformed granitic rocks. Some batholith/plutons in this area host Cretaceous granitic rocks. The deformed granite is weakly to strongly foliated equigranular and inequigranular to K-feldspar megacrystic biotite±muscovite granite (Ishihara et al., 1980; Cobbing et al., 1986; Kosuwan and Nakapadungrat, 1992), whereas undeformed granite is main phase of the Central Granitic Belt (Baum et al., 1970). The granite is medium- to coarse- grained porphyritic biotite granite intruded by minor phase of medium-grained two mica and fine-grained tourmaline-muscovite granites. Plutonic rocks in this area including Fang-Mae Suai (Baum et al., 1970; Cobbing and Pitfield, 1986), Doi Saket-Wiang Pa Pao (Nakapadungrat et al., 1984; Cobbing and Pitfield, 1986) Samoeng (Teggin, 1975; Cobbing and Pitfield, 1986). The age of granitic rocks from Rb-Sr whole rock isochron yielded 201±22 Ma (Beckinsale, 1979), and from K-Ar biotite ages for vinicity of the Samoeng mining range from 71-43 Ma (Teggin, 1975). Additional, the partial overprinting affected on these granitic plutons at Omkoi yielded age 46.2±2.3 Ma or Tertiary age (Department of Mineral resources, 1986). The main mineralizations in the Central Granitic Belt are tin, tungsten, and fluorite related to Late Triassic-Early Jurassic granitic rocks and/or affected by hydrothermal alteration in Cretaceous (Beckinsale, 1979; Cobbing et al., 1986, 1992). Mineral deposits in northern Thailand associated with the Central Granitic Belts are occurred in Pai district, Mae Sariang district, Mae Hong Son province, Samoeng batholith, Omkoi pluton in Chiang Mai province, Li pluton, Khutan batholith in Lamphun-Lampang province, Wiang Pa Pao pluton/Mae Suai pluton 5 in Chiang Rai province. The deposit types in Late Triassic – Early Jurassic are mainly hydrothermal veins and contact metamorphism (Department of Mineral Resource, 1985; 1986; Nakapadungrat and Putthapiban, 1992). However, mineralization associated with Cretaceous and/or Tertiary granite is also occurred in north Samoeng pluton, Chiang Mai Province (Beckinsale, 1979; Cobbing et al., 1992). Thermal activity of Cretaceous/Tertiary granite was dated during 71-43 Ma (from K-Ar method) by Teggin (1975) and 49±1 Ma biotite K-Ar age by Cobbing et al. (1992). Tin-tungsten deposit occurs as hydrothermal type, pegmatite, quartz- feldspar veins, aplite, and contact metasomatic zone (Deparment of Mineral Resource, 2003). Samoeng tin-tungsten mineralizations are