Geochemistry and Magma characteristics of Granitic rocks in Sn-W mine area, Samoeng district, Chiang Mai province, Thailand

Abstract

The Granitic rocks in the Bo Kaeo mine area, Samoeng district, northern Thailand, from part of the Central Granitic Province and are associated with tin-tungsten mineralization in the Bo Kaeo mine, Chiang Mai province. They include light-grey porphyritic biotite granite, biotite granite, leucogranite, aplite, and pegmatite. Geochronological analysis of the porphyritic granite indicates a Late Triassic age. Despite fewer studies on granitic rocks compared to mineralization, this research examines their petrographical and geochemical characteristics, enhancing our understanding of granitic magma sources. The Bo Kaeo porphyritic biotite granites consist mainly of monzogranite, slightly syenogranite, quartz syenite, quartz monzonite, and granodiorite, showing porphyritic holocrystalline textures with phenocrysts of microcline, orthoclase, and plagioclase in a medium- to fine-grained groundmass of quartz, alkali feldspar, and plagioclase. They also contain minor amounts of biotite, zircon ±apatite, ±rutile, ±tourmaline, and ±allanite. Bo Kaeo biotite granites are mostly monzogranite but also include tonalite, quartz monzonite, and granodiorite, with inequigranular textures composed of quartz, orthoclase, microcline, plagioclase, biotite, zircon, apatite, ±titanite, ±allanite, ±rutile, and ±hornblende. Bo Kaeo late-stage granites (aplite and pegmatite) are monzogranite, quartz syenite, syenogranite, alkali feldspar granite, quartz alkali feldspar syenite, and quartz monzonite, displaying inequigranular textures and made up of quartz, microcline, ±orthoclase, ±plagioclase, ±biotite, ±muscovite, ±zircon, ±apatite, ±rutile, ±tourmaline, ±titanite, ±allanite, ±garnet, and ±opaque minerals. The Geochemical analysis categorizes them into two groups: (1) granites with high MgO granite (1.40-3.58 wt.% MgO) and low MgO granites (0.91-1.35 wt.% MgO), and (2) late-stage granites (0.01-0.59 wt.% MgO). They exhibit a chemically classified as magnesian peraluminous characteristics. Furthermore, high MgO granites are classified as I-type granitoids with some S-type affinities (Aluminum Saturation Index = 1.00-1.30), while low MgO granites are primarily I-type (Aluminum Saturation Index = 0.99-1.10). Although they exhibit characteristics of calc-alkalic magmas and enrichment of light rare earth elements with relatively flat in heavy rare earth elements ((La/Yb)N = 1.21-39.31), their chondrite-normalized rare earth element patterns and the trends in bivariate diagrams (Nb-Zr and Y-Zr) are discontinuous, indicating they do not form a co-magmatic suite. Moreover, the late-stage granites in Bo Kaeo fall within the subalkaline to alkaline series. Considering the correlation between TiO2, Al2O3, Total FeO, MgO, and CaO, the high MgO granites are derived from melt associated with amphibolite/metabasic to tonalitic sources, while the low MgO granites are derived from melts associated with greywacke/metagraywacke sources. Additionally, the relationship among Nb, Th, and Y indicates geochemical properties consistent with those found in volcanic arcs, with the Nb/Th ratio resembling that observed in crustal source (Nb/Th = 1.1-1.3). This suggests that the granitic magma likely did not originate from the partial melting of basaltic magma or mantle peridotite, where Nb/Th ratios typically exceed 15.