Petrochemistry of volcanic rocks in Q-prospect of The Chatree Gold Deposit, Changwat Phichit

Abstract

Q-prospect area is situated in the northern Chatree gold deposit in which is generally composed of porphyritic andesite, andesitic tuff, rhyolitic tuff that are cross cut by andesite dike. Petrographically, the porphyritic andesite samples usually show hypocrystalline and microporphyritic to porphyritic textures. Their groundmass usually contains plagioclase microlite and glass. Andesitic tuff contains 0.05-2 mm up to 20 % volcanic fragments embedded in volcanic ash and glassy materials. Rhyolitic tuff usually present 0.5-2 mm quartz grains and wedge-shaped rock fragments, mainly replaced by chlorites; these crystals and rock fragments are set in microcrystalline quartz and glass. Andesite dike samples normally show subophitic texture, which represented by the occurrence of small plagioclase enclosed partly by pyroxene crystals. Geochemically, twenty-five rock samples were analyzed for major oxides and some trace elements using XRF and ICP-MS techniques, respectively. Plots of Zr/TiO₂ versus SiO₂ diagram indicate that these rock samples are equivalent to andesite, sub-alkaline basalt to rhyodacite/dacite and rhyorite. They appear to have derived from tholeiitic magma for host porphyritic andesite, andesitic tuff and rhyolitic tuff, and calc-alkaline magmas for late stage andesite dike, based on Zr-P2O5 and Zr-Y diagrams. Regarding tectono-diagrams, such as Th-Zr-Nb, La-Y-Nb, Zr-Ti, La/Yb-Sc/Ni and Th/Yb-Nb/Yb, suggest that these rocks appear to have originated in relation to island arc. MORB normalized patterns show enrichments of Sr, K, Rb, Ba and Th with depleted of HFS elements indicating volcanic arc environment.