SUMARRY

U-Pb age and geochemistry of zircons from granitic rock in the G18 gold mine in the Tam Kỳ - Phước Sơn suture zone and implication for geology and related mineralization

Trần Mỹ Dũng, Ngô Xuân Thành, Vũ Quang Lân, La Mai Sơn,

Trần Hải Nam, Chu Văn Dũng, Đặng Hương Giang, Nguyễn Quốc Hưng

Currently, 98 gold mines and gold deposits have been recorded along the Tam Kỳ - Phước Sơn (TKPS) zone, Central Vietnam. Of which the G18 gold mine is one of high industrial value in the area. In this study, we presented some structural characteristics of the G18 mine and one granite sample intruded by Cu-Au sulphide bearing veins was collected to separate zircon grains for U-Pb age dating and determination of their geochemical composition. The mineral composition of granite is quartz, K-feldspar, plagioclase, biotite, and accessory minerals of zircon, apatite and ilmenite. The main ore minerals are chalcopyrite, pyrite, galenite, arsenopyrite and native gold, which were formed along with dykes of calcite and quartz that cross-cut through the magmatic rocks. The emplacement of granite was determined by LA-ICPMS zircon U-Pb isotope is 441±6 Ma, corresponding to the early Paleozoic magma series identified in the study area, possibly related to the magmatic activity of the subduction zone along the TKPS during the early Paleozoic time. Analyzed zircon grains have high contents of U, Th and Ce elements with positive anomalies of Ce (Ce/Ce*=15-300), low contents of Nb, Ti and Eu elements with negative anomalies of Eu (Eu/Eu*=0.1-0.32). The internal structure of zircon together with high Th/U ratios >0.1 (0.33 to 0.48) indicating magmatic origin. Furthermore, The ratios of Dy/Yb and 1000x(Eu/Eu*)/Y in zircons are low, ranging from 0.18 to 0.31 and 0.1 to 1.0, respectively. These low chemical ratios of zircon implied that themagmatic melt, from which the study granite formed, had low H₂O content and oxygen fugacity,therefore have no prospect of hydrothermal Cu-Au sulphide mineralization.

Keywords: Tam Kỳ - Phước Sơn, Cu-Au sulphide, granite, U-Pb zircon