Sources and genesis of the Chinkuashih Au-Cu deposits in northern Taiwan: constraints from Os and Sr isotopic compositions of sulfides

The Chinkuashih district at northern Taiwan hosts one of the largest Au deposits in the western Pacific gold province. Gold were precipitated from hydrothermal solutions as native gold or incorporated into sulfides at a temperature range of 200-350 °C. The sulfides in ore mines have ¹⁸⁷Os/¹⁸⁸Os ratios varying from 0.139 to 0.249. The positive ¹⁸⁷Os/¹⁸⁸Os-1/Os correlation is consistent with derivation from the hybrid fluids containing various proportions of mantle and crustal components. The crustal component was the meteoric water that had acquired its Sr and Os isotopic signatures from the local sedimentary formations and dacitic intrusions. The mantle component was the magmatic fluid segregated from the dacitic magma by fractional crystallization. Based on the ¹⁸⁷Os/¹⁸⁸Os-1/Os correlation, the hybrid fluids forming the Chinkuashih sulfides contained less than 30% magmatic fluid, except for one sulfide sample from Hsumei, which required >40% magmatic fluid. Compared to meteoric water, the magmatic fluid contained a higher Os content (130 times higher) and was enriched in Os relative to Sr with an Os/Sr ratio two orders higher than that of the crustal fluid. Consequently, the Os budget in the hybrid fluids was controlled by the magmatic fluid, although the meteoric water was volumetrically dominated. If gold and Os behave similarly in chemistry, the Chinkuashih gold deposits are of mantle origin and the area where sulfides with the greatest mantle Os signature may host undiscovered gold deposits. Finally, the ¹⁸⁷Os/¹⁸⁸Os ratios of sulfides show no relationship with the mineral assemblages of sulfides, implying that the sulfide mineral assemblages reflect local surfacial redox conditions rather than the chemical characteristics of parental fluids.