Apatite U-Pb geochronological and geochemical constraints on the Mazhala Au-Sb deposit in South Tibet, China

Abstract

Due to lack of suitable datable minerals, timing of the Mazhala Au-Sb deposit in South Tibet remains unresolved. Also, previous geochemical studies suggest an ambiguous conclusion on the source of ore-forming fluid, with diverse viewpoints of magmatic water, mixed magmatic–meteoric water or mixed metamorphic-meteoric water, respectively. In this study, hydrothermal apatite U-Pb dating constrains the Mazhala Au-Sb deposits to have formed at ca. 35 Ma, consistent with the timing of Eocene leucogranite intrusions in South Tibet. The Mazhala Au-Sb mineralization occurred during the main collision stage to late collision stage in Eocene to Oligocene, and predates the Cuonadong Sn mineralization in South Tibet which occurred during the crustal extension stage in Miocene. An EPMA analysis shows the apatite belongs to fluorapatite that have F contents in the dominant range of 2–4%. The Mazhala apatite displays an overall MREE enrichment and LREE- and HREE depletion distribution pattern with positive Eu anomalies, and have δ18O values dominantly in the range of 16-19‰ with a mean δ18O‰ value of 17.74 ± 0.15‰ (2σ), which suggest a relatively homogenous chemical composition for the ore forming fluids with a relatively stable temperature regime. In combination with previous temperature estimates, the ore forming fluids are inferred to have a dominant δ18Ofluid value of 13-16‰, which overlap with metamorphic water.The combined age and geochemical data suggest Eocene leucogranite intrusion induced metamorphic fluid circulation within Paleozoic-Mesozoic metamorphic sedimentary rocks around the study area, which prompted precipitation of fluorapatite together with Au-Sb bearing pyrite and arsenopyrite in the Mazhala Au-Sb deposit. This study puts an emphasis on the contribution of metamorphic water as the dominant ore-forming fluid origin in the Mazhala Au-Sb deposit. In contrast, the close spatial and temporal association of the Cuonadong Sn polymetallic mineralization with the Miocene leucogranite intrusion-induced hydrothermal activity in the Cuonadong dome demonstrates that their genetic links, as was also evidenced by C-H-O-S-Pb isotope data that point to a dominant contritution of Miocene leucogranite to ore-forming fluid and materials for the Cuonadong Sn polymetallic ore deposits. As such, from a perspective of ore-forming model in South Tibet it be seen that during the main collision stage to late collision stage ore-forming fluid and materials were mainly derived from metamorphic water, whereas during the crustal extension stage, ore-forming fluid and materials were dominantly sourced from magmatic water.