Genesis of the Zhaxikang epithermal Pb-Zn-Sb deposit in southern Tibet, China: Evidence for a magmatic link

Highlight
- The Zhaxikang Pb-Zn-Sb deposit resembles typical IS epithermal deposits.
- The Pb-Zn-Sb mineralization is closely related to Fe-Mn carbonation and silicification.
- Ar-Ar age, FIs and H-O-C isotopes imply genetic link between magmatic fluids and mineralization.
- High regional geothermal gradient facilitated long distance magmatic fluid transportation.
- Long distance fluid transport formed wide alteration zone and distal Pb-Zn-Sb mineralization.

The Zhaxikang Pb-Zn-Sb deposit is one of the most important deposits in the Southern Tibet metallogenic belt. Based on field geology, petrography, melt- and fluid inclusions and C-H-O isotopes, we describe and discuss the mineralization, alteration, and their possible link with magmatic fluids. Our results show that the Zhaxikang deposit shares many geological and geochemical similarities with typical intermediate-sulfidation (IS) epithermal deposits. The Pb-Zn-Sb mineralization is closely related to Fe-Mn carbonate- and silicic alterations, which formed the outer rim around the greisen in the Cuonadong Dome. Orebodies occur mainly as structurally-controlled veins and breccia dikes, with major minerals include sphalerite, galena, pyrite, arsenopyrite, and Fe-Mn carbonates. Main stage ore-forming fluids were of medium temperature (214-292 °C), low salinity (2.6-5.3 wt.% NaCl eqv.) and CO2-bearing.Melt/fluid inclusions in beryl and quartz from the pegmatite indicate that the primary magmatic fluids were derived from the melt-fluid immiscibility. The magmatic fluids were of low salinity (0.2-7.9 wt.% NaCl eqv.), high temperature (298-457 °C) and CO2-rich, and contained minor CH4, N2, C2H6, C3H8 and C6H6. The presence of Mn-Fe carbonates and daughter gahnite minerals in the beryl-hosted inclusions indicates high Mn, Fe and Zn contents in the parental magma and related magmatic fluids. This implies a genetic link between magmatic fluids and the Pb-Zn-Sb mineralization, as also supported by Ar-Ar dating and H-O-C isotopic evidence. We suggest that the Zhaxikang is best classified as an IS epithermal deposit, and the ore-forming fluids are likely to be magma-derived. Boiling of the magmatic fluids led to high-salinity fluids and metal enrichment. High regional geothermal gradient caused by the thermal doming event may have facilitated long distance transportation of magmatic fluids, and led to the formation of a wide alteration zone and distal Pb-Zn-Sb mineralization. The temperature drop and meteoric water involvement may have precipitated the Pb-Zn-Sn minerals in the distal fault systems.

Homogenization temperature vs. salinity diagram of the Zhaxikang fluid inclusions. Dashed line denotes isodensity line