Mechanism and age of gold remobilization in orogenic gold deposit from primary laminated ore to the high-grade ore: A case study from Cenozoic Bangbu gold deposit, Tethyan Himalaya

The formation of high-grade gold ore has always been attractive, mostly located in Precambrian gold deposits with various mechanisms. Yet, in the orogenic Bangbu gold deposit, Tibet, we discovered that the coupled dissolution-reprecipitation (CDR) process and low melting point chalcophile elements (LMCE)-related melting are responsible for the formation of high-grade ore at ca. 38 Ma constrained by in-situ U-Th-Pb dating of hy-drothermal monazite. Three generations of pyrite have been recognized, metamorphic barren pyrite (Py I) in the host schist, hydrothermal pyrite (Py II) in the preceding primary laminated ore with an Au grade of similar to 8 g/t, and the hydrothermal coarse pyrite (Py III) in later massive ore with a grade of up to 29 g/t. In the primary laminated ore, the Py II in the sulfide ribbons is intergrown with native gold and colloform quartz and carries abundant trace element contents, such as Au, As, Cu, Pb, Zn, Te, Se, and Bi. These suggest that the rapid precipitation and local supersaturation occurred during the episodic crack-seal process. In the later high-grade massive ore, Py III exhibits distinct zonation, ranging from Py III-1 in the core to Py III-3 in the rim. The Py III-1, porous and rich in inclusions, has sharp boundaries and successive lattice orientation with the mantle Py III-2 suggesting a CDR process. Higher Au and As in Py III-2 than the Py III-1, and the close affinity between visible gold and LMCE melt inclusions at the boundary suggest that gold was remobilized and facilitated by the LMCE during the CDR process. The electron backscatter diffraction (EBSD) mapping result shows little crystallographic misorientations of Py III, excluding the deformation process that has significantly contributed to the remobilization of gold. The decrease of the delta S-34 ratio of pyrite between Py II (delta S-34 = 2.0%0 to 4.3%0, mean 3.4%0) and Py III (delta S-34 = - 2.5%0 to 3.8%0, mean-0.2%0) most likely reveals a more oxidized condition in the open space which may trigger the CDR process and gold remobilization of the pyrite in the primary ore. Within the zonation of Py III, an increasing trend of the 834S ratio represent the evolution of the redox state of fluids towards relatively reducing conditions during the healing of quartz veins. The newly obtained ca. 38 Ma age is comparable to regional amphibolite-to granulite-facies metamorphism during the continental subduction, which was considered to trigger the modifi-cation of primary gold orebody to form high-grade ore.