Multistage processes linked to tectonic transition in the genesis of orogenic gold deposit: A case study from the Shanggong lode deposit, East Qinling, China

摘要

The Shanggong lode deposit represents one of the largest Mesozoic orogenic gold deposits in the East Qinling metallogenic belt of central China. Here we investigate the texture, trace element and sulfur isotopic composition of pyrite from this deposit to gain insights in the ore genesis. Three types of pyrite are recognized by its texture: Py1 is represented by coarse-grained cubic and pentagonal dodecahedron belonging to the stage I quartz-pyrite vein; Py2 comprises flee-grained disseminated pentagonal dodecahedron belonging to stage II quartz-sulfides vein; Py2 can be further divided into core and rim: porous cores (Py2c) carrying abundant, visible mineral inclusions and significantly higher arsenic concentration, and clean rims (Py2r) with low arsenic. Systematic LA-ICPMS trace elements and SIMS sulfur isotopic analyses reveal two distinct stages of mineralization with diverse source materials. 1) The early stage is represented by Py1 which contains 0.65-70 ppm Au (average 11.6 ppm), with relatively low and variable concentrations of Cu, Ag, Te, Pb, Zn, and Bi. The delta S-34 values of Py1 range from - 2.7 parts per thousand to 7.2 parts per thousand, suggesting that the sulfur source for the early stage fluid was mainly derived from the Taihua and Xiong'er Groups and that the depleted delta S-34 values reflect changes of pH, f(O2), and fluid-rock interaction. 2) The main stage is represented by Py2 which contains Au in the range of 0.14-189 ppm (average 39.7 ppm). The Py2 carries significantly higher contents of compatible elements (Ni, Co, V, and Cr) and ore metals (e.g. Au, Ag, Pb, Zn, As, Te) than those in Py1, with the ore metals occurring as invisible lattice bound or nanosized particles and visible micron-sized inclusions. The delta S-34 values of Py2 show marked depletion with a range of -16.3 parts per thousand to - 11.7 parts per thousand, attributing to the fractionation between barite and sulfide minerals. Together with previous studies and trace element characters in Py2, we identify a contribution of Au from mantle source into the ore-forming fluid system during the main stage. We propose that the two stages of gold mineralization in the Shanggong deposit, in response to the collisional assembly of Qinling Orogenic Belt, preserve evidence for diverse metal sources with the early stage derived from partial melting of the lower crust (Taihua and Xiong'er Groups) in the compressional stage, and the main stage characterized by mantle contribution in a post-collisional extensional setting. Our findings may have important implications for the exploration of regional Triassic gold and molybdenum deposits.