Hydrothermal evolution and gold precipitation mechanism of the Hekou gold deposit, North China Craton: Insights from pyrite texture, composition and in situ S isotope

Pyrite, as one of the most important Au carriers, can well document ore-forming processes. In this study, we select the Hekou gold deposit (HKGD), a newly discovered deposit in the North China Craton (NCC), to rebuild its ore-forming process and gold precipitation mechanism with pyrite. The gold-bearing pyrite was used to perform a combined micro-textural petrography, elemental and S isotope analyses. The micro-textural petrography reveals two distinguished generations of pyrite, including coarse-grained euhedral Py1 and fine-grained xenomorphic Py2. The fine-grained xenomorphic Py2 likely results from a fast nuclear rate responding to the rapid thermochemical changes of hydrothermal fluid. Trace elements of pyrite constrained by LA-ICP-MS reveal that the concentrations of metals such as Au, Ag, Cu, and Pb dramatically increase from Py1 to Py2, suggesting that the fluid evolved to oversaturation. The concentrations of As notably decrease from 284.35 to 23.27 ppm from Py1 to Py2, supporting that As escape from the ore solutions. Moreover, the in situ S isotopes by LA-MC-ICP-MS reveal a minor delta 34S fractionation between Py1 (6.29 +/- 0.22%o, 2SD) and Py2 (5.95 +/- 0.22%o, 2SD), which also agree with the isotopic behavior of H2S escape. All of them, together with fluid inclusion observation, we explain that the two-stage pyrite formation in the HKGD records a hydrothermal system evolution from nonimmiscibility (Py1) to immiscibility (Py2). Fluid immiscibility likely effectively facilitated the Au precipitation in the HKGD, via significantly destabilizing the Au-HS complex in fluid and incorporating Au into pyrite. Meanwhile, it also led to an unusual decoupling of Au and As in pyrite. This study highlights that the pyrite texture and chemistry can be excellent tools to reveal trace hydrothermal system evolution and gold precipitation mechanism.

Automated summary

In this study, the Hekou gold deposit in the North China Craton was selected to investigate its ore-forming processes and gold precipitation mechanism using gold-bearing pyrite. The micro-textural petrography revealed two generations of pyrite, and trace element and S isotope analyses provided further insights into the evolution of the hydrothermal system and the gold precipitation mechanism.