Sulfide micro-textures and in-situ compositions from the Wulong gold deposit, northeastern North China Craton: Implication for ore-forming processes

Abstract

Lode gold deposits in the North China Craton (NCC) form the only known giant late Mesozoic gold province in the world’s Precambrian cratons. Understanding the ore genesis for these deposits is of great scientific and economic importance. In order to provide constraints on the ore-forming processes, this study presents a combination of micro-textures and in-situ compositions of sulfide from the Wulong gold deposit (80 t @ 5.35 g/t Au; 127–122 Ma), northeastern NCC. Five sub-generations of pyrite (Py1–Py5) are identified based on micro-textural observations and EPMA results. In-situ trace elemental concentrations show that pyrite from the stage 1 (Py1) has high Co and Ni contents but no Au (<0.02 ppm). In contrast, euhedral Py2 and porous Py3 from stage 2 exhibit much higher Au (average 3.08 ppm and 1.66 ppm), As (average 1400 ppm and 946 ppm), and Ag (average 2.81 ppm and 3.21 ppm). Anhedral Py4 from stage 3 shows lower Au (average 0.31 ppm) and As (average 337 ppm) concentrations when compared to Py3. However, Py5 from stage 3 exhibits significant increases of As (average 7102 ppm) and Au (average 5.46 ppm) again and is featured by the alternation of Co-Ni- and Au-As-rich zones. Other sulfides, including pyrrhotite, chalcopyrite, and sphalerite, are poor in Au and As. Apart from visible gold grains related to Bi-rich melts, LA-ICP-MS spot and mapping analyses reveal that invisible gold hosted in Py2 to Py5 occurs dominantly as lattice-bound solid solutions (Au+). Direct gold precipitation from anomalously As-rich auriferous fluids induced by fluid immiscibility is proposed as the dominant mechanism of invisible gold enrichment. In-situ δ34S values of Py1 to Py4 exhibit narrow ranges of 1.0 to 3.8 ‰, indicating a deep-seated magmatic sulfur source. The elevated δ34S values (2.3–6.3 ‰) of Py5 are ascribed to some wall-rock contributions during fluid-rock interaction. These findings shed new light on the link between multistage fluid immiscibility and the chemical and micro-textural fingerprints of pyrites and the role of fluid immiscibility in the deposition of invisible gold.