Gold behavior in intermediate sulfidation epithermal systems: A case study from the Zhengguang gold deposit, Heilongjiang Province, NE-China.

Graphical abstract We have described the texture and Au distribution within pyrite, sphalerite and chalcopyrite from Zhengguang gold deposit. Pyrite formed through the ore-deposit evolution from early porphyry mineralization to the followed epithermal mineralization, to latest metamorphism event. Among all the sulfides, pyrite and sphalerite are the two important carriers for gold in Zhengguang. There is no doubt an undiscovered porphyry system below or near the Zhengguang gold deposit. Emplacement of the porphyry will result in the formation of Au-poor group of pyrite in pyrite zone. After the formation of an Au-poor group of pyrite in porphyry system, the later epithermal fluids alter and overprint the early pyrite, and give birth to Au-rich group of pyrite, Bi-rich group of chalcopyrite, and Au-rich group of sphalerites in Zhengguang deposit. This process results in the deposition of gold and base metal sulfides with overprinting, and altering the early formed pyrite. The gold element is most likely derived from magma system. There was a metamorphic event altered and deformed the early formed sulfides, and result in the remobilization of Au. Highlights • Porphyry emplacement results in formation of Au-poor pyrite and Bi-poor chalcopyrite. • Later epithermal fluids replace and overprint early pyrite. • Epithermal pyrite is relatively Au-rich pyrite; chalcopyrite is Bi-enriched. • Gold is most likely sourced from magma; pyrite and sphalerite both carry some gold. • Gold occurs as Au in Au-poor and Sb-rich pyrite, and Au+ and Au0 in Au-rich pyrite. • A metamorphic event led to sulfide deformation and associated Au remobilizations follows. Abstract The Zhengguang gold deposit, a typical intermediate-sulfidation epithermal deposit, is located in the southeastern part of the Duobaoshan orefield, west of the Hegenshan-Heihe suture zone, in the eastern part of the Central Asian Orogenic Belt. The deposit comprises five ore zones with total Au reserves exceeding 35 tonnes, with potential additional resources at depth. All vein-type orebodies are hosted by Paleozoic volcanic rocks and comprise multiple vein sets 1–100 cm in thickness. Although gold generally occurs in native form, or as electrum in epithermal deposits like Zhengguang, both pyrite and sphalerite are known to accommodate modest concentrations of invisible gold. This study employs a combination of petrography and sulfide chemistry to determine the role of invisible gold in the Zhengguang ores and the mechanisms of gold incorporation into epithermal sulfides. Three sulfide stages are identified: an early quartz + pyrite (Py1a, Py1b) ± chalcopyrite (Ccp1) stage; a subsequent quartz + sphalerite (Sph2a, Sph2b) + pyrite (Py2a, Py2b, Py2c, Py2d) + chalcopyrite (Ccp2a, Ccp2b) ± galena ± calcite stage; and a late stage containing deformed quartz + pyrite (Py3a, Py3b) ± sphalerite. Petrography and sulfide chemistry allow three groups of pyrite (Au-poor, Au-rich, and a distinct Sb-rich group) to be distinguished, alongside three groups of chalcopyrite (Bi-rich, intermediate-Bi, and Bi-poor), and two groups of sphalerite (Au-poor, Au-rich). A potential porphyry system is indicated beneath the epithermal system by the appearance of Au-poor pyrite and Bi-poor chalcopyrite. After precipitation of early Au-poor sulfides, inflow of relatively low temperature epithermal fluids led to alteration and replacement of early porphyry-related sulfides, and to precipitation of Au-rich pyrite, Bi-rich and intermediate-Bi chalcopyrite, and sphalerite. Gold-rich pyrite contains up to 140 ppm Au, interpreted as both as lattice-scale substitution (Au1+) and as included particles of native gold (Au0). Epithermal chalcopyrite is an important silver carrier but, although Au is measurable, it is a not a good carrier for gold. A strong positive correlation between Au and Cu in pyrite from the first two stages indicate that gold and other metals were likely sourced from magma-derived hydrothermal fluids. The deposit was formed in the Early Paleozoic but some gold ores appear deformed and partially destroyed by a later metamorphic event during which a distinct Sb-rich pyrite crystallized. This study should catalyze exploration in the orefield as it provides further support for an as-yet undiscovered porphyry system close to the Zhengguang deposit. [ABSTRACT FROM AUTHOR]