Genesis of the special sellaite-bearing orogenic gold deposit at Pianyanzi, western Yangtze Craton: Constraints from fluid inclusions, S-Pb-H-O isotopes, and quartz trace elements

• The Pianyanzi gold deposit is classified as a special sellaite-bearing orogenic gold deposit. • The ore-forming fluids comprise mixtures of metamorphic and magmatic fluids. • Fluid phase separation and fluid-rock interaction serve as key mechanisms driving gold and sellaite co-precipitation. The Daduhe gold belt, located in the western part of the Yangtze Craton, is well-known for the widespread orogenic gold deposits. As one representative gold deposit in this belt, the Pianyanzi deposit differs from other typical orogenic gold systems due to the specific sellaite-bearing mineral assemblage. The orebodies are controlled by the faults within the Ediacaran Dengying Formation dolomite. Three hydrothermal stages were distinguished: native gold-pyrite-quartz stage (I), native gold-sellaite stage (II), and native gold-sulfide-sulfosalt minerals stage (III). Five types of fluid inclusions (FIs) were observed in the quartz, sellaite and sphalerite from three stages: mono-phase CO 2 (type 1), bi- or tri-phase aqueous–carbonic (type 2), halite-bearing tri-phase aqueous-carbonic (type 3), and solid-bearing multiple-phase aqueous-carbonic (type 4) and liquid–vapor bi-phase aqueous (type 5) inclusions. The CO 2 -rich type 2b FIs homogenize to the liquid CO 2 phase, whereas H 2 O-rich types 2a, 3 and 5 FIs homogenize to the liquid H 2 O phase, with consistent homogenization temperatures. Type 4 inclusions commonly decrepitated before the dissolution of the solid phase. This indicates that fluid immiscibility occurred continuously throughout the mineralization process. The FIs in stage I quartz are dominated by types 2 and 3, with minor types 1 and 5, with high temperatures (concentrated at 300–380 °C) and variable salinities (2.0–37.0 wt% NaCl eqv.). Both the multiple types of FIs with high temperatures and salinities and δ 18 O H2O - δ D H2O values (4.5 ‰–6.1 ‰ and −86.7 ‰–-63.9 ‰) of fluids in stage I are consistent with those of magmatic fluids. The FIs in stages II and III are dominated by types 1 and 2, which exhibit relatively consistent homogenization temperatures (240–320 °C) and salinities (0.4–8.1 wt% NaCl eqv.). Such fluids are similar to that of the typical metamorphic fluids with low salinity and high CO 2 contents. Given the abundant sellaite and fluorite within ores in stage II, it is inferred that the F-rich fluids in the main stage were likely sourced from the mixture of metamorphic and magmatic fluids. The δ 34 S values of sulfides in the ores range from 4.25 to 7.54 ‰, suggesting the mixed derivation of sulfur from magmatic and sedimentary rock reservoirs. The in-situ Pb isotopic compositions of galena indicate that Pb originated from the mixtures of lower crustal and upper mantle. Previous studies have shown that the Triassic Hanfengya Formation in the Pianyanzi area is enriched in Au, as well as Ag, Cu, Pb, and Zn, suggesting that the gold for the Pianyanzi gold deposit was likely sourced from the Hanfengya Formation. The widespread occurrence of sellaite and fluorite in stage II likely resulted from the reaction of F-rich fluids with the hosting dolomite. It is therefore proposed that the precipitation of gold and sellaite resulted from both the fluid-rock interaction and fluid phase separation. Quartz from three stages is characterized by low Ti, Li, and Ge contents and variable Al contents, which resemble those of typical orogenic gold deposits. Overall, the Pianyanzi deposit is classified as a sellaite-bearing orogenic gold deposit, with the initial fluids from the magmatic water and progressive input of metamorphic water over time.