Symplectitic intergrowths enriched in low-melting-point chalcophile elements and their implications for gold enrichment in the Dadiangou gold deposit, western Qinling Orogen, Central China

Abstract Low-melting-point chalcophile elements (LMCEs) such as tellurium (Te), bismuth (Bi), lead (Pb), and antimony (Sb) were identified in multiple minerals at the Dadiangou gold deposit in the Western Qinling Orogen, central China. Two distinct types of LMCE-rich symplectitic intergrowths, interpreted to have formed through interactions between metallic melts and hydrothermal fluids, were recognized. Symp-I comprises galena and tetradymite (Bi2Te2S) with associated native gold, and can be subdivided into two successive subtypes based on mineral assemblage and textural evolution. Symp-Ia is characterized by an assemblage of galena, tetradymite, and native gold, with minor altaite (PbTe), melonite (NiTe2), chalcopyrite, and hessite (Ag2Te); in contrast, Symp-Ib comprises galena, aikinite (CuPbBiS3), tetradymite, and chalcopyrite, with or without pyrite. Symp-II formed subsequently, and is composed of galena-matildite (PbS-AgBiS2) solid solution associated with native bismuth, electrum, and dulanggouite (Bi6Te3). Textural evidence—including vermiform tetradymite intergrown with altaite and melonite, melt-like microdomains, and curvilinear grain boundaries—indicates that LMCE-rich metallic melts were present during Symp-Ia, where Pb–Bi–Te melts coexisted with hydrothermal sulfides and acted as efficient scavengers for Au. Thermodynamic estimates indicate that, as the fTe2 decreased, the redistribution of Bi, Pb, and Cu among galena, tetradymite and chalcopyrite proceeded via a dislocation-mediated interfacial re-equilibration (DMIR) process, forming aikinite (CuPbBiS3) of Symp-Ib. Further decreases in fTe2 and fS2 drove coupled dissolution–reprecipitation (CDR) of galena-matildite solid solution, generating porous Symp-II and resulting in the late-stage Au-Ag enrichment. Collectively, these results demonstrate that Pb-Bi-Te metallic melts can form in orogenic hydrothermal gold systems. These melts can act as efficient, transient carriers of precious metals, and their evolution is governed primarily by fluctuations in fTe2–fS2, rather than by temperature alone.