Abstract:

The Zaozigou gold deposit is located in the Tongren-Xiahe-Minxian gold metallogneic belt in west Qinling orogen. The ore bodies are hosted in fine-grained clastic rocks intercalated with limestone of the Middle-Triassic Gulangti Formation and varied dikes. The gold ore bodies are strictly controlled by the NE, NW and SN trending tensional and shearing faults with high dipping angle. The ore-forming hydrothermal period is divided into four stages: pyrite-quartz, pyrite-arsenopyrite quartz, stibnite-quartz-calcite and quartz-calcite. Quartz, calcite and sericite are the main alteration minerals. Environmental scanning electron microscope and electron probe analyses show that the gold is micro-visible or invisible. Micro-visible gold is seen in the intercrystalline space or in the fissures of quartz and silicates. Invisible gold is captured in pyrite, arsenopyrite, stibnite and other sulfide mainly as nanoparticles. Gold concentration in sulphides of ore-forming stages is between 0.003% and 0.658% with an average of 0.257%. All the characteristics above lead us to draw the conclusion that the Zaozigou deposit is a typical carlin-type. Hydrogen and oxygen isotope compositions show that ore-forming fluids came from meteoric water while sulfur isotope compositions show that the sulfur came from the sedimentary strata. The meteoric water went deep into the crust through the deep cutting faults and was heated and circulated to extract the metals from the wall rocks. Gold may be transported in the hydrothermal fluids in forms of AuH₃SiO⁰₄,Au(HS)^-₂, H₂Au(Sb,As)S⁰₂ and HAu(Sb,As)S^-₃. In the early stage, the breakup of the AuH₃SiO⁰₄ complex resulted in the deposition of Au and Si, Fe, Na, Al etc; while in the later stages the breakup of  Au(HS)^-₂, H₂Au(Sb,As)S⁰₂ and HAu(Sb,As)S^-₃  complexes led to the simultaneous deposition of sulfides and gold nanoparticles in them.

Key words: Zaozigou, micro-disseminated, gold deposits, ore-forming period, stable isotopes, gold occurrence