Trace elements of pyrite and S, H, O isotopes from the Laowan gold deposit in Tongbai, Henan Province, China: implications for ore genesis

Abstract

The Laowan deposit is a large gold deposit in the Qinling-Dabie orogenic belt where pyrite is the main Au-bearing mineral phase. We present results from the occurrences of gold, trace elements and sulfur isotopes of pyrite, and hydrogen and oxygen isotopes of quartz and calcite to elucidate the sources of ore-forming fluid; the genesis of pyrite and the ore-forming process. From field observations, five generations of pyrite are identified; one formed in a metamorphic-diagenetic epoch (PyI), and the others during four mineralization stages: 1) the coarse-grained pyrite-quartz stage (PyII), 2) the quartz and medium- to fine-grained pyrite stage (PyIII), 3) the polymetallic sulfide stage (PyIV), and 4) the carbonate-quartz stage (PyV). Gold mainly occurs in PyIII and PyIV. We find that Au, Ag, Pb, and Cu are incorporated into pyrite as micro-/nano-inclusions and that Co, Ni, As, and Se enter the pyrite lattice via isomorphous replacement. The Co/Ni values and Se concentrations indicate that PyI formed from metamorphic hydrothermal fluids and that pyrites (PyII, PyIII, and PyIV) from the ore-forming stages typically reflect a hydrothermal genesis involving magmatic fluid. The δ34S values of PyI (1.45 ‰–2.09‰) are similar to that of plagioclase amphibole schist, indicating that S was primarily derived from wall rock, while those of PyII, PyIII, and PyIV (3.10‰–5.55‰) suggest that S was derived from the Guishanyan Formation and the Laowan granite. The four mineralization stages show a systematic decrease in δD (from −77.1‰ to −82.8‰, −84.7‰, and −102.7‰), while the $${{\rm{\delta }}^{18}}{{\rm{O}}_{{{\rm{H}}_2}{\rm{O}}}}$$ values showed a gradual decrease from 5.7 to 2.7‰, 1.0‰, and −1.3‰. These data show that the ore-forming fluid was similar to a mixture of magmatic and meteoric waters. Thus, we conclude that the Laowan gold deposit is related to magmatic-hydrothermal fluid.