Two-stage gold deposition in response to H2S loss from a single fluid in the Sizhuang deposit (Jiaodong, China)

Abstract

The material source, fluid evolution and mineralizing mechanism in the Jiaodong gold province have been hotly debated for several decades. The Sizhuang gold deposit provides an excellent object for studying these issues. Gold in this deposit mainly precipitated in the quartz–Au–pyrite (stage II) and quartz–Au–polymetallic (stage III) stages. Stage III can be further subdivided into the early stage III (chalcopyrite, galena with minor amounts of electrum, pyrrhotite, pyrite, siderite and sphalerite) and the late stage III (chalcopyrite, magnetite, siderite with a little pyrrhotite). Pyrite from stage II and stage III contains consistent δ34S signatures (8.1‰–9.6‰ and 7.5‰–9.8‰, respectively) as well as trace elements (e.g. Co: 8.19 vs. 2.63 ppm, Ni: 3.54 vs. 0.74 ppm, As: 29.95 vs. 5.78 ppm, and Bi: 3.18 vs. 7.81 ppm). These consistent geochemical features combined with previous fluid inclusion and C-H-O isotope studies argue for a common source for ore-forming materials and fluids of the stage II and III at Sizhuang deposit. During the stage II and III, reduced sulfur species may have been lost from ore-forming fluids in response to pyrite precipitation (HS- being scavenged by pyrite) and fluid immiscibility (H2S entering vapor phase), which led to the total sulfur concentration (∑Stotal) decrease in the ore-forming fluids. Thermodynamic calculations at different ∑Stotal and constant pressure (2000 bar) and temperature (300 °C) in Fe-Cu-O-S-H system reveal that ∑Stotal decrease can lead to first enlargement and then shrinkage of pyrrhotite stability field with magnetite stability field being broadened to more acidic field; that sulfur species is dominated by H2S (>90 mol%) under the bornite–pyrite phase boundaries in the acidic ore-forming fluids; and that decreases in H2S concentration can efficiently cause gold precipitation by reducing gold bisulfide solubility up to 2–3 orders of magnitude while ∑Stotal just decreases ~50 times. Pyrrhotite and magnetite precipitated due to enlarged stability field during this physicochemical process, which is in line with geologic observations. It is proposed that the two-stage gold deposition by H2S loss from a single ore-forming fluid could account for gold precipitation coupling with occurrence of pyrrhotite and magnetite in the mineral assemblage at Sizhuang deposit.