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• Stage II and III in the Sizhuang gold deposit share similar ore-forming material source. • H 2 S concentration affects stability of gold, pyrrhotite and magnetite. • Sulfidation and fluid immiscibility promote precipitation of gold, pyrrhotite, magnetite and barite. • Two-stage gold mineralization from a single ore-forming fluid is suggested. 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 (H 2 S entering vapor phase), which led to the total sulfur concentration (∑S total) decrease in the ore-forming fluids. Thermodynamic calculations at different ∑S total and constant pressure (2000 bar) and temperature (300 °C) in Fe-Cu-O-S-H system reveal that ∑S total 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 H 2 S (>90 mol%) under the bornite–pyrite phase boundaries in the acidic ore-forming fluids; and that decreases in H 2 S concentration can efficiently cause gold precipitation by reducing gold bisulfide solubility up to 2–3 orders of magnitude while ∑S total 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 H 2 S 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. [ABSTRACT FROM AUTHOR]

The Jiaodong Peninsula in eastern China is the third largest gold-mining area and one of the most important orogenic gold provinces in the world. Ore shoots plunging in specific orientations are a ubiquitous feature of the Jiaodong lode deposits. The Sizhuang gold deposit, located in northwestern Jiaodong, is characterized by orebodies of different occurrences. The orientation of ore shoots has remained unresolved for a long time. In this paper, geostatistical tools were used to determine the plunge and structural control of ore shoots in the Sizhuang deposit. The ellipses determined by variogram modeling reveal the anisotropy of mineralization, plus the shape, size, and orientation of individual ore shoots. The long axes of the anisotropy ellipses trend NE or SEE and plunge 48° NE down the dip. However, individual ore shoots plunge almost perpendicular to the plunge of the ore deposit as a whole. This geometry is interpreted to have resulted from two periods of fluid flow parallel to two sets of striations that we identified on ore-controlling faults. Thrust-related lineations with a sinistral strike-slip component were associated with early-stage mineralization. This was overprinted by dextral and normal movement of the ore-controlling fault that controlled the late-stage mineralization. This kinematic switch caused a change in the upflow direction of ore-forming fluid, which in turn controlled the orientation of the large-scale orebodies and the subvertical plunge of individual ore shoots. Thus, a regional transition from NW-to-SE-trending compression to NW-to-SE-trending extension is interpreted as the geodynamic background of the ore-forming process. This research exemplifies an effective exploration strategy for studying the structural control of the geometry, orientation, and grade distribution of orebodies via the integration of geostatistical tools and structural analysis. [ABSTRACT FROM AUTHOR]

The Sizhuang gold deposit with a proven gold resource of >120 t, located in northwest Jiaodong Peninsula in China, lies in the southern part of the Jiaojia gold belt. Gold mineralization can be divided into altered rock type, auriferous quartz vein type, and sulfide-quartz veinlet in K-feldspar altered granite. According to mineral paragenesis and mineral crosscutting relationships, three stages of metal mineralization can be identified: early stage, main stage, and late stage. Gold mainly occurs in the main stage. The petrography and microthermometry of fluid inclusion shows three types of inclusions (type 1 H2O–CO2 inclusions, type 2 aqueous inclusions, and type 3 CO2 inclusions). Early stage quartz-hosted inclusions have a trapped temperatures range 303–390 °C. The gold-rich main stage contains a fluid-inclusion cluster with both type 1 and 2 inclusions (trapped between 279 and 298 °C), and a wide range of homogenization temperatures of CO2 occurs to the vapor phase (17.6 to 30.5 °C). The late stage calcite only contains type 1 inclusions with homogenization temperatures between 195 and 289 °C. With evidences from the H–O isotope data and the study of water–rock interaction, the metamorphic water of the Jiaodong Group is considered to be the dominating source for the ore-forming fluid. The ore-fluid belonged to a CO2–H2O–NaCl system with medium-low temperature (160–360 °C), medium-low salinity (3.00–11.83 wt% NaCl eq.), and low density (1.51–1.02 g/cm3). Fluid immiscibility caused by pressure fluctuation is the key mechanism in inducing gold mineralization in the Sizhuang gold deposit. [ABSTRACT FROM AUTHOR]