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The Guilaizhuang gold deposit is composed of limestone- and breccia-type ores. In the limestone ores, gold is hosted by pyrite, As-bearing pyrite and arsenopyrite, whereas in the breccia ores, tellurides are the main gold carriers. Selenium often isomorphously substitutes for sulfur in sulfides in the limestone ores, but occurs as selenides in the breccia ores. Hematite is the only oxide in the breccia ores, associated with barite and other tellurides. In this paper, the thermodynamic parameters, especially the Gibbs free energies of formation and reaction, of related sulfides, tellurides, selenides and oxides are computed to explain the distinct mineral styles in the limestone- and breccia-type ores. We construct the phase relations among minerals delineated by phase diagrams at 250°C, the temperature for gold precipitation in the Guilaizhuang deposit. According to the phase relations between sulfides and selenides, we propose that gold has been transported as [Au(HS,HSe)2]−, and released during rapid formation of pyrite, As-pyrite and arsenopyrite, at logfS2(g) between −12.8 and −11.4 (250°C). In the breccia ores, logfTe2(g) and logfSe2(g) are constrained within −12.9 to −9.4, and −12.4 to −6.9 (250°C), respectively, based on the mineral assemblages observed in the ores. The phase relations between tellurides and selenides indicate that calaverite, the only stable gold telluride mineral in nature, was not stable in the breccia ores at 250°C during the gold precipitation. Based on EMPA analysis, the so-called “calaverite” revealed from microscopic observations is in fact Au–Ag telluride, probably derived from the decomposition of sylvanite (AuAgTe4) and petzite (AuAg3Te2), the only two stable Au–Ag tellurides in nature at high temperature. However, we do not exclude the possibility that calaverite occurs in this deposit, which may have precipitated in early relatively high tellurium concentration stage preceding gold mineralization. Selenium was not detected in gold-bearing tellurides, implying that selenide is unrelated with gold precipitation directly in the breccia ores, but can buffer the tellurium fugacity, and thus influence the gold precipitation indirectly. Furthermore, in logfO2(g) range of >−35.4 at 250°C, the majority of the tellurides and selenides can be stable in the breccia ores, but residual sulfides are proposed to be oxidized into sulfate minerals, which could also free some gold into ores, although less important relative to that formed from telluride decomposition. [ABSTRACT FROM AUTHOR]

Abstract: The Changyi banded iron formation (BIF) in the eastern North China Craton (NCC) occurs within the Paleoproterozoic Fenzishan Group. The BIF shows alternating quartz-rich light and magnetite-rich dark bands with magnetite (15–65vol.%), quartz (25–65vol.%) and amphibole (15–30vol.%) constituting the major minerals. Minor garnet, epidote, chlorite, calcite, biotite and pyrite occur locally. The BIF bands are interlayered with amphibolite, hornblende gneiss, biotite quartz schist, garnet biotite schist, biotite gneiss and leptynite, and are intruded by granites. LA-ICP-MS U–Pb dating on zircons separated from the BIF bands and the wallrocks constrains the depositional age as 2240–2193Ma and metamorphic age as ~1864Ma. The dominant composition of SiO2 +Fe2O3 T (average value of 92.3wt.%) of the BIF bands suggests their formation mainly through chemical precipitation. However, the widely varying contents of major elements such as Al2O3 (0.58–6.99wt.%), MgO (1.00–3.86wt.%), CaO (0.22–4.19wt.%) and trace elements such as Rb (2.06–40.4ppm), Sr (9.36–42.5ppm), Zr (0.91–23.6ppm), Hf (0.04–0.75ppm), Cr (89.1–341ppm), Co (2.94–30.4ppm), and Ni (1.43–52.0ppm) clearly indicate the incorporation of clastics, especially continental felsic clastics, as also confirmed by the presence of ancient detrital zircons in the BIF bands. When normalized against Post Archean Average Shale (PAAS), the seawater-like signatures of REE distribution patterns, such as LREE depletion, positive La and Y anomalies, and superchondritic Y/Ho ratios (average value of 36.3), support the deposition in seawater. Strong positive Eu anomalies (Eu/Eu*PAAS =1.14–2.86) also suggest the participation of hydrothermal fluids. In addition, the sympathetic correlation between Cr, Co and Ni as well as the Co+Ni+Cu vs. ∑REE and the Al2O3 vs. SiO2 relations further indicates that the iron and silica mainly originated from hydrothermal fluids. Combined with regional geological investigation and protolith restoration of the wallrocks, a continental rift environment is suggested for the Changyi BIF deposition. The appearance of negative CePAAS anomalies might suggest the influence of the Great Oxidation Event at the time of deposition. The Changyi BIF witnessed the major Paleoproterozoic rifting–collision events in the NCC and their unique distribution in the NCC contrasts with other examples elsewhere in the world. [Copyright &y& Elsevier]