Multistage gold mineralization in the Hadamengou gold deposit in the northern margin of the North China Craton: Insights from in-situ trace element contents and sulfur isotope analyses of pyrite

The Hadamengou gold deposit is one of the largest gold deposits in the Wulashan-Daqingshan gold mineralization cluster, in the northern margin of the North China Craton. The gold deposit, which is hosted by Archean gneiss, comprises potassic alteration and abundant quartz-pyrite veins, with five types of hydrothermal pyrite associated with gold, tellurides, and sulfides. The earliest stage of pyrite (Py1) occurs as random disseminations within the limited volume of modified quartz-pyrite veins and was formed during or prior to the last episode of metamorphism. Py2, from the potassic and chlorite-sericite alteration zones, is related to the activity of potassic fluids, and contains inclusions of native gold, galena, melonite, electrum, and petzite. Py3, without inclusions, occurs as randomly- disseminated grains within the limited volume of grey quartz-pyrite veins. A few native gold grains are found along fractures of quartz associated with Py3. Py4 occurs as veinlets or aggregates in the milky quartz-pyrite veins and contains the highest Au concentrations. Abundant native gold, calaverite, and electrum occur along the grain boundaries and fractures, or as inclusions within Py4 and quartz. Py5 occurs as rounded and cataclastic shapes within the limited volume of quartz-carbonate veins and is depleted in Au. LA-ICP-MS trace element analysis of the pyrite shows that Py1 has a low content of gold (median of 0.07 ppm) and other trace elements (Ag, Te, Pb, Cu, Bi, and As). Potassic alteration, grey quartz-pyrite veins, and milky quartz-pyrite veins make a very large contribution to the gold endowment, with a relatively high gold content in pyrite, including Py2 (median of 0.8 ppm), Py3 (4.3 ppm), and Py4 (2.5 ppm). Other trace elements (e.g., Ag, Te, Pb, Mo, and Bi) are also significantly enriched within Py2 to Py4. Little or no gold was detected in Py5. Thus the potassic alteration and milky quartz-pyrite veins were major contributors to gold mineralization, based on the volume and Au content of pyrite within the Hadamengou deposit. In-situ LA-MC-ICP-MS sulfur isotope analyses of pyrite show negative values and a narrow range of variation of delta 34S (averages of -11.4%o, -11.8%o, -10.5%o, and -9.4%o for Py2-Py5, respectively), indicating an oxidized magmatic sulfur source for the pyrites in Hadamengou. Additionally, the occurrence of tellurium minerals associated with various pyrites, and the high Te concentration of various pyrites, also suggest that ore-forming fluids were derived from an oxidized alkaline magma. These results, combined with published geochronological data, suggest that during two episodes, Aurich potassic and SiO2-rich fluids separated from the deep-lying Devonian alkaline magma during ascent along the EW-trending faults, and reacted together with the wall rocks to form potassic and chlorite-sericite alteration zones, resulting in gold precipitation.