Multi-stage hydrothermal processes at the Laozuoshan gold deposit in NE China: Insights from textures and compositions of sulfide assemblages

Abstract The Laozuoshan gold deposit is a large gold deposit which experienced Variscan skarn mineralization and Yanshanian hydrothermal mineralization in Jiamusi massif, Northeast China. The Jiamusi massif hosts numerous gold deposits that have the commonly considered as products of the metamorphic hydrothermal process during the twice oceanic subductions. We conduct detailed texture observations, determine the sulfides phases by the laser Raman spectrum, and obtain in situ composition data with electron probe micro analyzer to further understand the formation and metasomatism of multi-stage sulfides in the Laozuoshan gold deposit. The paragenetic relationship between pyrite and marcasite under nanoscale was also clarified by the combination of both focused ion beam scanning electron microscopy and transmission electron microscopy. According to the observed texture relationships among ore and gangue minerals, we divide the Variscan skarn mineralization period into three stages (Stage I to Stage III), and the Yanshanian hydrothermal mineralization period into four stages (Stage IV to Stage VII). The sulfides are mainly pyrrhotite in Stage I, with minor chalcopyrite, arsenopyrite, and pyrite. The native golds rich in silver (averaging 15.44 wt.%) are distributed in arsenopyrite which is rich in cobalt (averaging 2.69 wt.%). The pyrite is the main sulfide in Stage II, with minor chalcedony and calcite. The compositions of sulfides in both Stage I and Stage II indicate that the fluid was the sulfide-poor and weakly cobalt-rich system. The mineral stability relationships in both Stage I and Stage II confirm that the increase of sulfur fugacity and oxygen fugacity and the decrease of temperature. The sulfides are mainly arsenopyrite in Stage III, with minor native golds and tellurbismuth. The native golds contain averaging 8.02 wt.% silver. Arsenopyrite geothermometers indicate they formed at 420 to 390°C in Stage III. The sulfur fugacity and oxygen fugacity of ore fluids were below the pyrite-pyrrhotite buffer during the Stage III, revealing a reduced fluids system. In the hydrothermal mineralization period, multiple magmatic activities caused multi-stage hydrothermal fluids activities. From Stage V to Stage VII, the pyrrhotite replaced by marcasite and pyrite through the dissolution-reprecipitation mechanism, indicating the increase in pH value and the oxygen fugacity of the fluids, and alteration from the weakly acidic and weakly reduced conditions to neutral and weakly oxidizing condition. The mineralogical and compositional evolution of the sulfides in the Laozuoshan gold deposit constrain the physicochemical conditions of multi-stage ore-forming fluids, which is of considerable significance to understand the metamorphic-hydrothermal superimposed gold deposit fluids evolution in the Jiamusi massif.