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 the early hydrothermal mineralization and the late hydrothermal mineralization in Jiamusi Massif, Northeast China. The Jiamusi Massif hosts numerous gold deposits that are commonly considered as products of the metamorphic hydrothermal process during two oceanic subductions episodes. In this study, we report detailed textures observations, identify the sulfides phases by the laser Raman spectroscopy, and obtain in situ composition data with electron probe micro analysis to further understand the formation and metasomatism of multi-stage sulfides deposition in the Laozuoshan gold deposit. The paragenetic relationship between pyrite and marcasite at the nanoscale was also clarified by focused ion beam scanning electron microscopy and transmission electron microscopy. According to the observed textural relationships among ore and gangue minerals, the early hydrothermal mineralization period can be divided into three stages (Stage I to Stage III), and the late hydrothermal mineralization period into four stages (Stage IV to Stage VII). The sulfides are mainly pyrrhotite (Po-1) in Stage I, with minor chalcopyrite, arsenopyrite, and pyrite. The Ag-rich (averaging 15.44 wt%) electrum (Au-1) is hosted in arsenopyrite, which is also rich in cobalt (averaging 2.69 wt%). Pyrite (Py-2) is the main sulfide in Stage II, with minor chalcedony and calcite. The mineral stability relationships in both Stage I and Stage II confirm that there was an increase in sulfur and oxygen fugacities and a decrease in temperature. The sulfides are mainly arsenopyrite (Apy-2) in Stage III, with minor native golds and tellurbismuth. The native golds (Au-2) average 8.02 wt% silver. Arsenopyrite geothermometers indicate precipitation temperatures between 420 and 390 °C for Stage III. The sulfur and oxygen fugacities of the ore-related fluids were below the pyrite-pyrrhotite buffer during Stage III, revealing a reduced fluids system. In the late hydrothermal mineralization period, multiple magmatic activities caused multi-stage hydrothermal fluids events. In Stage VI, the pyrrhotite replaced by marcasite and pyrite through the dissolution-reprecipitation mechanism, indicating the increase in pH and oxygen fugacity of the fluids, and alteration from weakly acidic and weakly reduced conditions to neutral and weakly oxidizing conditions. 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 in understanding the metamorphic-hydrothermal superimposed gold-bearing fluids evolved in the Jiamusi Massif.