Hydrothermal evolution and ore genesis of the Laozuoshan Au skarn deposit, northeast China: Constrains from mineralogy, fluid inclusion, and O-C-S-Pb isotope geochemistry

The Laozuoshan mine, located in the central Jiamusi block, is the largest gold skarn deposit in Northeast China. It comprises three ore zones (East, Middle, West) with complex stratigraphic and structural controls. The skarn-type orebodies mainly occur in the East and Middle ore zones along the contact zones between Hercynian granite and Precambrian metasedimentary rocks. The earliest stage of alteration is characterized by potassium hornfels. Subsequently, prograde skarn formation occurred, characterized by garnet in the grossular-andradite series and pyroxene in the hedenbergite-diopside series. This was followed by a retrograde skarn overprint of hydrous silicates and ore minerals. Three types of garnet with distinct textures and elemental compositions are recognized, which indicate fluctuating physicochemical conditions in the hydrothermal fluids. The fluids associated to the prograde skarn are characterized by high temperature and high salinity, which underwent fluid boiling that led to precipitation of minor pyrite and arsenopyrite. The main gold and sulfide mineralization is related to the retrograde skarn overprint, where cooling and strong fluid-rock interactions occurred. Gold precipitation is estimated to have occurred at temperatures of 380-470 degrees C, based on the arsenopyrite geothermometer and fluid inclusion microthermometry. The fluid inclusion and C-O isotope data indicate that the ore-forming fluids were dominated by magmatic water during the prograde skarn stage and the main ore-forming stages. Mixing between meteoric and magmatic fluids occurred during the latest stages of the retrograde skarn formation. Both sulfur and lead isotope compositions of sulfides support a magmatic origin for the ore-forming materials. On the basis of the available geological and geochemical evidence, we propose that the Laozuoshan deposit is a reduced gold skarn deposit, genetically related to the granitic pluton.