Geology, fluid inclusions, 40Ar/39Ar geochronology and H-O-S-Pb isotopes of the Galale Cu[sbnd]Au deposit, Northwestern Tibet, China

Abstract

The Galale CuAu skarn deposit is located on the southern margin of the Bangong-Nujiang metallogenic belt, Tibet, and the CuAu mineralization is related to the Late Cretaceous granodiorite. The orebodies occurring as stratiform, lenticular and vein types are hosted within the skarn along the contact between the granodiorite and dolomitic marble or calcitic marble of the Jiega Formation. At least four hydrothermal alteration stages of the deposit have been recognized: prograde magnesian and calcic skarn stage (Stage 1), retrograde skarn stage (Stage 2), hydrosilicate alteration stage (Stage 3), and phyllic alteration stage (Stage 4).Fluid inclusions from skarn rocks and quartz veins are indicative of multiple fluid events. The prograde skarn was formed from a medium-salinity, hot (466–518 °C), high pressure (>1000 bar) homogenous magmatic-hydrothermal fluid, under its possible direct separation from crystallizing magma. The subsequent retrograde skarn was formed from a slightly lower temperatures (370–460 °C) and lower pressure (~800 bar) homogenous magmatic-hydrothermal fluid that could be supplied by crystallizing magma. Sharp drop of pressure (from 800 bar to 300 bar) and significant decrease of temperatures (low to 300 °C) may be related to fracturing and increase of porosity during alteration of skarns, and caused the fluid boiling in hydrosilicate alteration stage. The fluid from the latest phyllic alteration stage is characterized by dominant meteoric water and more lower temperatures (low to 178 °C). Decrease of fluid temperatures and pressures, fluid boiling and changes of redox regime are considered the crucial mechanisms for controlling the precipitation of the ores.The δ34S values of the sulfides range from −4.4 to 6, primarily from −0.9 to 1.7, that suggest a magmatic source. The sulfides yield narrow ranges of 206Pb/204Pb ratios (18.141–18.871) and 207Pb/204Pb ratios (15.588–15.701) and a wide range of 208Pb/204Pb ratios (38.359–39.008), indicating a mixed crust-mantle ore-forming metal source. The 40Ar39Ar plateau age of phlogopite from the chalcopyrite-bearing phlogopite tremolite skarn (Stage III) is 87.5 ± 0.6 Ma, which is consistent with the crystallization age of the Late Cretaceous granodiorite. Consequently, taking the data together suggests that the Galale CuAu deposit is a typical oxidized CuAu skarn deposit related to the Late Cretaceous magmatic-hydrothermal systems.