Multistage fluid sources and evolution of Qinglong Sb-(Au) deposit in northern margin of Youjiang basin, SW China: REE geochemistry and Sr-H-O isotopes of ore-related jasperoid, quartz and fluorite

Abstract

Silicic and fluorite alterations are widespread and spatially linked to Au and Sb mineralization in the Youjiang basin, SW China. However, metallogenic studies on the hydrothermal quartz and fluorite, and their implications on the fluid source(s) and evolution are very limited, leading to ambiguity about the Au-Sb ore formation. The large Qinglong Sb-(Au) deposit in the basin is characterized by an early-ore jasperoid + stibnite + fluorite assemblage, and a late-ore stibnite + Au-bearing pyrite + veined-quartz assemblage. Early-ore jasperoid-fluorite-stibnite precipitated mainly in open-space wallrock breccias. The relatively wide fluorite 87Sr/86Sr range (0.70776–0.70931) suggests the mixing of two 87Sr isotopically distinct end-members. The highly-radiogenic Sr (87Sr/86Sr > 0.70800) of the Qinglong fluorite suggests that the fluids were likely originated from the Lower Paleozoic basement rocks of the basin. MREE-enriched patterns of the fluorite samples suggest that MREEs were leached from these Lower Paleozoic sulfate-rich basement rocks by the fluids (which contain mainly metal sulfate complexes), as also supported by the positive Y anomalies (Y/Y* = 2.89 – 4.50) and elevated Y/Ho ratios (86.76 – 134.73) of the fluorite. Moreover, the depleted radiogenic Sr, absence of Eu anomalies, negative Ce anomalies (Ce/Ce* = 0.72 – 0.87), and the presence of hydrocarbon-rich fluid inclusions indicate basinal fluid (paleo-seawater/formation water) incursion into the ore fluids. Hydrogen and oxygen isotope compositions of the fluid inclusions (in fluorite) also indicate that the ore fluids have had basinal carbonic fluid contribution. A compilation of the published H-O-S isotope compositions of coeval Carlin-like gold deposits in the Youjiang basin suggests that the initial fluid may have had a magmatic origin. A general fluid δ18O drop may have occurred during the ore formation, and was likely attributed to the varying degree of fluid dilution. Taken together, the ore fluids may have mixed with the circulating basinal fluids at shallow depths, causing significant temperature and fO2 drop, which further lowered the stibnite solubility and triggered its precipitation (together with jasperoid and fluorite). At the late-ore stage, the lack of fluorite suggests that the basinal fluid incursion had waned, and the hydrothermal fluid was again dominantly magmatic-hydrothermal. Quartz veins/stockworks were developed in non-brecciated tuffaceous wallrocks and have disseminated sulfide mineralization. This, and the presence of CO2-rich fluid inclusions, could be caused by the dissolution of wallrock carbonate minerals by the magmatic-hydrothermal fluids. Therefore, we proposed that the mixing between magmatic-hydrothermal and basinal fluids had formed the early-ore Sb mineralization, whilst fluid-rock interactions had formed the varying fluid compositions and late-ore Au mineralization.