Ore genesis and fluid evolution of the Qiaomaishan Cu-W deposit, in the Middle-Lower Yangtze River Metallogenic Belt: Evidence from in situ analyses of apatite and scheelite

The newly exploited Qiaomaishan skarn-type Cu-W deposit, located in the Xuancheng ore-cluster district in the Middle-Lower Yangtze River Metallogenic Belt (MLYRB), is one of the few tungsten polymetallic deposits in this region. This study conducted a series of in situ analyses of elements and isotopes for apatite, scheelites and sulfides, aiming to provide new insights into the source, hydrothermal ore-forming processes, fluid evolution history and genesis of this deposit and important knowledge to investigate regional tungsten mineralization events in the MLYRB. Apatite geochemistry shows that the early magma source of the ore-related rocks in Qiaomaishan obviously displayed adakitic affinity with high oxygen fugacity and featured fluorine-rich fluids originating from a W-rich mixed source. The tungsten mineralization is dominated by scheelite in the Qiaomaishan deposit. Based on microscopic observations and in situ analysis, two types of scheelite with different geochemical characteristics were recognized. Two types of scheelite in the Qiaomaishan have controlled by different mechanism of REE substitutions. Scheelite-I occurs in the sulfide stage and displays high Na and Sigma REE concentrations, downwards-sloping REE fractionation trends with negative Eu anomalies, and Sr-87/Sr-86 ratios (0.70945-0.71033) similar to those of ore-related granodiorite porphyry. However, scheelite-II shows relatively low Sigma REE concentrations, high Na contents, MREE-rich patterns with positive Eu anomalies, and high Sr-87/Sr-86 ratios (0.71477-0.72352). Distinct Eu anomalies and Sr concentrations of the two types of scheelite and S isotopes of sulfides indicate that the early-stage fluids were inherited from the primary granodioritic magmatichydrothermal fluids; however, the late-stage fluids were modified by intense fluid-rock interactions and inputs from the host gypsum-bearing carbonates. Collectively, the Fand W-rich sources, intense fluid-rock interactions and changes in oxidation-reduction conditions favoured tungsten mineralization, which resulted in the formation of the Qiaomaishan tungsten deposit.