Mineral compositions and fluid evolution of the Tonglushan skarn Cu-Fe deposit, SE Hubei, east-central China

The Tonglushan Cu-Fe deposit (1.12 Mt at 1.61% Cu, 5.68 Mt at 41% Fe) is located in the westernmost district of the Middle-Lower Yangtze River metallogenic belt. As a typical polymetal skarn metallogenic region, it consists of 13 skarn ore-bodies, mainly hosted in the contact zone between the Tonglushan quartz-diorite pluton (140 Ma) and Lower Triassic marine carbonate rocks of the Daye Formation. Four stages of mineralization and alterations can be identified: i.e. prograde skarn formation, retrograde hydrothermal alteration, quartz-sulphide followed by carbonate vein formation. Electron microprobe analysis (EMPA) indicates garnets vary from grossular (Ad(20.2-41.6)Gr(49.7-74.1)) to pure andradite (Ad(47.4-70.7)Gr(23.9-45.9)) in composition, and pyroxenes are represented by diopsides. Fluid inclusions identify three major types of fluids involved during formation of the deposit within the H2O-NaCl system, i.e. liquid-rich inclusions (Type I), halite-bearing inclusions (Type II), and vapour-rich inclusions (Type III). Measurements of fluid inclusions reveal that the prograde skarn minerals formed at high temperatures (>550 degrees C) in equilibrium with high-saline fluids (>66.57 wt.% NaCl equivalent). Oxygen and hydrogen stable isotopes of fluid inclusions from garnets and pyroxenes indicate that ore-formation fluids are mainly of magmatic-hydrothermal origin (delta O-18 = 6.68 parts per thousand to 9.67 parts per thousand, delta D = -67 parts per thousand to -92 parts per thousand), whereas some meteoric water was incorporated into fluids of the retrograde alteration stage judging from compositions of epidote (delta O-18 = 2.26 parts per thousand to 3.74 parts per thousand, delta D = -31 parts per thousand to -73 parts per thousand). Continuing depressurization and cooling to 405-567 degrees C may have resulted in both a decrease in salinity (to 48.43-55.36 wt.% NaCl equivalent) and the deposition of abundant magnetite. During the quartz-sulphide stage, boiling produced sulphide assemblage precipitated from primary magmatic-hydrothermal fluids (delta O-18 = 4.98 parts per thousand, dD = -66 parts per thousand, delta S-34 values of sulphides: 0.71-3.8 parts per thousand) with an extensive range of salinities (4.96-50.75 wt.% NaCl equivalent), temperatures (240-350 degrees C), and pressures (11.6-22.2 MPa). Carbonate veins formed at relatively low temperatures (174-284 degrees C) from fluids of low salinity (1.57-4.03 wt.% NaCl equivalent), possibly reflecting the mixing of early magmatic fluids with abundant meteoric water. Boiling and fluid mixing played important roles for Cu precipitation in the Tonglushan deposit.