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

摘要

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 H_2O-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°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 (?~(18)O = 6.68‰ to 9.67‰, ?D = -67‰ to -92‰), whereas some meteoric water was incorporated into fluids of the retrograde alteration stage judging from compositions of epidote (?~(18)O = 2.26‰ to 3.74‰, ?D= -31‰ to -73‰). Continuing depressurization and cooling to 405-567°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 (?~(18)O = 4.98‰, ?D = -66‰, ?~(34)S values of sulphides: 0.71-3.8‰) with an extensive range of salinities (4.96-50.75 wt.% NaCl equivalent), temperatures (240-350°C), and pressures (11.6-22.2 MPa). Carbonate veins formed at relatively low temperatures (174-284°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.