Short wavelength infra-red (SWIR) characteristics of hydrothermal alteration minerals in skarn deposits: Example from the Jiguanzui Cu-Au deposit, Eastern China

The Jiguanzui Cu-Au deposit is an important skarn deposit in the Edong ore district of the Middle-Lower Yangtze River Valley metallogenic belt (MLYRB, Eastern China). The mineralization occurred dominantly in the Lower-Middle Triassic Jialingjiang Formation marble, either along interlayer fracture zones or along the intrusive contact with the Early Cretaceous quartz diorite. Based on the mineral assemblages and textural relationships, the Jiguanzui alteration/mineralization can be divided into five stages, namely (I) prograde skarn alteration (garnet-pyroxene), (II) retrograde alteration (homblende-epidote-actinolite), (III) hematite mineralization, (IV) quartz-sericite-pyrite-chalcopyrite mineralization (main Cu-Au mineralization stage) and (V) calcite-sulfide mineralization (pyrite-chalcopyrite-molybdenite-galena-sphalerite). Systematic short wavelength infrared (SWIR) spectral analyses show that alteration minerals at Jiguanzui include mainly sericite group minerals (montmorillonite, illite and muscovite), kaolinite, and carbonate minerals (calcite, ankerite and dolomite), with minor chlorite, halloysite and dickite. Spectral features of Stage IV sericite group minerals indicate that the Al-OH absorption feature wavelength positions (Pos2200) have shifted toward longer wavelengths (> 2209 nm) in the ore proximal area, which may be caused by the substitution of Fe and Mg in the octahedral Al site. The vertical distance of mineralization indication can reach up to ca. 450 m, but can be affected by the wall-rock lithology and the Jiguanshan fault zone. The Fe-OH absorption feature wavelength positions (Pos2250) of chlorite in the Middle Triassic Puqi Formation hornfels have also shifted toward longer wavelengths in the ore proximal area, which may be caused by the relative Mg-rich nature of the chlorite. We conclude that the high Pos2200 (> 2209 nm) of sericite group minerals and the high Pos2250 of chlorite can be used as prospecting tools for the Jiguanzui Cu-Au skarn mineralization.