Geology and geochemistry of the Changlongshan skarn iron deposit, Anhui Province, China

Abstract

The Changlongshan skarn iron deposit is located in the northeastern part of the Tongling-Fanchang Fe–Cu district, Anhui Province, People's Republic of China. It is a specularite (α-Fe 2O 3)-dominated iron deposit hosted in Middle Carboniferous to Lower Permian limestones, with a drill indicated resource of 28.84 million tons of Fe grading 35.9–58.6%. Both skarns and iron orebodies occur as tabular bodies along bedding-parallel faults, at some distance from the inferred granitic intrusions. Prograde skarn alteration consists mainly of andradite and hedenbergite whereas retrograde skarn is characterized by the development of actinolite, chlorite and calcite. The mineralogy of ores is dominated by specularite, magnetite, calcite and quartz. Primary fluid inclusions in garnet consist of aqueous liquid, a vapour bubble and a halite crystal. Microthermometric measurements yield a homogenization temperature range for skarn alteration of 423 to 478°C, with a salinity of 34 to 49 wt.% NaCl equivalent. Fluid inclusions in calcite associated with mineralization generally consist of a vapour bubble and a liquid phase with a rare occurrence of three-phase inclusions. Homogenization temperatures for two phase inclusions vary from 239°C to 303°C with a salinity of 13 to 22 wt.% NaCl equivalent. Homogenization of three phase inclusions was observed between 264° and 287°C, coupled with a salinity of 31 to 36 wt.% NaCl equivalent. Stable isotope data indicate magmatic water was involved in the early stage of skarn formation ( δ 18 O H 2O =3.7‰ to 7.6‰, δD H 2O =−71.3‰ to −51.2‰) and a mixed magmatic–meteoric water was responsible for the subsequent mineralization ( δ 18 O H 2O =2.1‰ to −3.1‰, δD H 2O =−40.1‰ to −28.1‰). Geological and geochemical evidence obtained in this study supports a metasomatic replacement origin for the Changlonshan iron deposit, a conclusion contrary to previous syngenetic-based interpretation.