Geochemistry of magnetite from theHuanggangskarn iron–tin polymetallic deposit in the southernGreat Xing'an Range, NE China

Abstract

The Huanggang iron–tin polymetallic skarn deposit is located in the southern Great Xing'an Range. According to the ore types and mineral assemblages, the paragenetic sequence of the Huanggang deposit can be divided into three stages, and these magnetite grains were mainly formed at the retrograde and sulphide skarn stages. The magnetite (sample HG12‐13) from the early retrograde stage is represented by fine‐grained magnetite cutting across coarse‐grained magnetite surrounded by quartz and calcite. The magnetite (sample HG12‐73) from the late retrograde stage is locally replaced by haematite along the margin or interior and is surrounded by calcite. The magnetite (sample HG12‐88) from the early sulphide stage is characterized by obvious core–rim textural features. The magnetite (sample HG‐62) from the late sulphide stage is featured by zone‐like magnetite occurring along the margin and interior of the primary magnetite.Laser ablation inductively coupled plasma mass spectrometry was used to obtain trace element concentrations of magnetite from the different mineralization stages in order to better understand the geochemical variations in the ore‐forming process. Some magnetite grains have abnormally high Mg, Al, K, Cu, Zn, and Sn due to the presence of numerous inclusions (e.g., chlorite, sylvite, chalcopyrite, sphalerite, and cassiterite). In general, magnetite grains from the different mineralization stages demonstrate similar bulk continental crust normalized trace elements patterns, suggesting that they share a similar origin. The increasing Si + Al/Mg + Mn ratios and decreasing Mg + Mn contents for magnetite show an increasing fluid–rock ratio from the retrograde to sulphide stage. Co contents of magnetite decrease abruptly from the retrograde stage to the sulphide stage, whereas Mn contents show the reverse trend, which is affected by minerals coprecipitating with magnetite. A zoned magnetite from the sulphide stage shows decreasing Ti and V contents from core to outer rim; the variation of Ti and V may be related to the temperature or oxygen fugacity. Magnetite compositions from the Huanggang deposit are similar to those from Fe, Cu‐polymetallic, or other skarn deposits, but display more variable compositions than previously presented. Our study demonstrates that the evolution of magnetite in the skarn deposit and trace element of magnetite could be a powerful tool in determining the origin of skarn iron deposit.