Contraints on Petrogenesis and Fe Fertility of Intrusive Complexes in the Han–Xing Region, North China Craton from Apatite Geochemistry

Abstract

The Han–Xing (Handan–Xingtai) region is famous for its endowment of skarn iron deposits in China. These deposits are mainly spatially and genetically associated with diorite rocks, but these rocks show different Fe ore potential. Major and trace elements composition of apatite from the Kuangshan and Fushan diorite complexes were investigated to explore the potential of apatite as a proxy of petrogenesis and Fe fertility of these rocks. All the investigated apatite grains are identified as fluorapatite, which is typical for magmatic apatite. The Sr, Y, Mn, and Heavy Rare Earth Elements (HREE) contents of apatite in the Kuangshan diorite complex are positively correlated with the increase of melt SiO2 content compared to that in the Fushan diorite complex. Apatite geochemistry indicates that magmas of the Fushan complex mainly experienced the fractional crystallization of hornblende in the deep crustal reservoirs, whereas the Kuangshan complex has experienced the fractional crystallization of hornblende in the deep and the shallow plagioclase fractional crystallization. The F, Cl and S content of the Kuangshan complex estimated by apatite volatile (F = 2632 ppm, Cl = 4100 ppm, SO3 = 140 ppm) is significantly higher than that of the Fushan complex (F = 2488 ppm, Cl = 3400 ppm, SO3 = 90 ppm). The Eu, Ce anomalies, Mn, and SO3 contents of apatite show that both of the two complexes have higher oxygen fugacity (Δ FMQ), but the oxygen fugacity of the Kuangshan complex calculated by Mn and SO3 content (Δ FMQ + 2.41) is higher than that of the Fushan complex (Δ FMQ + 1.77), which may also be one of the reasons for the great difference in ore-forming scale between the two complexes. Our results suggest that the high volatile contents and oxidation states of magma estimated by apatite, as well as the lower Sr/Y in apatite reflect favorable conditions for skarn iron mineralization. Therefore, our study shows that magmatic apatite geochemistry may be a useful tool to distinguish the Fe fertility of plutonic rocks related to skarn deposits.