Geology, geochronology, and fluid evolution of the Pingshun skarn iron deposit, southern Taihang Mountains

Abstract

The Pingshun skarn iron deposit is located in the southern Handan–Xingtai iron district of the southern Taihang Mountains, North China Craton. Skarns and ore bodies occur as lamina, lenses, and veins along the contact between a dioritic pluton and carbonates of the Middle Ordovician Majiagou Formation. The paragenetic sequence is albite followed by diopside, then garnet, magnetite, and finally calcite. Diorite and olivine-bearing hornblende gabbro in the Pingshun area contain zircon grains with SHRIMP U–Pb ages of 123.4 ± 1.7 and 125.5 ± 2.3 Ma, respectively. Based on the close spatial association between diorite and mineralization, we suggest that both magma activities and iron mineralization in the Pingshun area took place at 125–123 Ma. Three types of fluid inclusions identified in minerals from the Pingshun deposit include three-phase (type 1), two-phase (type 2), and pure liquid (type 3) inclusions. The ore-forming fluid evolved from high temperature (515–679 °C) with high salinity (39.8–77.6 wt% NaCl equiv.) in diopside, to medium–high temperature (365–496 °C) with moderate–low salinity (11.8–17.6 wt% NaCl equiv.) in garnet, and to low temperature (162–295 °C) with moderate–low salinity (6.7–35.5 wt% NaCl equiv.) in calcite. The presence of hematite, halite, and gypsum daughter minerals in type 1 fluid inclusions and the occurrence of andradite in skarns indicate that the pre-ore–forming fluid was oxidized. The δ34S values of pyrite separated from ores vary from 12.5 to 17.4‰, with 206Pb/204Pb = 17.26–19.03, 207Pb/204Pb = 15.42–15.69, and 208Pb/204Pb = 37.29–39.76, suggesting the ore-forming fluid was originated from a magmatic source and with additional crustal material during the mineralization process. This is consistent with H–O isotope data (δ18Odiopside-H2O = 10.8‰, δ18Ogarnet-H2O = 9.5–10.8‰, δ18Omagnetite-H2O = 9.8–13.4 ‰, δDgarnet varying from − 106 to − 86‰, δDdiopside = − 90‰). The microthermometry of fluid inclusions and isotope data reveal that the addition of gypsolyte and salt-bearing marlstone into the ore-forming fluid increases Na, Ca, and Cl and that it significantly increases the ability to transport Fe and enhances the capacity of Fe in the ore-forming fluid. Fluid boiling and additional meteoric water involved are two major key factors controlling the ore precipitation. Mantle materials involved in magma and high oxygen fugacity are considered to be crucial roles in the formation of numerous iron deposits in the Handan–Xingtai iron district.