Formation of the Jinshandian Fe skarn ore field in the Edong district, Eastern China: Constraints from U–Pb and 40Ar/39Ar geochronology

Abstract

The Jinshandian ore field represents one of the largest concentrations of Fe skarn deposits in the Edong district of Eastern China. Orebodies of these deposits occur in skarn along the contacts between quartz diorite or quartz monzonite and Triassic sedimentary country rocks. In this paper, we present new U–Pb geochronological data for magmatic zircon and hydrothermal titanite, as well as 40Ar/39Ar ages on hydrothermal phlogopite, to determine whether multiple magmatic-hydrothermal events associated with formation of the Fe skarn deposits in the Jinshandian ore field. The robust U–Pb zircon ages demonstrate that the quartz monzonite and quartz diorite were emplaced at 130Ma and 127Ma, receptively. The U–Pb and 40Ar/39Ar ages of hydrothermal titanite and phlogopite form the skarn indicate that Fe mineralization also formed at ca. 131–128Ma, but with an additional 118Ma hydrothermal event recorded by phlogopite. This geochronology confirms a causative linkage between granitic intrusion and skarn mineralization. The data also indicate that at least two hydrothermal events took place in the Jinshandian ore field, one at 131–128Ma and the other at ∼118Ma. Integrating these ages from the ore field with previous geochronological data from the broader Edong district, two regional magmatic-hydrothermal events are recognized in the Edong district after 135Ma, with the older one at 131–127Ma in excellent agreement with the ∼131Ma formation age of magnetite-apatite deposits in the eastern part of Middle–Lower Yangtze River region, and the younger one at 122–118Ma post-dating emplacement of the ca. 126Ma A-type granite and syenite in the eastern part of Middle–Lower Yangtze River region. A detailed comparison between the Fe skarn deposits and magnetite-apatite deposits in the Middle–Lower Yangtze River region suggests that these two deposit types not only share similar alteration assemblages and association with the evaporites within the ore-forming systems, but also may have formed during the same Early Cretaceous magmatic-hydrothermal event.