Ore Genesis and Ore-forming Age of the Tiemahabaqin Ultra-low-grade Iron Deposit in Chengde, Hebei Province, China

The ultra-low-grade Fe ore resource (content of total Fe is lower than 20%) hosted in mafic-ultramafic complexes in Chengde plays an important role in the iron production of Hebei Province, China. In order to investigate the ore-forming age and ore genesis of the Tiemahabaqin ultra-low Fe deposit, which makes a significant contribution to Chengde's iron production, this study presents electron microprobe and ⁴⁰Ar/³⁹Ar isotopic data of the hornblendes from iron-bearing hornblendites. The electron microprobe data shows that major elements of the hornblendes from hornblendites of the Tiemahabaqin complex are Ca-rich, Mg-rich, Na-rich and K-poor with limited aviation range, which were attributed to pargasites or magnesiohastingsites. The genetic mineralogy study indicates that the mantle source derived hornblendes were contaminated with crust materials. The ⁴⁰Ar/³⁹Ar plateau ages of the hornblende from hornblendites yielded 379-401 Ma, which indicates that the ore-forming process was formed in the Devonian period. Tectonically, the iron-bearing hornblendites are probably related to post-collisinoal extension after the arc-continental collision between the Bainaimiao arc belt and the northern margin of the North China craton during the latest Silurian period.