Element geochemical characteristics and sedimentary palaeoenvironment of the Yanqianshan iron deposit in Liaoning Province

Abstract: Located in the central eastern part of Liaoning Province, the Yanqianshan iron deposit is a typical iron deposit of this area. The authors studied the petrographic and mineralogical characteristics of the Yanqianshan iron deposit, and analyzed the major and trace element geochemical characteristics of the iron formation and rocks. The iron formation is mainly the banded iron formation (BIF) and contains a small amount of lump ore, with its roof and floor rock as well as orebody interbed being mainly Archean Anshan Group phyllite. Analyses of major elements show that the iron ore is mainly composed of SiO2 and iron oxides, with low Al2O3 content and high LOI. The trace element distribution curve of the ore is of the right-oblique type, with the enrichment of the highly incompatible elements and LILE. There is a significant positive Eu anomaly, which resulted from the mixture of the seafloor hydrothermal fluid (> 300℃) with the seawater. It is shown that the BIF formation of this area was closely related to the seafloor hydrothermal activity. The HREE are abundant in the BIF in this area, the Y / Ho ratios are greater than 26, and La and Eu have positive anomalies. Due to lack of terrigenous material, the authors infer that the BIF formed in shallow water with the depth greater than 200m, and the iron-rich hydrothermal fluids migrated to relatively shallow water area and then experienced oxidation and precipitation to form a stable banded structure. A-C-FM restoration of the original rock in combination with Zr/TiO2-Ni diagram shows that phyllite is a kind of metasedimentary rock, and Sm / Nd ratios also suggest the characteristics of sedimentary rocks. Th values vary from 7.9 to 22.5×10-6 with an average of 15.8×10-6, La values vary from 17.9 to 71.7×10-6 with an average of 38.4 ×10-6, the average of La / Sc ratios is 2.3, the average of Th / Sc ratios is 1.0, and the average of La / Th ratios is 2.4 in the phyllite of this area. These data suggest the continental island arc sedimentary characteristics. It is thus concluded that the metallogenic environment of the Yanqianshan iron deposit was a back-arc basin around the Archean craton.