Genesis of Huoqiu banded iron formation (BIF), southeastern North China Craton, constraints from geochemical and Hf-O-S isotopic characteristics

Abstract

Abstract The Huoqiu iron deposit is one of the few Superior-type BIF ore fields in China. To clarify the genesis of Huoqiu BIFs, we give out the geochemical and O-S isotopic data of the BIF samples and zircon Hf isotopic data of the wall-rock. The Huoqiu BIFs show dominant composition of SiO2\u202f+\u202fFe2O3T, and varying contents of Al2O3, MgO, CaO, Rb, Sr, Zr and Hf, suggesting their formation mainly through chemical precipitation and incorporation of terrigenous felsic clastics. Shale-normalized rare earth element patterns of the BIF samples show the features characteristic of other Archean and Paleoproterozoic BIFs, with HREE enrichment relative to LREE, positive La and Eu anomalies, and superchondritic Y/Ho ratios comparable to modern seawater. Positive Eu anomalies are attributed to an imprint of high-temperature hydrothermal fluids. None to negative Ce anomaly shown by the BIFs imply the Huoqiu BIFs was precipitated from the mildly oxidized seawater in a nearshore setting. The quartz separated form Huoqiu BIF has δ18OV-SMOW values of 11.8‰–14.8‰, within the extent of the siliceous rocks deposited from high-temperature hydrothermal. The δ34S values of pyrite grains vary from −2.84 to +4.38‰, which is similar as the sulfur derived from mixtures between mantle fluids and sediments; The Δ33S values varied from −0.08‰ to +1.03‰, imply that the Huoqiu BIFs deposited distally from the volcanic center. The Hf model ages of these detrital zircons from the wall rock give three prominent peak at 3.1–3.0\u202fGa, 2.9–2.7\u202fGa and 2.5\u202fGa, respectively, it suggest that there were three episodes of the crust growth in the Huoqiu area. Integrated these data with the pioneering studies on the Huoqiu BIFs and wall rocks, we confirmed that the Huoqiu BIFs were sourced from the high-temperature hydrothermal related with the volcanics and precipitated from the mildly oxidized seawater in a nearshore setting and distally from the volcanics at 2.5\u202fGa.