Temporal, environmental and tectonic significance of the Huoqiu BIF, southeastern North China Craton: Geochemical and geochronological constraints

Abstract

The Huoqiu iron deposit with a typical banded iron formation, located in the southeastern margin of the North China Craton (NCC), and hosted by Precambrian high-grade metamorphic rocks, mostly occurred as metasedimentary rocks. Eighty-eight detrital zircons have been collected from three BIF samples, and they displayed clear oscillatory zoning, high Th/U ratio and low to variable luminescence, indicating magmatic origin. There are two peaks of 2753 Ma and 2970 Ma in concordant 207Pb/206Pb ages distribution plot, which is well consistent with the protolith ages of wall rocks (e.g., TTG gneiss and amphibolite) in this region. These appearances in association with the relatively good linear correlations between Al2O3 and TiO2 for BIF and their wall rocks, suggest that at least some terrigenous clastic sediments contributed to BIF formation, which thus constrained the upper limited age of BIF deposition at 2.75 Ga. In addition, no detrital zircons record 2.56 Ga magmatic event, or recently reported 2.71 Ga magmatic event in this region and neighboring blocks, probably constrained the lower limited age of BIF formation at 2.56 Ga or 2.71 Ga. The flysch rhythmic structure occurrence in BIF-bearing strata, abundant carbonate minerals such as primary breunnerite association with magnetite, and no volcanic record present within BIF ore body or adjacent rocks inferred that the Huoqiu BIF deposited in continental marginal sea or back-arc basin environment, thus belonged to Superior-type. They show similar REE patterns to seawater, however, the positive Eu anomalies and reduced Y/Ho ratios relative to seawater indicates a possible mixture of hydrothermal fluids.