Paleoproterozoic hydrothermal overprinting over Neoarchean banded iron formation produced high-grade iron ores in the giant Gongchangling deposit of North China: Evidence from O-S-B isotopes

The Gongchangling iron deposit in North China hosts abundant high-grade (>50 wt% total Fe) magnetite ores and represents one of the largest BIF-hosted magnetite deposits worldwide. These high-grade magnetite ores are generally accepted to be formed by replacement of banded iron formation (BIF) (similar to 30 wt% total Fe), but the mechanism of iron enrichment remains controversial. Here we report new O-S-B isotope data of BIFs, high-grade iron ores, and related altered wall rocks, from the Gongchangling iron deposit, together with the geological and geochronological data to constrain the formation of the high-grade iron ores, and proposed new genetic model. The high-grade iron ores have positive delta Fe-56 and negative delta Si-30 values, similar to the BIFs, whereas the delta O-18 values of the high-grade iron ores and altered wall rocks are significantly lower than those of the BIFs. The delta S-34 values of pyrite from the high-grade iron ores are much higher than those of the BIFs. The delta B-11 values of tourmaline in the altered wall rocks of the high-grade iron ores are anomalously high, which is distinct from those of the BIFs and wall rocks, but similar to those of the evaporites from the Liaohe Group. At ca. 1.85 Ga, intense hydrothermal activity was initiated by post-orogenic extension and uplift with O-18-poor meteoric water circulated along crustal structures. Compositions of Na+, K+, Mg2+, Cl-, CO32-, S-34-rich SO42-, B-11-rich borate, and other soluble salts minerals were leached from the evaporitic rocks of the Liaohe Group, which driving the ore-forming hydrothermal fluids toward slightly alkaline and weakly oxidized as system matures. As the alkaline fluids migrated along the structures and fractures in the BIFs and reacted with the wall rocks, SiO2 was mobilized from the low-grade iron ores and consequently formed chlorite, garnet, and tremolite in the wall rocks due to reactions with clay minerals. These Si-rich hydrothermal fluids was also related to vein type quartz and massive quartz-bearing rocks. Most Fe was retained in situ and resulted in the formation of magnetite-rich iron ores. The SO42- in the hydrothermal fluids was reduced to form pyrite while B in the hydrothermal fluids reacted with the wall rocks to form B-11-rich tourmaline.