Banded iron formations (BIFs) are marine chemical sedimentary rocks comprised of alternating siliceous- and iron-rich bands and deposited from Eoarchean to early Paleoproterozoic. Due to their geological antiquity, BIFs normally have been overprinted by postdepositional tectono-thermal events, leading to large uncertainties with respect to their depositional age and field occurrence. The studied Gongyiming BIF-type iron deposit, which is a typical example of its metamorphosed Archean counterparts, is preserved within the Guyang Greenstone Belt, the North China Craton (NCC). The formation age of this BIF and effects of postdepositional tectono-thermal events on this BIF have not yet been well determined, limiting our understanding of its geological implication and the current occurrence of its orebodies. In this study, we provided new geological and zircon U-Pb geochronological evidence for the Gongyiming BIF that supports a possibly early Neoarchean depositional age (>2.66 Ga). This finding not only helps to fill the early Neoarchean age gap in BIF records in China, but also supports the previously documented multi-stage crustal growth model for the NCC. Furthermore, a metamorphic age of ~2.50 Ga is recorded by the BIF-bearing plagioclase amphibolite and monzogranitic gneiss that intruded into the plagioclase amphibolite. This metamorphic age is consistent with the time for an extensively identified late Neoarchean tectonic event in the NCC. The identification of a 1.90 Ga old potassium feldspar granite within this BIF indicates the plausible influence of the regional late Paleoproterozoic tectono-thermal event. This event is likely to have caused the development of a large-scale scale dextral shearing in the Gongyiming mining area, which ultimately shaped the field occurrence of its No. 2 and No. 3 ore bodies. Collectively, a structurally controlled exploration model was established for the Gongyiming BIF-type iron deposit, which facilitates the understanding of its ore body reworking processes and guides further regional iron deposit exploration and prospecting efforts.
Wang et al. (Tue,) studied this question.