Age, origin and significance of the Wugang BIF in the Taihua complex, Southern North China Craton

摘要

The Wugang BIF hosted in a Neoarchean Taihua complex (TH) in the Wugang area, at the southern margin of the North China Craton (NCC), is one of the important iron ore in Central Plain. In this study, we report new data on whole-rock compositions of iron ore sample, U-Pb isotopic compositions of detrital zircon from metasedimentary rocks (one BIF sample and two associated garnet paragneisses). In combination with literature data, they yielded a minimum age peak of 2.60 Ga for 110 detrital zircons of magmatic origin from the Tieshanmiao Formation (dominantly BIF-bearing strata) that we interpret as an upper limit age for the Wugang BIF depositional age. Thirty-six detrital zircons of metamorphic origin gave a maximum metamorphic age peak of 2.45 Ga, reflecting the lower limit age. Thus the depositional ages of Wugang BIF can constrained at 2.60-2.45 Ga. The presence of proximal detrital zircons in BIF samples, river-flux contributions, and some BIF samples have true negative Ce anomalies suggest that they were deposited in near-shore continental-shelf or back-arc basin environments, coinciding with that they are typically interbedded with marble, quartz arenite and other metasedimentary rocks, but lack of volcanic material. In particular, both hanging wall and footwall for Tiegukeng BIF orebody from Tieshanmiao Formation are marble, except for several diopside dykes (1943 Ma), orebody is dominantly composed of pyroxene-rich BIF, quartz-rich BIF and other metasedimentary rocks. The Wugang BIF displayed the weak positive Eu/Eu*((SN)) anomalies (average 1.74), associated with the low Cr (similar to 10ppm) and V ( 5ppm) contents, similar to other Superior-type BIFs worldwide. All lines of evidence suggest that the Wugang BIF belongs to Superior-type. In combination with features of coeval counterpart, such as Huoqiu BIF and Xincai BIF in its southeast, it can be assumed that a relatively large Superior-type BIF belt presented at proto-continental margin, they deposited on proto-continental shelf or back-arc basin environments during Neoarchean-Paleoproterozoic. Geochemical evidence propose that the Wugang BIF originated from chemical deposition of carbonate and silica facies, its iron sourced from the mixture of seawater, hydrothermal fluid and river-flux, with trace continental detritus. Consequently, it is reasonable to predict that a great variety of BIFs may distribute between Wugang and Huoqiu area at the southern margin of NCC.