Origin of the Neoarchean VMS-BIF Metallogenic Association in the Qingyuan Greenstone Belt, North China Craton: Constraints from Geology, Geochemistry, and Iron and Multiple Sulfur (δ33S, δ34S, and δ36S) Isotopes

Abstract

Abstract The association of volcanogenic massive sulfide (VMS) deposits and Algoma-type banded iron formations (BIFs) in many Precambrian terranes indicates a link between submarine hydrothermal processes, seawater chemistry, and chemical sedimentation. The Neoarchean (~2.55 Ga) Qingyuan greenstone belt VMS-BIF metallogenic association, located on the north margin of the North China craton, is a typical example of such an association. The stratigraphy of the Qingyuan greenstone belt includes three units (from the oldest to youngest): (1) the Shipengzi Formation, composed of tholeiitic-transitional arc basalts with negative Nb anomalies, interlayered normal mid-ocean ridge basalts (N-MORBs) and FI-type dacites, and BIFs; (2) the Hongtoushan Formation, consisting of polycyclic bimodal suites of N-MORB-type basalts and FII-type dacites, as well as VMS mineralization and minor BIFs; and (3) the Nantianmen Formation, composed of schist, quartzite, and marble with minor basalts and BIFs. Positive Fe isotope compositions (δ56Fe of 0.48–0.69‰) for magnetite in the silicate BIF of the Shipengzi Formation indicate partial oxidation of aqueous Fe(II). Using a dispersion-reaction model, the relatively high δ56Fe values (0.72–1.04‰) estimated for primary ferric (oxyhydr)oxides in this BIF constrain local dissolved O2 contents of the Neoarchean surface ocean to 10–4 to 10–3 μmol/L. By comparison, negative δ56Fe values for magnetite (–0.83 to –0.65‰) in silicate BIFs of the Hongtoushan Formation and the Nantianmen Formation suggest deposition from a residual water column that was depleted in 56Fe. Following the formation of the bulk of the VMS deposits in the Hongtoushan Formation, a significant change to positive magnetite δ56Fe values (0.79–1.04‰) occurs in the youngest sulfide-bearing BIF in the Nantianmen Formation. This implies that the VMS-related hydrothermal vents injected a large mass of unfractionated ferrous iron into the ocean. Negative Δ33S anomalies in sedimentary pyrite of bedded VMS ores (avg of –0.08 ± 0.007‰, n = 6) and sulfide-bearing BIFs (avg of –0.06 ± 0.007‰, n = 3) of the Qingyuan greenstone belt, along with mass-independent fractionations (with an average Δ36S/Δ33S ratio of –1.1 ± 0.3), are best explained by incorporation of seawater sulfate of atmospheric photochemical origin during their formation. The systematic differences in whole-rock geochemistry and Δ33S values for different types of VMS ores imply variable seawater sulfate contributions to their mineralization. Our results are consistent with global anoxic conditions during the Neoarchean to Paleoproterozoic transition (i.e., at 2.5 Ga), and confirm that formation of the VMS-BIF metallogenic association took place in dominantly anoxic, ferruginous basins at different depths, with the VMS-related hydrothermal system contributing significant Fe to the deposition of BIFs.