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Mafic–ultramafic intrusions comagmatic with Proterozoic massif-type anorthosites can provide insights into the parental magma from which large volumes of hyper-feldspathic rocks are produced. Recent deep drilling has unveiled a large olivine–Ti-magnetite-rich layered intrusion (named Dawusunangou) beneath the Damiao massif-type anorthosite complex in the North China Craton. The layered intrusion is composed of alternating olivine–Ti-magnetite-rich dark layers and plagioclase-rich light layers (ca. 35–80% plagioclase), with the latter also containing pod- or lens-shaped pyroxene–Ti-magnetite-rich aggregates. This layered intrusion shows low Mg# and REE patterns similar to the overlying Damiao anorthosite complex. Baddeleyite Pb–Pb geochronology yielded indistinguishable crystallization ages of ca. 1735 Ma for both the Dawusunangou layered intrusion and the Damiao anorthosite complex, suggesting coeval emplacement. Using the average bulk compositions of the two intrusions, mass balance calculations assuming 30–40% Dawusunangou and 70–60% Damiao would give a composition similar to high-Al basaltic magma. Collectively, these features indicate that the Dawusunangou layered intrusion represents the mafic residues after the segregation of the Damiao anorthosites from high-Al basaltic parental magma. A short-lived magma chamber is thought to have supplied the two intrusions. In situ crystallization with variable nucleation rates for plagioclase combined with the mafic minerals crystallizing in equilibrium proportions resulted in the formation of repeated dark and light layers of the Dawusunangou layered intrusion. The two intrusions are interpreted to have formed by multiple magma injections, instead of continuous differentiation of one melt. The parental magma was derived from a depleted mantle source with significant crustal contribution during magma evolution. The large Nd–Hf isotopic variations suggest contamination by Paleoarchean to Neoarchean crust.

The Damiao Fe–Ti–P deposit offers a rare opportunity for studying late-stage Fe–Ti ore-forming processes in Proterozoic anorthosites. The orebodies are hosted in anorthosite and commonly show chlorite-dominated alteration in the contact zone on both sides, but the nature and origin of fluids in Fe-Ti-P mineralization remains contentious. Baddeleyite is a common accessory mineral in anorthosites and Fe–Ti–P orebodies, and typically occurs as blebs and lamellae in primary ilmenite reflecting decreasing solubility of Zr in ilmenite during slow cooling and consequent exsolution of ZrO2. Two types of zircon are identified in the Fe–Ti–P orebodies and altered anorthosite at Damiao, and both are related to hydrothermal replacement of baddeleyite by Si-rich fluids. The type-I zircon shows subhedral to anhedral shapes with variable sizes (5–50 μm) in Fe–Ti–P orebodies, and coexists with magnetite–rutile symplectite formed by ilmenite breakdown. In contrast, the type-II zircon typically occurs as tiny aggregates in chlorite–quartz–titanite replacement fronts of altered anorthosite, indicative of a hydrothermal origin. The type-I zircon yielded an age of 1739 ± 16 Ma, similar to the age of baddeleyite previously reported for the orebodies. Formation of the type-I zircon is related to the replacement of baddeleyite in the presence of Si-enriched hydrothermal fluids evolved from magma. Ti-in-zircon geothermometry indicates a fluid temperature of >700 °C for the formation of the type-I zircon. However, homogenization temperature of fluid inclusions in co-precipitated apatite suggests that the type-II zircon in altered anorthosite may have formed by later hydrothermal fluids at temperature of ~350 °C. Our results indicate that the Fe–Ti–P mineralization of the Damiao anorthosite complex involved hydrous melts and magmatic–hydrothermal processes, with the Fe–Ti oxides being formed at the magmatic stage and apatite at the hydrothermal stage.

BIFs (banded iron formations) in China, which account for approximately 64% of the total identified resources in the country, belong to two categories: a predominant Neoarchean Algoma-type and a few Paleoproterozoic Superior-type. A minor group of BIFs developed in the Neoproterozoic is also representing, having possible correlation with the “Snowball Earth” scenario. Spatially, the BIFs in China are mainly distributed in the North China Craton. They experienced intense metamorphism and deformation, which led to the majority of iron oxides changing into coarse-grained magnetites. Though their ores generally contain only 30% TFe, the BIFs are potential targets of industrial exploitation by magnetic beneficiation. On the basis of ore-forming age, host formations, mineral assemblages, and grade of the ores, the BIF-related iron deposits in China are classified into seven types. (1) Anshan-type: Neoarchean volcanic sedimentary formations with magnetite as the main ore mineral (20% ≤ TFe

The Damiao type iron deposit is hosted in a typical Proterozoic anorthosite complex in the northern North China Craton. The types of ores in Damiao mainly comprise massive Fe ores, massive Fe–P ores, and disseminated Fe and Fe–P ores. The disseminated Fe and Fe–P ores formed by fractional crystallization are generally hosted in oxide-apatite gabbronorite and account for 70% of the proven reserve of the Damiao type iron ore. The massive Fe and Fe–P ores account for 30% of the proven reserve of the Damiao type deposit iron ore and generally occur as irregular dykes or veins filling vertical fractures of the previously consolidated anorthosite, showing typical features of hydrothermal mineralization. The contact between the massive orebodies and wall rocks is sharp and straight. The anorthosite comprises white and dark varieties, with the former resulted by the alteration of the latter that occurs as relicts. Petrographic observation and electron microprobe analyses show abundant Fe–Ti oxide inclusions in plagioclase which impart the dark color to the rock. The similar spider diagram patterns between fresh and altered plagioclase and between dark- and white-colored anorthosite imply a genetic relationship between the dark and white types. During the alteration of anorthosite, CaO and MgO were slightly decreased, the SiO2, Al2O3 and Na2O were significantly increased, and the TFe2O3 and TiO2 were significantly decreased. The TFe2O3 and TiO2 in the dark-colored anorthosite have a range of 4.86–12.18 wt.% and 0.37–1.65 wt.%, respectively. However, The TFe2O3 and TiO2 in the white-colored anorthosite have a range of 1.67–3.1 wt.% and 0.14–0.31 wt.%, respectively. These features suggest that the alteration of the anorthosite led the Fe element by leaching from the dark-colored anorthosite at highly oxidized condition, and then precipitated within the fractures of the anorthosite, thus forming the massive Fe and Fe–P orebodies. Because the estimated amount of transported Fe is much more abundant than the proven ore reserve, we infer that there should be huge potential for prospecting Damiao type iron ores.

The origin of the Zhaoanzhuang serpentine-magnetite deposit in the southern North China Craton (NCC) is highly disputed, with some investigators having proposed an ultramafic origin, whereas others favor a chemical sedimentary origin. These discrepancies are largely due to the difficulty in determining the protolithic characteristics of the highly metamorphosed rocks. Sulfur, magnesium, and boron isotope geochemistry combined with detailed petrography was carried out in this study to constrain the original composition of the Zhaoanzhuang iron orebodies. Anhydrite is present as coarse crystals intergrown with magnetite, indicating that the anhydrite formed simultaneously with the magnetite during metamorphism rather than as a product of later hydrothermal alteration. The anhydrite has a narrow range of positive &delta, 34S values from +19.8 to +22.5&permil, with a mean value of +21.1&permil, These values are significantly higher than that of typical magmatic sulfur (&delta, 34S = 0 &plusmn, 5&permil, ) and deviate away from primary igneous anhydrite towards mantle-sulfur isotopic values, but they are similar to those of marine evaporitic anhydrite and gypsum (~+21&permil, ). The sulfur isotopic compositions of several samples show obvious signs of mass-independent sulfur fractionation (&Delta, 33S = &minus, 0.47&permil, to +0.90&permil, ), suggesting that they were influenced by an external sulfur source through a photochemical reaction at low oxygen concentrations, which is consistent with the Neoarchean-Paleoproterozoic atmosphere. Coarse-grained tourmaline from the tourmaline-rich interlayers of the orebodies occurs closely with Mg-rich minerals such as phlogopite, talc, and diopside, indicating that it has a metamorphic origin. The &delta, 11B values of the tourmaline range from &minus, 0.2&permil, to +3.6&permil, with a mean value of +2.0&permil, which is much positive relative to that of magmatic tourmaline but is consistent with that of carbonate-derived tourmaline. The magnesium isotopic analyses of the serpentine&ndash, magnetite ores and the magnesium-rich wall rocks revealed a wide range of very negative &delta, 26Mg values from &minus, 1.20&permil, to &minus, 0.34&permil, with an average value of &minus, 0.80&permil, The value is higher than that of ultramafic rocks (&delta, 26Mg = &minus, 0.25&permil, ) and exhibits minor Mg isotopic fractionation. However, these values are consistent with those of marine carbonate rocks, which have lower &delta, 26Mg values and larger Mg isotopic variations (&delta, 0.45&permil, 4.5&permil, ). Collectively, the S&ndash, Mg&ndash, B isotopic characteristics of the Zhaoanzhuang iron orebodies clearly indicate a chemical sedimentary origin. The protoliths of these orebodies most likely reflect a series of Fe&ndash, Si&ndash, Mg-rich marine carbonate rocks with a considerable evaporite component, indicating a carbonate-rich superior-type banded iron formation precipitated in an evaporitic shallow marine sedimentary environment.