Mineralization and magmatic origin of the 2.1 Ga Zhaoanzhuang magnetite-apatite deposit, North China Craton: Constraints on the formation of the Paleoproterozoic iron oxide-apatite deposits

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Iron oxide-apatite (IOA) deposits provide large amounts of iron for steel industry worldwide, but their origins are still in dispute. Although IOA deposits were commonly genetically related to intermediate-mafic igneous rocks, the occurrences of IOA deposits in ultramafic rocks are scarce. The Zhaoanzhuang iron deposit in the North China Craton is characterized by comprising abundant low-Ti magnetite, apatite and ultramafic minerals (e.g., olivine, orthopyroxene), but its origin has long been controversial. These ultramafic minerals mainly occur as aggregates surrounded by magnetite in the iron ores and display typical cumulate textures and high Fo values, indicative of magmatic origin. The δ56Fe and δ18O values of magnetite from the ores range from 0.22 ± 0.03‰ to 0.32± 0.03‰ and from 2.6‰ to 4.5‰, respectively, akin to those in magmatic rocks and IOA deposits but distinct from those of metamorphic BIFs. Both types of apatite occurring in the ultramafic aggregates and the iron ores contain 2.07 to 2.69 wt% F, less than 0.65 wt% Cl, up to 7900 ppm rare earth elements (REE) and greatly variable Sr-Nd isotopic compositions, reflecting that both types are of magmatic-hydrothermal origin. The crust-like εNd(t) values (-7.1 to −5.2) of all apatite grains record reset of their Sm-Nd system, which is resulted from the involvement of carbonate fluids based on their similar REE patterns. In situ Sm-Nd isotope analyses of apatite from the ultramafic aggregates and the iron ores have yielded similar Sm-Nd isochron ages at 2100 ± 81 Ma (MSWD = 0.84) and 2096 ± 240 Ma (MSWD = 0.85), respectively, which can constrain the minimum age for the Fe mineralization. Combined with sedimentary age of the strata, the ore-forming age of the deposit was 2.3–2.1 Ga. The negative εNd(t) values of the ultramafic aggregates (-1.2 to −1.0), the iron ores (-1.8 to −1.5) and magnetite (-1.6 to −0.8) from the ores indicate that the ore-forming magma has a slightly enriched signature. Combined with linear relationship between their εNd(t) values and MgO contents, an involved mantle-derived ultramafic magma is most likely to be contaminated by the crustal material during ascending. Therefore, the Zhaoanzhuang deposit is a good Paleoproterozoic case of the rare IOA deposits associated with ultramafic magmas.