Petrological and Geochemical Constraints on the Protoliths of Serpentine-Magnetite Ores in the Zhaoanzhuang Iron Deposit, Southern North China Craton

The uncommon Mg-rich and Ti-poor Zhaoanzhuang serpentine-magnetite ores within Taihua Group of the North China Craton (NCC) remain unclear whether the protolith was sourced from ultramafic rocks or chemical sedimentary sequences. Here we present integrated petrographic and geochemical studies to characterize the protoliths and to gain insights on the ore-forming processes. Iron ores mainly contain low-Ti magnetite (TiO₂ ∼0.1wt%) and serpentine (Mg^#=92.42–96.55), as well as residual olivine (Fo=89–90), orthopyroxene (En=89–90) and hornblende. Magnetite in the iron ores shows lower Al, Sc, Ti, Cr, Zn relative to that from ultramafic Fe-Ti-V iron ores, but similar to that from metamorphic chemical sedimentary iron deposit. In addition, interstitial minerals of dolomite, calcite, apatite and anhydrite are intergrown with magnetite and serpentine, revealing they were metamorphic, but not magmatic or late hydrothermal minerals. Wall rocks principally contain magnesian silicates of olivine (Fo=83–87), orthopyroxene (En=82–86), humite (Mg^#=82–84) and hornblende [X_Mg=0.87–0.96]. Dolomite, apatite and anhydrite together with minor magnetite, thorianite (Th-rich oxide) and monazite (LREE-rich phosphate) are often seen as relicts or inclusions within magnesian silicates in the wall rocks, revealing that they were primary or earlier metamorphic minerals than magnesian silicates. And olivine exists as subhedral interstitial texture between hornblende, which shows later formation of olivine than hornblende and does not conform with sequence of magmatic crystallization. All these mineralogical features thus bias towards their metamorphic, rather than magmatic origin. The dominant chemical components of the iron ores are SiO₂ (4.77–25.23wt%), Fe₂O₃^T (32.9–80.39wt%) and MgO (5.72–27.17wt%) and uniformly, those of the wall rocks are also SiO₂ (16.34–48.72wt%), MgO (16.71–33.97wt%) and Fe₂O₃^T (6.98–30.92wt%). The striking high Fe-Mg-Si contents reveal that protolith of the Zhaoanzhuang iron deposit was more likely to be chemical sedimentary rocks. The distinct high-Mg feature and presence of abundant anhydrite possibly indicate it primarily precipitated in a confined seawater basin under an evaporitic environment. Besides, higher contents of Al, Ti, P, Th, U, Pb, REE relative to other Precambrian iron-rich chemical precipitates (BIF) suggest some clastic terrestrial materials were probably input. As a result, we think the Zhaoanzhuang iron deposit had experienced the initial Fe-Mg-Si marine precipitation, followed by further Mg enrichment through marine evaporated process, subsequent high-grade metamorphism and late-stage hydrothermal fluid modification.