Genesis Of The Early Precambrian Zhaoanzhuang Iron Deposit,Southern Margin Of The North China Craton

The Zhaoanzhuang iron deposit in the Paleoproterozoic Taihua Group,southern margin of the North China Craton,consists of abundant serpentine-magnetite ores,showing low Ti and Cr contents.It remains unclear whether the original material of this type of ores were sourced from ultramafic rocks or Mg-rich sedimentary rocks.In this dissertation,we present integrated mineragraphic,petrographic,mineral chemical,rock geochemical and isotope geochemical studies on iron ores and wall rocks?serpentinite,amphibolite,orthopyroxenite,etc.?,in an attempt to characterize their protoliths,and further to gain insights on special ore-forming mechanism.The Zhaoanzhuang iron deposit had experienced upper amphibolite-granulite facies regional metamorphism.The orebodies occur as bedded and lenticular,showing metamorphosed genesis,rather than hydrothermal origin.Iron ores contain low-Ti magnetite?TiO₂⁰.1%?and Mg-rich serpentine?Mg^#=92.42-96.55?,as well as residual olivine?Fo=89-90?and orthopyroxene?En=89-90?in the serpentine.It shows that the gangue minerals,including anhydrite,apatite,dolomite,phlogopite and hornblende,are intergrown with serpentinized olivine and magnetite.And metamorphic detrital zircons are seen as inclusions within anhydrite,phlogopite and hornblende.In the wall rocks,the subhedral and anhedral olivine grains occur as interstitial minerals between euhedral hornblende grains?contrary to Bowen reaction sequence?,indicating they formed through metamorphism,rather than magmatic process.The magnetite in this deposit shows lower Al,Sc,Ti,Cr,Zn relative to that from ultramafic Ti-?V?iron ores.Rock geochemistry reveals that the dominant chemical components in the iron ores are SiO₂?4.77-25.23%?,Fe₂O₃^T?32.9-80.39%?and MgO?5.72-27.17%?and uniformly,in the wall rocks are also SiO₂?16.34-48.72%?,MgO?16.71-33.97%?and Fe₂O₃^T?6.98-30.92%?,showing features of striking high Fe-Mg-Si and low Ti-Al-Cr-Co-Ni contents.Besides,the wall rocks show high contents of B?up to125-225 ppm?,which are enriched in some layers and show strata-bound features,indicating the Zhaoanzhuang iron deposit has the composition and distribution characteristics of chemical sedimentary rocks.Anhydrite and hydrothermal activated gypsum display very positive and narrow?³⁴S values from 19.8‰to 22.5‰with a mean value of 21.1‰,significantly higher than typical magmatic sulfur isotopes(?³⁴S=0±5‰),but in proximity to sulfur isotopes of marine sulfate?²1‰?,suggesting the original materials of the Zhaoanzhuang iron deposit should be formed in marine environment.Besides,sulfur isotope compositions show obvious mass-independent sulfur fractionation(?³³S=-0.47‰to 0.90‰),denoting a low oxygen concentration at the atmosphere during the sedimentation of the Zhaoanzhuang iron deposit.It is inferred that original materials of the Zhaoanzhuang iron deposit could be formed before 2.3 Ga.?Carbonate-bearing?serpentine-magnetite ores and magnesium-rich wall rocks?olivine-orthopyroxenite,amphibolite and anhydrite-serpentinite?all exhibit very negative and wide?²⁶Mg values from-1.20‰to-0.34‰with an average value of-0.80‰,distinct from ultramafic rocks which have constant chondritic Mg isotopes(?²⁶Mg=–0.25‰)with minor Mg isotopic fractionation in high-temperature process,but in accord with marine carbonate rocks with lower?²⁶Mg values and larger Mg isotopic variations(?²⁶Mg=–0.45‰to–4.5‰),which suggests that the ores and wall rocks in the Zhaoanzhuang obviously show magnesium isotope features of marine carbonate,deviating from those of ultramafic rocks and related magnesian silicates.In situ boron isotopic compositions of tourmaline from intercalated tourmaline-rich rocks in the Zhaoanzhuang orebodies show a range of?¹¹B values from-0.2‰to 3.6‰with a mean value of 2.0‰,which are very positive relative to that of magmatic tourmaline(?¹¹B⁹‰in average),but fall within boron isotopic spectrum of marine evaporate-associated tourmaline.Consequently,we suggest that the protolith of the Zhaoanzhuang iron deposit was not mafic-ultramafic rocks,but marine chemical sedimentary rocks.The typical enrichment features of anhydrite,magnesium-rich carbonate and borate minerals are consistent with Wengquangou ludwigite deposit of marine evaporated genesis in the Lieryu Formation of Liaohe Group.Therefore,we infer that the original materials of the Zhaoanzhuang iron deposit had also possibly experienced marine evaporated sedimentary process.Its ore-forming processes could be involved in initial shallow-seawater Fe-Mg-Si precipitation,followed by further Mg enrichment through evaporated environment in the closed or semi-closed basin,subsequent high-grade metamorphism and late-stage hydrothermal fluid modification.