The Mechanisms And Key Factors In Forming High-grade Iron Skarn Deposits In Handan-Xngtai District,North China Craton

The Handan-Xingtai district located in the Trans-North China Orogen in the North China Craton?NCC?contains numerous iron skarn deposits.The district host more than 100 deposits/mineralized sites with proven reserve of more than 800 million tons.The deposits are characterized by their high-grade iron ore.Previous studies mainly have focused on deposit features,geochronology and geochemistry of the intrusions,however,little study has been done on the ore-forming fluids.The composition of the ore-forming fluids and how magnetite ore formed in such a large scale with such a high grade and within such a short amount of time are still not well understood.In the Handan-Xingtai district,iron skarn deposits are closely related to the middle Ordovician evaporite-bearing carbonate strata and no economic iron deposits have been found in the Cambrian to early Ordovician carbonates or Carboniferous to Permian shales and sandstones where evaporites are lacking.This observation indicates that the evaporitic rocks have played a critical role in the formation of the magnetite deposits.However,how evaporites have contributed to the formation of high-grade iron ore still remains obsecure.This dissertation mainly focuses on Baijian and Xishimen iron skarn deposits from the Handan-Xingtai district and the goal of which is to understand and explore the nature of the ore-forming fluids,the role of evaporite and the genesis of high-grade iron ore and establish a deposit model based on a combination of comprehensive field work,mineralogy,geochemistry and fluid inclusions studies.Albitization alteration is typically observed in ore-related intrusions from Handan-Xingtai district.During alteration,igneous amphibole is altered by albite and diopside and igneous magnetite is extensively reequilibrated.Four types of magnetite have been identified in the samples studied: pristine igneous magnetite?type 1?,reequilibrated porous magnetite?type 2?,reequilibrated nonporous magnetite?type 3?,and hydrothermal magnetite?type 4?.Pristine igneous magnetite contains abundant well-developed ilmenite exsolution lamellae that are largely replaced by titanite during hydrothermal alteration.The titanite has a larger molar volume than its precursor ilmenite and thus causes micro-fractures in the host magnetite grains,facilitating dissolution and reprecipitation of magnetite.During albitization,the igneous magnetite is extensively replaced by type 2 and type 3 magnetite via fluid-induced dissolution and reprecipitation.Porous type 2 magnetite is the initial replacement product of igneous magnetite and is subsequently replaced by the nonoporous type 3 variety as its surface area is reduced and compositional equilibrium with the altering fluid is achieved.Hydrothermal type 4 magnetite is generally euhedral and lacks exsolution lamellae and porosity,and is interpreted to precipitate directly from the ore-forming fluids.Hydrothermal reequilibration of igneous magnetite leads to progressive chemical purification,with coupled loss of trace elements?Ti,Al,Mg,Mn,Cr,Zn,Ga and Co,etc.?and gain of iron.Results presented here confirm that magnetite is much more susceptible to textural and compositional reequilibration than previously thought.The reequilibrated magnetite has geochemical patterns that may be distinctively different from its precursor,making existing discrimination plots questionable when applied to genetic interpretation.The Fe versus V/Ti diagram is proposed that can be used to discriminate between pristine igneous magnetite,reequilibrated magnetite,and hydrothermal magnetite.There are three magnetite mineralization stages observed in Baijian iron skarn deposit:?1?Prograde skarn stage;?2?Retrograde skarn stage;?3?Magnetite-pyrite stage.Magnetite formed from different stages typically has distinct trace element compositions.Magnetite from prograde stage contains high Ti,V,Cr,Ni and Ga which generally decreases progressively through retrograde skarn stage to magnetite-pyrite stage.This trend indicating that temperature plays a key role controlling the incorporation of these elements into magnetite.Later stage fluid has high oxygen fugacity which may contribute to the lower concentration of Ti and V in the magnetite from magnetite-pyrite stage.Cobalt and Ni concentration are more enriched in pyrite than the coexisting magnetite and thus the precipitation of pyrite likely decrease the concentration of these two elements in co-precipitate magnetite.Incompatible elements like Si and Ca are enriched in oscillatory zoning magnetite which generally represent an un-equilibrated formation due to fast growth of the mineral and the incorporation of these incompatible elements into magnetite is likely controlled by surface absorb processes.Fluid-induced dissolution and reprecipitation textures are typically observed during which magnetite is chemically purified with trace elements like Si,Ca,Mg and Al being expelled from the mineral.There are distinctive differences on both textures and trace element concentration when comparing magnetite from ore body 1 in marble with igneous magnetite from Baijian intrusion.The geochemistry and texture features of magnetite indicating a hydrothermal origin rather than iron-oxide melt from ore body 1 in marble.Pyroxene in endoskarn from Baijian is typically zoned.From core to rim,Na and Fe increase but Mg drops significantly.Pyroxene from exoskarn is basically pure diopside and is partly replaced by later Fe-rich pyroxene.Transition metals like Ni,Co,V,Cr,and Zn are typically controlled by Fe concentration of pyroxene.High field strength elements like Nb,Ta,Zr and Hf show no correlation with major components of pyroxene and are likely controlled by temperature and fluid composition.Rare earth elements?REE?are positively correlated with P concentration in pyroxene and apatite precipitation is likely the cause of REE decrease in pyroxene.Apatite from Baijian is highly enriched in LREE and apatite precipitation likely causes LREE depletion from the fluids,which is characterized by a decrease of La/Sm ratio in diopside.Based on texture characterization and chemical analysis,we consider the Mg-rich core of pyroxene from endoskarn and Mg-rich pyroxene from exoskarn are formed under diffusive metasomatism that is characterized by high temperature,low salinity,low ater/rock ratio and lithostatic pressure.However,the Fe-Na rich pyroxene is like formed during pervasive infiltration which is characterized by lower temperature,high salinity,high water/rock ratio and hydrostatic pressure.The main ore body from Baijian is hosted in the marble and controlled by fault and fractures and is more likely formed during pervasive infiltration stage rather than diffusive metasomatism stage.Magmatic quartz and amphibole from the diorite and hydrothermal diopside from the Xishimen deposit contain abundant primary or pseudosecondary fluid inclusions,most of which have multiple daughter minerals dominated by halite,sylvite,and opaque phases.Scanning electron microscopy?SEM?and laser Raman spectrometry confirm that pyrrhotite is the predominant opaque phase in most fluid inclusions,in both the magmatic and skarn minerals.These fluid inclusions have total homogenization temperatures of 416–620 °C and calculated salinities of 42.4–74.5 wt% NaCl equiv.The fluid inclusion data thus document a high-temperature,high-salinity,ferrous iron-rich,reducing fluid exsolved from a cooling magma likely represented by the Xishimen diorite stock.Pyrite from the iron ore has ?34S values ranging from 14.0 to 18.6 ‰,which are significantly higher than typical magmatic values??34S=0±5‰?.The sulfur isotope data thus indicate an external source for the sulfur,most likely from the evaporitic beds in the Ordovician carbonate sequences that have ?34S values of 24 to 29‰.We suggest that sulfates from the evaporitic beds have played a critically important role by oxidizing ferrous iron in the magmatic-hydrothermal fluid,leading to precipitation of massive magnetite ore.12Fe²⁺+SO₄^2-+12H₂O = 4Fe₃O₄ + H₂S+22H⁺According to the reaction,magnetite deposition is accompanied by a significant increase of H⁺ in the fluids,which may suppress the formation of magnetite.Nevertheless,H⁺ produced by reaction would be effectively neutralized by reaction with the surrounding carbonate rocks,driving the reaction continuously toward the right.Consequently,large amounts of magnetite were likely precipitated to form the iron oxide ores.A synthesis of available data suggests that oxidation of Fe2+-rich,magmatic-hydrothermal fluids by external sulfates could have been a common process in many of the world's iron skarn deposits and other magnetite-dominated ores,such as iron oxide-copper-gold?IOCG?and iron oxide-apatite?IOA?systems.Result from this study shows that the early stage ore-forming fluid from iron skarn deposits in Handan-Xingtai district likely went through boiling.The fluid is formed at high temperature with high salinity and low oxygen fugacity,which is capable dissolving significant iron in it.The fluid interacted with the cooling intrusion and is able to extract significantly amount of iron from the solid rocks and cause large scale albitization.Microthermometry date of fluid inclusions from boiling assemblage indicates iron skarn deposits from Han-Xing district formed at a relative shallow environment?<4km?.Under shallow environment,the formation of garnet and pyroxene are typically inhibited and thus iron in the fluid has not loss significantly during the early stages.Mixing of sulfate-rich,oxidizing fluids with ferrous iron-rich,reducing magmatic-hydrothermal fluids would trigger the deposition of magnetite under favorable temperature conditions.On the other hand,the dissolved evaporates leave an open space for fluid infiltration and precipitating massive magnetite ore.The early formed trace element-rich magnetite has extensively reequilibrated through fluid-induced dissolution and reprecipitation,during which trace elements is leached and magnetite greatly enhance its purity and thus like improve the iron grade of magnetite ore.