| The Chengchao iron deposit,located in the noutheastern Hubei ore cluster,is the largest scale high-grade iron deposit.To investigate the mineralization conditions in different stages,origin of ore-formation fluid and material,and the enrichment mechanism,in this contribution,detailed field investigation,coupled with multiple analysis methods have been carried out,such as EMPA,fluid inclusion,stable isotope,noble gas isotope,radio isotope.Through all these works,we now obtain several conclusions,as follows:1)The ore-forming process can be divided into five stages: prograde skarn stage,retrograde skarn stage,veinlet skarn stage,sulfide-sulfate stage,carbonate stage.The large scale magnetite was deposited during the second ore-forming stage.2)Systematic skarn mineral analysis has discovered that many minerals,such as garnet,pyroxene,magnetite,epidote,have several generations and are rich in oscillatory texture.Garnet is formed mainly during the prograde and veinlet skarn stage.Obvious compositional differences are exited among different generations.From the first to the third generation,the andradite end-member is becoming higher,which indicates the increasing of oxygen fugucity and PH.However,inverse trend happened during the formation of the fourth generations.Pyroxene is formed during the prograde skran stages,and can be divided into two generations.The EMPA analysis result show that they are belonging to the series of diopside-hedenbergite.Generally,pyroxene in the endoskarn and the second generation are rich in Fe,poor in Mg than the exoskarn and the first generation.The large scale magnetite was deposited during the retrograde skarn stage,and few were formed in the sulfide-sulfate stage.Based on the detailed field and microscope observation,four generations of magnetite have been identified in different types of ores and mineralized skarn.The earliest phase of magnetite(Mt1)undergone dissolution-reprecipitation process.The second phase magnetite(Mt2)is homogeneous and rich in oscillatory texture.EMPA data reveal that big composition varaitons are existed among different locations and generation's magnetite,predominantly for the elements Si,Al,Ca,Mg,while differences for elements,such as Ti,Cr,V,Zn,Ni,are relatively small.The composition difference is associated with many factors,such as fluid oxygen fugacity,temperature,concentration of elements,water-rock reaction.Compared with the magnetite within intrutions on composition,textute,we porposed that the magnetite in Chengchao is of hydrothermal origin rather than ore magma.Multiple superimposed mineralization is likely the key enrichment mechanism for Chengchao high-grade iron deposit.Quantitative calculation results show that the former three generations may contribute more than 96.00% Fe and paly a crucial role in the whole mineralization process.3)Comprehensive study of fluid inclusions,Hydrogen-Oxygen,Helium-Argon isotope,we draw the conclusion that the fluid of the prograde skarn stage is predominant magmatic water with high temperature(>500?),high salinity(>40wt%NaCleq).Involvement of meteoric water in retrograde skarn stage leads to the dramatic decreasing of temperature and salinity.From sulfide-sulfate to carbonate stage,the temperature are gradually decreased(average 253 ?,224 ?),however,the salinity show small difference(average 13.94wt%NaCleq,17.04wt%NaCleq),and both of them show “oxygen shift” towards rainwater line.The carbon is derived mainly from magma,and the sulfur are the mixture of magmatic and salt-layer.4)Mineralogical and Sr-Nd-Pb isotopic characteristics indicate that the intrutions were originated from partial melts of an enriched mantle source,and followed by significant contamination of lower crust material during the evolution of magma and the earlier diorite contain a greater contribution of mantle material than the later granite and quartz monzonite porphyry.Composition of biotite also reveals that the corresponding magma are all have higher oxygen fugucity. |
PDF Full Text Request