Super－efficient Enrichment Mechanism Of Iron In The Intermediate－basic Magmatic System

In this dissertation, four deposits were selected, including the plutonic PanzhihuaFe-Ti-V oxide ore-bearing layered intrusion in Emeishan Large Igneous Province,Cihai hypabyssal diabase-related Fe skarn deposit in the Beishan region, NW China,Gushan ultra-hypabyssal dioritic porphyry-related Fe deposit, Eastern China andYamansu submarine volcanic-related Fe deposit in Eastern Tianshan, NW China. Onthe basis of the discussion of petrogenesis, some new lights shed on the super efficientenrichment mechanism of iron in the intermediate basic magmatic system includingmagma differentiation, magmatic-hydrothermal transition and hydrothermal fluids.Zircon U-Pb dating on the co-magmatic picritic porphyryin Panzhihua, and thehigh Mg olivine (Fo90) discovered in the latter, indicate the formation of world-classFe-Ti-V oxide deposit is closely related to the high temperature magmatism inducedby the Permian Emeishan mantle plume. The estimated primary magma isferropicritic, combined with the He-Ar and Re-Os isotopes show enriched signatures,implying the lithospheric mantle had been affected by the ancient subduction-relatedmetasomatism and contains an eclogite or pyroxenite component which could interactwith mantle plume and could have produced the parental Fe-rich magma. Plagioclaseshow higher contents of Fe and Ti in the rim which could be explained by interstitialliquid immiscibility. Except this mechansim, several times of the Fe-rich magmareplenishment are also probably key factors to facilitate the formation of massive ores.The petrogenesis studies on the Cihai diabase suggest the parental basaltic magmawas formed by partial melting of the enrich lithospheric mantle and experiencedfractionation of relatively Fe-poor minerals. Moreover, the shallow emplacementdepth are probably the key factor that triggered the release of post-magmatichydrothermal fluids and facilitate Fe skarn mineralization. Mass balance calculationsuggests that the high grade iron ores in Gushan probably were formed from theimmiscible iron-rich liquid which separated from the ferrodioitic magma. The latterwas derived from the enriched lithospheric mantle and MELTS simulation suggeststhe parental magma may experienced fractionation of clinopyroxene and plagioclase,and contamination of P-rich strata. LAICP-MS U-Pb zircon dating of the basalts and skarns at Yamansu yields almost coeval ages, suggesting that the skarn formation isrelated to subaqueous volcanism. The hydrothermal fluids that generated the skarnscould be a mixture of evolved magma-derived fluids and sea water. The Yamansubasalts provided the source of iron for the skarn mineralization.In this study, some common characteristics of key factors were elucidated asfollowed:1) the iron is exlusively from magma, thus large scale magmatism is thebasis of formation of large scale iron deposit;2) Extensive fractional crystallization isone of the requirement for the iron enrichment in the residual magma and possibleliquid immiscibility, and in the high salinity fluids;3) the decompression duringmagma differentiation is favorable for magmatic hydrothermal fluids, and thecarbonate wall rocks is the most favorable environment for the deposition of Fe.