Metallogenesis And Metallogenic Pedigree Of The Fe-Ti-P-Cr Deposits In Chengde,Hebei Province,China

The Chengde area in Hebei Province is the most important ore-concentrated district in North China,with a proven resource reserve of ca.1 billion tons.The Damiao-type Fe-Ti-P deposits at Mesoproterozoic,Tiemahabaqin ultra-low-grade Fe-Ti-P deposit at Late Paleozoic and Gaositai Cr-Fe-Ti-P deposit at Early Mesozoic are typical in general.All of them were products related with mafic-ultramafic complexs that fromed at tectonic setting of regional extension,and thus providing good examples for metallogenic pedigree study and further exploration.In order to gain more information about the origin of these Fe-Ti-P-Cr deposits and associated metallogenic pedigree,we carried out geochemical studies on the basis of field observations in this study.The new achievements and highlights are listed as follows:Large-scale low-Ti nelsonites associated with the Damiao anorthosite complex(DAC)on North China Craton exhibit unequivocal late-stage features in the magmatic evolution.Field observation and petrography indicate two types of nelsonites:the banded/brecciated nelsonites and the massive nelsonites(the dominated type).Apatites of all the rock types formed at different stages of crystallization show similar REE distribution which indicate a common source.Apatites of the banded/brecciated nelsonites display stronger negative Eu and Sr anomalies than apatites of the spatially related oxide-apatite gabbronorites,suggesting that sufficient plagioclases had crystallized before apatite crystallization;and such apatites also display remarkably low-F and high-REE contents,which are interpreted the geochemical break as an evidence of liquid immiscibility.It can be concluded that the banded/brecciated nelsonites and associated Fe-Ti-(P)oxide-rich clinopyroxenites crystallized from Fe-Ti-P-rich immiscible liquid,with mangerites as the corresponding Si-rich immiscible conjugate.In contrast to the nelsonites associated with anorthosites worldwide,the massive nelsonites in the Damiao anorthosite complex are characterized by only small amounts of TiO2.This feature makes up a surprisingly large abundance and connects with widespread cogenetic alteration products,thus showing unequivocal evidence for the hydrothermal mineralization.The strong similarity of REE contents and distribution for apatites between the massive nelsonites and spatially related oxide-apatite gabbronorites indicate a cogenetic relationship.Combined H-C isotope data of the fluid and O isotope data of the minerals for apatites of the massive nelsonites and quartz of the altered zone demonstrate that H2O and CO2 contents of the fluid were probably originated from a magmatic system with mixing of significant proportion of meteoric water(or groundwater or seawater)channeled by the Hongshila-Damiao fault.The Fe-Ti-P materials for generating the massive nelsonites were precipitated from supersaturated hydrothermal fluid and then injected into the open fractures of the anorthosites or zone of contact between anorthosites and oxide-apatite gabbronorites.with corresponding alteration widely developed near the contact.In summary,at late-stage magmatic evolution of the Damiao anorthosite complex,liquid immiscibility accounts for generating small volumes of banded/brecciated nelsonites,while hydrothermal mineralization supported by earlier Fe-Ti-P enrichment of liquid immiscibility is responsible for generating large-scale massive nelsonites in the Heishan iron deposit.New zircon U-Pb age of the massive nelsonite(1797±6 Ma)indicates that the magmatism of the DAC and associated mineralization had lasted over a relatively long period from 1.80 Ga to 1.68 Ga.This study presents geochemical and and 40Ar/39Ar isotopic data of the hornblendes from iron-bearing hornblendites.The40Ar/39Ar plateau ages of the hornblende from hornblendites yield 401?379Ma,which indicate that the ore-forming process was fromed at the period of Devonian.Tectonically,the iron-bearing hornblendites are probably related to post-collisinoal extension after the arc-continental collision between the Bainaimiao arc belt and the northern margin of the North China Craton during the period of latest Silurian.Genetic minerageny studies of hornblendes and apatites have shown that crustal materials account for significant part for the parent magma of the Tiemahabaqin complex as well as mantle materials.Occurrences of nelsonites associated with ultramafic rocks are very rare.Recently,small concentrations of nelsonites were discovered in close association with Fe-Ti-P hornblendite from the Tiemahabaqin clinopyroxenite-hornblendite complex,northern Hebei Province,China.This study present major and trace element data of the apatites from rocks that formed at different stages of magmatic evolution including clinopyroxene hornblendite,hornblendite and nelsonite.Spider diagrams of apatites form different rock types are very similar in shape.Trace element compositions such as F,U,Th,Sr and?REE of apatites from the nelsonite are significantly higher than those from the clinopyroxene hornblendite and hornblendite and exhibit a continuous trend.It is indicated that the nelsonites were separated at the latest stage of magmatic evolution when Fe-Ti-P components were saturated in the parent magma of the hornblendites.Located on the northern margin of the NCC(North China Craton),the concentric zoning Gaositai ultramafic complex show features of chromite-bearing dunite in the core,surrounded subsequently by wehrlite and clinopyroxenite,and minor hornblendite in the rim.Chemically,the Gaositai ultramafic complex display systematic decrease of Mg#,negatively sloped REE patterns with no Eu anomalies and high LILEs with low HFSEs such as Nb and Zr from core to rim,implying a fractional crystallization trend during magma evolution.LA-ICP-MS zircon U-Pb ages of three chromite samples and one clinopyroxenite sample show that emplacement of the Gaositai ultramafic complex was 213±1 Ma,which occurred in Late Triassic,implying the geodynamic setting of lithospheric extension after final collision of the Mongolian arc terranes in the NCC.The ?Hf(t)values of the zircons aged from 214 Ma to 211 Ma for the clinopyroxenite range form-2.0 to+1.6,implying that the parental magma was derived from a mixed source composed of depleted mantle source and enriched lithospheric mantle source,with significant crustal contamination during its ascending.The mineralization of iron in Chengde area has long lasted from Neoarchean to Early Mesozoic which constitutes a typical mineralization pedigree of Fe-Ti-P.The Fe-Ti-P mineralization pedigree in Chengde was closely related with tectonic setting and featured by the long-term concentration of metal elements or mineralization materials.The detailed members of the mineralization pedigree are as follows:? Banded iron formations related with Early Craton Basin at Neoarchean;? Damiao-type Fe-Ti-P deposits related with Damiao massif-type anorthosite cassemblages;? Ultra-low-grade Fe-Ti-P deposits related with ultramafic complexes which resulted from post-collisionalomplex at Mesoproterozoic and marine sedimentary-type deposits related with sedimentary/post-orogenic extension at Late Paleozoic;and ?Cr-Fe-Ti-P deposits related with ultramafic complexes which resulted from post-collisional/post-orogenic extension at Early Mesozoic.As constrained from comparative study of Hf isotope data,the ore-forming materials of the mineralization of iron in Chengde were from a mantle source or mafic lower crust which formed at 2.8?2.7 Ga.Guided by theory of mineralization pedigree,the potential areas for prospecting high-grade iron deposits in Chengde area are as follows:? The East Body of the Damiao anorthosite;and? The contact zone of the clinopyroxenite and horblendite in ultramafic complexes.