Geochemical Characteristics And Genesis Of Daxigou Magnetite-apatite Deposit In Southeast Chifeng

Daxigou magnetite-apatite deposit in Chifeng,Inner Mongolia located on the nouth ofInner Mongolia axis of northern borderland of North China Platform,,which belongs to thebinding sites of the Northern Margin of North China Platform and the Xingmeng geosynclinalarea. Divided by the Regional Geology of Inner Mongolia Autonomous Region,"(1991), theore deposit is located in the northeast of Nuluerhu uplift zone,which stands at the northernmargin of the North China platform. It is an extremely low content magnetite-apatite ores ofvolcano sedimentary-metamorphic type, and its host rocks are medium-fine grained (biotite)amphibolite and its altered rocks.The deposit which can comprehensive utilization ofmagnetite and apatite with lower-grade magnetite-apatite has the characteristics of large-scale,low-grade, harmfulcomponents, easy to select and a variety of elementsassociated,etc.Because of less effort for its exploration and research, developing the study ofgeology and geochemistry condition of the deposit to explore the ore-formingmechanism,comprehensive utilization of the beneficial elements as well as the guidance of themine and regional prospecting and exploration of ore deposits of similar types have a veryimportant significance.In terms of its genetic mechanism, there hasn’t any special field investigation. Somescholars in accordance with its occurring in Archaean greenstones, defined it as GreenstoneBelt phosphorus (iron) Deposits.Trough entire field investigation, the ore body of apatite-magnetite was hosted inmetamorphic supracrustal rocks.It has a certain position in ore-body and mainly occurs inamphibolite containing apatite. The ore body shows transitional relationship with the thecountry rocks. The boundaries of the ore body is delineated by the analysis of P2O5contenthigher than2percent. The morphology of ore body mainly are stratiform-like and veinlet.Great changes took place widely following the strike and dip appeared the phenomenon ofexpansion and contraction.Restoration of original rocks of amphibolite shows that metamorphic supracrustal rocks of the Archean are volcanic sedimentary rocks which are mainly tholeiite with a small amountof calcareous sedimentary rock；They were formed in a tectonic setting of continental islandarc or oceanic island. The study on the mineralogical genesis of amphibole and biotite showsthay have been subjected to regional metamorphism and the metamorphic grade isamphibolite facies with metamorphic reaction temperature of500℃or so. The study on themineralogical genesis of amphibole and apatite shows that ore-forming material such as Fe、P、Ca mainly came from basic volcanic rock. Parts of P come from biological and phosphorusfrom the continent which were brought through the upwelling currents. Parts of Ca come fromcalcareous sedimentary rock. Intermittent volcanic eruptions and vertical movement ofoceanic crust made the metallogenic material such as Fe, P showed a heterogeneousdistribution.Regional metamorphism superimposed into the ore-forming materials of volcanicsedimentary rocks rich in Fe, P made ore-forming materials into a local enrichment ordepleted, and the deformation and reorientation of ore bodies.Analysis of major elements through EMPA、trace elements and race earth elementsthrough LA-ICP-MS show that in the process of regional metamorphism, siderophileelements V、Co、Ni、Ti enter the magnetite as isomorphic, REE and Sr mainly enter theapatite as isomorphic; Rb、Ba、Ga and a small part of P enter the biotite.Due to large-scale Indosinian-Yanshanian magmatism,silicification、 sericitization、carbonation and chloritization which are hydrothermal alteration occurred in amphibolite.Hydrothermal alteration leads to the content of metallogenic material such as Fe、P in alteredamphibolite decreased,metal sulfide and Au,etc enrichment, and can comprehensiveutilization.In summary, Daxigou magnetite-apatite deposit is an extremely low content ofmagnetite-apatite ores of volcano sedimentary-metamorphic type magnetite-apatite deposit.