Geological Characteristics And Metallogenic Regularity Of Nihe Iron Deposit In Lujiang, Anhui Province

Nihe iron ore deposit is located in the Yangtze River metallogenic belt of Luzong ore district, which is one of five key mineral exploration areas identified by the Ministry of Land and Resources. Luzong region is a typical continental volcanic-igneous activity area in easten China, where continental volcanism-magmatic activity related to iron ore deposits is an important genetic type and the main producer in China's iron ore resources. Nihe iron is a significant new discovery which is in the implementation of land resources survey project "Sheng Bridge-Zongyang Hengbu area iron-copper exploration in Lujiang, Anhui province."by Geological Survey of Anhui Province in 2007. The discovery of Nihe iron ore deposit not only breakthrough Luzong area past confined to only 400 to 500 meters depth exploration, but also indicates further deep prospecting work has very broad prospects in the Yangtze River. Nihe iron ore deposit as an typical object to do research into metallogenic characteristics and metallogenic regularity summarized, which has important theoretical and practical significance on the promotion of the Yangtze River metallogenic belt deep prospecting work.This paper is targeted on Nihe iron ore deposit as the main anatomical object, Luo River iron ore deposits as a supplementary study, which to focus research on the content of characteristics of geological metallogenesis, structure of ore field, characteristics of mineralization and mineralization signs study. Then having systematic study on metallogenic geological setting and metallogenic geological features, geochronology, fluid geology and tectonics ore field and so forth, which to limit the age of diagenesis and mineralization, and to confer the source of fluid, the source of minerals, enrichment rules of metallogenesis, etc. Ultimately determining the Nihe iron ore deposits geological body and deliberating the genesis. On the basis of the above-mentioned study, dealing the research with the regional structure-construction-fluid-mineralization as the main line, the iron ore deposits and structural-volcano-hot events in time-space configuration and the coupling relationship as the primary context, to explore and summarize characteristics of mineralization, space-time structure of mineralization and metallogenic regularity, and to further deepen knowledge of the continental sub-volcanic hydrothermal iron ore-forming mechanism and mineralization model. Through the comprehensive study of indoor and outdoor, the paper have made made the following main understanding and results: (1) Though rock microphysiography identification, rock geochemical analysis shows that Luzong region volcanic-sub-volcanic rocks are mainly a set of alkaline-sub-alkaline basaltic trachyte, trachyandesite, rough dacite, tephrite and so on. Geochemical features indicate that Luzong volcanic rocks with medium to high SiO2, alkali-rich, potassium-rich, high-aluminum and titanium-poor characteristics. Potassium was significantly to be metasomatism, mobilization and migration out from rock in the mineralization stage of alkali feldspar. Volcanic rock-sub-volcanic-intrusive rock evolution from morning till night, SiO2 and TiO2, Fe2O3, FeO, MgO, CaO,P2O5, etc.was negatively correlated, but SiO2 and A12O3, K2O, Na2O showed a certain positive correlation.In Luzong region, the volcanic and intrusive rocks differentiation index showed both of them have the same differentiation trend, they are the homologous products, and also has the same magma evolution characteristics with mesozoic era volcanic-subvolcanics in Nanjing-Wuhu basin.(2) Luzong volcanic trace elements relatively primitive mantle, significantly enriched in Rb, Pb, Hf, Th and other large ion lithophile elements, but Nb, Ti, Y,et al. high field strength elements and P element are relatively depleted, the variation characteristics between volcanic and intrusive rocks trace elements relatively primitive mantle-normalized spider diagrams is similar,which showed that sub-volcanic body and syenite-monzonite intrusions trace elements in Nihe mining area have characteristics of succession with volcanic features of crystallization differentiation, and the advanced degree of the late magmatic differentiation enhancing. It is the homologous heteropic phases product with a complete magmatic evolution series.(3)The REE curves shape about volcanic,subvolcanic and intrusive rocks in Nihe mining area is similar,which all showed a relatively smooth Right type. LREE and HREE clear differentiation, LREE enriched, HREE loss, and all rocks have the characteristics of light rare earth fractionation significantly higher than the heavy REE fractionation. REE distribution patterns may be related to partial melting of enriched mantle and crustal material of different degree of contamination.(4) the Sr-Nd isotopes and Pb isotopes showed that both volcanic and intrusive rocks are derived from the deep mantle, and both are the homologous heteropic phases product of fractional crystallization of magma evolution.(5) According to field survey, combined with the systematic indoor petrographic identification,summed up the characteristics of rock alteration zoning and the corresponding alteration mineral zoning in Nihe iron ore deposit, bottom-up as follows:the lower light-colored alteration zone (pyroxene-potassium feldspar-albite-apatite-anhydrite)→dark alteration zone (pyroxene- apatite-magnetite- anhydrite,and local shows garnet)→superimposed alteration zone (apatite-magnetite- anhydrite- hematite-pyrite)→upper light-colored alteration zone (quartz- kaolinite- chlorite- carbonate- Hydromica,etc.).(6) Through analyzed and summarized the physical and chemical conditions of mineralization, revealing that the magnetite of Nihe and Luo River iron ore deposits have the features of multi-stage mineralization. The formation of late magmatic fractional crystallization of magnetite crystallization temperature (T) is:775～1152℃, oxygen fugacity (fo2) is: 10-8.96～10-15.27 (according to Buddington and Lindsley,1964); T=711.04～1114.39℃, aO2=1.86×10-8～3.61×10-16(according to R. Powell and M. Powell,1977). Both methods obtained almost the same range of temperature and oxygen fugacity, which significantly higher than the formation temperature of hydrothermal magnetite. The main magnetite mineralization stage is pyroxene-anhydrite-magnetite mineralization stage, forming temperature is 350～506℃. Oxygen fugacity (fo2), sulfur fugacity (fs2) and acidity-basicity (pH) and other physical and chemical parameters indicate that pyroxenite and magnetite are the product for the same period,which formed under the combined action of high sulfur, high oxygen and acid environment. From the late magmatic to late hydrothermal alteration,ore-forming environment evolution from weak acidic environment of high oxygen sulfur into hypoxia-low sulfur and strongly acidic environment.(7) Using high-precision zircon dating technique U-Th-Pb method to determine the diagenetic age of magmatic mineralization, preliminary limit the metallogenic chronology is between 128～134Ma. To synthesize diagenetic mineralization theory at home and abroad and the metallogenic evolution sequence of mineralization types and associated chronology data in Luzong region can be further constrained reasonable time limit for Nihe iron ore deposit mineralization of 132～131Ma. So as to establish the temporal structure of metallogenic Luzong basin.(8) By the previous data analysis and isotope geochemistry, the oxygen isotope characteristics of magnetite in the magnetite ore deposit and magnetite in volcanic rocks of Luzong Basin show that iron oxide in iron ore and iron oxide in volcanic rocks may be of the the same source,the iron mineral were extracted from volcanic rocks by metasomatism. Lead isotope showed a characteristics of deep fluid source. Nihe-Luo River iron ore deposits of ore-forming hydrothermal sulfur may be part from rift basins the underlying strata,especially marine anhydrite layer in Zhou Chong village Triassic, the rest is from the deepen carried by magma. According to quartz-water oxygen isotope fractionation as a function of temperature work out that the water stage in the dark alteration stage and generation phase of magnetite stage is in the range of magmatic water, and the light-colored alteration pyritization stage may have participated in meteoric water.(9) According to the output features of Nihe iron deposit ore body, ore fabric characteristics, wall rock alteration and mineralization alteration features, forming ages, ore resouses,etc., discussion and analysis that Nihe-Luo River iron ore deposits are the continental subvolcanic magmatic-hydrothermal type deposit, the ore host rock of iron ore deposit is diorite porphyry.(10) According to the macro-geological field investigation and indoor micro-identification test and analysis, summarized and analyses the geological features and mineralization geological factors of Nihe iron ore deposits, ore body position significantly integrated control by stratum-tectonic-magmatic construction, mineralization is relevant to construction-magma-hot events, local wall rock hydrothermal alteration plays an important role in promoting mineralization enrichment.(11) Summarized the distribution law of spatial and temporal related to continental sub-volcanic iron deposits in Luzong region, clarify structure-magma-liquid-mineralization relationship, and establishment comprehensive metallogenic model for Luzong Basin continental sub-volcanic-type iron deposits,which provide important theoretical basis for study area prospecting.