| The Bayan Obo deposit is a giant Fe,Rare Earth Elements(REEs)and Nb(Nb)deposit located in the southern-central part of Inner Mongolia,China.The Bayan Obo deposit has complicated mineral associations,structural controls and multi-stage genesis and the mineralisation is rich in Nb,REEs and Fe resulting from multiple mineralising events.The Bayan Obo West deposit is located at the western edge of the mineral field,and the Bayan Obo West Mine is the main mining production area now in Bayan Obo area.However,the metallogenic study of useful minerals in the mining area,especially the enrichment characteristics of Nb,is not clear,and the relationship between metallogenic elements is confusing.Based on previous studies,detailed petrology,mineralogy and geochemistry studies were carried out on samples from four trial trenches and borehole data from the 2-48 prospecting line from the Nb orebodies in the West Mine.Based on the geochemical characteristics and correlation mineralization of elements,the enrichment mechanism and the characteristics of deposit and Nb orebody were studied,which provided new theoretical evidences for the enrichment of Nb.Moreover,it was illustrared the Bayan Obo superlarge deposit formation genesis and the metallogenic modelling,the combination relationship of elements in the symbiosis minerals,the material sources and metallogenic conditions,etc.In this paper,the distribution differences of three types of orebodies and the main types of rock mineralization in the Fe-REE-Nb deposit of Bayan Obo West Mine were systematically studied.It shows that the mineralization area distribution of Nb in is roughly parallel to the strata of REE and Fe orebodies,which is strictly controlled by syncline and is irregular distribution.Moreover,Nb and REE,Fe orebodies showed a common enrichment characteristics but respectively mineralization.The tectonic system,magmatic rocks and the influence of metamorphism on mineralization,and metallogenic types of dolomite in the mining area were comprehensively analyzed.It illustrates the complicated structure formed by the late fracture inheritance and cutting the early fracture.Magmatic rocks and metamorphism have important influence on mineralization of Nb.The dolomite rich in Nb commonly are occurred amphibolization,micacization and fluoritization.The orebody is often near or sandwich in the REE and Fe orebody in the dolomite.The research target was directed to the Nb orebody,and the type of Nb mineralization was classified,and the petrographic characteristics,mineral assemblage and ore-bearing status were analyzed.Sulfide minerals often appeared with Nb minerals,thus the characteristics of sulfide in the West Mine were summarized.It reveals that the West Mine contains various types of Nb-bearing rocks which have complicated chemical compositions.The various species of mineral and types of ores show obvious different Nb content.Nb mainly exists in the form of Nb oxide and silicate minerals,while the minerals containing Nbare mainly niobite,a small amount of ilmenorutile,fersmite and baotite,etc.REEs are distributed in fluorocarbonate and phosphate minerals,and the rare earth minerals are mainly monazite and bastnaesite.The content of Fe and REEs in Nb-bearing orebody is generally low.The main type of sulfide is pyrite in the form of massive or fine veins,and other sulfides include a small amount of pyrrhotite,galena,sphalerite,chalcopyrite,etc.Gangue minerals are mainly dolomite,biotite,fluorite and amphibolites,with apatite,diopterite,calcite,and barite,etc.According to the analysis of the Nb-bearing dolomite and its surrounding rock,the element geochemical analysis characteristics show that the Fe-REE-Nb deposit of dolomite has the igneous characteristics of carbonatite.It is suggested that the characteristics may come from the mixing of crustal material when the magmatic emplacement occurs.It indicates that the different types of rocks in Bayan Obo West Mine have the same source of elements and similar mineralization processes in the same metallogenic environment by the late hydrothermal metasomatism and alteration.The dolomite has been mostly affected by the replacement of metasomatism as various kinds of minerals.Moreover,it suggested that the fractionation process of the light and heavy rare earth and the REE metallogenic process are simultaneous.At the same time,it is believed that the enrichment of REE and Nb is influenced by the exogenous fluid rich in F,P,S,et al.,but the concentration of REE and Nb in orebodies show relatively distinct mineralization.It is believed that the orebody originated from the original ore-bearing carbonatite,and the process superimposed by the large amount of hydrothermal solution formed the massive Fe-REE-Nb polymetallic deposit.For the metallogenic correlation analysis of the Nb-REE-Fe mineralization in the Bayan Obo west mine,a combined method that integrates factor analysis and Back Propagation(BP)neural network technology into geological data processing and modeling is presents.To provide the required data,systematic geochemical sampling is performed,after which the Nb and REE contents of the samples are transformed into discrete values for factor analysis,and the correlations among the metallogenic elements are analyzed.The results show that the mineralization correlations between Nb and the REEs are weak.Nb and U are closely related to the geochemical patterns,while Fe is closely related to both Th and Mn.LREEs are an important factor for the mineralization of the Bayan Obo deposit,while Fe and Nb can be considered the results of passive mineralization.This is consistent with the observations of field exploration surveys and the geological characteristics of the Bayan Obo deposit.On the basis of a metallogenic correlation analysis,the factors affecting the Fe-REE-Nb mineralization are extracted,and the geochemical Nb mineralization model is established by the BP neural network.At the same time,based on geological data,the metallogenic correlations among the Fe-Nb-REE geochemical patterns in a complex geological setting can be obtained by an analysis of the relationships among mineralization elements.The parameters selected during metallogenic correlations analysis as the model inputs are applied to predict the Nb enrichment.Based on the BP network,the mineralization prediction model provides reliable results regarding the Nb enrichment in different rock types.The integrated method consisting of factor analysis and BP neural network technologies in this study can extract the mineralization controlling factors,which are beneficial for predicting the extent and location of mineralization,and further predict the mineralization enrichment of different elements that may exist throughout the Bayan Obo area.By the BP neural network data mining,the variability in the Nb concentration displays a coupled multi-factor nonlinear relationship that can be used to determine the inherent metallogenic elemental regularities and predict the degree of element mineralization enrichment in the mining area.The influences of a variety of factors that lead to the enriched behavior of Nb mineralization display a type of nonlinear relationship.This relationship can be used to exploit the variations in the concentrations of Nb through data mining to obtain the metallogenic pattern of enrichment of Nb.The characteristics of the distribution,structural controls,and spatial variation of Nb,REE,Fe,S in the West Mine of Bayan Obo mineralisation are discussed.These characteristics have been used to characterise the chaotic attractor(i.e.mathematical definition of chaos)and fractural natures of sulfur and its spatial variation in the mineralisation,which are used to analyses the indicative significance of the mineralising environment.The results show that the spatial variation of the sulfur grade has a multi-fractal nature forming a chaotic series.Furthermore,the results confirm that the evolution of the ore-forming fluid and the enrichment of the mineralisation followed chaotic dynamics.The multifractal nature of the sulfur indicates that the mineralisation went through at least two major stages involving an initial mineralization followed by complex and chaotic metasomatic events.The characteristics of the sulfur distribution at Bayan Obo West Mine indicate that a least two generations are present.The first generation is syngenetic,followed by remobilisation during the deformation event forming Bayan Obo West Mine with mineralisation concentrating in the axial trace of the fold and along small faults.The fractal dimension variations show that the remobilisation of the sulfur-bearing minerals focused into the synclinal axis of Bayan syncline.The variation indicates that the ore-forming fluid was relatively complex during the first mineralising event.The second events gradually intensified from the syncline's limbs to its core where it stabilised.The spatial distribution of sulfur,Nb and REEsat Bayan Obo West Mine is a chaotic,and a chaotic strange attractor(i.e.an attractor with a fractal structure)exists related to the deposit's evolution.Furthermore,the evolution of the mineralising fluid and the deposition define a chaotic dynamic process.This can be explained by invoking multiple events that have remobilised and concentrated the mineralisation during complicated responses to tectonic stress,mineralised fluid flow,and fluid-rock reaction during hydrothermal events.Evidence from fractal analysis,chaotic characteristics,mineral aggregates,and tectonic structures illustrates that the distribution of sulfur at Bayan Obo West Mine is related with a combination of various complicated processes.Based on the complex geological background of Bayan Obo deposit,the research of the formation mechanism and metallogenic model associate with geological characteristics,petrology,mineralogy,geochemical analysis,and the demonstration of fractal and chaotic dynamics,collection of the previous isotopic evidence,combined with the comparative study of trace element containing Nb ore enrichment mechanism of magma type,shows two main mineralization process.Mineralization is closely related to magmatic rocks from aspects of time,space and material circulation.The developmented fault provides the fluid passage,and the anions in the hydrothermal fluid activate the metallogenic elements in the rock,and the continuous transformation leads to the redistribution of the metallogenic elements.Bayan Obo enrichment of metallogenic elements may be due to igneous carbonatite magma intrusion or extrusion in a shallow sea environment in the period of Mesoproterozoic.The carbonate magma is enriched in Nb,REEs,Fe and other rare metals and volatile.With the evolution of magma,the late carbonatite rocks are more abundant in these components,forming a comprehensive deposit of rare metals such as iron,rare earth and Nb.The hydrothermal superposition and transformation of Caledonian period have important influence on mineralization.Moreover,for Nb-bearing ore prospecting,it is very meaningful to find the deep fault zone and a series of secondary fractures.Carbonatite is often associated with igneous alkaline rock formed a complex rock mass,it is also an important prospecting indicator.The region which has the metasomatism of Li Na,K,P,F and Cl such as amphibolization,feldspar with potassium,fluorite and micaalteration effect and with the characteristics of mixed lithification also has a great potential for prospecting. |
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