Study On The Mineralization Of Coal-Derived Graphite In Lutang,Hunan Province

Taking the coal-derived graphite in the mining area of Lutang Town,Chenzhou City,Hunan Province as the research object,this thesis adopted the theoretical knowledge of coal geology,geochemistry,petrology,mineralogy,structural geology,and the analytical testing methods of X-ray diffraction,transmission electron microscope,electron-probe and Raman spectroscopy,respectively.This thesis systematically studied the evolution patterns of geochemical compositions,microstructures and metamorphism of coal-derived graphite and its wall-rocks,identified its carbon source,established the strain-induced graphitization model of tectonic-magmatic coupling,clarified the mechanism of mineral catalytic graphitization,and elaborated the metallogenic mechanism of coal-derived graphite.（1）Based on the comprehensive study of stable carbon isotopes of coal-derived graphite in Lutang,its major carbon source was identified as an organic one.This was proven by comparing theδ¹³C values of Lutang coal-derived graphite mostly ranged from-23‰to-15‰with theδ¹³C values of the organic carbons in the sediments of Precambrian and later periods to today.Although the positive biases of some comparison results remained within the identification range of organic carbon sources,it was still inferred that the graphite might also be affected by the inorganic carbon sources from the deep magmatic-rock activity and carbonate decomposition to a certain extent,because the isotope fractionation effect was closely related to the complex geological activity history.（2）The effect mechanism of tectonic activity on coal graphitization was clarified as below.On the fold belt with the same distance from the pluton,the graphitization degree of the coal-beds at or near the axis of the fold was often relatively higher,indicating that the coal-bed was subjected to strong shear stress.The increasing energy not only promoted the removal of macro-molecules’side-chains in the coal and enhanced their preferential orientation,but also advanced the formation and perfection of graphite structure.（3）The catalytic effect of minerals on the graphitization was revealed as follows.Chlorite was a metamorphic mineral widely distributed in coal and its wall-rocks.With the changes of temperature and pressure,its chemical contents would have ion-replacement reactions through Fe-Mg replacement,Si+（Mg,Fe）-Al^Ⅳ+Al^Ⅵreplacement（Tschermark replacement）and（Al^Ⅳ+Al^Ⅵ）-3（Fe,Mg）replacement（di-trioctahedral replacement）.By analyzing the chlorite’s electron-probe data,it was found that the iron content presented a positive correlation with the XRD’s Full Width at Half Maxima（FWHM）of adjacent graphite.It indicated that the higher the iron content in wall-rocks,the higher the degree of graphitization in adjacent ore-bodies,which defined the catalytic mechanism of chlorite on the graphitization process.（4）The wall-rock alteration zones generated by magmatic thermal metamorphism were differentiated as below.According to the intensity of this metamorphism,the study area could be divided into four specific alteration zones from west to east:the biotite zone,andalusite zone,garnet zone and cordierite zone.The stabilization conditions of mineral assemblages were 450～500℃,0.2～0.4GPa;500～600℃,0.2～0.35GPa;550～650℃,0.5～0.7GPa;590～630℃,0.4～0.5GPa,respectively.The garnet belt with abnormal temperature and pressure often indicated the higher quality of coal-derived graphite nearby.（5）By studying the temperature and pressure of wall-rocks’metamorphic zone,the initial temperature of magmatic rocks’intrusion,the metamorphic temperature of chlorite,given the analysis results of Raman spectrum geological thermometer,the graphitization temperature in Lutang area was determined as 400～540℃.The metamorphic evolution of coal graphitization under natural conditions was revealed to be mainly influenced by the magmatic thermal effect.And in the metamorphic process,the stress-induced graphitization mechanism and mineral catalysis effect also lowered the temperature of coal transformation to graphite.（6）This thesis also discovered the mechanism of coal graphitization with the coupling of tectonism-magmatic thermal metamorphism under mineral catalysis.Macroscopically,tectonism led to not only geological phenomena like folding and torsion but also cavities and pores making space for magma intrusion.While magma intruded into the stratum from the deep underground along the channel,it brought lateral compression which intensified the strength of tectonic activity.Microscopically,the increase of tectonic activity strength lifted the energy level of coal-derived graphite ore-bed,which promoted the aromatic macro-molecules’side-chain removal and structure rearranging.Furthermore,the magmatic heating not only elevated the coalification energy level but also promoted the integration of iron-contained elements and side-chain-oxygen atoms in wall-rocks;This facilitated the purification of graphitized basic materials and advanced the ion exchange of clay minerals’elements with outside.The precipitation or saturation of iron atoms also played a catalytic role in such graphitization.