Genetic Mineralogy Of Mapeng Instrusion From Mid-Taihang Mountain And Its Implication On Gold Mineralization

Mapeng intrusion lies in the northwest 2～3 kilometers of the large gold deposit in Shihu, which is from Mid-Taihang mountain. The relationship between the intrusion and mineralization hasn't been studied systematically from the Genetic Mineralogy. Mapeng intrusion has an instrusive contact with metamorphic rocks of Archaeozoic Fuping group, which is composed of quartz diorite (border facies), granodiorite (transitional facies), granite (central facies). This paper studies granodiorite and granite, and the conclusions are as follows.The study on Genetic Mineralogy of rock-forming mineral shows that: plagioclase zonings are abnormal, Fe₂O₃ and TiO₂ are high in feldspar, which reflect the oxidation environment and mixing characteristics of magma. Amphiboles are the calc-amphibole subtribe, and are rich magnesium mineral, located in the area of MC in TiO₂-Al2O3 diagram, all of those characteristics on amphiboles indicate magmatic mixing; On the other hand the characteristics of the changing alkaline largely and the rich Fe3+ are the sign of the oxidation environment. All characteristics of rich Mg, low AlVI and the average of MF (0.56) on biotites indicate marke of syntexis type; MnO in biotites are higher, the quantity of MgO in biotites are the quantities between shell source and mantle source, Biotites are located in the area of MC in Fe3+- Fe2+- Mg 2+ diagram, quartzes are wrapped in biotites and amphiboles, all of those characteristics on biotites indicate magmatic mixing. The indicator of hornblende-biotite and the former reseaches show: the main temperatures of the intrusion are 740℃～760℃, the main pressure of the intrusion are 2×108～3×108Pa, the main oxygen fugacities lgfo2 of the intrusion are -14.60～-10.77.The study on Genetic Mineralogy of accessory mineral shows that: zircons are divided in the typical magmatogene zircons, the study of zircon shape shows crystallizing liquidus temperature of the intrusion is 900℃, and the magma are richer in water and alkaline; Trace elements of zircon indicates a relative oxidation environment of magmatic crystallization, that the source region is main with shell source and mixed by mantle source. U-Pb zircon dating indicates the formation age of Mapeng intrusion is about 130Ma; the relationships between the intrusion and dykes are primarily considered to be homologous. Intergrowth of a large number of magnetite and spineline and rich Mg in amphiboles indicate the higher oxygen fugacities of magma, and also indicate that the intrusion belongs to magnetite series. Aptities are long and thin needle-like and some are short prismatic, the quantity of MgO,FeO,TiO₂ of aptities are respectively 0.07%,0.27%,0.20%, the characteristics of aptities indicate the magmatic mixing.The study of geochemistry shows that: with the contents of the SiO₂ and whole alkali increasing gradually from the border facies, transitional facies to central facies of the Mapeng intrusion, the value of∑R"O and Al presents decreasing inversely. The intrusion is rich in Al, alkali and divalent ion but poor in Si generally, which belongs to calc-alkaline series, and has the characters of high K; The geneses of the intrusion is I-type granite, and the intrusion has the high value of the mineralization elements and Sr, Ba but low value of Rb; The origin source of the magma located at the lower crust and maybe mix partial mantle material, and its evaluation is not thorough.In conclution, Mapeng intrusion has the homoeologous relationship with the dykes, and it is divided into I-type granite which is come from lower crust with mantel material mixing. Its formation age is 130Ma±, and during the process of the intrusion, the magmatic environment is under the oxidation conditions and rich in alkali so that it is advantageous for migration and enrichment of Au.