The Study On Metallogenic Model Of Bainaimiao Copper-Gold Ore Field, Inner Mongolia

The Bainaimiao Copper-Gold Ore Field is the only large Copper-Gold ore field in the middle section of the northern margin of the North China platform. The ore field is composed of the Bainaimiao large Copper-Molybdenum deposit and middle Gold deposit. It has experienced the complex orogenic process with the subduction of the Paleo-Asian Ocean and the continental accretions. The ore field has been studied by many geologists on its fundamental geology, geochemical, ore-forming fluid, tectonic evolution, age of petrogenesis and mineralization, and so on. However, its age and ore-forming evolution are still argued.It is still needed to study the ore-forming evolution of the Bainaimiao Copper-Gold Ore Field. This work chooses the Inner Mongolia Bainaimiao Copper-Gold ore field as the research area. By the systematical study of the volcanic rocks in the Bainaimiao formation, the Early Pleozoic magma, geochemistry, petrogenic age, origin and evolution, forming environment, and so on, Bainaimiao copper gold orefield of sedimention、deformation and metamorphism from marine volcanic eruption-magmatic hydrothermal superimposed complex type of porphyry will be built. Meanwhile, the mineralization can be recognized as a significant product in the process of arc-continent orogeny and the tectonics-magma-fluid evolution. Based on these works, the metallogenic mechanism of the ore field would be further revealed.The research progress and achievements of this work are concluded as follows:(1) It is a comprehensive conclusion about the regional geology of the Bainaimiao area. We make the research of the Bainaimiao Copper-Gold ore field by compising the Bainaimiao Cu-Mo deposit and Au deposit as a unit. It is the result of different tectonic evolution stages under the uniform geological background, and recognized as the result of the subduction of the Paleo-Asian Ocean plate to the North China plate.(2) The petrology and petrochemistry features in the Bainaimiao Formmation are studied, and its diagenetic and metallogenic mechanism analyzed. The Bainaimiao Formation is compised by the low metamorphic greenschist. Its original rocks is the eruptive basic and intermediate-acidity volcanic lava and tuff rocks from submarine with some clastic and carbonate rocks, which is considered to be the shallow-volcano-sedimentary formation. While, the Bainaimiao Formation belongs to the Cambrian. It is related to the late alternation as Na2O+K2O is 1.15%～7.42%, and some samples up to 18%～27%. The A/CNK is among 0.43～1.04 together Mg# among 27.03～74.66, which represents the partial melting of continental and oceanic crust material. The rocks mainly belong to the calc-alkaline series, then high K calc-alkaline series and low K series. It is obtained that the volcanic rocks of the Bainaimiao Formation are sourced from the continent with joining of oceanic crust components.The large ion lithophile elements are rich in K, Rb, while deficit in Ba, Sr. The deficit high field-strength elements has the Nb-Ta loss, together with the rich of Th, U, Zr, Hf. These reflect the characteristics of continental and oceanic crust materials. The magmas from the mental related to the subduction are contaminated by continental materials.The total amount of the REE is low, and of a right type curve. It is enriched the LREE and with a high fractional degree. Some have a clearly Eu anomaly, which indicates that plagioclases have a poor fractional crystallization assimilation. Meanwhile,δCe is among 0.78～1.05, and Ce has week negative anomaly.Pb isotopic characteristics shows that the origin of the Bainaimiao Formation is related to the magma during the progress of oceanic plate subduction. It belongs to the volcanic arc rocks with contamination of mental and crustal materials. The diagenetic environment is adjacent to the subduction zone in the continental margin.(3) The petrology and geochemical characteristics of granodiorite porphyry are found out, the formation environment discussed, and its relationship with mineralization analyzed.The granodiorite porphyry, intruding into the chlorite-actinolite-plagioclase-schist, are grey-pale red, similar gneissic structure, granitic porphyroclastic texture, and metasomatic relict texture. Altered mental minerals are pyrite, chalcopyrite, molybdenite, and so on.A/CNK of the granodiorite porphyry is 0.797～1.61, which belongs to the high K calc-alkaline series, shoshonite series, calc-alkaline series, and has the characteristics of continental granite re-melting.The total amount of the REE is 113.97-199.08. The fractional degree of the LREE is higher than the HREE.Pb isotopic characteristics show that the origin of the granodiorite porphyry, a medium deep to ultra shallow intruding body, is related to the volcanic arc environment in the collided continental margin. The pluton spreading direction is coincided with the regional tectonic direction. The origin environment is different. The magma source is from the mental, with capturing some components of sediments. The radiogenic lead is also mixed at the late stage.The ore body in the north ore belt is mainly in porphyry plutons, takes up 60% of mineralized bodies, in which molybdenum ore is 70% and copper ore is 50%. The plagioclases in granodiorite porphyry are rich in Cu and W. However, the plagioclases in the adjacent plagioclase granite, muscovite granite and migmatized granite has less Cu and W. The plutons are the mineral resources, and also supply the space for ores.(4) The magmatic-mineralizing events of the Bainaimiao Copper ore and Gold ore are accurate dated in this work. By LA-IPC-MS zircon U-Pb dating, the granodiorite porphyry is determined. Most of zircons have clear girdles, with high Th/U. Its intruding age is the Early Silurian, and the weighted mean ages are 435.5±1.9 Ma,439.8±2.5Ma and 443.0±1.0Ma.The time trellis of the copper-molybdenum deposit and gold deposit in the Bainaimiao area is built. Meanwhile, the metallogenic events related with the Early Peleozoic tectonics-magma activities are concluded. By Re-Os isotopic dating, the age of molybdenite, filling fissures in the greenschist and granodiorite porphyry, is limited to 440.5±4.4Ma, which is recognized as one important mineralization event. The U-Pb dating of zircons from the quartz vein boring gold limits the age of veins to be 437.0±6.0Ma～445.6±5.0Ma, belonging to the Early Silurian. Two mineralization events correspond with the intruding of granodiorite porphyry, which composes one united copper-gold metallogenic system.(5) With the comprehensive study of stable isotope characteristics of copper-gold ore field, it is discussed the sources of ore-forming material. Lead isotope composition characteristics of Bainaimiao gold ore field is similar to the south ore belt and north ore belt which reflect that Bainaimiao gold deposit and Bainaimiao copper-molybdenum ore have a similar metallogenic environment.In the lead isotope evolution figure, south ore belt reflects the lead mixture of shell and mantle source.Tectonic environment diagram shows that lead is from multiple sources, which form in the continental margin arc volcanic environment.The characteristics of Strontium isotope reflects that shell source material mixs in the metallogenic process of copper (gold) ore, the ore-forming materials of the north ore belt and gold deposit originate from granite diorite porphyry.Nd isotopic features indicate that metallogenic material has the similarity among the south ore zone, north ore belt and the gold ore field. Because of rock intrusion for the north ore belt, tianshui influenced for gold ore field, there is some difference in 143Nd/144Nd ratio. So the south ore belt slightly higher than the north ore belt and the lowest value of north ore belt is lower than gold ore field.Sulfur isotope characteristics indicate that’S values of sulfide between the south and north ore belt are similar, a little changed, which explain sulfur source of the south and north ore belt mineral is the same, and from the mantle sulfur.The sulfur isotope of Bainaimiao gold deposit issimilar to copper deposits, and the two ore-forming materials source are homologous, all from the upper mantle, with arc genetic background.Characteristics of hydrogen and oxygen isotopes suggest that the south ore belt mineralization material of Bainaimiao copper-molybdenum ore is given priority to deep source igneous rock, accompanied by shell source material. The metallogenic hydrothermal of Bainaimiao gold ore is from atmospheric precipitation.(6) The paper analyzes the trend of copper-molybdenum-gold mineralization. Choosed Cu, Mo, Au grade related data of Ⅲ ore block Ⅲ-2 copper body, analyses its thickness of ore body and the correlation of Cu, Mo, Au grade. Their overall distribution is broadly similar, Their enrichment center position roughly consistent, and there are some strong positive correlations between each other. Cu, Mo grade and thickness of ore body correspond to the EW distribution of regional structure, and have the obvious trend of deep to extension.It suggest a Good ore prospects in deep area.(7) The paper discusses the type, distribution and composition of fluid inclusion and the evolution of ore-forming fluid. Fluid inclusions is given priority to gas liquid inclusions, followed by gas inclusion and mineral inclusions. Primary inclusions are nearly uniform distribute, and secondary inclusions’ distribution is mainly linear along the fracture, with superposition and conjoined phenomenon. The fluid inclusion’uniform temperature is 219.9℃～248.1℃ in quartz vein of Bainaimiao gold ore, which is narrow. Their salinity is 5.41%～8%, so the fluid inclusion belongs to the medium temperature low salinity. While the uniform temperature of Bainaimiao copper ore’ fluid inclusion have two ranges:One is 208.3 ℃～243℃, the temperature is relatively narrow.Salinity is 3.55%～9.34%, Belongs to the medium temperature low salinity; another is 168.4 ℃～178.3 ℃, the temperature is also narrow, corresponding to the salinity is 4.96%-8.56%, belong to low temperature and low salinity. The result reflects that the field has at least two stages ore-forming fluid:the medium temperature low salinity fluid and low temperature low salinity fluid.The gas phase composition of Bainaimiao copper ore’fluid inclusion are CO2、H2、N2 and H2S, liquid phase composition is H2O. Its chemical composition enriches HCO3-、Ca2+ which is not conducive to the migration of copper and good for copper precipitation. The gas of Bainaimiao copper-molybdenum ore are CO、H2、N2. and the liquid phase composition is H2O.Early stage of primary fluid inclusions in quartz of uniform temperature is 248 ℃～380 ℃, salinity is 4.34%～6.59%;middle stage of uniform temperature is 215℃～241℃, salinity is 2.90%～4.18%; later stage of uniform temperature is 137℃～181℃, salinity is 0.50%-2.00%.(8) The paper analyzes the diagenetic and metallogenitic evolution process, and establishes the metallogenic model of copper -gold ore field.At the end of the neoproterozoic,the ancient Asia oceanic plate subducted to the north China plate. During the Cambrian-Ordovician, there are some large-scale multiphase volcanic eruptions in the near east-west Wenduermiao subduction zone south side on the edge of north China. The mantle-derived magma which mixed with continental crust, provide mineral source for initial mineralization. In the Ordovician, the intense subduction plate produced nearly east-west deformation and metamorphism. It made copper, molybdenum, gold and other useful elements in volcanic rock activate, and migrate to the favorable tectonic position and precipitation mineralization. In the early Silurian Epoch, the invaded granite diorite porphyry along the east-west tectonic belt and the late hydrothermal superimposed transformation enriched the copper deposits. At the same time, magmatic hydrothermal drive atmospheric precipitation cycle and form auriferous hydrothermal. Finally, it formed the gold deposit in Bainaimiao fault and circumferential crack.According to the main ore-controlling factors, metallogenic physicochemical conditions of the ore deposit formation, it is established the metallogenic model of Bainaimiao ore field.Starting from the proterozoic to early Paleozoic, the ancient Asia oceanic plate subducted to the north China plate, and Gradually formed the Wenduermiao ancient trench, Polycyclic Neutral-base and acidic volcanic eruption, and at last formed the island arc. Volcanic material carrying some Cu, Au, etc metallogenic material, which formed the initial source bed.In the Ordovician, the volcanic rocks of Bainaimiao group generally produced deformation and metamorphic. Metamorphic hydrothermal fluid activated copper, molybdenum, gold and other useful elements in volcanic rock, which formed the embryonic of the copper and gold mine field.In the early Silurian, the ancient Asia oceanic plate intensely subducted to the north China plate, and the east-west tectonic belt of ore field gradually formed. Acidic magma invaded passively along the east-west tectonic belt, blended in part of the continental crust material, the magma and late ore-bearing hydrothermal produced superimposition and transformation, enrichment for the lean ore layer of the prior period. Part of the hydrothermal assaulted on along the Bainaimiao fault, evoluted into gold-bearing hydrothermal, and formed the north ore belt and gold deposit. Part of the hydrothermal affected the south ore belt either. The pattern of Bainaimiao copper-gold ore field basically completed. Metalogenic model of Bainaimiao copper gold orefield of sedimention、deformation and metamorphism from marine volcanic eruption -magmatic hydrothermal superimposed complex type of porphyry had refomed.The superimposed reformation of the Devonian, Permian and its later, didn’t produce damage to early ore body at all.