Ore-forming Fluid And Material Sources’ Research Of The Copper-Gold Ore Bodies Meishan Iron Deposit Peripheral, Jangsu

Ningwu basin, is the birthplace of "porphyrite iron" metallogenic theory, located in the northern margin of Yangtze plate. As a typical large iron ore deposits among porphyrite iron, Meishan iron ore deposits is characteristic of high grade and large reserves. Therefore, there is a good mineralization potential in this region.In 2009, the prospecting work around the periphery of Meishan Iron ore made great breakthrough. That is, a copper-gold polymetallic deposits with medium-sized gold reserves was found, which provided a new direction for the prospecting of porphyrite-type ore. However, the source of ore-forming materials is important in the research of metallogeny, especially the source of copper and gold of Meishan deposits and its relationship with the iron ore deposits, which is valuable to the comprehensive understanding of Meishan deposit and significant in further improving "porphyrite iron" metallogenic model. In this paper, we conducted corresponding research in respect of the ore-forming fluids and the source of ore-forming materials of the Meishan copper-gold polymetallic deposits.In this paper, on the basis of detailed field investigation of Meishan Iron ore, we also carried out petrographic observation, fluid inclusions, element geochemistry and isotope geochemistry work in order to make a comprehensive research in respect of the geology, geochemical characteristics and metallogeny of Meishan copper-gold deposits. Therefore, we have harvested some new understandings.By hand specimen and microscope observation, the ores of copper-gold polymetallic ore body are divided into three categories:siliceous rock type ores, pyrite ores and diorite breccia type ores. However, the gold content of siliceous rocks and pyrite ore is higher, so most of them are 1～2g/t. When it comes to the dioritic breccia ores, most of the gold is enriched in the pyrite cementation between the breccias. No visible gold was found in each type of ores. Electron probe analysis showed that most of gold exists as micro gold or lattice gold in the micro cracks or crystal lattice of pyrite and chalcopyrite.Fluid inclusions are selected from the calcite veins that are in symbiosis with the copper gold polymetallic ore. Most of the inclusions are gas-liquid H2O two-phase inclusions that are characteristic of simple structure, scattered distribution and most of them are oval shaped, elongated or irregular shaped. The inclusions are generally small with the size of 5-10μm and homogenization temperature concentrated in 200 degrees and 250 degrees mainly.Combined with previous research results, we believe that the gold and copper ore bodies may have experienced two stages of hydrothermal activity and the metallogenic fluid are mainly middle-low temperature fluid.The lead isotope studies indicate that the lead in pyrite that is closely related to the gold mineralization all plotted in the orogenic belts but the ones in the host rock of tonalitic porphyrite plotted in both sides of orogenic belts, which is consistent with the overall characteristics of Ningwu area. The lead in Meishan copper-gold polymetallic sulfide ore should be sourced of crust-mantle mixed but the lead in magnetite is more close to the upper crust, which indicates that a lot of crustal sourced lead are mixed.C and O isotopic studies of carbonatite vein in copper-gold ore body indicate that the δ13CCO2 and δ18OSMOW value of copper-gold,iron ore in Meishan and copper-gold orebody Ningwu area are between magmatic rocks and carbonate rock, which suggests that continental and marine sedimentary and igneous rocks all provided materials for the mineralization.When compared with the δ13CCO2 and δ18OSMOW value of the Meishan iron ore body,the δ13CCO2 and δ18OSMOW value of Meishan copper-gold ore body are more close to the sedimentary rocks of continental facies, the 813CCO2 value of which similar to that of andesitic rocks and different from that of iron ore. Therefore, we believe the C of copper-gold ore body mainly comes from the lower Triassic continental sedimentary carbonate.The sulfur isotope composition shows that there are obvious differences in the sulfur isotope composition of Meishan iron ore and copper-gold ore. The δ34S value of pyrite in iron ore body is 8.9‰-14.3‰ with the average of 11.9‰,while the δ34S value of gypsum is 18.5‰-23.3‰ with the average of 21.43‰. However, the 834S value of wall rock and alteration zone is 8.7‰-10.7‰ with the average of 10.0‰, which is similar to that of pyrite in magnetite orebodies. So there were sulfur from the sedimentary stratas added in the rising process of fluid from the rocks and iron ores. The δ34S value of pyrite in copper-gold polymetallic deposits is -1.4‰-2.1‰ with the average of 0.73%o, indicating that the sulfur are mainly deep sourced sulfur.The rare earth elements analysis shows that the total content of rare earth elements in tonalitic porphyrite changes in a big range.That is, the ΣREE of the rocks are 55.75×10-6～270.93×10-6 with an average of 126.45×10-6 and the LREE/HREE value is 4.94～19.67 with an average of 10.1.In addition,δ Eu of the rocks are 0.84-1.25 with an average of 1.06 and the rock is characteristic of rich in LREE, but its Eu anomaly is not obvious.Therefore, the rock was formed in a relatively stable environment, and was affected by magmatic fractionation. The ΣREE value of copper-gold ore body varied greatly. That is, the ΣREE value ranges between 26.01×10-6 and 3222.20×10-6 with an of 126.45×10-6. However, the LREE/HREE value is 15.81～218.98 and δEu is 1.79～5.44 with an average of 2.89. Therefore, the obvious LREE enrichment and positive Eu anomalies suggest that they formed in the reducing conditions. While the obvious negative Eu anomaly suggest that they were formed under oxidizing conditions, which is different from the reducing environment of copper-gold ore body. Therefore, they were the result of different ore-forming fluids.Above all, we conclude that the ore-forming materials of Meishan copper-gold polymetallic ore came from pyroxene diorite porphyrite rock. The fluids associated with mineralization can be divided into two stages. Therefore, the mineralization process is as follows:First, the andesitic rocks formed and they experienced silicification when the iron ore formed so the auriferous cherts formed. Then, the lower layers of auriferous cherts were disturbed by the later mantle hydrothermal fluid so gold entered the hydrothermal fluids again. During the process of cooling, gold entered the crystals of pyrite in the form of lattice gold and super micro-gold, so the volcanic breccias-bearing sulfide ore and gold-bearing pyrite ore formed.