| 其他摘要 | The production of the Mayang Copper Mine has a long history, which can be dated to the Spring and Autumn Period. This mine not only provided a large number of copper to the ancients of South China, but also is still the largest copper mine in Hunan Province today. The Mayang copper deposit is located in the border between Mayang County and Chenxi County at the Central of YuanMa Basin, and neighbor with the Xuefeng Mount in the south. The ore-bodies only occur in the redbed of the Cretaceous.
For a long time, the Xuefeng Mount district is a key area to study the tectonic framework in South China. The YuanMa Basin as the secondary structural unit of the Xuefeng Mount poorly paid attention. Only in the 1960s, several research institutions conducted some exploration for copper, oil and gypsum, and found 12 copper deposits or mineral occurrences including Mayang, Huangshuang and Shanmuxi. Since the exploitation of Mayang Copper Mine，the research on the ore deposit had almost been at a standstill until 1990s. When the copper became depleted, lots of scientific researches have been performed for the geologic setting, the sedimentation analysis of Yuanma Basin, but the geochemical study for the deposit still is little. It is debatable in the source of the copper, the mineralizatic condition and the age of ore-formation. Detailed studies on the regional geology and the local geology of the Mayang deposit have been carried out in this study. And the systematic microscopic observation of ores and the quantitative electron-probe analysis of the minerals have been done. Meanwhile, the contents of major elements, trace elements, rare earth elements and the Pb, S, C, O isotopic compositions of the ores and rocks have been analyzed. Based on these data and previous works, several conclusions as follows:
(1) The major elements and trace elements study show that the genesis of the sedimentary rock in Yuanma Basin and the basal metamorphic and magmatic rock in Xuefeng Mount are from the crust belonging to the tectonic setting of island arc or active continental margin. The weathering history of the sedimentary rock in the basin is weakly, only experienced the basic plagioclase weathering.
(2) The Cu mineralization in Yuanma Basin only occurs in the Cretaceous sandstone near the Xuefeng Mount, and the ore bodies never cross to the redbed. The mineralization is multistory, and a single ore-body is stratified or lentoid, the mineralization almost disappears with the phase change from river course to others.
(3) The clastic material in the mining area derived from the Xuefeng Mount region, including a large quantity of native copper containing siliceous debris, which is considered the prime source of copper. The structure of the siliceous debris like that of sedimentary siliceous rocks, but the lead isotopic composition of native copper is rather similar with granitic rocks in Xuefeng Mount and distinctly different with the Pre-Cambrian metamorphic rocks and Cambrian black shales with sedimentary siliceous rocks. It can be concluded that copper may come from granitic rocks, and these native copper siliceous debris may be the product of hot water or hydrothermal solution related to granitic rocks. A little of native copper in basin was directly from leaching the mineralizing siliceous debris in situ, and the vast majority may be dissolved at the weathering area and transported into the sedimentary basin by water (surface water or groundwater).
(4) The exotic copper concentrated in sandstones and a part formed chalcocite at the layers containing organic sulfur, the vast majority of copper in sulfur absent environment formed the native copper mineralization. The ore-formation occurred due to the change of physico-chemical condition when the fluids rapid across the sedimentary sands. The main mineralization formed during the deposition period, which environment is low-temperature, weak-reducted and high-salinity conditions. The mineralizing fluid may be with chloride and sulfate ion, and organic matter may play an important role for the ore-formation.
(5) After the formation of native copper mineralization, the ore-bodies were reconstructed by the synsedimentary faults, the later faults and drape. At the weak bands, such as fracture and hinge line of drape, found the copper mineralization re-enrichment. And in some faults accessing ore-bodies can found hydrothermal gypsum vein with large size native copper aggregate. Although tectonic hydrothermal activation formed copper re-enrichment, the formation of the most ore-bodies are still layered and controlled by the sandstones of Cretaceous.
(6) The possible genetic model for this native copper mineralization may be a simple model: copper was dissolved in weathering area, and transported in the form of ion or complex by the water into Yuanma Basin, then concentiated the mineralization during sedimentary process or early diagenesis. |
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