| To explore ore-forming mechanism of a porphyry copper deposit in Yunnan, the magnetic anomaly characteristics and geoelectric structural properties have been used to analyze and study in the area.Firstly, magnetic and electric model test analyses are applied to verify the validity and reliability of magnetic treatment method and the MT-soft2 D software developed by Chengdu University of technology. Combined with the known properties of the model and the forming anomalous characteristics, we summarize the rules, which can provide references for the interpretation of the observed datas.In order to study the distribution of magnetic characteristics, upward continuation and reduction to the pole are utilized to process the ground magnetic survey datas, and the interpolated cutting method is used to intercept the residual magnetic anomaly. On this basis, adopting linear enhancement method, we obtain the boundary location of six pieces of cubic shaped or ribbed magnetic anomaly, which are induced by deep intrusive granite rock mass. And according to the magnetic anomaly characteristics of the plane, we determine the deep main fracture structure(Fc1), as well as the secondary structure of the faults(Fc2- Fc6).The two audio magnetotelluric sounding profiles are adopted impedance tensor decomposition and inversion of two-dimensional pseudo section and we get following cognitions: Whether a larger frequency(shallow) or smaller frequency(deep),all the impedance tensor of electrical structures show obvious bidimensionality, and the electrical spindle direction is north east trending. In the AMT two-dimensional inversion pictures, there are 8 normal and low resistance channels(I- VI;I2、II2), controlled by both tectonic fracture zone and deep rock fracture. And there exist 17 electrical gradient belts(T11- T112;T21- T25),which are nearly parallel and perpendicular to the main faults(Fc1).And there exists a layer of low resistivity and high conductivity(S13,S24), caused by the ore-forming fluid of the porphyry copper deposit in the deep.According to the characteristics of magnetic anomalies and geological structure properties of research district, we speculate the metallogenic mechanism as follows: The main fracture(Na Majing fault Fc1), which was throughout research district, nearly erect and North East—North North East trending, cutting down deeply, provided channels(that these eight low-resistivity zones) for the deep low resistivity material. Driven by the fluid pressure, the ore-forming material of the metallogenic fluid layer migrated upward, whose apparent resistivity was 400-700Ω ? m. The main structure was affected by post-secondary faults, leading to form 17 electrical gradient zones, secondary fissures and fracture zones, which provided the storage space for ore minerals by repeatedly overlayed and accumulated. When the magma evolved to late stage,the magmatic crystallization gradually transformed into the role of hot water. With the change of the physical and chemical conditions and the concentration of the components in hot water, the ore-forming elements(copper, molybdenum, etc.) of magma dissolved in hot water. And chalcopyrite, pyrite, molybdenite, bornite and other metal sulfide precipitated in the vicinity of magnetic anomalies with granite cascade to form the body of disseminated porphyry copper and molybdenum mineralization. |
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