| The Nanling region, located in the southeast end of the Eurasian plate, adjacent to the east of the Pacific plate and to the west of Indian Ocean plate, geographically refers to the Wuling and the Jiulianshan areas. Nanling metallogenic belt is one of the five most important of the16key metal mineralization belts which our country set off metal exploration. This study area, abundant in non-ferrous and rare metals related to acidic magmatic rocks, is called the capital of tungsten, antimony and tin mineral in the world. Since the last century, many metallogenic theories were emerged in Nanling region, such as the theory of transforming granite crust source, the theory of metallogenic series and construction of geodepression theory etc.. In recent ten years, with the development of continental mineralization research, Nanling become a hot spot again to research related theory. This paper, based on the12-five-years research special project of Land and Resources department "Research on deep magmatic activity in Yanshan period in Nanling region and its metallogenic complexity (N:1212011121101)", using geophysical exploration as the main research means, with the help of method and the technology used in wavelet multi-resolution analysis and boundary detection of potential field to extract the information about granites and structures from different layers of deep source in Nanling region, identify the distribution of buried granite in the areas, invert the spatial features including buried depth and spatial shape of granite intrusions, reveals the structure characteristic of lithosphere, discusses and analysizes problems about the structure of magma system and granite formation and research the polymetallic metallogenic prediction in Nanling region. Paper’s main research results are as follows:(1) We Multistaged1:200000bouguer gravity and1:50aeromagnetic data by Wavelet decompsition and wxtractd the1th～5th order details from the source in the depth of about1km,3km,8km,15km and25km respectively. The research results show that the the1th wavelet order details of gravity anomaly mainly reflects the distribution of small granite rocks in shallow and exposed areas and the small granites often are favorable belt for forming mineralization; Such as granites in Qianli shan whose exposed area is not large, but surrounded by Dongpo, Hongqiling, Yaogangxian, Shizuyuan, Yejinwei, etc.; The2th order details of Gravity anomaly mainly reflects the distribution of concealed granites in shallow layers, and orefields are often distributed in this area with larger negative gravity anomaly. This paper studies granite local anomalies caused by the sum of1th～5th order details in Nanling region, and information about the granites’residual gravity anomaly has assumed obvious zoning characteristics. Bounded by Chaling-Chenzhou fault, granites in the north west of the study area has a lager anomaly,ranged from14to27mgal, and south east area is smaller, ranged8to20mgal, which explain that rock in north west is thick or less density; And information in different periods about granites’residual gravity anomaly also has certain differences, the value of the early granites’negative of gravity anomaly is larger than that of late granites, and the early granite are mainly distributed in the north west.(2) By using the different direction of horizontal derivative and the multi-scale wavelet fault identification methods, we divided the fracture system in Nanling region, pointed out distribution characteristics of the concealed faults and deep faults; combined with the previous results of magnetotelluric and seismic prospecting, we pointed out2lithosphere faults in the study areas, including Chengbu-Taojiang fault, Changde-Anren fault; and5Moho fracture, such as Cha ling-Chenzhou fault, Ganzhou-Shaoguan fault, Dayi shan-chenzhou-Dabaoshan fault, etc. Ancient mantle wedge in subduction zones and Moho fault sites are an important controlling factor of granites instrusion. Magma mainly upwelled along the deep big faults to the earth’s crust, then near the faults distribution, and corresponded to the gravity anomaly gradient belts, for which magma intrusion is most favorable place in the interchange of linear fracture, and a block of granite for fault is the biggest thickness, such as the granites in Zhuguang shan and Jiuyi shan,etc.(3) Using the method including the level total gradient modulus. Tilt gradient, Theta map and normalized standard deviation to conduct edge recognition to gravity anomaly in Nanling, we mapped the forecast result by studying the buried distribution of granite rock mass in Nanling region. The research results show that the concealed granite rocks distributed in the areas of Sancaijie. Xianghualing, Zhongshanshanhu and other places. Only small rock strain was found in some of this kind of rock mass surface, but the larger rock was underground concealed granite rocks where often is the key area of the metallogenic; Half concealed rock mass such as QitianLing, Wawutang, Mengzhuling and Mengzhu shans, Yangming shan and Dayi shan and so on. This kind of rock further expands into deep rock, and part of which is linked together in deep, such as Qianli shan-Qitianling-Xianghualing Haiyang shans-DouPangLing, Yangming shan-Dayi shan and so on.(4) By technique of3D density imaging, we revealed3D density and structure of granites in Qianlishan-Qitianling and Jiuyishan, further understood the findings of magnetotelluric according to the results of MT prospecting and2.5D interactive inversion of gravity anomaly profile. The results show that the Qianlishan-Qitianling-Xianghualing ore concentration area links together in deep, and may extend to the north east of Wanyangshan-Zhuguangshan along the north east of Chenzhou-Chaling fault, which forms a passageway of concealed magmatic emplacement to Qianlishan-Qitianling-Xianghualing, while the Jiuyishan granites is characterized for thickness in west and thinness in east. The thickness of Granite rocks in Nanling is about6-15km, commonly a few close to30km. Thin rock mass is generally larger than thick roc k in the width, such as rock mass in YueChengLing, the width is45km, and thickness is only6km; The granites of Dayishan, the width is26km, and thickness can be up to22km.(5) According to the result of gravity, magnetic, MT and seismic data, and discussing the causes of formation of Nanling granite, we got the following understanding:most of granite rock mass in Nanling region originated in depth less than30km, which are belong to no root granites, and provide direct geophysical evidence reasoning for geological inference of the topic that granites is resulted from continental crustal remelting. This paper believes that in the early movement of Yanshanian, in the process of subduction of the Pacific plate to the Cathaysian plate, lithosphere became thin, and the heat provided by the upwelling of the asthenosphere and the basaltic magma invasion led to partially melt the original rock which formed the early Yanshanian Nanling granite belts. We speculate that there is a residual magma chamber in the top of the upper mantle in Chenzhou area due to the asthenosphere upwelling. This residual magma chamber provides heat source for the partial melting of lower crust protolith, and help form the continental crust remelting area as the center of Zhuguangshan-Wanyangshan, is the birthplace of Mesozoic magmatic explosion, also offers mine source and heat source for mineralization in large areas of this region, where is the best Nanling mineralization zone centered Qianlishan-Qitianling. In addition, the middle and lower crust in Miaoershan-Yuechengling area may be in large continental crust remelting zone.(6) According to deep tectonic features in Nanling, such as Moho rolling, concealed fault and concealed granites distribution, etc., researching their corresponding relation with the known deposits, we got the following understanding:the mantle uplift area has few relationships with endogenous metal mineral, but usually corresponds to the shallow structure in the Mesozoic and Cenozoic basin; Mantle concave area tends to correspond to shallow tectonic uplift, although there is miberal deposits, not the most favourable metallogenic location. Between the variation area (belt) of mantle uplift and Mantle concave area, in alternating belt of crustal thickening and thinning, shallow tectonic activity is strong, and there are many mineral deposits distribution.The in N-W trend structures have the function of constructing ore and mine, and is the main structure factors about the distribution of orefields. the N-E trend structures control both the granites and ore, and it is the center of the magmatic activity where the different depth and different direction faults intersect, and it is also the best metallogenic place, especially the intersection between NNE trend structures and previous.(7) We summarized the geological metallogenic regularity, the characteristics of geophysics and geochemical about the famous mineral deposits in Nanling region, summed up the model of geophysical and geochemical mineral prospecting-two lows and three highs, two lows refer to negative gravity and lower resistivity, three highs refer to high AT anomalies and high ηS resistivity, and high geochemical anomalies such as Pb, Zn, Ag, As. |
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