Application of audio magnetotelluric method in prospecting for deep hidden manganese ore: A case study in the Pujue manganese mine in Songtao, Guizhou, China
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摘要: 普觉锰矿是黔东地区典型的南华纪"大塘坡式"沉积型锰矿床。区内一系列后期构造对早期形成的沉积型锰矿的埋藏和保存具有重要影响,深部构造格架及其与含锰岩系之间关系的勘查分析对区内找矿预测和深部锰矿勘查十分关键。为了查明普觉锰矿区成矿地质体和断裂构造的空间分布及其相互关系,采用音频大地电磁法(简称AMT)对区内地下电性结构特征和构造格架进行研究。在物性分析的基础上,构建了与测区地电结构大致相似的构造理论模型,通过开展不同模式的理论模型反演计算,优选出了适宜于研究区的Occam-TE一维反演为初始模型的非线性共轭梯度TM模式二维反演方法。运用上述反演方法对区内实测数据进行反演,识别了区内构造格架和含锰岩系的空间展布特征及二者的接触关系,查明了断层F0在深部未对含锰岩系造成破坏。应用研究结果表明,AMT对指导普觉锰矿区及毗邻地区开展深部找矿预测意义重大。Abstract: The Pujue manganese deposit is a typical Datangpo-type sedimentary deposit of Nanhua period in eastern Guizhou. In the Pujue mining area the early-formed sedimentary manganese deposits were significantly influenced by the subsequent structures in aspects of burial and preservation. Analyzing the deep tectonic framework and its relationship with the manganese-bearing rock series is of great importance to the ore-prospecting prediction and deep exploration of the manganese deposits in the area as well. Our aim is to find out the spatial distribution of metallogenic geological bodies and fault structures and their relationship in the Pujue Manganese mining area. We used audio magnetotelluric method to study the characteristics of underground electrical structure and tectonic framework in the area, and built a theoretical model similar to the geoelectric structure of the study area based on the physical property analysis. Through inversion calculation under different schemas, we gave preference to a two-dimensional inversion method of nonlinear conjugate gradient TM model, which is suitable for Occam-TE one-dimensional inversion as the initial model. We used the above inversion method to invert the measured data from the area, and thus identified the spatial distribution characteristics between the tectonic framework and the manganese-bearing rock series and their contact relationship. It demonstrates that the fault F0 hasn't damaged the manganese-bearing rock series in the deep. Our research results show that AMT is of great significance in guiding the deep prospecting prediction in the Pujue manganese mining area and its adjacent areas.
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Key words:
- Pujue manganese deposit /
- audio magnetotelluric method /
- inversion /
- tectonic framework
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图 1 黔东及毗邻区南华纪早期武陵次级裂谷盆地结构与构造古地理图(周琦等, 2016b)
1—控制Ⅲ级地堑盆地和地垒的同沉积断层; 2—控制Ⅳ级地堑盆地和地垒的同沉积断层; 3—Ⅳ级地堑盆地及所控制的锰矿床名称; 4—Ⅲ级地堑盆地范围; 5—Ⅲ级地垒范围; 6—研究区大地构造所在位置; 7—同沉积断层编号; 8—研究区在石阡-松桃-古丈(Ⅲ级)地堑盆地中的位置
Figure 1. Structural and tectonic paleogeographic sketch of the Wuling secondary rift basin in the Early Nanhua period in eastern Guizhou and its adjacent areas (Zhou et al., 2016b)
1-Synsedimentary faults that controlling 3rd-order graben basin and horst; 2-Synsedimentary faults that controlling 4th-order graben basin and horst; 3-Name of the Mn deposits controlled by 4th-order graben basin; 4-Range of 3rd-order graben basin; 5-Range of 3rd-order horst; 6-Geotectonic location of the study area; 7-Number of synsedimentary faults; 8-Location of the study area in the Shiqian-Songtao-Guzhang graben basin
图 2 黔渝湘毗邻区南华纪两界河-大塘坡期南华裂谷盆地复原(周琦等, 2016b)
1—两界河期-铁丝坳期砂砾岩沉积; 2—大塘坡早期黑色炭质页岩; 3—大塘坡中晚期粉砂质页岩; 4—同沉积断层; 5—菱锰矿体
Figure 2. Reconstruction of the Nanhua rift basin in Liangjiehe-Datangpo stages of the Nanhua period in the Guizhou-Hunan-Chongqing border area, South China(Zhou et al., 2016b)
1-Glutenite deposits in the Liangjiehe-Tiesiao stages; 2-Black carbonaceous shale in the early Datangpo stage; 3-Silty shale in the middle and late datangpo stages; 4-Synsedimentary fault; 5-Rhodochrosite
图 9 不同初始模型及参数的NLCG反演成果图
a—初始模型Occam-TE数据+TE反演; b—初始模型Occam-TE数据+TM反演; c—初始模型Occam-TE数据+TE & TM反演; d—初始模型Bostick-TE数据+TE反演; e—初始模型Bostick-TM数据+TE反演; f—初始模型Bostick-(TE+TM)数据+TE反演;
Figure 9. NLCG inversion results of different initial models and inversion methods
(a) Initial model Occam-TE data+TE inversion; (b)Initial model Occam-TE data+TM inversion; (c)Initial model Occam-TE data+TE & TM inversion; (d) Initial model Bostick-TE data+TE inversion; (e) Initial model Bostick-TM data+TE inversion; (f) Initial model Bostick-(TE+TM)data+TE inversion
图 10 900号剖面地质模型图
1—红子溪组; 2—清水江组; 3—两界河组; 4—铁丝坳组; 5—大塘坡组第二段; 6—南沱组; 7—震旦系; 8—牛蹄塘组; 9—九门冲组; 10—变马冲组; 11—杷榔组; 12—清虚洞组; 13—寒武系第三统至芙蓉统; 14—板岩; 15—变余砂岩; 16—推测含锰岩系; 17—粉砂质页岩; 18—含砾砂岩; 19—砂岩; 20—炭质页岩; 21—灰岩; 22—白云岩; 23—第四系松散堆积体; 24—推测断层
Figure 10. Geological model of the profile 900
1-The Hongzixi formation; 2-The Qingshuijiang formation; 3-The Liangjiehe formation; 4-The Tiesiao formation; 5-The second section of the Datangpo formation; 6-The Nantuo formation; 7-The Sinian system; 8-The Niutitang formation; 9-The Jiumenchong formation; 10-The Bianmachong formation; 11-The Palang formation; 12-The Qingxudong formation; 13-Cambrian third system to Furong system; 14-Slate; 15-Blastopsammite; 16-Supposed manganese-bearing rock series; 17-Sand shale; 18-Pebbled sandstone; 19-Sandstone; 20-Coaly shale; 21-Limestone; 22-Dolomite; 23-Quaternary loose deposits; 24-Supposed fault
图 11 900号剖面NLCG反演成果图
a—寒武系娄山关组和清虚洞组岩性组合层; b—寒武系第二统至南华系中上统岩性组合层; c—青白口系地层; d—南华系与青白口系过渡带岩性组合层; 1—推测地层界线; 2—推测断层及编号; 3—推测含锰地质体
Figure 11. Results of the profile 900 from the NLCG inversion
(a) Lithologic combination layers of the Cambrian Loushanguan formation and the Qingxudong formation; (b) Lithologic combination layers from the second Cambrian to the middle-upper Nanhua system; (c) The Qingbaikou formation; (d) Lithologic assemblage layer in the transitional zone between the Nanhua system and the Qingbaikou system
1-Supposed stratigraphic boundaries; 2-Supposed faults and numbers; 3-Supposed manganese-bearing geological bodies表 1 岩(矿)矿物性特征统计
Table 1. Statistics of physical properties of rocks (minerals)
岩性名称 地层代号 样点数/点 电阻率平均值/(Ω·m) 极化率平均值/% 地表黏土 Q 31 135.71 1.62 白云岩 €3-4ls、€2q、Z1d、Nh2l 52 3203.29 1.86 灰岩 €2q 38 5121.47 1.67 砂岩 €2p、€2b 40 1677.38 1.88 页岩 Nh2d2、€2jm 35 540.98 2.30 炭质页岩 Nh2d1 41 24.41 20.28 含砾砂岩 Nh3n、Nh2t 45 1662.41 2.76 板岩 Pt3h 68 1382.70 2.22 氧化锰矿石 Nh2d1 30 49.86 8.27 块状锰矿石 Nh2d1 35 19.04 11.98 -
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