Journal of Geoscience and Environment Protection, 2014, 2, 35-45 Published Online June 2014 in SciRes. http://www.scirp.org/journal/gep http://dx.doi.org/10.4236/gep.2014.23005 How to cite this paper: Lap, T. T. et al. (2014). Application of Audio-Magnetotelluric Method for Exploration the Concealed Ore-Bodies in Yuele Lead-Zinc Ore Feild, Daguan County, NE Yunnan Province, China. Journal of Geoscience and Environ- ment Protection, 2, 35-45. http://dx.doi.org/10.4236/gep.2014.23005 Application of Audio-Magnetotelluric Method for Exploration the Concealed Ore-Bodies in Yuele Lead-Zinc Ore Feild, Daguan County, NE Yunnan Province, China Tran Trong Lap1, Chuandong Xue1, Aiying Wei1, Lv Liu1, Wenyao Li1, Qiquan Hu2, Jingjie Li2, Dafeng Luo2, Shaoy ong Zhu2, Tiangui Zhang2 1Department of Ear th Sciences, Kunming University of Science an d Technology, Kunming, China 2Daguan Chihong Mining Co. Ltd., Yunnan Chihong Zinc and German ium Co. Ltd., Daguan, China Email: tronglaptran @imgg.vast.vn, cdxue001@aliyun.com Received February 2014 Abstract The results of recent mineral exploration in the Yuele lea d -zin c mining area of Daguan County, northeastern Yunnan province, showed that t he re ar e much e arl y Paleozoic strata under thick l ate Paleozoic strata in northeastern Yunnan provin ce, where developed some hidden salt structures (SSs), often with lead-zi nc p olyme tallic mineralization varying degrees along the tension tor s i ona l fault (belts) or fracture (joint). The ore-bodies belong to the epigenetic hydrothermal filling vein-typ e d epo sit , and the prospecting potential is gre at . In this a r e a, th e su per fic ial min erali za- tion information displayed clear, but the deep mineralization is unknown, so the exploration work is restri cte d. The audio -megnetotelluric (AMT) surv eyin g is an advantageous method to charac- terize the size , re si st iv i ty and s ki n depth of the polarizable mineral deposit concealed beneath thi ck overburden. This paper presents the s urve ying results using AMT method to evaluate the concealed lea d -zinc mineralization in Yuele lead-zinc ore fie ld, Daguancounty, NE Yunnan prov- ince, C hin a. Afte r comparing the interpretation result of AMT surveying data with the geologi cal data and the drilling data, it is found that there is some distinct differen ce in resis tiv i ty and po- larizable between ore-bodies hosted strata, upper strata and gypsum strata. The results sh ow that AMT method is helpful to identify lea d-zinc mineralization under this geo logi cal condition. Keywords Audio-Magnetotelluric Method (AMT), Physical Anom aly , Concealed Ore-Bodies Predicting, Salt Tectonics (SSs), Yuele Lead-Zinc Ore Field, NE Yunnan Province 1. Introduction Due to the complexity of geological conditions and mineralization , it is very difficult to obtain good resu lts for the mineral prospecting research ing only through the geological metallogenic law s. Currently, mineral pros-
T. T. Lap et al. pecting methods based solely on geological theory haven’t been met the requirements of modern mineral explo- ration (Wang et al., 2009; Xue et al., 2010, 2012). So that, it is necessary to arrange reasonably the prospecting projects, along with re search and application th e geophysical methods to predict the location of th e concealed ore-bodies. Along the metallogenic belt in northeastern Yunnan province that is important composition of the lead-zinc polymetallic mineralization area in Sichuan-Yunnan-Guizhou, the re has found more than 130 lead-zinc deposits and mining areas with different large and medium scale “Fig u r e 1(A)”. Bu t this region exist the minor axis anti- cline outcrop area with early Paleozoic strata and underlying very thick late Paleozoic strata. And in the ten- sion-torsional faulted and shattered zone or intersection of different direction faults, it developed many lead-zinc polymetallic deposits or minera lizating points with different poor and rich degr ee. In man y areas , the minerali- zation bodies are relatively obvious on ground shallows, but it is very difficult and not clear for the prospecting potential in the depth, the prospecting p rog res s is slow. It has great prospect for copper-lead-zinc mining pros- pecting in Yuele lead-zinc ore field. Recently, the projects for prospecting evaluation and mineral geological observation in this area found that the mineralization pattern s and geological characteristics of the stratiform and steep dipping vein ore-bodies (Xue et al., 2010, 2012) . Because the developing of the folds and faults in this area, geochemical observation has found some mineralization anomalies, but the vein shape ore-bodies have great variations, and the occurring of the salt tectonics in the depth. So the deep concealed ore-bodie s pr ediction and exploration have been limited largely. This article took the le ad-zinc deposit in the Yuele mining ar ea as the object to s tudy the metallogenic pros- pecting method. To help characterizing the size, electrical resistivity and skin depth of the polarizable mineral deposits concealed beneath thick overburd en, so audio-megneto tellur ic (AMT) method surveying (Abedi & No- rouzi, 2012; An & Di, 2007; Chen et al., 2009; Van Tuyen, 2011; Manzella & Zaja, 2006; Sampson & Rodr i- guez, 2010) was selected. To obtain an obvious resu lt and with more reliability, we hav e measured electrical re- sistivity and polarizable of the geological samples and the drilling samples for the AMT surveying data analyz- ing. Further studies will attempt to determine if induced polarization parameters extracted from the AMT sur- veying data, also can be used to determine the size and electrical resistiv ity of the mineralized area. 2. Geological Setting The Yuele lead-zin c mining area belongs to the northern part of the lead-z inc polymetallicmetallogenic belt in the northeastern Yunnan province “Figure 1(A)”. Its special geological structural position and complex struc- tural evolution history, has dec isively inf luenced on the lead-zinc mineralization. The regional outcropped is made up of a su ite of Paleozoic volcanic rocks, terrigenousclastic rocks, and carbonate rock. The continuous outcrops include the Ordovician, Silurian and Devonian strata in this region, the total thickness is more than 3000 meters. And the copp er, lead and zinc or e-bodies almost exist in the carbon ate roc ks in d i f ferent str ata “Table 1, Fig- ure 1(B)”. The mainly magma rocks are the amygdaloidal and vesiculate basalt of Emei Mount Formation of Upper Permian (P2β), erupted in Late Hercynian and widely distributed in the regional area of Yangtze platform in southweste rn China. The influence of magma in this f or mation on the lead-zin c miner alization surro unding the Yang tze platform, has many different disputes. In NE Yunnan region there are some main faults, including near SN trending Xiaojiang fault, NE trending Qiaojia-Lianfeng fault, NW trend ing Kangding-Daguan-Shuicheng fault, and SN trending Zhaotong-Q ujing burial fault, and many developed secondary faults. T here la y some dome structures near SN trending Mohan- Qinglin belong to the northeastern section of Sayu River fold syncline eastern wing of Zhenxiong-Zhaotong fault, the main p art position is secondary extensive wide and gently monoclinal structure of anticline eastern wing. The F1 an d F2 thrust faults across anticline core is located in near SN trending, alternate NE NEE trending secondary folds, NW-NNW and NE-NNE trending faults and interlayer fracture zon e, and formed the structure pattern with grid shape “Figure 1(B)”. The uncovering projects determined three types of copper, lead and zinc deposits in this mining area, the up- per part are vein-type Pb-Zn (-A g) ore-bodies and stratiform-type Cu-Pb-Ag ore-bodies, the under part is strati- form-type Pb-Zn (-Ag) ore-bodies. The main ore -bearing strata are limestone, carbonaceous debris argillaceous limestone, dolomitic limestone, calcite dolomite, and quartz sandstone in Ordovician and Silurian formation. Three types of ore-bodies exist together with similar geological features and obvious forming origin relationship. The
T. T. Lap et al. Figure 1. The regional and ore field geological map of Yuele lead-zinc deposit. A show the distribution of lead-zinc deposits in Sichuan-Yunnan-Guizhou border area (modified from Liu and Lin, 1999). B shows the structure outline map in Yuele lead-zinc deposit. The low Panel shows the AB geological profile. known ore-bodies (ore mineralization bodies) are produced from in flanking plume-like fractures and steep dip- ping tension-tors iona l faults and fracture zon e s that is n e ar SN trending and eastern p late of the F2 fault. O rd er of faults and ore-bodies (mineralization bodies) can be arran ged as follows, the F2, F3, F4 and F5 fault produced the V16 (Cu-Pb), V10 (P b-Zn ) , V8 (P b-Zn ) a nd V9 (P b-Zn ) or e-body (mineralization body). The forms of ore minerals are simple. The main ore minerals have galena, sphalerite, pyrite and little chal- cocite, chalcopyrite. The main gangue minerals have calcite, dolomite, quartz, barite, gypsum and little fluorite. The main ores have layer, stratiform, vein, stockwork, and massive str uctur e. Concr etization is automorphic, hypautomorphic, xenomorphic, metasomatic corrosion and metasomatic relict. In thi s mining area, the copper, lead and zin c ore -bodies are vein-type deposit, filled on epigenetic by low temperature hydrothermal, controlled by salt tectonics related to thrust fold. Salt-related structu re s are spatial-temporal relationship well with copper, lead and zin c polymetallic mineralization. 3. Method and Geophysical Data Analysis 3.1. Field Work Two geophys ical sur v eys had been done in the Yuele mining area, and two different geophysical instruments of EH4 instrument and GMS-07e instrument are used. Both of the instruments are audio-magnetotellur ic sound ing “Figure 1(B)”. The EH4 instrument has been surveyed in April 2011, sum of survey line are 13 lines, sum of survey station are 226 stations, sum of survey line’s length are 7580 m. The distance between two adjacent survey stations is 30 m. And the GMS -07e instrument has been surveyed in December 2012 , sum of survey line are 5 lines , sum of survey station are 224 stations, sum of survey line’s length are 7200 m. The distance between two adjacent sur-
T. T. Lap et al. Table 1. The stratum characteristics profiles in Yuele lead-zinc ore field. vey stations is 30 m. 3.2. AMT Method 3.2.1. Basic Equations Basis of the magnetotelluric theory are Maxwell’s equations, reduced for quasi stationary fields in the frequency domain, as the bas is of magnetotelluric theory: (1) (2) (3) (4) with (positive exponent, harmonic wave). Electric fields are produc ed by electric charges and time-var ying magn etic fields; magne tic field s are pro- duced by curr ents and time-varying electric fields. Further basic assumptions are that the material is isotropic and normal polarisable, and that no extrinsic electric curr ents are flowing (no unknown sources). The response function of the rocks is not frequency dependent nor influenced by pre s sure or temperature. This leads to the Helmholtz equation in the frequency domain (5) then the transformation in the wave number domain follows
T. T. Lap et al. () ( ) ikR F z,k,ωF z,R,ωe dR − = ∫ (6) with and ; the Laplace operator becomes a factor (7) (8) we obtain the complex vertical wave number and the complex skin depth (9) and with ≈ 0 (quasi homogeneous source field) the real sk i n depth. (10) Simply spoken, the source of the magnetotelluric field s is in the magnetosphere and ionosphere, separated by the non conductive atmosphere. The field is varying slowly, so that wave terms can be neglected and the diffu- sion equations are valid. The inducing fields are plane waves and do not vary in a range of 500 to 1000 km: dif- ferences in trans fer func tions are caused by fields from the subsurface and not by different inducing fields from above. The conductivity of the rocks is not frequency dependent: a change of conductivity at a certain frequency is caused by different conductivities in different rocks. The limitation is listed as follows: • In polar and equator ial r egions the field fro m the polar and equatorial electrojet is not plane . • At high frequencies (above 20 kHz ) over resistiv e material (e.g. gr anite) wav e terms become more and more important. • Bay variations might not be correlated up to 500 or 1000 km. • At very high indu ction depths where the skin depths reach the dimension of k. • The plane and far field assumption can be invalid during AMT recordings if thunderstorms are nearby. • Controlled source methods have to be evaluated by other means. 3.2.2. Impedance Func ti on The impedance function describes the relation between the electric and magnetic fie ld is: xxx xyx yyx yyy E ZZB E ZZB = (11) where, in the 1D case (homogeneous half space or plain, layered earth) . The apparent resistiv ity is defined as: 22 22 00 00 2 x a xyxyxy y Ep ZC B µµ ρωµ ωµ ωω ===≈ (12) it can be necessary to multiply Z with 100 0 b efo re taking the absolute value in case the electrical field was in mVkm ⁄. with its error: xy 0 a xya xyxyxy xy Z2μ ρ2ρZZ ω Z ∆ ∆== ∆ (13) and the phase: ( )() ( ) xy xy xy xy Im Z φarg ZarctanRe Z == (14) with its error:
T. T. Lap et al. (15) Hence that, the phase should be in the range of 0 - 90˚ and in the range of −90˚ - 180˚ degrees be- cause Z is complex and one has to respect the quadrant in th e complex plane: arctan 0 0 Re arctan 0 0 Re arctan 0 0 Re Im ifReand Im Im ifReand Im Im ifReandIm ϕπ π >≥ =+< ≥ −< < The erro r of C is 3.2.3. More Dimensi ons In the 1D case we had and . In the 2 and more dimensional case all elements of the impedance function are containing values and . 3.2.4. Swift’s Angle and Rotation The function can be rotated with the matrix coscos sinsin sinsin coscos D αα αα = − (hence that in the MT coord inate system x is North, y is Ea st an d z is positive d own w ards ). The function can be rotated using Swift’s formula to minimize and ( )( ) () 22 2Re 1arctan 4 xyyx xx yy xx yyxy yx ZZZZ ZZ ZZ α +− = − −− (16) with the error ( )( ) ( ) ( )( ) ( ) ( ) 22 22 2 1 42Re xx yyxyyxxy yxxxyy xx yyxy yxxx yyxy yx ZZZ ZZZZ Z ZZ ZZZZZZ α −∆ +∆+−∆+∆ ∆= −−−− + (17) The Swift angle has a periodicy of 4 ⁄because of the arctan function. If you test f or the second derivative ( )( ) () 22 2Resinsin 4coscos40 xy yx xx yyxx yyxy yx ZZZZZZ ZZ αα +−+ −−+> You should get the correct angle, where by convention . The Swift’s angle can be different for every freq uency if no 2D structure is dominant. The angle is undefined and unstable if th e anisotropy is low . When you evaluate a complete MT profile it can be a good idea to set an anisotropy limit; if the anisotropy after rotation is below that value you skip the rotation and let the angle be 0. In the cross section show in g your Swift’s angle it is then simple to find a dominating angle. 3.2.5. Anisotropy and Skewness The anisotropy is defined as (18)
T. T. Lap et al. with the error 22 22 xy yx xy yx ZZ AAZZ ∆∆ ∆= + (19) as mentioned above, the anisotropy is dependent on the rotation angle of the impedance tensor. The skewness can be a parameter of the dimensionality of the impedance tensor; the skewness is defined as (20) with the error 2 22 2 44 xxxx yyyyxyxyyxyx xx yyxy yx Z ZZZZ ZZZ SZZ ZZ −∆ −∆−∆ −∆ ∆= + −− (21) The skewness should be <0.3 to interpret the structure as 2D. However, even on 3D structures you find posi- tions with a skewness which is almost zero . 3.2.6. Induction Arrows, Tipper Induction arrows describe the transfer function between the horizontal components of the magnetic field and the vertical component (22) It is useful to draw an arro w w ith the length ()( ) Re Re real lengthHD Z ZZ= + (23) and direction (from North over East). ( ) ( ) Re arctan Re D realang H Z ZZ = (24) The arrowhead of the real induction arrow is pointing away f ro m a conductive structure The errors of length and angles are ( ) ()( ) ( ) ( ) ()( ) () 22 real length22 22 2Re 2Re Re ReRe Re HH DD HD HD ZZ ZZ ZZZ ZZ ∆∆ ∆= + ++ (25) ( ) ()( ) ( ) ( ) ()( ) ( ) 22 22 22 Re Re Re ReRe Re HH DD real ang HD HD ZZ ZZ ZZZ ZZ ∆∆ ∆= + ++ (26) Schmucker’s transformation gives a first resistivity-depth interpretation of the acquired data ( ) 22 0 *0 22 2 2cos cos245 145 2sin sin1 2 a xyrxyxy r xy a xyxy xy xy orImCif or if Im iC ρϕωµ µϕ µµ ρρϕ ϕωω ° ° ≥ =< (27)
T. T. Lap et al. Figure 2. The electrical resistivity 2D inversion profile of line C25 and its geological interpretation, the survey line position is line C25 with dotted violet line in “Figure 1(B)”. (28) with the errors ( ) ( ) * * * 2 45 Re 2 45 xy xy xy xy xy xy xy xy xy Zif Z Zif Im Z ρϕ ρ ρϕ ° ° ∆ ≥ ∆= ∆ < (29)
T. T. Lap et al. Figure 3. The electrical resistivity 2D inversion and its geological interpretation, the survey line position is the line with dotted green line in “Figure 1(B)”. a—line 1550; b—line 1400; c—line 1800.
T. T. Lap et al. ( ) * Re Re xy xy xy CZ zC ∆ ∆= (30) 3.3. Analytical Met hods The software used to analysis ATM surv eying data is th e software Mapros. 1) Using the fas t F our ier transform algorithm (FFT) to obtain the corresponding spectru m for the collected data, that is the correspond ing relatio n between the frequ ency and amplitude in electr ic f ield or magn etic field . 2) Using the impedance equation to obtain the corresponding relation between the Cagni ard resistivity and frequency. 3) Using the software MT Pioneer to display the two-dimensional electrical resistivity inversion profile from the collected data, fitting the collected data and establishment model. 4) Colligate the one-dimensional inversio n and two-dimensional inversion to obtain the inversion finally. The collected data after passing these processes, get the resistivity inv ersion se ction of each survey line. Combined with the geological data, drilling and other data to inference and interpr etation. 4. Inference and Interpretation Results The analysis of 2D electrical resistivity inversion profile of the AMT method, combination of known geological data and borehole data, bring into comparisons with strata and structure of electrical resistivity inversion profile has been shown. Analysis of all the electrical resistiv ity inversion profiles to anomaly division of geophysical exploration and interpretation of geophysical exploration inversion profiles. Because of mine for low tempera- ture hydrothermal vein type deposit, the ore mainly belong to epigenetic mineralization product filling, ore-body is significantly affected by the fracture zone and the ore-beari n g strata combined. According to the borehole d at a have known obtainable orebody output area mostly associated with carbona- ceous mudstone, limestone and that is corresponding to low electrical resistivity site of sounding inversion pro- file. Therefore, when anomaly body divided according to the reflec tion of sou nding electrical resistiv ity pr ofile, combined with geological data and borehole data to inference and afte r that comprehensive analysis was carried out to determine “Figure 2, Figure 3”. 5. Conclusions 1) The interpretation results of the AMT method have identified the location of geological structure, deforma- tion forms, and their spatial occurrence and depth variation. 2) Comparing the interpretation results of the AMT method with the known borehole data, it is shown tha t the location of medium electrical resistivity anomalies is the location of lead and zinc mineralization, the value of medium electrical resistivity anomalies is about 1000 - 3000 (Ω·m). 3) Based on the interpretation results of the AMT method, combined with the geological conditions and the known borehole data, the anomalies area can be designed as the drill verification to carry out the mineral pre- dicting. Acknowledgements This study is financially supported by the Doctoral Fund Projects of Education Ministry of China (20125314 110006 ), the Natural Science Fund Project of Yunnan Province (2009CI030) and the Science and Technology Project of Yunnan Chihon g Zinc and Germanium Co. Ltd. We thank Mr. Xing Li, Wei Yao, JieNiu, Yanqing Hu and other geological engineers for their help during the field work. We sincerely thank Dr. Yurong Gong and other re- viewer for t heir co mme n ts and suggestions. References Abedi, M., & Norouzi, G. H. (2012). Integration of Various Geophys ical Data with Geological and Geochemical Data to Determine Additional Drilling for Copper Exploration. Journal of Applied Geophysics, 83, 35- 45. http://dx.doi.org/10.1016/j.jappgeo.2012.05.003
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