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Podiform chromite exploration using audio magnetotelluric at Luobusa Ophiolite in Southern Tibet Lanfang He*, Nanjing University and BGP,China, Xiumian Hu, Nanjing University , Xuefeng Zhao , Rujun Chen, Central South University, Duoji, Tibet Geological Survey
Summary The Luobusa Ophiolite, Southern Tibet, lies in the eastern portion of Indus-Yarlung Zangbo suture zone that separates Eurasia from the Indian continent. Podiform chromites exploration proved difficulty during the past several decades, for most chromite deposits pinch out and reappear in the same survey direction. Several ground-based geophysical methods including gravity, magnetic and CSAMT have been used for searching chromite deposits. However, most of them failed to delineation the favorite area. A successful case history of chromite exploration using magnetic and geoelectical methods in Luobusa, Southern Tibet, is presented in this paper. Detailed geophysical survey including gravity, magnetic, and geo-electrical methods such as audio magnetotelluric (AMT) and IP based on spread spectrum technology for exploring chromite were conducted in a 10 km2 area within the Ophiolite belt. The results from magnetic and geo-electrical methods indicates several favorite areas. Some of them was verified by drilling in the past the two years. One drilling projected based on the geophysical survey result discovered the largest chromite deposit in China up to date. Several other verified drills met chromite deposits with a thickness range from 0.3~32 meters. The geophysical exploration results gave a new recognition on geophysical methods to chromites mining geologists in China. Introduction The Luobusa Ophiolite, Southern Tibet, is located in the eastern portion of Yarlung Zangbo Ophiolites which is controlled by Indus–Yarlung Zangbo suture zone. Given that it hosts the largest chromitite resources in China and has perfectly outcropping ophiolite and mantle peridotite, Luobusa Ophiolite has attracted great interest of the geologists from all over the world (He, et al, 2014). Progresses have been made on understanding the mineral characteristics (Li., et al, 2012), deposit features and the regional tectonics about the podiform chromite deposits in this Ophiolite belt. Luobusha chromite deposits, which was first found by a herdsman some 50 years ago, are bearing in ultra-mafic magma or Ophiolite lies in collision area between Gangdisi block and Himalaya block. Schematic geological map of the Luobusa Ophiolite is shown as Figure 1. Most of the host rocks of the chromite deposits are mantle peridotite including dunite and harzburgite. In few cases, the chromite ore directly contract the laminite of Triassic. Zhou et al provided a possible petrogenetic model for the formation
of the Luobusa mantle sequence. Yang et al.reported the occurrence of diamond and confirm the presence of ultrahigh pressure minerals( He, 2014). Exploration of the concealed deposit remain proving difficulty during the past several decades. Several geophysical methods were used but the exploration result could not convince the geologists because very few ore deposits were discovered by the drillings projected on the geophysical results. Meanwhile, drillings projected on the geological analysis acquired few economic discovery. As a result, risk of the resource shortage disturbs the chromite mining enterprises in Luobusa. In order to explore and evaluate the concealed and potential chromite deposits at Luobusa Ophiolite, a comprehensive exploration project, which was financed by the mining companies, was conducted during 2012 to 2013. Detailed gravity, magnentic, AMT and IP based on spread spectrum technology for exploring chromite were conducted in a 10 km2 area within the Ophiolite belt with survey grids of 40X20m, 80X20m and even 5X5m. Result of geophysical exploration indicated several favorite area and most of them has been verified by the projected drillings. One of the drilling, which was named as ZKWT02, met 4 layers chromite deposits with a total thickness of 49.18m, this drilling led to the the discovery of the largest ore deposits in China. Xi (2013) presented the result of IP and He (2014) discussed the distribution and origin of high magnetic anomalies at Luobusa Ophiolite in Southern Tibet. In this paper, we focus the operation and the application of AMT in Luobusa podiform chromite exploration. Data acquisition and processing We carried out the survey at more than 8000 stations with a grid of 80*20m or 40*20m in a 10 km2 area within the Ophiolite belt. 4 sets of MTU-5A (Phoenix Geophysics) and 20 sets of PMT-2 (Champion Geophysics) multi-function AMT receivers was used for data acquisition. The tensor survey mode with two magnetic channels and two electric field channels in one set was used in AMT method. 4 Pbcl-Pb electrodes and a pair of coils was used to measure the orthogonal components of the electromagnetic field on each station. the electric and magnetic field time-series are first transformed into the frequency-domain by a Fourier transform(He et al., 2006). The frequency band ranged from 11.5 Hz to 11250 Hz with 40 frequencies.
©The Society of Exploration Geophysicists and the Chinese Geophysical Society GEM Chengdu 2015: International Workshop on Gravity, Electrical & Magnetic Methods and Their Applications Chengdu, China. April 19-22, 2015 *The corresponding author:
[email protected] . GEM 2015 314
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Chromite exploration using AMT at Luobusa Ophiolite
The spectrum data was recorded during the field data acquisition. The data were corrected using the calibration data stored in the console of the data acquisition unit. We use EMAP to remove both the topographic effect and static shift effect (He et al., 2006). Two interpretation methods, Bostick conversion and nonlinear conjugate gradient inversion (NLCGI), were used for data processing. In order to select to the optimal site separation for filed operation, a field test of site separation with 10, 20, 40 and 80m was carried out in the same profile. The results are shown as figure 2, as we can see from the figures, few differences between the result of 10 and 20-m separations, both of them are clearly different from those of 40 and 80-m separations. Based on the test result, site separation of 20m was used for AMT data acquisition.
been measured to discover the relationship between the resistivity and composition of Oliver(Ol), clinopyroxene (CPX) and Orthopyroxene (OPX), the result is shown in figure 4. It indicates that a higher resistivity characterize as high percent content of OL+CPX+OPX. In other word, the fresh peridotite has higher resistivity. Note that the major metamorphism form of peridotite is serpentinization. We can reason the low resistivity of rock in results from the serpentinization of peridotite. In addition, the AMT survey results indicate that the upper portion of Luobusha Ophiolite is fresher than the lower portion.
Figure 4: Relationship between the resistivity and percent content of OL+CPX+OPX.
Figure 2: Comparison of AMT results with site separation of 10, 20, 40 and 80 meters.
Result and analysis More than 100 sections have been conducted in the whole survey and the AMT result unveils the interior of the Luobusha Ophiolite. Figure 3 shows two AMT inversion resistivity section on the western portion and middle of the Ophiolite. As we can see from the inversion results, the resistivity in one section ranges from less than 10 to more 2000 ohm-m. The shallow portion of the section features as relatively resistant and the lower portion features as conductive. Most of the other AMT sections has the similar feature. In order understanding the means of resistivity features in Luobusha Ophiolite. Some fifty rock samples have
Figure 5 shows the contour map of inversion resistivity based on AMT results. It shows that there develop two lower resistivity belt with two higher resistivity belt along the middle portion of the Ophiolite. This is the middle portion was first uncovered because most of this portion are cover with glacial deposit and the surface geology fail to acquire any useful information. Figure 6 shows the 3D resistivity imaging result of the middle portion of the Luobusha Ophiolite. It further demonstrates the interior of the Ophiolite through AMT survey. It also shows that upper portion is more resistant or fresher than the lower portion. The geological implication of the geo-electrical feature of the Luobusha Ophiolite is a long story to discuss. Research of He (2014) indicated there bear relation between the chromite deposits and the geo-electrical feature, three geo-electrical models were proposed for searching economic chromite deposits (CrD) : the first one is CrD and it lies in the contact zone of the resistant and conductive area, the second one is the CrD lies close to the top the conductive trap, the third one is CrD lies in the conductive fracture. Two boreholes were drilled to
©The Society of Exploration Geophysicists and the Chinese Geophysical Society GEM Chengdu 2015: International Workshop on Gravity, Electrical & Magnetic Methods and Their Applications Chengdu, China. April 19-22, 2015. GEM 2015 315
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Chromite exploration using AMT at Luobusa Ophiolite verified the first model, and both of them met CrD. One of which, ZKWT02, met the largest CrD in China. 6 borecoles were drilled to verified the second model, 4 of them met CrD. 10 borecoles were drilled to verified the third model, 2 of them met CrD.
References
Conclusions
He, L.F., 2014, Petro-electricity and its origin: Examples from Luobusha ultramafic rock and Upper Yangtze black shale, Ph.D thesis, Nanjing University
Data from AMT survey significantly refine understanding of the interior of the Luobusha Ophiolite. The result of AMT survey provide important information which could help to target favorite area for chromite exploration, the largest chromite deposit in China up to date has been discovered based on this model. Field testing on the site separation for chromite exploration indicate the optimal separation is 20m. Sample measurement result show that the fresher peridotite has higher resistivity, these provide us a new way elevate the fresh degree of the Ophiolite by using geo-electrical survey result. Acknowledgements This work was supported by the National Natural Science Foundation of China (U1262206), Chinese Geological Survey geological prospecting fund (12120113095200) and the National Science and Technology Program (2011ZX05019-007).
Bai, W.J., Fang Q.S., Zhang, Z.M., et al., 1999, The genesis of Luobusa mantle peridotites in the Yarlung Zangbo River ophiolite zone, Tibet: Acta petrologica et minerelogica, 18, 193-216
He, L.F., Hu, X.M., Zha, Y.B., et al., 2014, Distribution and origin of high magnetic anomalies at Luobusa Ophiolite in Southern Tibet: Chin. Sci. Bull, 59(23), 2898–2908 He, L.F., Feng. M.H., and He, Z.X., et al., 2006, Application of EM methods for the investigation of Qiyueshan Tunnel, China: JEEG, 11(2), 151-156. Li, J.Y., Yang, J.S., Ba, D.Z., et al.,2012, Origin of different dunites in the Luobusa ophiolite, Tibet. Acta Petrologica Sinica 28: 1829－1845 (in Chinese) Xi, X.L., Yang, H.C., He, L.F., et al., 2013, Chromite mapping using induced polarization method based on spread spectrum technology, SAGEEP 2013, paper 015, Denver, Colorado USA, http://www.eegs.org
Figure 1: Schematic geological map of the Luobusa Ophiolite (He et al., 2014, modified after Li et al., 2012). The legends are: 1, harzburgite ( bearing dunite and lherzolite) ; 2, dunite; 3, chromitites ore; 4, cumulus bojite; 5, cumulus consist of bojite, wehrlite; 6, Scope of the surface magnetic survey; 7, Luobusa group conglomerate; 8, upper Triassic Formation; 9, quartz diorite, quartz monzonite; 10, biotite granite; 11, lithostratigraphic boundary; 12, unconformable contact; 13, reverse fault; 14, strike-slip fault; 15, sample number and its position
©The Society of Exploration Geophysicists and the Chinese Geophysical Society GEM Chengdu 2015: International Workshop on Gravity, Electrical & Magnetic Methods and Their Applications Chengdu, China. April 19-22, 2015. GEM 2015 316
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Chromite exploration using AMT at Luobusa Ophiolite
Figure 3: Two AMT sections of inversion resistivity VS depth
Ohm-m
Figure 5: AMT inversion resistivity contour map at depth of 200m in middle portion of Luobusa Ophiolite
Ohm-m
Ohm-m
Figure 6: 3D imaging result of the middle portion of Luobusa Ophiolite based on AMT inversion data
©The Society of Exploration Geophysicists and the Chinese Geophysical Society GEM Chengdu 2015: International Workshop on Gravity, Electrical & Magnetic Methods and Their Applications Chengdu, China. April 19-22, 2015. GEM 2015 317
