Integrated geological and geophysical exploration for concealed ores beneath cover in the Chaihulanzi goldfield, northern China |
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| Authors: | Hongtao Liu Jianming Liu Changming Yu Jie Ye Qingdong Zeng |
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| Institution: | Institute of Geology and Geophysics, Chinese Academy of Sciences, PO Box 9825, Beijing 100029, PR China |
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| Abstract: | An integrated geological and geophysical investigation was carried out in the near‐mine areas of the Chaihulanzi gold‐field, Inner Mongolia, northern China, to determine the south‐eastern strike extension of the principal mineralized shear system, in the search for new resources. In this case study, surface geophysical surveys played an important role in defining the mineralized structures beneath cover. A re‐investigation of the mine geology by observing the underground exposures of mineralization and its hosting structures, coupled with re‐evaluation and re‐examination of previous exploration data, revealed that the mineralization style at the Chaihulanzi gold‐field is not the skarn‐type as previously assumed, but is structurally controlled by an oblique‐sinistral shear structure. Moreover, a south‐eastward shallow‐plunging mineralized corridor within the principal mineralized shear system was also identified, implying that the ore‐forming fluid‐flow might move through the mineralizing system, from the south‐east at depth to the north‐west. These new geological findings imply that the mineralized shear system should extend much farther south‐eastwards along strike beneath cover, and thus a conceptual target area was proposed. A detailed geophysical survey program, involving the application of VLF‐EM, Stratagem EH4, CSAMT and gradient IP measurements, was carried out to test the validity of the conceptual target. Orientation surveys on the geologically known Line‐18 traverse revealed that VLF‐EM and Stratagem EH4 were the most effective methods of detecting the unseen mineralized system, whereas CSAMT could only provide low‐resolution data, and IP proved to be unsuitable in this environment due to the ubiquitous presence of graphite‐bearing schists in and around the mineralized system. A follow‐up systematic VLF‐EM survey highlighted the principal mineralized shear system as a linear conductive belt, tracing the principal mineralized system for a further 750 m of strike length beneath cover from the previous exploration limit (Line 18). Stratagem EH4 soundings over six parallel traverses perpendicular to the mineralized trend revealed that the principal mineralized shear structure extends for more than 500 m in the dip direction, a strong indication of the presence of potential deep mineralization under the surveyed area. Detailed modelling of the Stratagem EH4 sounding images provided well‐defined targets for test drilling. Subsequent test drilling on some of these targets returned encouraging results as several core‐intercepts of economic gold mineralization were encountered. This led to a more extensive drilling and underground prospecting program, focusing on deep mineralization south‐eastwards along the mineralized trend. |
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