Geologic and geochemical results from the Ni-Cu deposits of the Central Asian orogenic belt show that carbon from carbonaceous sediments was incorporated in the magma that formed the Ni-Cu deposit. Thus, the presence of an Ni-Cu deposit may be hypothesized at localities where carbonaceous sediments are in contact with mafic intrusions. However, the widely developed carbonaceous layer in the Central Asian orogenic belt has low-resistivity and high-chargeability characteristics similar to those of massive and semimassive orebodies, which makes it difficult to detect Ni-Cu deposits by conventional geophysical methods. To test whether the frequency-domain controlled-source electromagnetic (FDCSEM) method can circumvent this problem, a survey was conducted using horizontal electric fields to map the spatial distribution of rock types around gabbro intrusions containing an Ni-Cu sulfide deposit and barren gabbro intrusions of the Central Asian orogenic belt. The recovered resistivity structure resolved two low-resistivity layers: one was located at an elevation range of 850 to 950 m (the elevation of the surface is about 1,000 m) and widespread along the survey lines, whereas the other was present between 300 and 600 m and was limited to only part of the survey line. These structures are interpreted to represent carbonaceous layers. Gabbro intrusions are present as high-resistivity formations. Based on the drilling results, in the area where the deep carbonaceous layer exists, the Ni-Cu orebodies are associated with gabbro intrusions, whereas where the gabbro intrusions are not associated with a carbonaceous layer, no Ni-Cu orebodies are present. These associations can be detected using FDCSEM with a horizontal electric field and could be used to explore Ni-Cu deposits.

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