Example 2. Exploration for Cu-Ni deposits at the Ungra site in southern Yakutia

The Ungra site is located in the interfluve of the Left and Right Ungra rivers in southern Yakutia (Siberia). Integrate 1:25000 geological and geophysical surveys, including TDEM soundings, were carried out over an area of 50 km2 for regional- and local-scale Cu-Ni exploration.

The site lies in the northern flank of the Zverev HP granulite block allochthonous with respect to Cretaceous clastics that fill the Chulman basin. The block belongs to the Kalar-Jugdjur granulite belt in the northern Stanovik block of the Aldan-Stanovoy shield and encloses mafic-ultramafic complexes, among which the Burpaly complex in the central Ungra field. According to lithology and chemistry data, it bears Ni-Co-Cr-Cu secondary mineralization in aureoles.

TDEM surveys with a 250 m × 250 m coincident loops revealed zones of high longitudinal conductance (700 S). Fig. 1 shows more than 10-fold difference in apparent resistivity, with the highest conductance at stations 2 and 1.

apparent resistivity ρ(t) curves
Fig. 1. Apparent resistivity ρ(t) curves of profile 3, Ungra site. Numerals at the curves are station numbers.

Resistivity of host rock estimated with reference to a priori geological data is from a few hundreds to a few thousands Ohm*m, while that of Cu-Ni deposits is as low as ~0.2 Ohm*m. In the inversion, either both thickness and resistivity or only thickness of sulfide-bearing layers were allowed to vary. In the latter case, resistivity was locked at 0.2 Ohm*m. Resistivity was the lowest at point 2 of line 3 (Fig. 2a) where the 0.2 Ohm*m layers had a total thickness of 140 m. This resistivity corresponds to massive sulfide deposits. Resistivities at points 3-5 were notably higher (to 3.8 Ohm*m) indicating Cu and Ni depletion. See Fig. 2b for geological interpretation of the EM-derived model.

Geological interpretation of TDEM data
Fig. 2. Geological interpretation of TDEM data at the Ungra site: resistivity section (a) and geological section (b).
In b: 1 - massive sulfide deposits; 2 - disseminated sulfide mineralization; 3 - host rocks.

Thus, inversion of TDEM data with subsequent geological interpretation is an efficient tool for detection of sulfide deposits and mineral potential appraisal. The Ungra site includes zones of high potential for Ni-Co-Cr-Cu mineralization, where the thickness of deposits may reach 140 m.

See the inversion process in a video.