Experience in Applying Geophysical Research Methods in the Search for Residual Titanium Deposits

L. I. Ivanchura

(Submitted by the Department of Geophysical Prospecting Methods)

Of the titanium minerals, rutile and ilmenite are of practical importance. The latter is the principal raw material for the titanium industry. Its deposits are often formed through the weathering of gabbroid rocks, which almost always contain an elevated amount of titanium minerals. In this case, during formation of the weathering crust, it becomes enriched in titanium owing to the removal of alkalis, calcium, magnesium, and iron, and partly owing to an increase in the titanium content in the ilmenite itself (leucoxenization). Such areas of ancient weathering crust, under favorable conditions, are successfully mined for ilmenite.

In the area studied, the gabbro massif is represented mainly by pyroxene, amphibole, and olivine gabbro.

The massif is almost everywhere covered by a weathering crust. Its thickness varies widely and averages 10–15 m, with a depth of occurrence beneath sandy-clayey formations ranging from a few meters to one hundred meters and more.

Macroscopically it is represented by greasy kaolin clay, predominantly bright white, bluish-white, and green in color.

The weathering crust is everywhere enriched in ilmenite. At the same time, the highest ilmenite content is observed where the parent rock has an increased dissemination of dispersed magnetite and titanomagnetite.

The numerous boreholes drilled in 1954–1955 on a 2000 × 800 m grid did not provide a clear picture of the titanium mineralization of the massif. To guide further prospecting and prospecting-exploration drilling, geophysical work was carried out in 1955–1956: magnetic surveying on a 100 × 40 m grid and vertical electrical sounding (VES) every 200 m. In this work, the VES stations were placed selectively, mainly in zones of intense magnetic anomalies and along their strike, extending into the peripheral parts of the anomalous magnetic field.

In selecting the methods, it was assumed that the zones of greatest ilmenite accumulation in the weathering crust of the gabbro would be accompanied by an increased magnetic field and reduced electrical resistivity in comparison with the barren peripheral weathered horizons; that is, electrical sounding was expected to provide a qualitative assess-

…to magnetic anomalies. As a result of the geophysical work performed, these assumptions were, for the most part, confirmed.

As an example, Fig. 1 shows one of the magnetic anomalies with a field intensity of up to 35 mE. In plan view the anomaly is characterized by a latitudinal extent. Its area along the 20 mE isoline is \(600 \times 200\) m.

Fig. 1. Plan of \(\Delta Z\) and graphs of \(\Delta Z\) and \(\rho_k\) above the ore horizon.
1. Sandy-clayey deposits. 2. Gabbro rocks. ○ VES points.

Sounding here indicates the presence of a weathering crust of gabbro rocks of considerable thickness. The depth to unweathered gabbro rocks in the magnetic epicenter, according to VES, is 20 m. With electrode spacings of \(AB = 300\) m, the apparent resistivity at the site of the magnetic anomaly decreases from 140 to 115 ohm·m. At the same time, this is not observed in the higher horizons, i.e., in the overburden.

Thus, the geophysical indications—the decrease in resistivity at depth, the increase in the magnetic field, and the deepening into the underlying rocks noted by VES—provided favorable indicators for the presence here of a zone enriched in ilmenite. Subsequently, a borehole drilled here showed the presence, in the gabbro weathering crust, of ilmenite with a content of up to 23 kg/m³. The total thickness of the productive sequence is 13 m. The depth according to the borehole was determined as 18.5 m.

Another analogous example is shown in Fig. 2, characterizing one anomalous field, where an average of up to 32 kg/m³ of ilmenite was obtained in the weathering crust, with a maximum concentration of 68 kg/m³.

The indicated regularities are also observed along a number of other profiles—graphs run through the epicenters of magnetic anomalies.

Subsequent verification by drilling in all cases showed the presence, in the gabbro weathering crust, of one concentration or another of ilmenite.

In contrast, negative indications were obtained for a number of magnetic anomalies.

For example, along the profile shown in Fig. 3, according to the VES results, at the places where the magnetic field increases there is observed not a decrease but an increase in apparent resistivity with depth at all spacings. Inter-

Interpretation of VES indicates the absence of depressions in the relief of the basement.

When this anomaly was checked by a borehole, no appreciable ilmenite content was found in the weathering crust of the gabbro, given its insignificant thickness.

Fig. 2. Graphs of $\Delta Z$ and $\rho_k$ above an ore-bearing horizon.
1. Sandy-clayey deposits. 2. Gabbro rocks.

Fig. 3. Graphs of $\Delta Z$ and $\rho_k$ above a barren horizon.
1. Sandy-clayey deposits. 2. Gabbro rocks.

The observed regularities in the character of the magnetic and electric fields over areas of ilmenite concentrations made it possible to identify the most promising places within the entire area investigated.

The work carried out established that a complex of geophysical methods consisting of magnetic surveying and vertical electrical sounding, in combination with drilling, is the most rational method for prospecting residual titanium deposits under geological and geophysical conditions analogous to those of the areas studied.

If, before the geophysical work was carried out, it was difficult on the basis of drilling results to draw any concrete conclusions about the area under study—even though more than 150 boreholes had been drilled here—then after the geophysical work and a small amount of targeted drilling only in promising areas, it became possible to obtain more definite results and to draw more correct conclusions regarding the possibility of ilmenite concentration in the weathering crust of the gabbro intrusion under study.

In addition, the use of geophysical work makes it possible to reduce significantly the volume of drilling work, the overall time required, and the funds expended on prospecting and exploration of these deposits.