Search for a gold quartz vein in Eastern Yakutia

15.11.2022

Using the method of deep georadar we studied a gold ore deposit at an early stage of geological exploration to a depth of up to 300 m and localized prospective zones for exploration drilling.

*Field works using deep GPR GROT type with 10-meter-long antennas.*

The Vyun gold deposit is located 250 km southeast of the Batagay.

The deposit is represented by a mineralized zone formed by a core granodiorite-porphyry dike that controls the position of gold-quartz veins that lie parallel along both contacts, sometimes crossing the dike.

The host rocks are represented by Upper Triassic sandstones and siltstones, less often clayey shales of the Nyakunya Formation.

Morphologically, the ore bodies are characterized by a fairly consistent along strike and dip slightly sinuous shape. Their thickness is on average 1 m with swells and narrowings in the range of 0.2–5 m.

The ores of the deposit belong to the low-sulfide gold-quartz type. The main, dominant ore mineral is vein quartz with inclusions of vein-silicified siltstones, mudstones, and beresitized granodiorite porphyries.

Fig. 1. Morphology of the ore zone and ore bodies of the deposit in sections along lines А-Б and В-Г*: 1 – gold-quartz vein; 2 – granodiorite-porphyry dike; 3 – discontinuous disturbances of reverse-thrust kinematics; 4 - deposits of the basement terrace; 5 - underground mine workings; 6 - boreholes of underground core drilling.

Quartz in its electrical properties is a dielectric mineral with a very high resistance (10^12-10^15 Ohm∙m). Therefore, in the deep georadar study, the main attention was paid to the identification of anomalous zones with increased electrical resistivity and with a subvertical direction, similar to the morphology of the ore bodies of the deposit.

According to the results of the conducted georadar study, structural features of the upper part of the studied section were revealed up to signal time delays of 6500 ns. To convert the scale of time delays to the scale of depths, the averaged relative permittivity ε=9 is used. On the radargrams, the profile number is in the center at the top, on the left - the depth scale in meters, horizontally - the distance along the profile from the starting point of measurements in meters, on the right - the scale of signal time delays in nanoseconds.

Figure 2 shows a radargram of a profile passed across the supposed strike of the ore zone after the low-frequency filtering procedure, which allows removing high-frequency noise and tracing the dynamic characteristics of the signal. The red marks show the centers of anomalies in the projection onto the surface.

*Fig. 2. Radargram of the profile passed across a deep anomaly.*

A general examination of the radargram in Figure 2 shows the following main structural features of the site. The near-surface layer is relatively consistent in depth and consists of several interlayers down to depths of 25-30 m. Below is a layer that is heterogeneous in depth and horizontal (shown here in a yellow-green palette) with a relatively high electrical resistivity, the signal attenuation in it slows down. This layer corresponds to the permafrost layer, the bottom of which provides a high contrast on all radargrams. Starting from 60-70 m from the surface, reflections from steeply dipping contrasting boundaries appear, which limit the zone with high resistivity from host rocks with higher signal attenuation.

Figure 3 shows radargrams from profiles parallel to the ore zone. Blue marks additionally marked the projections onto the surface of the centers of the identified anomalies in the upper part of the section. The radargram of profile b4 clearly shows signs of a deep anomalous zone under the permafrost layer (Fig. 3a), while there are no signs of an anomaly at depth on profile b7 (Fig. 3b).

*Fig. 3. Radargrams from profiles parallel to the ore zone with signs of a deep anomaly (a) and without anomaly (b).*

Profile b4 (Fig. 3a) passes above the anomaly in several places; it can be concluded that it is mostly coaxial with the extended anomalous zone. On profile b7, signs of a deep anomaly are not observed, while less contrasting inhomogeneities and reflections from different horizons are traced. On the last meters of the profile (Fig. 3b) a relatively calm horizontally layered pattern of the site structure without contrasting unconformities is observed.

The main results of the study using deep georadar:

• We found out the main structural features of the gold deposit site and indicated promising areas for exploratory drilling.

• The permafrost layer with increased resistivity was revealed in the near-surface part of the section up to 70 m, overlapping steeply dipping anomalies with high resistivity.

• An anomalous area with signs of an ore zone was discovered, displaced to the northwest relative to the previously assumed location of the ore body.

The contour of the ore zone below the bottom of the permafrost, found from the data of deep georadar, was confirmed by the further drilling (Fig. 4). Drilling was carried out at a shallower depth than suggested for the study of the anomalous zone, however, Figure 4 shows that the intersection of the granodiorite-porphyry dike with a thickness of 2 m by the borehole is located on the axis of the deep anomaly identified according to GPR data. The error in determining the distance to the anomaly here is associated with a lower value of the effective permittivity of rocks.

*Fig. 4. Section along a borehole crossing a quartz vein 2 m thick.*

Thus, the application of the method of deep georadar in the deposits of solid minerals makes it possible to increase the success rate of geological exploration and reduce losses in the development of ore deposits.

We thank Interrascan LLC for organizing the expedition!

* Anisimova G.S., Protopopov R.I. [2009] Geological structure and composition of ores of the Vyun gold-quartz deposit, Eastern Yakutia. Ore and Metals Publishing house. 5, 59-69. (In Russian)