Modern airborne geophysical technologies in gold deposit exploration

XI INTERNATIONAL SCIENTIFIC AND PRACTICAL CONFERENCE OF TsNIGRI 2022

“Modern airborne geophysical technologies in gold deposit exploration”

Presentation description

Progress in airborne geophysical technologies in recent years has contributed to a marked increase in information content and exploration effectiveness. One consequence has been a significant increase in the volume of airborne geophysical surveys for gold deposit exploration. Experience in applying modern airborne geophysical technologies in gold-prospective areas under diverse geological and geographical conditions is summarized.

In terms of the application of geophysical methods, primary and placer gold deposits can be classified as low-contrast geological objects, characterized by the absence of direct exploration indicators, sharp physical boundaries and, consequently, pronounced geophysical-field anomalies [1]. Given the geological characteristics of most gold deposits and their associated expression in geophysical fields, the suite of airborne geophysical methods should include as many information channels as possible. This substantially improves the reliability and credibility of target identification and reduces ambiguity in the geological interpretation of survey results.

In recent years, extensive experience in airborne geophysical technologies for surveying gold-prospective areas has accumulated both in Russia and abroad. Traditionally, the suite included aeromagnetic surveying and airborne gamma-ray spectrometry, but in recent years one of the airborne electromagnetic variants has also been included, substantially expanding the range of geological tasks addressed.

Modern Russian frequency-domain electromagnetic equipment surpasses the best Western equivalents in sensitivity and resolution. The system currently in use is based on a frequency-domain airborne electromagnetic system (a variant of dipole inductive profiling), combining simple and efficient measurements with high resolution and a large effective investigation depth. Development of a new instrument configuration with a suspended, towed transmitter loop has now been completed, allowing a wider range of aircraft, including light helicopters, to be used.

Airborne transient electromagnetic surveying (ATEM) is considered the most effective current technology for direct mineral exploration at the detailed prospecting and appraisal stage. The method is effective for target resistivities up to the low tens of Ω·m, in which case investigation depths can reach hundreds of metres. At high resistivities, however, its resolution is significantly poorer and its mapping capabilities are therefore limited. An important advantage is its ability to determine not only the conductivities (resistivities) of formations in the upper section but also their chargeability.

In recent years, ATEM has been in greatest demand among clients; however, this is not always justified, and selection should take the following factors into account.

1. The investigation depth of frequency-domain electromagnetic surveying is approximately 150 m, compared with up to 300–500 m for ATEM under favourable conditions.
2. ATEM has unsatisfactory resolution where section resistivities exceed 100–200 Ω·m.
3. In addition to resistivity, ATEM can determine formation chargeability.
4. An ATEM survey costs 2–3 times as much as a survey using frequency-domain electromagnetic variants.

Thus, transient electromagnetic surveying is effective for exploring massive sulphide ores (for example, pyritic ores), complex deposits (such as porphyry copper deposits with epithermal gold), and targets associated with sulphide mineralization. If the objective is to locate overprinting alteration zones, structural and compositional complexes of a particular generation, and similar features, the less expensive frequency-domain variant is preferable. It should also be preferred when both variants are expected to produce approximately the same effect.

Selection of the survey scale (regular survey line spacing) is an important issue. Our experience shows that, to fully support prospecting and appraisal for ferrous, non-ferrous and precious metals, the airborne geophysical survey scale must be no smaller than 1:10 000 [2].

For interpreting airborne geophysical data aimed at gold exploration, two complementary approaches appear most effective [4]: sequential mapping of factors controlling mineralization and probabilistic prediction based on machine learning methods. Specialized data analysis and modelling methods play the decisive role in the interpretation of geophysical methods in general and airborne geophysical methods in particular. A key condition for effective use of airborne geophysical data is the incorporation of the full range of available prior geological and geophysical information.

Based on studies of the localization conditions of gold deposits, several researchers [3, 5] have identified predictive and exploration criteria (factors) controlling endogenous gold mineralization. Factors common to the principal genetic types of mineralization are structural-tectonic, lithological-stratigraphic, magmatic, hydrothermal-metasomatic, metamorphic and geophysical. All are reflected to some degree in geophysical fields and can be localized using specialized analytical methods. Results from the Southern Yenisei gold district, the Vitim mining district and other areas of Transbaikalia illustrate how these predictive factors are expressed in airborne geophysical data.

The characteristics of airborne geophysical surveys—high productivity, integrated coverage, low cost and high information content—make them most effective as advance surveys, so that their interpretation can guide the exploration process rapidly during the main work stage. Airborne geophysical technologies therefore optimize exploration operations by reducing the time needed to prospect an area and positioning expensive excavation and drilling work more precisely, making their application particularly relevant under present conditions.

References: 1. Babayants P. S., Kontarovich O. R., Trusov A. A. Airborne geophysical technologies in gold deposit exploration: current trends // Gold and Technologies. – 2018. – No. 11. – pp. 106–112. 2. Babayants P. S., Vorgacheva E. Yu. Effectiveness and feasibility of advance detailed integrated airborne geophysical surveys // Ways to Realize the Oil, Gas and Ore Potential of the Khanty-Mansi Autonomous Okrug–Yugra. 12th Scientific and Practical Conference. – Khanty-Mansiysk. – 2009. 3. Benevolsky B. I., Blinova E. V., Brazhnik A. V. et al. Methodological Guidelines for Estimating Prognostic Resources of Diamonds, Precious and Non-ferrous Metals. “Gold” issue. – Moscow: TsNIGRI. – 2002. – 182 pp. 4. Kalmykov B. A., Lyovin F. D., Trusov A. A. Capabilities of modern airborne geophysical methods in predicting and exploring gold deposits // Gold and Technologies. – 2017. – No. 2. – pp. 64–70. 5. Gold Deposits of Russia / Ed. M. M. Kontantinov. – Moscow: Akvarel, 2010. – 374 pp.