Main results of integrated airborne geophysical surveys in the Vitim ore district

In 2007–2010, GNPP Aerogeofizika conducted an integrated airborne geophysical survey in the Vitim mining district. The work was performed under State Contract No. 13-80 dated 02.05.2007. The suite of methods included aeromagnetometry, airborne gamma-ray spectrometry and DIP-A airborne electromagnetic surveying.

The geological terms of reference specified a 1:50 000 survey over 34 000 km2 and detailed 1:10 000 surveys of selected prospective areas totalling 6 800 km². The objective was to establish a geophysical basis for 1:200 000 geological mapping and assessment of the area’s gold and uranium potential, with completion scheduled for the fourth quarter of 2010.

The survey used an An-2 aircraft (2007) and a Mi-8 MTV helicopter (2008 and detailed survey areas), following parallel east–west lines. Regular survey line spacing was 500 m (100 m for detailed surveys). The mean flight height was 150 m.

State-of-the-art instrumentation and technical equipment were used for the survey. The Exploranium (USA) gamma-ray spectrometer digitally records the full gamma-ray spectrum in 256 channels at 1 Hz, with an energy-channel interval of 10 keV. Spectrometer resolution at the 0.662 MeV (137Cs) line was no worse than 10%, and total detector volume was 32 L. Advanced instrumentation and data-processing technology achieved survey root-mean-square errors, calculated from repeat lines, of 0.2% for potassium, 0.7×10-4% for thorium, 0.35×10-4% for uranium and 1.5 μR/h for dose rate.

DIP airborne electromagnetic surveying uses an arrangement with the transmitting dipole onboard the aircraft and receiving dipoles in a deployed gondola on a 70 m cable. This arrangement achieves a transmitting magnetic dipole moment of at least 40 000 A·m², allowing digital recording of three components of the secondary electromagnetic field at a sampling frequency of 6.67 Hz at four frequencies: 130, 520, 2080 and 8320 Hz. The relative measurement error achieved, based on repeat lines, was no worse than 12% at each frequency.

The magnetic field was measured using a modern quantum magnetometer with a caesium sensor, providing a recording rate of up to 100 Hz and an instrumental error of 0.001 nT. The survey root-mean-square error, calculated at intersections of a specially flown diagonal line with regular survey lines where the horizontal gradient was below 50 nT/km, was no worse than 2 nT.

Horizontal and vertical positioning of observation points and guidance of the aircraft along survey lines use a Javad (USA) GPS satellite navigation system, providing real-time aircraft positioning accuracy no worse than ±3 m in all three coordinates at a recording frequency of 10 Hz.

Data processing included physical and mathematical modelling and geophysical interpretation of the original physical fields from the 1:50 000 survey and subsequent detailed surveys of four areas. In addition, a new, original airborne gamma-ray spectrometric data-processing technique was used, sharply improving the reliability of identifying local uranium anomalies in the presence of strong natural interference many times greater than the required survey accuracy level.

The geological interpretation of the acquired data incorporated available prior geological and geophysical information and was performed jointly with staff of the State Federal Unitary Enterprise Buryatgeotsentr and the Buryat Branch of Sosnovgeologiya. The following main results were obtained.

1. Integrated analysis of airborne geophysical data, incorporating satellite-image interpretation, enabled mapping of the tectonic framework of the study area and determination of the kinematic nature and hierarchy of the principal faults.
2. Physical and mathematical modelling of the magnetic field, incorporating State Gravity Survey and airborne gamma-ray spectrometric data, enabled mapping of the principal structural and compositional complexes in the upper section. The granitoid massifs widespread in the survey area were differentiated by composition, including identification of highly radioactive granitoid intrusions that are potential source rocks for uranium deposits, including those of hydrogenic origin.
3. Integrated airborne geophysical data refined the distribution boundaries of Cenozoic basalt covers that preserve uranium accumulations, primarily those of hydrogenic origin.
4. DIP airborne electromagnetic data identified and delineated post-Palaeozoic (Mesozoic and Cenozoic) erosional-tectonic depressions, including those buried beneath basalt covers.
5. Within the depressions beneath the basalt cover, electromagnetic data mapped preserved palaeovalley fragments, possibly controlled by faults. Their infill deposits were differentiated compositionally into predominantly sandy and clayey types. Drilling data show that almost all known hydrogenic uranium deposits and occurrences are associated specifically with the sandy deposits, which have the highest conductivity.
6. Airborne gamma-ray spectrometric data identified local uranium anomalies within the basalt cover that spatially coincide with anomalies of increased lineament density derived from satellite-image processing. In other words, the basalt cover is permeable and uranium can seep to the surface with water; such anomalies are often observed around known uranium deposits.
7. Specialized processing of airborne gamma-ray spectrometric data identified inferred local zones of potassium metasomatism, including zones associated with faults and fractured zones, spatially coinciding with almost all known gold deposits and occurrences.

The work resulted in structural-tectonic and predictive geophysical maps (for uranium and gold) of the survey area at 1:200 000, as well as predictive geophysical maps at 1:50 000 for the detailed survey areas.