Introduction
Since the discovery of the first kimberlite pipe in the Arkhangelsk diamondiferous province (ADP) (North of the East European Craton, Russia) many traditional geophysical methods have been tested with the purpose to search new kimberlite bodies (Bezborodov et al., 2003; Kontarovich & Babayants 2011). Aeromagnetic methods have shown the most effective (Mwenifumbo & Kjarsgaard 1999; Reed & Witherly 2007). However, at the present time crisis of the efficiency aeromagnetic survey to searching kimberlite bodies influences on the searches results increasingly. Thus, in the early eighties of the last century the efficiency verification magnetic anomalies was about 20 %; at the end of the eighties, 12.5 %; in the nineties it was already below 1 % (Stognij & Korotkov 2010). Today, this figure is less than 0.5% for the whole territory of the Arkhangelsk diamondiferous province. It is connected with a decrease of contrast magnetic anomalies which are tending to geological noise. Therefore, development of new approaches for exploration and prediction of kimberlite pipes are an important task.
One such approach is a complex use of surface geophysical methods such as a highprecision gamma-spectrometric survey, radon emanation survey in soil air and microseismic sounding. According to the microseismic sounding (MSM) method, kimberlite pipes appear as vertical zones with values of relative microseismic intensity lower than 5 dB. Kimberlite pipes are accompanied by faults with higher values of the relative intensity of MS (Popov et al. 2014). Gamma-spectrometric studies are used widely to search for uranium ore with high efficiency (IAEA, 2003; Babayants et al., 2015; Killeen et al., 2015). Also, radiometric studies have shown that total gamma-ray radiation and concentration of radioactive elements are increased in near surface layer. The boundaries of kimberlite pipes are accompanied by anomalous radon emanations.
Furthermore, as an example at Arkhangelskaya kimberlite pipe it was found that secular radioactive equilibrium of the even uranium isotopes is disturbed in enclosing rocks of the near-contact zone. Similarly, the radioactive elements concentrate inside that zone (Yakovlev et al., 2016).
The objective of the present work is to study Chidvinskaya pipe of the ChidvinskoIzhmozerskoe kimberlite field of ADP. This pipe was selected because of its relatively large size and low overburden thickness that is important for radiometric measurements. The pipe’s dimensions exceeded the resolving power of the microseismic sounding method (MSM) and allowed to expect a more reliable determination of the deep structure.
General characteristics of the research object
Chidvinskaya pipe is part of the ChidvinskoIzhmozerskoe kimberlite field, located 30 kilometers north-east from Arkhangelsk, and belonging to a membership of six pipes, constituting a chain with about 20 kilometers length to the North-North-East (Figure 1).
Chidvinskaya pipe is 1810x580 m. The pipe has low-grade ores without commercial value. However, a small amount of tests casts doubt on the accuracy of these estimates.
The complex overburden rock presented quaternary loose sediments, made mainly of sands, loams, peat and sand-pebble formations. Overburden thickness change from 1.5 to 35.3 m northern and southern part of the pipe. The average thickness of quaternary sediments is about 9.4 m.
The vertical section of Chidvinskaya pipe is two-root diatreme with a common trumpet in the surface portion. The upper part of the pipe is formed by rocks of crateral facies which are divided into two bench, the top and bottom bench with total capacity of about 123.5 m.
Top bench is composed of tuffaceous sedimentary rocks represented by coarse sandstones with an admixture of magmatic material, tuff-sandstones, tuffites and tuffs. Bottom bench is distributed in the northern part of the pipe and is made by tuffites with rare interlayers of tuffs and tuff-sandstones.
Diatreme facies is composed by autolit breccias, tuff breccias that form two ore pillars. The boundary between them is held in the middle bottleneck zone. Contacts between the diatreme rocks and the surrounding sedimentary deposits are quite clear.
Experimental
Surface geophysical measurements including high-precision gamma-spectrometric survey, radon emanation survey in soil air and microseismic sounding were used.
Gamma-spectrometric survey
During the opening of the first diamondbearing pipes of ADP gamma-spectrometric survey was used in combination with traditional geophysical methods, but it didn’t receive further development, although it has been shown that kimberlite bodies were accompanied by radiation field anomalies in well landscape and geological setting (Babayants et al., 2015). Kimberlite pipes of Yakut diamondiferous province were characterized by increased concentrations of thorium according to the airborne gamma-spectrometric measurements (Tsyganov et al., 2004). Diamond deposits of tuffizit type were characterized by highradioactivity in areas with low thickness of covering deposits (Rybalchenko et al., 2011).
Surface gamma-spectrometric studies in the area of Chidvinskaya pipe were performed in pedestrian options using high-precision mobile scintillation gamma spectrometric complex RS-700 (Canada). Complex RS-700 is an advanced high resolution (1024 channels) digital spectrometer (ADS) allowing real-time to perform measurements of total radioactivity (Bq m-3), as well as a separate measurement of the concentration of total uranium (µg/g), thorium (µg/g) and potassium in percentages (%). The complex is equipped with an integrated GPS-receiver, enabling precise measurement of each binding. Complex RAD Assist software is used, that may directly conduct processing of the data in the field.
The traverse survey was carried out at a relatively uniform network with increments of 50 - 100 m. The measurements were made at 1.2 m above the ground. In total more than 16800 measurements, were performed using the gamma-spectrometric complex RS-700.
Radon emanation survey
222Rn is a member of the 238U radioactive series (Abumurad & Al-Tamimi 2001). Radon in the earth’s surface depends on the characteristics of geological structure of the territory: development of plutonic rocks, permeable zones (faults), groundwater dynamics, presence of hydrocarbon accumulations, etc. (Bossew 2003). Heightened concentrations of radon are associated with tectonic faults, where it enters on the system of joints and tiny fractures and with fracturing persilicic intrusive mass (King, 1980). Zone crushing rocks surrounding these faults are feeders for radon (Kikaja et al., 2016). Fractured rocks can increase the in situ local radon concentration, hence increasing the radon signal.
Measurements of radon emanation in soil air above Chidvinskaya pipe were conducted on the profile that crosses the north pole of the pipe in north-west - south-east direction using an automatic radium emanation radiometer RRA-01M-03 (Figure 2).
For the measurement of radon in soil air, drilling of pits to 0.5 m depth and 0.2 m diameter using soil portable drilling rig were conducted. The gas collector was placed to pit, then gas was pumped out from soil into the ionization chamber of the radiometer. After that radon measurement was conducted. The values of radon emanation were obtained for the 16 observation points.
Microseismic sounding method
The microseismic sounding method (MSM) was implemented on the assumption that the vertical component microseisms are determined mainly by the contribution of the fundamental mode of Rayleigh waves (Bath 1979). According to the results, two regularities were noted (Gorbatikov & Tsukhanov 2011; Gorbatikov et al., 2013):
1) power spectrum amplitudes of Rayleigh waves increases when passing through lowvelocity heterogeneities and decreases when passing through high-velocity heterogeneities;
2) the greatest change in intensity is observed for wavelengths twice the size of the cover thickness of the heterogeneity. This can be explained by the fact that a shear stress zone of Rayleigh wave is located at a depth equal to half of the length of a wave. As this takes place, the shift zone is located closer to surface.
The first statement on a qualitative level was predicted theoretically by Savarenskii and Kirnos, (1955). The close conclusions were reached at independent research of Rayleigh waves amplifications (Lin, et al., 2012; Eddy and Ekström, 2014)
In the MSM it is necessary to define an indicative spectrum of microseisms for each point of measuring. For this purpose microseisms are accumulated during the period of stationarity equal to 1.5 hours duration (Gorbatikov & Stepanova, 2008).
Processing of the data starts with the construction of power spectra. Next, each frequency is studied individually. The relative intensity (Ii) of microseisms between the reference and mobile stations is calculated as
where Aip and Aio are the spectral amplitudes for recordings at the ith point of the mobile and reference stations at the considered frequency.
The next step is the determination of the depth to be sounded which is equal to a half of the wavelength. The wavelength is determined based on the most probable velocity model of the investigated object.
The result of this processing is a distribution diagram of relative intensity of microseisms along the profile by depth. Zones with a higher relative microseism intensity represent an area with relatively reduced velocity properties, and vice versa.
Due to the fact that the microseismic sounding method is aimed at the vertical and near-vertical boundaries, it can be used for the allocation of various geological objects, including faults and kimberlite pipe.
The microseismic sounding method was successfully tested on a number of tubes of the kimberlite pipes (Gorbatikov et al., 2009; Popov et al., 2014)
Microseisms were measured along the profile that crosses the northern block of the pipe Chervinskaya. The central part of the profile of (MSM) coincided with the route of radon emanation survey (Figure 2). Microseisms were measured by the seismic equipment of production Guralp (Great Britain).
Records of microseisms were processed in a complex DAK program (Popov et al. 2014).
Results and discussion
Gamma-spectrometric studies
Figure 2 shows the map of total gammaray distribution of near surface layer above Chidvinskaya pipe. The total radioactivity of rocks varies widely from 172 to 1585 Bq m-3. The distribution of high gamma-field has a slightly wide configuration than the pipe contour. The most contrast and the biggest area of increased gamma-ray values is confined to the northern part of the pipe and localized within the north pole, where thickness overburden of quaternary sediments is minimal and is about 1.5 m. Value of total radioactivity reached 1500 Bq m-3.
Spatial distribution of high values of total gamma-ray radiation of near surface layer above Chidvinskaya pipe forms a linear anomaly of northeast trending which goes beyond contour of the pipe. Average values of radioactivity are about 800-900 Bq m-3 outside this area.
Concentration of thorium in Chidvinskaya pipe area varies within 0.5 - 14.0 µg/g. The maximum values of thorium are localized at small areas and are arranged substantially within the contour of the pipe. The potassium content ranges from 0.3 to less than 3.8%. The spatial distribution picture of potassium content in general is identical to the index of total gamma-ray radiation and forms an area of high values within pipe contour, especially in the northern part of the pipe where thickness overburden of quaternary sediments is minimal. Uranium concentration ranges from less than 0.4 to 4.6 µg/g. Distribution picture of uranium isn’t unambiguous. Explicit spatial confinement to the contour of the pipe wasn’t observed.
In general, it can be noted that Chidvinskaya pipe have increased values of gamma radiation, as well as potassium. Distribution of thorium and uranium concentration doesn’t have areal disposition that apparently is a consequence of erosional feature of top bench craterial facies Chidvinskaya pipe.
Radon emanation results
Traverse of radon method included 16 observation points. Points of observation are shown in Fig. 2. The results are shown in Table 1.
Radon radioactivity in the soil air ranges from 114 to 1312 Bq/m3 on the profile that crosses the north pole of Chidvinskaya pipe. Variation of values radon emanation are insignificant within the contour of the pipe from 185 to 312 Bq/m3. Anomalous increase of radon concentrations was observed at the boundaries of Chidvinskaya pipe.
Maxima of radon radioactivity in soil air were obsewrved at the north-western and south-eastern borders of the profile, 1156 Bq/m3 and 1312 Bq/m3, respectively. 222Rn radioactivity falls sharply and decreases to a minimum beyond the boundaries of the pipe.
Revealed regularities of radon changes on the profile can be elucidated by the fact that the contacts of kimberlite pipe with the enclosing rocks are fractured faults zones with high permeability.
Microseismic sounding results
The results of MSM are shown in Figure 3 were the high-velocity tube body is allocated at observation points 6 - 15. The border dividing the western and eastern blocks of a pipe has an inclination of about 45 degrees at depths up to 400 m. Deeper than 400 m all borders remain almost vertical. Eastern block is traced up to the depths more than 1000 m despite the small sizes in deep part of a section. The pipe differ from environment in 2-7 dB by relative microseism intensity. The western and east blocks of pipe differ from each over about 2-5 dB by relative microseism intensity. According to (Gorbatikov & Tsukhanov 2011) these relative microseism intensity changes correspond to velocity properties changes in 1.5-2 times and 1.2-1.5 times.
The allocated pipe is limited by contrast linear, vertical low-velocity zones from the West and the East. These zones are likely caused by faults in endocontact rock. These faults are traced at the depths of 200-1000 m under points 5 from the West and 15-16 from the East. Also, the vertical low-velocity zones can be traced for 300-400 m off pipe to the west and east.
In near-surface area the most contrast lowvelocity body is observed under points 2-5 at depths from 40 to 200 m and has a coneshaped structure. According to (Gorbatikov & Tsukhanov 2011). this contrast approximately correspond to velocity decreasing of 2-3 times. Also vertical low-speed zones are traced on removal from a pipe body on 300-400 m to the West and the East.
The changes in the configuration consistent with the changes in the intensity of the allocated zones allow to place the layer in the crust at about 200 m depth, and corresponds to boundary of Padun and Mezen suites of the top of Vend.
When comparing the results we observe an increase of the total gamma gamma-ray radiation consistent with MSM, that corresponds to a high-velocity anomaly. Intensity of gamma radiation on the high-speed anomaly is greater than 2-4 times for background values of enclosing rocks. At the same time, there is a decrease of total gamma radiation from western to eastern blocks of the pipe body. Such a significant change of the gamma radiation level and high-speed characteristics may indicate different material composition of the Western and Eastern pipe blocs. This difference can indicate either different implementation phases, or that data block was subjected to various (physical, metamorphic, erosive) processes after the formation of the tube.
Anomalous radon concentrations at the boundaries of Chidvinskaya pipe are appropriate contrasting linear low-velocity zones. As a dedicated fault structures, there is a possible influx of radon and other gases. High-velocity nature of the pipe body indicates a consolidated structure and a minimum influx of 222Rn. High velocity nature of the pipe body can be explained by the sharply different composition of the pipe and enclosing medium.
The proposed set of methods can be used in further studies, since the methods showed a good convergence of results.