Searching for Economic Minerals

CHAPTER 9

Searching for Economic Minerals Chapter Outline 9.1 Hydrocarbon Deposits 9.1.1 9.1.2 9.1.3 9.1.4 9.1.5

9.2 Ore Deposits 9.2.1 9.2.2 9.2.3 9.2.4 9.2.5

233

Gravity 235 Magnetics 236 Thermics 239 Resistivity and Self-Potential Integrated Analysis 241

241

244

Gravity 244 Magnetics 254 Thermics 256 Self-Potential 258 Integrated Investigations

9.3 Other Kinds of Deposits

263

269

9.3.1 Yakutian Diamond Province (Siberia, Russia) 269 9.3.2 Makhtesh Ramon Complex Ore Deposit (Northern Negev, Israel) 270

9.4 Underground Geophysics 9.4.1 9.4.2 9.4.3 9.4.4 9.4.5

270

Gravity 271 Magnetics 272 Temperature Survey 272 Self-Potential Survey 275 Examples of Integrated Underground Observations

References

276

278

9.1 Hydrocarbon Deposits In regional investigations, one and the same set of methods can in principle be employed to study objects of various classes that control different types of mineral resources. Of special interest is a shared approach to predicting the presence of ore and oil and gas on the basis of geophysical data, by relating discoveries of nonconventional oil-and-gas deposits in mountainous regions to igneous and metamorphic findings (Eppelbaum and Khesin, 2012). Table 9.1 illustrates these fundamental potentialities. Some qualitative and quantitative criteria for integrated interpretation are considered in the previous Chapters (mainly in Chapter 4). It is simply worth noting that routine qualitative Geophysical Potential Fields. https://doi.org/10.1016/B978-0-12-811685-2.00009-6 Copyright © 2019 Elsevier Inc. All rights reserved.

233

234 Chapter 9 Table 9.1: Revealing common controls of ore and oil-and-gas deposits by geophysical methods.

Geological Controls (Fluid Conductors and Hosts of Mineral Deposits)

Predominant Location

Typical Geophysical Methods and their Results

Mineralization

Hydrocarbons

Methods

Deep faults

Pinnate joining

Faults zones to neighborhood

Gravity and magnetic prospecting

Overlap-overthrust structures

Volcanogenic and sedimentary rock masses

Sedimentary rock masses

Seismic exploration Gravity prospecting

Intersection of longitudinal and transcurrent fractures Igneous rocks concealed highs, their contacts with sedimentary rocks

Local structures

Regional structures

Gravity and magnetic prospecting

Exomorphic and endomorphic zones of intrusives Scattered bodies

Eroded volcanic structures, sedimentary rock pinching-out zones Continuous filling

Magnetic and gravity prospecting Seismic exploration Magnetic and gravity prospecting Electric prospecting

Hydrothermal alteration and pyritization zones

Pyritization zones

Below pyritization zone

Brachy-anticlines flexures in terrigenous ecarbonate deposits

Northeastern flanks of structures

Eastern and Northeastern flanks of structures

Magnetic prospecting Electric prospecting Electric prospecting, Seismic exploration

Porous and fractured metamorphites (secondary quartzite, serpentinites, shales)

Field Patterns and Geometry of Sources Gradient zones, local anomaly chains, linearly elongated anomalies Gently sloping reflecting boundaries Extrema on cover edges Interference pattern

Anomalies of circular and ring shapes Zones of zero reflection Field decreasing (excluding magnetic maxima over serpentinites) Conductivity anomalies Linear field decrease Increased polarizations Contact flexures of media with various physical properties

criteria are also integrated, since geological problems that can be solved using one geophysical method are very rare, whereas a combination of only two methods considerably expands the possibilities of geophysical data interpretation. A number of papers dealing, for the most part, with the principles of joint interpretation of gravity and magnetic fields have made this point clear-cut. For instance, it is expedient to consider

Searching for Economic Minerals 235 Table 9.2: Magnetic and gravity fields over faults. Types of Fractures

Geological Characteristics

Faultsdchannels for magma flow

Fractured zones filled with basic rocks Intrusions or volcanic centers localized along fractured zone

Contemporaneous (growth) faults Faults fixing vertical displacement of blocks

Abrupt changes in lithological composition, facies, and deposit thickness on both sides of a fault Abrupt changes of boundary positions separating a section into individual structural facial complexes Crush zone due to differentiated movement of conjugated blocks along the fault

Faults fixing horizontal displacement of blocks

Horizontal rock displacement determined by comparing age, and structural and facies features of rocks on both sides of a fault

Reflections in Fields Linearly elongated positive anomalies Chain of near-isometric maxima or sometimes minima Change of sign or behavior of the same sign field at the fault Zone of high field gradients (an additional criterion is an abrupt change in occurrence depth for upper or lower anomalous mass edges) Chain of linearly elongated magnetic and gravity minima coincident in plan Rupture and echelon displacement of zones with linearly elongated anomalies, abruptly inflected isolines

qualitative criteria to single out and trace faults, which are among the main targets of geophysical investigation. It is possible not only to isolate and trace the zones of fracture by some typical indicators in the magnetic and gravity fields but also to assess their types. The main indicators of various faults reflected in magnetic and gravity fields are summarized in Table 9.2. Thus, the results generated from a model of the medium should be as follows: (1) specification of classes of objects under investigation; the definition of the geological, petrophysical, and geometrical characteristics of typical objects in these classes; and corresponding characteristics of host media; (2) identification of the features from the observed field (and from other geophysical fields) and a set of indicators associated with the targets; determination of the methods and estimates of their anomalous values; and (3) interpretation criteria for singling out the targets according to the given field (and other geophysical fields) and/or to a set of indicators.

9.1.1 Gravity Tzimelzon (1965) carried out effective gravity data analysis in the Middle Kur Depression, Azerbaijan. His gravity data analysis delineated the Muradkhanly anticline structure (Fig. 9.1) where later an oil deposit was discovered. More recently, Gadirov (2009)

236 Chapter 9

1 10 km

0

1 4

5

3 2

1 0

1 2

a

-1

1

b

1

2

Figure 9.1 Gravity method as a successful tool for delineation of the buried Muradkhanly structure (Middle Kur Depresion, Azerbaijan). (1) difference in gravity field Dg0-10, mGal: (a) positive and (b) negative (subscripts “0” and “10” designate gravity field observed at the earth’s surface and analytically continued to the level of 10 km, respectively); (2) contour of Muradkhanly uplift according to seismic prospecting data and drilling. After Tzimelzon, I.O., 1965. Earth’s crust deep structure and tectonics of Azerbaijan by geophysical data. Soviet Geology, (4), 103e111 (in Russian).

presented a more detailed physicalegeological model (PGM) of the Muradkhanly deposit on the basis of gravity (DgB) and magnetic (DZ) modeling.

9.1.2 Magnetics Magnetic surveys were used to classify a Mesozoic section into magmatic (magnetic) and carbonaceous (nonmagnetic) complexes (for example, Fig. 9.2) according to their composition. As a result, oil and gas traps of a previously unknown type were revealed in zones of carbonaceous rock pinch-outs near the stocks of igneous rocks and in eroded roofs of these stocks (Khesin et al., 1996). Results of numerous investigations indicate that physicalechemical reactions of hydrocarbon deposits with the host media often create precursors for detecting directly

Searching for Economic Minerals 237

Figure 9.2 Geologicalegeophysical section of a profile across the ShirinkumeMuradkhanlyeJarly areas (Middle Kur Depression) (Khesin and Eppelbaum, 1997). (1) DZa curves: (a) and (b) observed, (c) calculated; (2) deep boreholes; faults revealed by the data from: (3) drilling and seismic prospecting, (4) gravimetric and magnetic prospecting; (5) zones of complicated seismic recording; (6) location of magmatic rocks roof determined by modeling; (7) conventional seismic horizons: (a) Mz roof, (b) Mz volcanogenic rocks roof, (c) Mz carbonaceous rocks roof; (8) carbonaceouseterrigenous rocks; (9) magnetized magmatic rocks (in the figure the magnetization J is given in mA/m; (10) oil-bearing layers.

magnetic signals from the oil & gas deposits even in the cases of large depths (e.g., Donovan et al., 1984; Saunders et al., 1999; Perez-Perez et al., 2011; Gadirov and Eppelbaum, 2012). For instance, extraction of low-signal magnetic anomalies generated by hydrocarbon deposits is demonstrated on examples of several deposits from the Middle Kur Depression (Azerbaijan) and Dnieper-Donets depression (Ukraine) (Gadirov et al., 2018).

238 Chapter 9 The results of magnetic prospecting indicate that the anomalous magnetic field DT in the Tarsdallar region (Kur Depression, Azerbaijan) varies within 300e400 nT. Magnetic field behavior clearly divides this area into southern, northern, and central parts. The Tarsdallar magnetic field anomaly acquires a sublatitudinal direction, and the field intensity decreases up to 50 nT to the north. Along the profiles crossing the Tarsdallar area the field DT is manifested by ruggedness, nevertheless, local magnetic maxima can be distinguished from the regional field. Local maxima with intensities of up to 60 nT have been singled out at all parallel profiles and a scheme of local maxima reflecting the zones of development of volcanic rocks (Fig. 9.3) has been constructed. Note that almost in all wells drilled in this area, tuffaceous rocks of

Figure 9.3 Scheme of distribution of local magnetic maxima (values are given in nT) in the Tarsdallar area (Middle Kur Depression). (1) positive local magnetic anomalies, (2) arch of the structure, (3) exploration wells, (4) observation profiles (Gadirov et al., 2018).

Searching for Economic Minerals 239 Eocene and Upper Cretaceous with high magnetic susceptibility (2500e3700) , 10
5 SI were discovered, and in well No. 9 basalts with a magnetic susceptibility of 6000 , 10
5 SI were revealed. The zone of the most intense maxima covers the Tarsdallar area, passes between the western and northern Tarsdallar, and stretches to the west (Fig. 9.3). Against the backdrop of local maxima, local magnetic minima with an intensity of 20e30 nT are detected. These anomalies are associated with the presence of hydrocarbon accumulation.

9.1.3 Thermics An interesting thermal anomaly in the northern Dead Sea basin was observed by BenAvraham and Ballard (1984). The amplitude of the anomaly exceeded 0.2 C (the accuracy of these observations was about 0.01 C). Its form suggested that the anomalous body (or its upper surface) had enhanced thermal conductivity and could be approximated by a horizontal circular cylinder (HCC). Application of quantitative methods determined the position of the cylinder’s center (Fig. 9.4); it was predicted to be at a depth of about 60 m. Here both the characteristic point method (parameters d1 and d2) and the tangent method (parameters d3 and d4) (Khesin and Eppelbaum, 1994) were used (see Section 5.3).

Figure 9.4 Quantitative interpretation of temperature anomaly in the northern Dead Sea basin (observed temperature curve after Ben-Avraham and Ballard, 1984; interpretation after Eppelbaum et al., 1996). (1) sea bottom and (2) position of center of a horizontal circular cylinder.

240 Chapter 9 Parameter d3 is the difference in the abscissa of the points of intersection of an inclined tangent with horizontal tangents on one branch; parameter d4 is the same on the other branch; d3 is selected from the plot branch with conjugated extrema (see also section 5.3). It was suggested that the anomaly was caused by the upper surface of a small salt dome (there are numerous salt domes in this area). The temperature moment was found to be approximately 9 C m. It is well-known that salt domes are directly associated with hydrocarbon prospecting. The next example illustrates application of the near-surface thermal prospecting which was applied in the Muradkhanly oil deposit in Central Azerbaijan (Fig. 9.5), where the temperature was measured in 3 m deep wells (data thus obtained were smoothed by the use of a sliding interval average of three points). A fault was discovered by a field crew from the “YuzhVNIIGeofizika” (Baku) using gravity and magnetic methods, which was confirmed by drilling data. Thermal observations were also conducted by the “YuzhVNIIGeofizika” (Sudzhadinov and Kosmodemyansky, 1986). The position of the upper edge of the fault was calculated using the tangents and characteristic point method

Figure 9.5 Interpretation of the temperature anomaly in the district of the Muradkhanly oil field in the Middle Kur Depression (Khesin and Eppelbaum, 1994).

Searching for Economic Minerals 241 (Eppelbaum et al., 2014); its projection in plane coincided exactly with the fault position indicated by the independent geophysical and geological data. Thermal density dependence of sedimentary rocks discovered by Gadirov and Eppelbaum (2015) may demand reinterpreting results of some detailed gravity surveys.

9.1.4 Resistivity and Self-Potential An interesting example of cooperative presentation of resistivity (blue) and self-potential (SP) (red) graphs observed in boreholes Mishovdag 58 and Babazanan 43 (Kur Depression of Azerbaijan) are presented in Fig. 9.6.

9.1.5 Integrated Analysis At times, integrated geophysical field analysis is extremely useful even at the qualitative level. To calculate the parameter Iintegr (see Eq. 4.21 in Chapter 4) in the area of Bulla-Sea (Bay of Baku) three different fields were employed: local magnetic anomalies DT (marine survey data), the second horizontal derivative of the gravity potential Wxz (data from the bottom gravity survey were utilized), and DH/H (relative changes of the sea bottom topography) (Fig. 9.7). As can be seen from the graph of Iintegr, both faults are reflected. The large amplitude of the SW anomaly of the Iintegr parameter compared to NE may be explained by the proximity of the SW fault to the surface of the sea bottom. An interesting example of integrated seismic-gravity-magnetic analysis was carried out in the Zhdanov Shoal (Fig. 9.8) located in the southeastern part of the Caspian Sea. Besides conventional seismic and magnetic surveys, detailed bottom gravimetric investigations were performed (Malovitsky et al., 1977). Quantitative analysis of the bottom gravimetric data revealed a significant local gravity maximum against the regional gravity minimum. It was assumed that this maximum was caused by a gravitational effect of the ChelekenLivanov high, and other small local anomalies associated with the fault zones (Fig. 9.7). Later in this area the Djigalybek oil deposit was discovered. The Kusar-Divitchi Basin is located in eastern Azerbaijan. Integrated analyses of gravity, magnetic, vertical electric sounding, and seismic data are shown in Fig. 9.9 (the applied methodology is presented in subsection 4.4.1). Note that parameter SI clearly reflects the position of the predicted basement structure Pz. A quantitative estimation of changes in acquired magnetization, density, and temperature data was applied to develop a detailed PGM of the Muradkhanly deposit (Middle Kur Depression, Azerbaijan) (Fig. 9.10) (Gadirov and Eppelbaum, 2012). Geometry, stratigraphy, lithology, and hydrocarbon deposit location (17) in the model were taken from the drilled deep well cores (15). Data on density, porosity, and magnetization were

242 Chapter 9

Figure 9.6 Representative well log traces through two oil deposits in the Kur Basin, Azerbaijan (fragment of figure presented in Vincent et al., 2010).

Searching for Economic Minerals 243

Figure 9.7 Qualitative delineation of faults by computing parameter Iintegr at the area of Bulla (Baku Archipelago, Caspian Sea). (1) deep wells and (2) faults delineated by combined analysis of geological and geophysical data (Eppelbaum, 2014b).

utilized from the works. The density contrast within the subvertical zone over hydrocarbon deposit was calculated using the new approach. As was shown in Fig. 9.10, the physical parameters of the zone over hydrocarbon deposit (16) differed considerably from the surrounding medium (Gadirov and Eppelbaum, 2012). At increased depths there were increased contrast magnetization (up to 200  10
5 SI) and temperature (up to 17 C) parameters, and a decreased density contrast (up to 3.6 kg/m3) in the sedimentary deposits in the hydrocarbon deposit zone (Gadirov and Eppelbaum, 2012). Analysis of selected cores also showed a sharp decrease in magnetization in effusive associations in the structure arch in the vicinity of the hydrocarbon deposits.

244 Chapter 9

Figure 9.8 Integrated geophysicalegeological section across the Zhdanov Shoal, Caspian Sea (Malovitsky et al., 1977). (1) sea bottom (50m only marks the position of the sea bottom), (2) reflective seismic horizons, (3) fault zones. (I) DgB, (II) graph of regional gravity background (parabolic interpolation with average radius of 3 km), (III) graph of regional gravity background (parabolic interpolation with average radius of 4.5 km), (IV) residual anomaly IdII, (V) residual anomaly IdIII, (VI) computed gravity effect from fault zones, (VII) computed gravity effect from ChelekenLivanov high, (VIII) graph DT.

Computed gravity and magnetic anomalies (curves 6 and 11, Fig. 9.10) from subvertical zone (16) indicated z
0.35 mGal and z
35 nT, respectively. Comparison of the theoretical curves of the gravity and magnetic fields (graphs 1 and 9) with the observed curves (graphs 2 and 10) indicated that they had a similar form. Both in observed and theoretical graphs relative decreases in the gravity and magnetic fields over the hydrocarbon deposits were found. Retrieved maxima on gradient zone changes (4) clearly pinpointed local gravity and magnetic minima.

9.2 Ore Deposits 9.2.1 Gravity The model example below (Fig. 9.11) illustrates the rapid interpretation of a gravity anomaly revealed in rugged terrain relief by the improved quantitative methods (see section 5.2). In the Northern Caucasus several skarn deposits were discovered and exploited (e.g., Tyrnyauz, Upper Chegem, etc.). Unfortunately, during the Soviet era most of the geophysical data on these deposits were prohibited from publication. A generalized PGM of the skarn deposit is presented in Fig. 9.11, where the DgB plot was computed by the improved version of the GSFC program (Eppelbaum et al., 1992). It is obvious that the application of a conventional approach to gravity field analysis (e.g., Mironov, 1980; Parasnis, 1986; Telford et al., 2004) would be unfeasible not only due to disturbances from

Searching for Economic Minerals 245

Figure 9.9 Integrated interpretation of geophysical data using information units for the delineation of hidden fault zones under thick sedimentary cover in the Precaspian-Kuba oil-and-gas area (KusarDivitchi Basin, Azerbaijan). (A) graphs of amount of specific information derived from the results of various geophysical methods and total information SI ¼ IDg þ Iz þ IS þ If (Dg is the gravity field, Z is the vertical component of the magnetic field, S is the series admittance according to vertical electric sounding data analysis, and f is the observed frequency of reflections (visible frequency of reflected waves) through comparison with the cross-correlation function Kz(x), (B) distribution of reflected surfaces along seismic profile No. 85-10-03 and diagram of the proposed basement structure, (C) graphs of the total amount of information along 14 profiles including profile No. 85-10-03. After Pototsky, E., Khesin, B., 1971. Probabilistic-informational approach to the revealing of fracture zones in the Near-Caspian region. Azerb. Oil Industry (5), 15e16 in Russian.

the rugged terrain relief but also to the superposition of gravity effects from different geological bodies. The results (an approximation model of the HCC was employed) indicate that without calculating the terrain relief disturbance effect, the inversion results are not sufficient. However, application of the terrain relief correction can yield satisfactory results (see Fig. 9.11). The gravity method can be illustrated by a comprehensive gravity field examination in the Katekh deposit. The Katekh pyrite-polymetallic deposit is situated in the southern slope of the Greater Caucasus (Northern Azerbaijan) under conditions of severe rugged topography.

246 Chapter 9

Figure 9.10 Results of gravityemagnetic modeling in the Muradkhanly area (Kur Depression, Azerbaijan). (1) computed gravity effect from geological model; (2) observed field DgB; (3) regional trend; (4) restored maxima due to changes in the gradient zones; (5) local minima; (6) computed gravity

Searching for Economic Minerals 247 According to the data of the “Azerbaijangeologiya” Association, the geological section of the area is composed mainly of interstratifications of sandyeclay associations of the Upper Aalenian. Two subparallel stratified sheetlike bodies make up the Katekh deposit; tectonic faults control the space dislocation of all known economic ore bodies. The ore bodies in this deposit are primarily characterized morphologically as lenticular. However, a combination of latitudinal and longitudinal faults complicates this type and they acquire the form of steam-chest beds. The Katekh deposit was investigated by mining and drilling up to a depth of 500 m. However, some experts note that due to the extremely complicated tectonics these operations failed to delineate the ore bodies completely. The following types of texture ores were identified in the Katekh deposit: (1) massive, (2) veiny-clastic, and (3) spotty-disseminated. The main ore minerals of the Katekh deposit are pyrite, sphalerite, chalcopyrite, and galena. The secondary ore minerals are represented by hepatic pyrite, wurtzite, arsenopyrite, and melnikovite; rare minerals are silver and gold (Mekhtiyev et al., 1976; Zaitseva et al., 1988). A 3-D combined modeling of the field DgBouguer (gravity field in the Bouguer reduction) and the magnetic field DZ (vertical component of the total magnetic field) was performed using the following procedure. A detailed PGM of the Katekh deposit with a length of 800 m and depth of 400 m was constructed from the Mekhtiyev et al. (1976) and Zaitseva et al. (1988) generalized data. Then, all the available data for this area on density (Gadjiev et al., 1984) and magnetic susceptibility (Ismailzadeh et al., 1983a,b) were utilized. For enhanced calculation of the surrounding terrain topography a digital terrain relief model (DTRM) was created. The expressed SWeNE regional topography trend in the area of the Katekh deposit led to the selection of a rectangular DTRM with a length of 20 km and a width of 600 m (the physicalegeological profile with a length of 800 m was located in the geometric center of the DTRM) (Fig. 9.12). On the whole, 1000 characteristic points describing the DTRM (with these points located close to the center of the DTRM and more rarely on these margins) were utilized. The results of the first iteration of the gravity and magnetic fields modeling are presented in Fig. 9.13A. As indicated in this figure, the plots of DZ and DUSP (SP anomalies)

=

effect from subvertical hydrocarbon zone; (7) and (8) local gravity anomalies extracted from graphs 1 and 2; (9) computed magnetic effect from the geological model; (10) observed magnetic field; (11) computed magnetic effect from subvertical oil zone; (12) and (13) local magnetic anomalies extracted from graphs (9) and (10), respectively; (14) points where gravityemagnetic effects were computed; (15) deep boreholes; (16) subvertical hydrocarbon zone; (17) oil pool; (18) sedimentary deposits; (19) fault; (20) effusive associations. After Gadirov, V.G., Eppelbaum, L.V., 2012. Detailed gravity, magnetics successful in exploring Azerbaijan onshore areas. Oil and Gas Journal 110 (11), 60e73.

248 Chapter 9

Figure 9.11 Rapid interpretation of Dg anomaly over the model of the North Caucasian skarn deposit occurring in a homogeneous (A) and heterogeneous (B) medium. (1) loose deposits; (2) limestone; (3) acid volcanites; (4) andesites; (5) contour of the skarn deposit; (6) density, g/cm3; location of the HCC center according to the results of Dg interpretation: (7) fictitious, (8) real. After Eppelbaum, L.V., Khesin, B.E., 2012. Geophysical Studies in the Caucasus. Springer, pp. 411.

Searching for Economic Minerals 249

Figure 9.12 Location of computed physicalegeological model and profiles used for calculation of the surrounding terrain relief influence. After Eppelbaum, L.V., Khesin, B.E., 2004. Advanced 3-D modelling of gravity field unmasks reserves of a pyrite-polymetallic deposit: a case study from the Greater Caucasus. First Break, 22, No. 11, 53e56, with small modifications.

oscillate around zero and cannot provide useful information about the buried targets. This is due to the peculiarities of the mineralogical composition of ores in the Katekh deposit. The almost total absence of magnetic mineral pyrrhotite causes the virtually nonmagnetic nature of the ores. On other hand, the fairly large lead content impedes the normal course of oxidationereduction reactions needed to trigger intense SP anomalies. Thus, essential geophysical information can only be derived from the DgBouguer curve (Eppelbaum and Khesin, 2004). The analysis of the observed and computed gravity fields (Fig. 9.13A) shows that the initial PGM has a certain deficit of anomalous masses. 3-D modeling of gravity field was carried out using about 30 sequential iterations. It yielded the following results (Fig. 9.13B). Two ore bodies of massive composition (not reflected in the previous geological constructions) were identified in the southwestern and northeastern segments of the deposit (Eppelbaum and Khesin, 2004). It should be noted that the conclusion as to the presence of a hidden ore object in the southwestern portion of the profile is consistent with the results of independent investigations; namely, underground geothermal observations and a ground geochemical survey. A temperature anomaly of 0.5e0.8 C was recorded in adit 8 during the underground geothermal investigations at 250e300 m; the surface zone containing a large amount of lead and zinc was revealed at 150e200 m (according to Ginzburg et al. (1981) and data from the “Azerbaijangeologiya” Association).

250 Chapter 9

Figure 9.13 (A) Computation of a geophysical effect based on a known geological section in the Katekh pyrite-polymetallic deposit (southern slope of the Greater Caucasus) (Eppelbaum and Khesin, 2004). (B) Revised geological section model after comprehensive gravity field analysis (Eppelbaum and Khesin, 2004). (1) Quaternary loose deposits; (2e4) Middle Jurassic deposits: (2) massive fine- and meso-grained sandstones, (3) interstratification of clay shales and sandstones, (4) rhythmical alternation of aleurolites and clay shales; (5) disjunctive dislocations; (6) upthrust-overthrusts; (7e9) pyrite-polymetallic ores: (7) spotty, (8) stockwork-veiny, (9) massive; (10) contour of orebodies introduced during selection; (11) prospecting boreholes: (a) on the profile, (b) projected on the profile; (12) adits: (a) in the plane of the geological section, (b) projected onto the plane of the geological section; (13) curves of gravitational and magnetic fields: (a) observed, (b) selected; (14) physical properties: numerator ¼ density, g/cm3, denominator ¼ magnetization, mA/m (1e9 and 11e12 according to mining and drilling data).

Searching for Economic Minerals 251

Figure 9.13 cont’d

The following example demonstrates a novel method of the terrain correction calculation in complex mountainous environments. Gyzylbulagh deposit situated in the Mekhmana ore district of Nagorno-Karabakh, Azerbaijan (Lesser Caucasus). The deposit has been sufficiently investigated by mining and drilling operations and can thus be used as a reference for testing our method of interpretation. The deposit is composed of the Jurassic rocks and characterized by complex lithofacies, magmatism, and tectonics. The geological section is represented by andesite-dacite porphyrites and tuffstones of the Bathonian, silicified calcareous sandstones and tuffaceous gravelstones (Callovian-Oxfordian), and quaternary alluvial and diluvial deposits (Fig. 9.14A). Common for the deposit are subvolcanic formation of the Bajocian (liparite porphyries) and the Bathonian (liparite porphyries, dacites, and andesites), minor intrusions and a dike complex (dioriteporphyrites, liparite-dacite porphyries and andesite basalts) of the Upper Jurassic. The

Figure 9.14 Comparison of corrections in gravity prospecting under complex environments. (A) Geological map of the central portion of the Gyzylbulagh deposit; (B) and (C) Fragments of DgB field charts obtained by conventional and special technique, respectively. (1) Quaternary deluvial deposits, (2) lens of tuffaceous conglomerate from the Upper Jurassic, (3) metachert lenses, (4) tuffs and lavas of andesite porphyrites, (5) tuffs of liparite-dacite porphyrites of the Upper Bajocian, (6) subvolcanic body of andesite porphyrites, (7) dikes of andesite basalts of the Upper Jurassic, (8) tectonic dislocations, (9) zones of brecciation and cruch with weak pyrite-chalcopyrite mineralization, (10) zones of brecciation with pyrite-chalcopyrite ore content, (11) outcrop of ore body represented by oxidated pyrite-chalcopyrite ore, (12) dead rock and ore component piles, (13) terrain relief isolines, (14) brook bed, (15) isoanomalies, mGal: a ¼ positive, b ¼ zero, c ¼ negative. After Khesin, B.E., Alexeyev, V.V., Eppelbaum, L.V., 1993. Investigation of geophysical fields in pyrite deposits under mountainous conditions. Journal of Applied Geophysics, 31, 187e205, with modifications.

Searching for Economic Minerals 253 mineralization is represented by streaky impregnated and massive copper-pyrite ores (in the ores in a definite percentage of gold). More than 2500 observation points were carried out at the deposit on a network of 10 m  25 m. Density and magnetic sussceptibility of 620 samples which had been taken from the surface and drill cores were studied. The accuracy of measurement was 0.03 mGal. Small ore objects and their low excess densities are expected to produce gravimetric anomalies of 0.2e0.3 mGal. The errors due to neglecting the effect of very rugged topography are, therefore, comparable with the expected anomalies. Taking this fact into consideration, the gravimetric results were processed by two methods: a conventional technique (Berezkin, 1967; Nemtsov, 1967; Veselov, 1986) and by using a special processing scheme (Eppelbaum, 1989; Khesin et al., 1993). An isoline chart of (DgB)0-50, that is, Bouguer anomalies with topographic correction within the zones of 0e50 km (the heights of the surrounding terrain relief elements vary from 0 to 4 km within a radius of 50 km) was compiled using the conventional terrain relief correction method. In accordance with the obtained accuracy, the isolines on the chart are drawn every 0.2 mGal (Fig. 9.14B). It is seen from the figure that the isoline chart of gravity anomalies fails to reflect the known elements of the geological structure with desired accuracy. According to the novel method, the terrain effect is considered simultaneously with the interpretation. The terrain relief is simulated using six calculation profiles 2c-7c situated in the central portion of the Gyzylbulagh area (see Fig. 9.14C) and crossing the deposit of the same name. Trial calculations were carried out using the GSFC program to investigate the relief digital description range. For a 3-D model the relief was described for all the calculation profiles and, additionally, for another four profiles two of which are to the left of profile 2c and the other two to the right of profile 7c while the border profiles coincided with those observed in the Gyzylbulagh area. The bodies the upper edges of which described the topography of these profiles were considered as semiinfinite. In this case the Dg-values were calculated along profiles 2c-7c with unified digital description of the terrain relief. From this it follows that devising a 3-D model terrain relief model presents no problems where there are many calculation profiles because the terrain heights data have to be prepared simultaneously for each calculated profile (Khesin et al., 1993). On executing the above experiment, the further gravity data processing for the Gyzylbulagh area involved the following steps. A 3-D terrain relief model with an interval of its description of 80 km in points of profiles 2c-7c (about 100 points for each profile) was employed to calculate, using the GSFC program, the gravitational effect of a medium with an average density of 2.67 g/cm3. This effect was an incomplete topographic correction of opposite sign (-DgITC). These corrections were subtracted from, that is, added with their sign to, the free-air anomalies and the Bouguer anomalies (DgB)m were thus obtained for constructing an isoanomaly map (see Fig. 9.14C).

254 Chapter 9 The accuracy of calculating (-DgITC) is considerably higher than that of relief corrections Dg, for which ε ¼ 0.105 mGal, due to the more accurate approximation of relief forms in the novel technique. The chart of the (DgB)m isoanomalies is much more differentiated than the basic chart of the DgB isoanomalic obtained by the conventional technique (Fig. 9.14B) and in better agreement with the available geological data (see Fig. 9.14A).

9.2.2 Magnetics The presence of pyrrhotite in the copper-pyrite ore provides redundant magnetization of this deposit occurring in low-magnetic sand-shale associations which an aeromagnetic survey along straight profiles over this deposit failed to detect (Fig. 9.15A). By contrast, a flowing airborne survey clearly showed the magnetic effect of the buried ore body (Fig. 9.15B). Two examples of vertical components of magnetic field Za examination in the Filizchay ore field (southern slope of the Greater Caucasus, Azerbaijan) are shown in Fig. 9.16.

(A) 1.

3

1. 5

1.6

1.

4

(B)

0.6

0

0.2 0.4

0.8 0.6

0.2

0.2

0.2

1

2

0.

4

3

4

Figure 9.15 Results of a helicopter magnetic survey in the area of the copper-pyrite deposit of Kyzyl-Dere in the Dagestan Mountains (Greater Caucasus): (A) along straight profiles, (B) topography flow. DT isolines in hundreds nanoTesla: (1) positive, (2) negative, (3) zero; (4) location of the ore deposit (Eppelbaum and Khesin, 2012).

Searching for Economic Minerals 255

Figure 9.16 Quantitative analysis of anomaly Za observed on an inclined relief. (A) by the use of preliminary reduction to the horizontal plane with Logachev’s method (Logachev and Zakharov (1979)), (B) using the methods presented in (Khesin et al. (1996) and Eppelbaum and Khesin (2012)). Graphs (1e4): (1) Za (x,z), (2) Za (x,0), (3) Xa (x,0), (4) Quaternary deposits; (5) interbedding of sandstone, siltstone, and shales; (6) shales with flysch; (7) faults; (8) pyrite-polymetallic ore; (9) oxidized ore; (10) veiny ore; (11) horizontal level of magnetic anomaly reduction; (12) location of the middle of the upper edge of the ore body by interpretation of Za (x,0) and Xa (x,0) on the level of reduction, results of interpretation of observed curve (13e14): (13) middle of the upper edge and direction of dipping of real body; (14) middle of the upper edge of fictitious body; (15) corrected line of the normal background of curve Za (x,z), (16) Reford’s point (Reford and Samner, 1964); (17) boreholes. 0f and 0r are the calculated origins of the coordinates for fictitious and real bodies, respectively.

Fig. 9.16A illustrates the application of Logachev’s method (Logachev and Zakharov, 1979). As shown in Fig. 9.16A, reducing Za by the use of Logachev’s method and interpretation of the obtained graphs Za and Xa by vector methodology defined the upper edge of ore body above the Earth’s surface. Thus, this method is unacceptable in complex geological conditions (oblique magnetization, inclined rugged relief, and an unknown level of the normal magnetic field).

256 Chapter 9 Application of the methodology as described in Eppelbaum and Khesin (2012) (see also section 5.1) determined the position of the buried ore body with high accuracy (Fig. 9.16B). The inverse probability method (IPM) was used on the Filizchay pyrite-polymetallic deposit which is the largest in the Caucasus. This deposit was revealed by geophysical methods. Application of IPM with the purpose of singling out weak magnetic anomalies showed that the ore deposit can be detected using this method (Fig. 9.17) even in that part of the area where it is not fixed in the observed field. The effectiveness of IPM may be further improved, if we take into account the conditions which partially manifest themselves during the analysis of other methods of this kind.

9.2.3 Thermics Near-surface temperature survey has a great potential in ore geophysics (Khesin and Eppelbaum, 1994). Fig. 9.18 presents the results of the interpretation of a temperature anomaly (applying the techniques described in Eppelbaum et al. (2014)). In the figure, the results are given for a district with the steeply inclined relief of the Katekh deposit (southern slope of the Greater Caucasus). The temperature was measured in 1.0 m deep blastholes. The temperature plot

Figure 9.17 Inverse probability and cross-correlation methods testing on the Filizchay polymetallic deposit (southern slope of the Greater Caucasus, Azerbaijan). (1) upper edge projection of the deposit on the earth’s surface by geological data; (2) profiles; (3) disjunctive dislocations; (4) isopotential lines of SP, mV; (5) isolines DZ, nT; (6) DZ anomaly axes obtained with the use of crosscorrelation; (7) areas, where after the inverse probability method application the probability of detecting the targets exceeded 0.9 (Khesin et al., 1996).

Searching for Economic Minerals 257

Figure 9.18 Quantitative interpretation of the temperature anomaly at the Katekh pyrite-polymetallic deposit (southern slope of the Greater Caucasus, Azerbaijan). The observed temperature and geological sections are taken from Zverev et al. (1982) and an unpublished report of the “Azerbaijangeologiya” Association. The “þ” symbol marks the position of the upper edge of the thin body obtained from the analysis of the anomaly profile (after Eppelbaum et al., 2014).

shows two anomalies, one of which is due to the subhorizontal ore deposit and is less pronounced. The thermal conductivities for the host rock and thick pyrite-polymetallic ore were 1.45  0.35 and 3.87  0.57 W/m C, respectively (Zverev et al., 1982); that is, their ratio exceeded 2.5. The upper part of ore body was completely oxidized and did not differ in thermal conductivity from the host medium. The results made it possible to localize the upper edge of the subvertical ore body within an acceptable error.

258 Chapter 9

Figure 9.19 Quantitative interpretation of temperature anomalies in the area of the Kvaisa pyrite-polymetallic deposit (Southern slope of the Greater Caucasus, Georgia) (observed data and geological section after Ginzburg et al., 1974; interpretation after Eppelbaum et al., 2014). (1) volcanoclastic medium, (2) fault, (3) position of mine, (4) temperature graph, (5) determined position of the middle of upper edge of ore body.

Fig. 9.19 illustrates the temperature anomaly observed along the profile across the Kvaisa pyrite-polymetallic deposit (the Greater Caucasus). The anomaly amplitude exceeds 2 C, which is traceable to the additional effect of the fracture located at the edge of the ore body. The temperature was measured in 1.0 m deep blastholes. The ore was pyritesphalerite in composition and there was a subvertical occurrence in volcaniclastic host rocks. Thermal conductivities for sandy-argillaceous host rocks and thick pyritepolymetallic ore were 2.0  0.5 and 5.0  1.0 W/m C, respectively. Quantitative interpretation was carried out using the characteristic point method. This served to locate the upper edge of the ore body. The results were confirmed by mining.

9.2.4 Self-Potential SP measurements are often applied for searching and localization of ore deposits (e.g., Semenov, 1980; Mendonca, 2008; Eppelbaum and Khesin, 2012)

Searching for Economic Minerals 259

Figure 9.20 Displacement of SP isolines during exploitation of a new shaft of the Chiragidzor sulfur deposit (Lesser Caucasus, Azerbaijan) (after Khesin, 1969). (1) shaft contour, (2) isolines of SP field (in milliVolts).

SP studies were carried out over the Chiragidzor sulfur deposit (central Azerbaijan) for several years (Fig. 9.20). This figure shows that the mining works in the underground shaft distort the observed SP field strongly at the earth’s surface. This testifies to the tight correlation between mining processes and SP anomalies. Clear isometric SP anomaly with intensity up to 100 mV was observed over graphite body at the Bender deposit in India (Fig. 9.21) (Bhattacharya et al., 2007). SP observations carried out in the ore district Tashkazyk (Mt. Pamir) indicate complex pattern (Fig. 9.22A). Application of well-known SaxoveNygard transformation allowed to detect several ore zones (Fig. 9.22B). It was identified that in this area the most effective is employment of R1 ¼ 10 m and R2 ¼ 40 m (R1 and R2 are the radius of circles). SP surveys were frequently applied on the pyrite-polymetallic deposits of the southern slope of the Greater Caucasus (e.g., Alexeyev, 1971; Eppelbaum and Khesin, 2002; Eppelbaum and Khesin, 2012). Four cases of SP interpretation are presented in Fig. 9.23A,B. A very intensive SP anomaly (about 600 mV) was observed in the Filizchay copper-polymetallic field (Fig. 9.23A). Use of the approach described in Eppelbaum et al. (2004) determined the position of upper edge of the ore body in conditions of sharply inclined relief. Three SP anomalies were successfully interpreted in the Katsdag copper-polymetallic deposit (Fig. 9.23B). Yu¨ngu¨l (1954) published the results of the survey in the Sariyer area (Istanbul). Since this time many authors reproduced this example in the various reviews and books, however, without any quantitative interpretation. The performed interpretation indicates that the obtained position of HCC center is in the line with geometrical and physical parameters of the sulfide-pyrite ore body (Fig. 9.24).

260 Chapter 9

Figure 9.21 Results of SP method application at the Bender graphite deposit (Orissa state, India). After Bhattacharya, B.B., Jardani, S.A., Bera, A., 2007. Three-dimensional probability tomography of self-potential anomalies of graphite and sulphide mineralization in Orissa and Rajasthan, India. Near-Surface Geophysics 5, 223e230.

Fig. 9.25 presents the results of SP anomaly interpretation by the developed techniques in the Potentsiaalne deposit of the Rudny Altai. The inclination angle of the natural polarization vector fp is calculated (see Section 5.4) from the expression 4p ¼ 900
q;

(9.1)

4p ; s ¼ 900
q þ u0 ;

(9.2)

on an inclined relief

Searching for Economic Minerals 261

Figure 9.22A SP map in the high-mountain ore district Tashkazyk (Mt. Pamir). After Markov, V.A., 1983. SaxovNygard transformation in SP electric prospecting. Applied Geophysics 102e108 in Russian.

It should be noted that the investigation of this area under fields conditions in order to get information about an ore object called for measuring the SP in the wells (Shatrov and Dun Tsun-in, 1984), while the developed technique permits to determine the ore body’s parameters by the SP measurements on the earth’s surface. Fig. 9.26 depicts the position of the HCC center, which evidently fixes the undrilled edge of a flat-lying ore body.

262 Chapter 9

Figure 9.22B Results of SaxoveNygard transformation application (after Markov, 1983). (1) mineralized ore zone and (2) gravels.

Figure 9.23 Quantitative interpretation of SP anomalies at the Filizchay (A) and Katsdag (B) copperpolymetallic deposits on the southern slope of the Greater Caucasus (Azerbaijan). IdIV: numbers of interpreted SP anomalies. (1) interbedding of sands and clay schists; (2) clay schists with the flysh packages; (3) clay sandstone; (4) sand-clay schists; (5) diabases, gabbro-diabases and diabasic porphyrites; (6) andesites and andesite-porphyrites; (7) dacitic porphyrites; (8) faults; (9) massive ore of pyrite-polymetallic composition; (10) oxidized ore; (11) zones of brecciation, crush, and boudinage with lean pyrite-polymetallic ore; (12) SP curves; location of anomalous source (13) without calculation of inclined relief influence; (14) after introducing correction for relief. After Eppelbaum, L.V., Khesin, B.E., 2012. Geophysical Studies in the Caucasus. Springer, 411 pp., with minor modifications.

Searching for Economic Minerals 263 d4

d3

d1

+20 0

0

-20

d5

-40

d2

δ α

25m

Devonian Schist Senonian Andesite Sulphide with Cu%

-60 P

Pyrite

-80

N-W

-100

S-E

δ δ δ δ δ δ δ δ δ δ δ δδ δ α α α α α δ δ α α δ α α Adit δ α 1-2% 7% P α δ α α δ α 14% α α α α α α α α α α α α α α α α α Drill hole

-120 mV

δ

δ

δ

180 170 160 150 140 130

m

Figure 9.24 Quantitative interpretation of SP anomaly by the characteristic point and tangent methods in the Sariyer area, Turkey. The “◉” symbol marks the obtained position of the ore body center (approximated by an HCC). Observed SP curve and geological section are taken from Yu ¨ngu ¨l (1954) (interpretation after Eppelbaum and Khesin, 2002).

9.2.5 Integrated Investigations The above techniques of processing and interpretation form a continuous sequence of procedures aimed at a final geological productda resultative map, a section, etc., that is, a plot representing a final PGM of the investigated medium. For example, summing up the amounts of information contained in Dg, DT fields and in the vertical component of VLF magnetic field Hz, we get the anomaly of the parameter (see section 4.4.1) 3 1X Ji ; 3 i¼1

which is more marked and reveals a gently sloping ore object on the profile 5c through the Gyzylbulagh gold-pyrite deposit in the Lesser Caucasus (Fig. 9.27A). Earlier we considered a novel method of the terrain correction calculation at this deposit (see Fig. 9.14).

264 Chapter 9

Figure 9.25 Interpretation by the developed techniques of SP anomaly in the area of deposit Potentsiaalne in Rudny Altai, Russia ((1)e(7) from Semenov (1975), (8) after Khesin et al. (1996). (1) soilvegetative layer; (2) alternation of lavas and tuffs of acid composition and chlorite-sericitic schists; (3) sulfide ores; (4) sulfide impregnation, pyritization; (5) level of groundwaters; (6) drilling wells (a) and adits (b); (7) plot of SP; (8) interpretation results: (a) midpoint of the dipping thick bed’s upper edge, (b) upper edge of the dipping thin bed, (c) center of a horizontal circular cylinder (arrow indicates the direction of the polarization vector obtained by the interpretation).

Searching for Economic Minerals 265

Figure 9.26 Interpretation of SP anomaly by the method of characteristic points in the area of the Uchambo ore field of the Adjar group of copper-polymetallic deposits (Georgia). (1) SP observed values; (2) heteroclastic tuff breccia and their tuffs; (3) cover trachyandesite-basalts with pyroclastic interbeds; (4) disjunctive dislocations; (5) zones of increased mineralization; (6) drilled wells; (7) location of HCC center according to the interpretation results ((1e6) from Bukhnikashvili et al. (1974) (7) after Eppelbaum and Khesin, 2002).

Figure 9.27 Stages in geophysical data interpretation at the Gyzylbulagh gold-pyrite deposit (Lesser Caucasus) (Eppelbaum and Khesin, 2012). (A) Singling out of the desired object (ore deposit) by summing up the amounts of information obtained by different geophysical methods. (B) Quantitative rapid interpretation of DT, Dg, Hz, and H4 fields. (C) Selection of a geological section on the basis of gravimetric and magnetic data. (1) Quaternary deluvial deposits; (2e8) Middle and Upper Jurassic rocks: (2) silicificated limestone lens, (3) tuffs and lavas of andesitic porphyrites, (4) tuffs of liparite-dacitic porphyrites, (5) deconsolidated tuffs of liparite-dacitic porphyrites, (6) lavas of dacitic porphyrites, (7) consolidated lavas of dacite-porphyrites, (8) dikes of andesite-basalts; (9) disjunctive dislocations; (10) zone of brecciation and crush; (11) zones of brecciation, crush and boudinage with lean pyrite-chalcopyrite ore; (12) zone of brecciation, crush and boudinage with rich impregnating mineralization; (13) massive pyrite-chalcopyrite ore; (14) drilled wells: (a) on the profile, (b) projected on the profile; (15) terrain relief in the profile (in Fig. 5.16B only); results of rapid interpretation; (16) location of the center of HCC from the interpretation of Dg and DT plots, (17) location of the HCC center from the interpretation of the Hz (a) and H4 (b) plots, (18) location of the upper edge of thin bed from the interpretation of Hz and DT (a) and H4 (b) plots;

Figure 9.27 (cont’d) (19) physical properties (numerator ¼ density, g/cm3; denominator ¼ magnetization, mA/m; (20) gravitational and magnetic fields: (a) observed, (b) selected; (21) body contours assumed in the process of 3D modeling.

268 Chapter 9

Figure 9.27 cont’d

Taking into account a limited sampling, Eq. (4.19b) was used. During this computation DUi was taken as a doubled standard deviation of Ui. The zero line of DT field is represented by the level of anomaly maximum. The values of delta T as Ui were calculated proceeding from this level in the direction of the field decrease (since pyrite deposits are characterized by very low magnetization with respect to volcanogenic host rocks).

Searching for Economic Minerals 269 The rapid interpretation of Dg, DT, Hz, and H4 plots (see Fig. 9.27B) confirmed the available geological ideas and permitted to form an initial approximation of the model of the medium for its further refinement in interactive PGM of Dg and DT fields (see Fig. 9.27C).

9.3 Other Kinds of Deposits 9.3.1 Yakutian Diamond Province (Siberia, Russia) Yakutian Diamond Province (e.g., Tolstov et al., 2009) occupies a giant territory (almost 1.5 mln km2) within the Siberian Platform of Russia. Magnetic prospecting is one of the main geophysical methods applied for searching kimberlite pipes. Kimberlite pipes occurring in limestones and dolomites, and are characterized by positive DT anomalies from 15 to 8000 nT, and negative anomalies range in the diapason from 0 up to
1500 nT. The positive anomalies in majority of cases display their mosaic character. Fig. 9.28A

Figure 9.28 Magnetic prospecting in searching kimberlite pipes in Yakutia. A: Schematic geologicalegeophysical section through the kimberlite pipes on the height of 200 m aeromagnetic T-survey of, B: Behavior of DZ curves over the largest kimberlite pipe “Mir”. (1) kimberlite pipes, (2) dikes of basic rocks, (3) predominantly carboneous rocks of the Lower Paleozoic, crystallic rocks of basement: (4) possibly of basic composition, (5) acid composition. I, II, III, and IV: DZ measurements performed at the levels of 1 m, 100, 200, and 300 m, respectively. After Nikitsky, V.E., Glebovsky, Y.S. (Eds.), 1990. Magnetic Prospecting. Geophysicist’s Manual. Nedra, Moscow (in Russian) Nikitsky and Glebovsky, 1990.

270 Chapter 9 shows a distribution of the field DT over kimberlite pipes by the aeromagnetic survey at the height of 200 m. Fig. 9.27B demonstrates attenuation of DZ land observations at 1m level (surface observations) to 100, 200, and 300 m airborne magnetic measurements. This figure indicates that the very expressive land (1 m) magnetic curve gradually turns into a difficult to interpret magnetic anomaly at the level of 300 m.

9.3.2 Makhtesh Ramon Complex Ore Deposit (Northern Negev, Israel) The Makhtesh Ramon erosionaletectonic depression (canyon), which is 40 km long and approximately 8 km wide, is situated in the Negev Desert (southern Israel), 65 km southwest of the Dead Sea. High-intensive magmatic acivity in the region occurred approximately 120 Ma ago, which corresponds to the epoch of global tectono-thermal activation. Larson (1995) termed this epoch the Middle Cretaceous maximum in the development of upper mantle hot spots. In the Makhtesh Ramon area, the Middle Cretaceous maximum stage is reflected by lava flows, eruptive breccia, and tuffs that suggest wide development of explosive volcanism in the Middle Cretaceous. Vent facies of the central-type Mahale Khahatsmaut depression contain peridotite xenoliths with spinel-bearing lherzolites. The presence of such rocks indicates significant depths of explosive volcanism. Magmatic rocks of the western Makhtesh Ramon Canyon are represented by picritic and olivine basalts, basanites, mantle xenoliths, melilites, and alkaline picrites. As a result of integrated geological, geophysical, mineralogical, and geochemical investigations along with geomorphological reconstructions, the Makhtesh Ramon Canyon was chosen as perspective area for the discovery of diamonds and some other economic minerals (gold and REE). The identified minerals-satellites of diamond include chrome-diopside, orange garnet, bright crimson pyrope, picroilmenite, chrome-spinel, olivine, moissanite, perovskite, anatase, corundum, titanomagnetite, and tourmaline (Eppelbaum et al., 2006). Petrological confirmation gives, for instance, discovered meimechite (Fig. 9.29B). A group of microdiamonds and five diamonds (with a size > 1.0 mm) were found (one of this diamonds is shown in Fig. 9.29C). One of geophysical evidences is presented in Fig. 9.29A. 3D magnetic field modeling indicates that the intrusive body of laccolith type may be reinterpreted as a kimberlite pipe.

9.4 Underground Geophysics Underground geophysics plays an important role in the investigation of ore deposits occurring in mountainous conditions. In the Caucasus, underground geophysics is employed in all three regions: the Northern and Southern Caucasus, and the Lesser

Searching for Economic Minerals 271 (A) T, nanoTesla

(B)

∇

250 200 150 100

NNW

SSE

50 0 -50

Total magnetic field:

-100

observed computed from an initial model (Bayer et al., 1989)

-150

computed from a new model (Eppelbaum et al., 2006)

-200 -250 0 0

Depth, m

200 400

200 m

(C)

Intial model (after Bayer et al., 1989)

2 A/m

600 800

1000

New model (after Eppelbaum et al., 2006)

Figure 9.29 (A): Results of magnetic data interpretation in the eastern part of Makhtesh Ramon area (initial data after Baer et al., 1989; revised modeldafter Eppelbaum et al., 2006), (B) recognized meimechite (this rock very closely associated with kimberlite), (C) one of the discovered diamonds (1.35 mm) (Eppelbaum and Katz, 2012).

Caucasus. Below the geophysical methods that are best suited for underground geophysics are discussed.

9.4.1 Gravity The methodology of underground gravity surveys is determined by the nature of the problem, as well as the physicalegeological and environmental mine technical conditions.

272 Chapter 9 The following conditions are favorable for searching ore bodies: (a) presence of contrast density and a sufficient volume of ore target causing a gravity effect of no less than 0.05e0.08 mGal, (b) comparatively small distance between the adit and ore target and its asymmetric position with respect to the adit (especially by dipping close to vertical), (c) comparatively uniform host medium (or presence of data about density inhomogeneities), (d) desirably dense net of operational mines at different horizons. An underground gravity survey was successfully carried out, for instance, in the mining openings of the Katsdag pyrite polymetallic deposit in the Southern Caucasus (Poltoratsky and Ginzburg, 1989) and in the Bazun iron-ore deposit in the Lesser Caucasus (Khesin et al., 1988). Interesting results by calculating the statistical reduction in an underground gravity survey in one of the Lesser Caucasian ore deposits (Armenia) is presented in Khesin et al. (1988).

9.4.2 Magnetics The estimation of accuracy and range of magnetic investigations in underground mines was shown, for instance, in Nikitsky and Glebovsky (1990). Preference of the positioning technique for underground magnetic investigations by searching economic deposits was displayed in Haverinen and Kemppainen (2011).

9.4.3 Temperature Survey A temperature survey in adits was performed by the use of following simple methodology. First of all, for this survey nonworking adits without any industrial activity (primarily, without air supply) were selected. As a rule, positive temperature anomalies (or thermal flow) and heightened values of geothermal gradients are measured over ore bodies. Hence, inside the target of heightened thermal conductivity, the geothermal gradient will be below the normal value. The width of the anomalous temperature interval gives definite information about the size of anomalous object, and the temperature gradient is indicative of the distance to the observation line. The distribution of the temperature graph can be used in many cases to draw conclusions about the location of the anomalous target. Temperature measurements in adit 10 of the Katsdag deposit (north-western Azerbaijan) were made in glasses filled with water at an observation step of 10 m (Fig. 9.30). Analysis of the temperature field indicated that the temperature anomaly observed in the right portion of this geological section in general corresponded to the anomalous body. At the

Searching for Economic Minerals 273

Figure 9.30 Graph of the anomalous temperature distribution along the main opening of adit 10 in the Katsdag polymetallic deposit, southern slope of the Greater Caucasus (this survey was carried out by L. Eppelbaum in 1978 under the supervision of Ginzburg et al. (1981). (1) sandstone, (2) liparite dacites, (3) massive copper-pyrite ore, (4) faults Eppelbaum et al. (2014).

same time, the temperature anomaly observed in the left portion of this section testified to the presence of some additional ore bodies not detected by geological means. An example of the influence of rugged terrain relief on adit temperature measurements is presented in Fig. 9.31. As is known, surface relief influences the temperature field at a distance of four to five amplitudes of the relief’s maximal amplitude (e.g., Cheremensky, 1972). A temperature survey was performed in blastholes of 1.5 m in the adit walls. The search targets in this case; namely, massive pyrite-polymetallic ore bodies, occurred in the sandy-argillaceous medium that provided a high thermophysical contrast. The observed temperature curve (Fig. 9.31B) was strongly distorted by the rugged relief influence. After application of the correlation approach (coefficients of linear regression were obtained by the least-square method), the corrected temperature graph was constructed (Fig. 9.31A). Quantitative analysis of the graph led to calculation of the position of the center of the anomalous target.

274 Chapter 9

Figure 9.31 Quantitative analysis of near-surface thermal investigations carried out in an adit of the Filizchay pyrite-polymetallic deposit (southern slope of the Greater Caucasus). (A) quantitative analysis of corrected temperature graph, (B) observed temperature field and geological section. The “þ” symbol designates the position of the middle of the upper edge of the anomalous body (Eppelbaum and Khesin, 2012). Observed temperature field and geological section are adapted from Borisovich, V.T., Eppelbaum, V.M., Ginzburg, S.N., 1988. Optimization of geophysical investigations in mountainous regions on the basis of integration of geophysical and mining and drilling works. In: Borisovich, V.T., Eppelbaum, V.M., (Eds.), Optimization of Ore Exploration in Mountainous Regions. Nedra, Moscow, 123e133 in Russian.

In the Katsdag pyrite-polymetallic deposit, an areal temperature survey was performed (L. Eppelbaum, as a student in the Geophysical Department of the Azerbaijan University of Oil and Chemistry, took part in these field works) in the underground drifts (Fig. 9.32A). The temperature map for the normal field (which incorporates the influence of the surface rugged relief, the depth of attenuation of annual temperature variations propagating from the earth’s surface and other factors) is shown in Fig. 9.32B. A residual anomalous temperature map is depicted in Fig. 9.32C. Clearly, the final anomalous temperature map (Fig. 9.32C) differs considerably from the initial observations (Fig. 9.32A).

Searching for Economic Minerals 275

Figure 9.32 Results of mine thermal prospecting in the Katsdag pyrite-polymetallic deposit (southern slope of the Greater Caucasus) (after Ginzburg and Maslennikov (1989), with minor modifications). Temperature maps: (A) observed values, (B) normal field, (C) anomalous values. (1) temperature isolines, (2) underground shifts.

9.4.4 Self-Potential Survey A classic example of an SP field distribution along a highly electrical conductive ore body is presented in Fig. 9.33. In the upper part of the massive ore body a value of (
600) mV was observed, and in the lower part the value was (þ400) mV (obviously, the absence of

276 Chapter 9

Figure 9.33 SP field distribution in a copper-polymetallic deposit in Armenia (Lesser Caucasus). (1) porphyritic tuff breccia, (2) diabasic dyke, (3) SP field distribution, (4) copper-polymetallic orebody. After Brodovoi, V.V., 1989. Searching and prospecting of useful deposits: copper. In: Brodovoi, V.V. (Ed.), Borehole and Mining Geophysics, vol. 2. Nedra, Moscow, 190e208 (in Russian), with minor modifications.

SP measurement in the outermost lower points precludes observing the equivalent value of þ600 mV).

9.4.5 Examples of Integrated Underground Observations An impressive integration of gravity, temperature, and VLF observations in the Katsdag deposit (Greater Caucasus) is presented in Fig. 9.34 (the author took part in these field observations and geophysical data analyses). Analysis of geophysical graphs in the lower part of this figure and comparison of these data with the geological section provide rich data for integrated analysis and prospection. Certain geophysical graph distributions testify to the presence of previously unknown ore bodies.

Figure 9.34 Integrated geophysical investigations in mines of the Katsdag deposit (southern slope of the Greater Caucasus) (Poltoratsky and Ginzburg, 1989). (1) shales, (2) interbedding of shales, sandstones, and siltstones, (3) dioritic porphyry, (4) oxidizing zone, (5) polymetallic ores of I and II zones, (6) faults, (7) apparent beds of redundant: density (a) and thermal conductivity (b), (8) current axes revealed by the VLF method.

278 Chapter 9

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