Underground temperature background of Saksagan iron-ore region in Kryvyi Rih basin

2019 The data from thoroughly selected 46 deep exploration boreholes drilled along the Saksagan iron-ore stripe located in the Kryvbas and a transverse section of the stripe, which included results of studies conducted in the Kryvyi Rih super-deep borehole SD-8, have served as the data bank for analyzing and calculating the tem perature backgro­ und param eters. The m ethodical procedures for ordinary m easurem ents averaging, de­ term ining and generalizing tem perature values and other therm al background param e­ ters were given. The distribution of tem perature along the strike of the Saksagan iron-ore stripe from the south-west to the north-east within the depth range from 0 m to 2000 m was shown. The whole territory of the Saksagan iron-ore stripe from the Rodina Mine in the south was divided into 7 sites, approxim ately equal in area, each of sites ned at least 4 boreholes. The area-averaged and the depth-averaged data were calcula­ ted for each of the sites. The averaging depth range was 500 m. The pattern of the ano­ malous isotherms has shown the anomalous field to be very diverse, and can be defined as the alternation of negative and positive therm al fields, com plicated by local anom ali­ es in the northern direction. The characteristic of the geothermal gradient after the ave­ raged data of 46 deep boreholes drilled along Saksagan iron-ore stripe was given. The given data allow us to state about high variations of GTG values. The obtained results will be greatly appreciated in interpreting the features of the overall structure of the Kryvbas, especially its western side and the central part tectonics, by hydrogeological and mining services in view of further deepening of the mines. The tem perature data will help in determ ining areas and zones of neotectonic and seismic activation.

Kryvyi Rih iron-ore basin is located within the Ukrainian Shield (US). Thermal b ack ground investigations of the US were car ried out in sufficient details by many rese archers [Kutas, Gordienko, 1971;Gordienko, 2000Gordienko, , 2005Gordienko et a l., 2002]. It was discovered that the Precambrian US has a basically low therm al background com par ed to its geologically younger framing. This is fully consistent with the studies conduct ed in other similar regions [Mechnikov, Vol kov, 2018]. At the same time, areas with in tense and diverse thermal anomalies were no ticed in the background of common stable low values in some areas, especially in those adjacent to the big deep faults and the e d ges of the shield [Gordienko, 2000[Gordienko, , 2005. In terms of this fact, Kryvyi Rih-Kremenchuk deep fault, in the zone of which Kryvyi Rih iron-ore deposit is located (Kryvyi Rig ironore basin), is of particular interest.
Temperature studies in Kryvyi Rih basin, which conducted together with the extrac tion of iron-ores, began in the late 40's of the tw entieth century. By the time of the fi nal analysis of therm om etric data in [1985][1986][1987][1988][1989][1990], almost all area covered by detailed geo logical surveys was also studied in terms of the subsoil tem perature observations. The analysis of the tem perature background of the deep layers of Kryvyi Rih iron-ore depo sit was carried out by the South-Ukrainian expedition of the super-deep drilling w hen correlating super-deep drilling data to the results of studies of the Kryvbas framework [Sheremet, 2011;Mechnikov, Volkov, 2013].
Subsequently, the data were analyzed by the authors in order to determine the correct averaged values of the underground thermal field, which would characterize the most dis tinctive features. To solve this task, 46 deep boreholes, located along the strike of the Saksagan iron-ore strip, were selected. They cha racterize the behavior of the temperature back ground along the strip. In addition, the tem perature background was analyzed across the strike of the Kryvyi Rih structure in the nor thern part, on the traverse of the super-deep borehole SD-8 and complementary boreholes.
The data of 46 deep boreholes were com bined and averaged within individual sec tions to determ ine the most likely tem pera ture indexes (Fig. 1). The averaging proce dure included determining the arithmetic me an value for all boreholes located within the site at identical abyssal elevations. 7 sites we re identified. They are evenly located from the latitude of the "Rodina" Mine in the so uth, to the latitude of the M ine nam ed af ter V. I. Lenin in the north. Each site has a different num ber of boreholes. This is due to the fact that complex logging, which in cluded thermal observations, was carried out to solve specific geological tasks being dif ferent for each site. Four deep boreholes we re the smallest am ount of those on a site. In other areas, the num ber of boreholes vari ed from 5 till 10.
The need to determine the most likely ave raged values of tem perature m easurem ents is due to several reasons. The first one is that recom m ended or calculated terms of bore holes calm standing have not always been respected during the industrial cycle of ex ploration, as well as in terms of the tech n i cal capabilities for saving the stability of the boreholes walls, from the m om ent of active drilling and run time to the m om ent of tem perature measuring. It means that timesca les of these terms were not always similar. The terms of calm standing of a borehole al- Fig. 1. Diagram of investigated areas within the Kryvyi Rih basin: 1 -rocks forming the Kryvyi Rih m etam orphic series of the Paleoproterosoic; 2 -granitoid com plexes of the Archaean; 3iron-ore strips within the Kryvyi Rih basin (L -Likhmanivka, CMS -closure of the main syn clinal, S -Saksagan, EH -East-Hannivka, WH -West-Hannivka); 4 -the zone of the W estern fa ult, which limits the Kryvyi Rih structure from the west; 5 -Kryvyi Rih super-deep borehole SD-8; 6 -deep boreholes; 7 -latitudinal (cross-sec tional to the Kryvyi Rih structure) geological pro file; 8 -location of the studied areas within the Saksagan strip.
Геофизический журнал № 5, T. 41, 2019 so changed depending on the drilling depths achieved, diameter and depth of the casing. The second im portant reason for the discre pancy of indexes in closely located boreho les at the same depths is the instability of the hydrogeological regime. On the one hand, this is due to the existence of large fault tec tonic zones, some of which represent the ways of transporting fluids of different tem pera tures. On the other hand, it is due to the exi stence of a large network of water pum ping from existing mines and open-pits. These fac tors led to the excluding of a large num ber of boreholes located within the limits of d e pressive hydrogeological reservoirs or close to them from the analysis.
A rather significant factor that causes in stability of the tem perature at similar depths and which is impossible to be neutralized is the high variability of therm al properties of the rocks of the Kryvyi Rih series. Under the conditions of the steep dip of the metamorphogenic volcanic-sedim entary strata of di verse composition, the probability of occur ring of different therm al param eters rocks at the same depth is very probable.
Averaging of specific data for certain depths was also carried out. This also had to be do ne due to the high variation of tem p eratu re param eters with the depth. Their growth is different in individual boreholes with the increase of depth. The data, that were ave raged over several boreholes in each cell, were also averaged within 500 m intervals to reduce the influence of local factors.
The temperature distribution along the strike of the Saksagan iron ore strip. The presented data were calculated after avera ge data from several boreholes, as well as average data within 500-meter intervals to the depth of 2000 m. The analysis shows the temperature to increase gradually with depth, but in a different way, in each of the studi ed areas the param eter pattern changes in dividually.
In the Fig. 2 schematic longitudinal pro jection of the tem perature background dis tribution is shown from the south-east to the north-west along the Saksagan iron-ore strip (depth interval 0-2000 m). The isotherms of the averaged temperature indexes are shown as well.
The behavior of isotherms clearly shows an uneven growth of the thermal field to the depth of 2000 m. This process is m anifest ed the most clearly in the upper horizons (in terval 0-1000 m). The isotherms have a w a velike shape with well-defined ups and downs. Also, the isotherm + 30 °C has a fluctuating shape, which characterizes significant tem perature changes in the deepest range of 1500-2000 m. Such configuration of the tem perature fi eld is typical to the depth of 1000 m (in the south) and up to 1500 m in the north. At lo wer depths, the local maxima slightly displa ce : from the first (so uthern) site , they m ig rate to the south, from the seventh (northern) site they migrate in the northern direction. This behavior of the tem perature field can indicate the possible presence of two outflows of therm al energy sources from the subsoil that are located at some depth to the south of the first site and to the north of the seventh (northern) site.
To determ ine the most com plete charac terization of the local therm al field anom a lies influence on the general nature of its re gional growth, the calculation of average va lues of tem peratures for each 500-meter in terval was conducted with the subsequent estimation of the difference between the cal culated mean of the param eter in the depth intervals and the prelim inary calculated va lues of the parameter in each cell. The results of the calculations are shown in the Fig. 3. A schematic longitudinal vertical projection of anomalous tem perature values along the Saksagan iron-ore strip in the depth range 0-2000 m is shown there.
The figure is presented in the form of ano malous isotherms that have positive and ne gative values as the deviation of the tem pe rature from the average values to the higher (+) or lower (-) side. The term "anom alous" is used here for tem perature values that are different from the average interval tem pe rature value calculated for each 500-meter horizontal interval in all seven sites.
The general layout of distribution of ano malous temperature values shows its heterogenity. Both the regional changes along the strike of the Saksagan strip and the peculi ar local features are observed here.
Regional features of the distribution of ano malous tem perature values include the in terchange of negative vertical accum ulati ons by positive ones along the Saksagan strip. Both negative and positive anomalies occu py the entire depth of research from 0 to 2000 m. The most clearly the m entioned fact is ob served in the southern and central parts of the described area.
Local anom alies are located w ithin the boundaries of the regional fields and have the same sign. W ithin the first site in the ex trem e southern negative anomaly, there is an anomalous minimum of the field (-1.57 °C) within the range of depths 0-500 m. In the next positive anomalous field, which repla ces the previous one in the north direction, there are two local anomalies. One of them, the upper anomaly, is also located in the third site within the range of 0-500 m. Its inten- sity reaches the value of + 2.48 °C. The se cond one is located within the second site and covers intervals from 1000 m to 2000 m. Its maximum value is +1.87 °C.
A clearly outlined field of negative anoma lous values occupies the central part. On the surface it occupies the sites 4 and 5. Getting down to the bottom it covers the sites 3 and 4 (it tends to incline toward the south). One clearly outlined and rather intense anomaly is noticed here, it's the central part is loca ted within the site 4. Its intensity is -3.35 °C.
A positive field of anomalous values is lo cated in the northern part of the area; it in cludes two local positive anomalies. One of them, having the intensity of + 2.42 °C, is lo cated w ithin site 5 and within the range of depths 500 -1000 m, the second one, the intensity of which varies from +1.58 °C to +1.81 °C, is recorded within the range of depths of 1500-2000 m.
The spatial location of anomalous thermal field within the Saksagan iron-ore strip shows that there are some reasons for its changes in the regional plan, which lead to the gen e ral wave-like nature, where negative values occurred along the strip are replaced by po sitive ones, which, in turn, change to nega tive values.
Local therm al anomalies occur within si milar regional fields. Centers of local anoma lies are noticed both in the upper intervals, within 0-1000 m, and in the lower intervals of 1000-2000 m.
The field of anomalous negative values in the central part of the Saksagan strip ( sec tion 3-5) is seen the most clearly, it tends to increase the intensity of negative values from -0.63 °C within the range of 0-500 m to -3.35 °C within the range of 1500-2000 m. This field also tends to have a steep south ern inclination.
Distribution of temperature in transver se section of the Saksagan iron-ore strip. It is possible to estimate the temperature dis tribution in the transverse latitudinal secti on of the Kryvyi Rih structure within the la titudinal d ep th section going through the Kryvyi Rih super-deep borehole SD-8. U n fortunately, the temperature background ana lysis can only be done within this profile af ter individual boreholes data, and not after those of a group of boreholes, as it was ful filled for the estimation along the iron-ore strip. The section includes the following boreholes from west to east: the super-deep SD-8 and the com plem entary deep borehole-2 (bore hole 22 350) to the super-deep SD-8, and two other deep boreholes (boreholes 19 929 and 17 752).
The average temperature is calculated as the average value of tem perature within the range of 500 m. The resulting value is con sidered to be the middle of the interval. Ac cording to these data, a section was compi led in isotherms (Fig. 4), as well as a secti on of anomalous tem perature values calcu lated according to the previous procedure (Fig. 5). The figures give a general idea of the distribution of actual tem perature and additional anomalous heat sources in the trans verse section of Kryvyi Rih structure.
The first of them (see Fig. 4) clearly shows a decrease in the tem perature in the middle of the section (borehole 19 929). It is the bo rehole 19 929 that is located within the Hdantsivka suite, in the area close to the contact with the Saksagan iron-ore suite in the Sak sagan thrust zone, which is one of the series of tectonic fault layers. It turned up that bo reholes SD-8 and 22 350 in their upper parts, namely to a depth of 1000 m, also encounter the rocks of the Gleyuvatka and the Hdantsevka suites. It means that therm al proper ties of crystalline rocks have a very small in fluence on the nature of temperature changes.
The structural position, the location clos er to the central part of the syncline and the fault zone, outlined by the negative anoma lous field, obviously reveals the greatest in fluence. Fig. 5 shows more precisely the distribu tion of positive and negative additions to re gional tem perature changes.
It should be noted again that the configu ration and the intensity of the therm al ano malies presented in the schem atic figures make it possible to estim ate only the gen e ral features of the therm al field. For further definition and rectification, therm al studies should be continued, they will provide new geophysical information on underground con ditions, which makes the biggest interest for many scientific researchers and for mining operations.
Geothermal gradient. In order to deter mine the geothermal gradient (GTG), we ana lyzed tem perature change data with incre ase in depth for all 46 boreholes studied. As was noted above, the data of the initial me- asurements of temperatures vary considerab ly at similar depths [M echnikov, 2016]. In order to determ ine a constant for the regi on, the m ethod of averaged data w ithin hori zontal intervals was used. 100 m was consi dered to be the main interval; it was avera ged over depth intervals of 500 m. Determi ning local geotherm al gradients in d e p en dence on the geological structure will be considered to be the purpose of further re search. The behavior of the averaged geothermal gradient, determ ined along the Saksagan iron-ore strip, is shown in Fig. 6.
The curve of geotherm al gradient °C / 100 m, shown in Fig. 6 can be divided into two identical parts.
The first, the upper part, which is locat ed within the range of 0-1000 m, has a re latively small deviation from the average va lue of the GTG, determ ined for the full range of depths 0-2000 m (its value is 1.056 °C / 100 m). The values of the GTG here are from 0.79 to 1.15 °C /1 0 0 m, the deviation from the above m entioned average does not ex ceed -0.27 and +0.09 °C /100m . The upper part of the curve, in turn, can also be divi ded into two parts -the first one, from 0 m to the depths of 500 m, is fluctuating, w he re the major deviations of the values of the GTG are seen, and the second one, within the range of depths 500-1000 m, where the gradient values are very close to each other (0.85-1.01 °C /100 m).
The second lower part of the GTG curve (depth intervals of 1000-2000 m) , is cha racterized by a large scatter of the GTG va lues. The biggest differences are noted w i thin the interval 1200-1400 m. The values of the GTG here vary from 0.38 to 1.72 °C / 100 m, the deviation from the average v a lue (1.056 °C/100 m) is -0.68 and + 0.62 °C / 100 m. Lower, starting at a depth of 1400 m, we see the values of GTG to exceed the indi cated average value for the interval of 200 m, except of one of them, which is equal 0.96 °C / 100 m within the range of depths 1700-1800 m.
The averaged data of 500 and 1000 m in tervals shows the main tendencies of the GTG behavior, which is about 1 ° C/100 m in the upper part, and in the lower one it gradual ly increases up to 1.31 °C /100 m.
The highly variable nature of the GTG va lues are indicative of quite active geologi cal processes, especially in the lower part of the studied depth interval.
Discution and conclusion. The results of therm al background studies within the Kryvyi Rih iron-ore basin both along the Sak sagan iron-ore strip, which is the richest in ore deposits, and on a transverse profile, which included the Kryvyi Rih super-deep boreho le (SD-8), have first shown the underground therm al field within the basin to be rather deformed. W ithin the range of depths 0 -2000 m, both regional changes along the Saksagan iron-ore strip and a num ber of local anomalous manifestations of positive and ne gative character were identified.
There is also a deformation of the thermal field across Kryvyi Rih structure. It is no t ed that in the central part (borehole 19 925) there is an intensive negative local anom a ly, which is not related to the change in the therm al properties of rocks, because the ad jacent borehole (22 350) encountered simi lar rocks, but has a completely different ther mal background. The presence of this ano maly may be explained by the existence of a large fault tectonic zone here with p re dom inant movements of the surface cooled waters down. The showing of a negative ano maly that is seen at the depth from the wes tern side is an important feature of this sche matic section (see Fig. 5). Spatially it coinci des with a massif of the Archean granitoids.
This granitoid massif caused the arched vertical bending of the w estern side of the Kryvyi Rih structure, acting as a cold apron [Sheremet, 2011]. The thick ancient metamorphic w eathering crust on the Archaean granitoids, that is crossed twice by Kryvyi Rih SD-8, evidences the fact that this was a very cold contact.
It is difficult to overestimate the knowled ge gained about the intensity and variabili ty of regional and local com ponents of the underground thermal field for solving a num ber of geological, geophysical and mining tasks.
The quality of iron-ore raw materials, which is very dependent on the ratio of unoxidi zed and oxidized iron-ore minerals, is a sig nificant problem in Kryvyi Rih basin. C ur rent oxidation processes are closely related to the oxidative-reducing potential of ground water, the value of which, in turn, depends on the tem perature. Thus, the tem perature factor should be taken into account when conducting geological and geophysical stu dies of the current oxidative processes deve lopm ent.
The determining of the underground tem perature background values along the Saksagan iron-ore strip is also of a great prac tical im portance for mining specialists and mine developers. Data about the actual tem perature values and their changes with the growth of the depths are very important, as operations are already being conducted at depths below 1500 m. In the long run, the depths of the mining operations will increase.
Previous studies discovered quite active neotectonic movements within the Kryvbas and its frames [Mechnikov, 2009;Mechnikov, Volkov, 2013]. There exists a widely de veloped scientific point of view stating the fact that areas of increased neotectonic ac tivity are closely associated with regions of a differential therm al background [Morgan, 1978]. A detailed coverage of neotectonic movements is topical for the Kryvbas, since they lead to local earthquakes of little inten sity so far, together with active deep mining developm ents using blasting. Data of tem pe rature studies can help in solving this prob lem w hen identifying the most activated si tes and zones.

Y. P. M ech n ikov, S. V. T ikh livets, O. G. V olkov, 2019
T he d ata from th o ro u g h ly selected 46 d eep ex ploration bo reh o les drilled along the S aksagan iron-ore stripe lo cated in th e K ryvbas a n d a transverse section of th e stripe, w hich in clu d ed results of studies co n d u c te d in th e Kryvyi Rih su p er-d eep bo reh o le SD-8, have served as th e d ata b a n k for analyzing an d calculating th e tem p eratu re b ac k g ro u n d p aram eters. T he m eth o d ical p ro c ed u re s for ordinary m easu rem en ts averaging, d e term in in g an d generalizing tem p eratu re values an d o th er th erm al b ac k g ro u n d p aram e ters w ere given. The distribution of tem p eratu re alo n g th e strike of th e S aksagan ironore stripe from the south-w est to the n o rth -east w ithin th e d ep th range from 0 m to 2000 m w as shown. T he w hole territo ry of th e S aksagan iron-ore stripe from th e R odina M ine in th e so u th w as divided into 7 sites, app ro x im ately eq u al in area, each of sites c o n tai n ed a t least 4 boreholes. The area-av erag ed an d th e d ep th -a v era g ed d ata w ere calcu la ted for each of th e sites. T he averaging d ep th ra n g e w as 500 m. T he p a tte rn of th e a n o m alous isotherm s has show n th e anom alous field to b e v ery diverse, an d can b e defined as th e altern atio n of neg ativ e an d positive th erm al fields, co m p licated b y local an o m ali es in th e n o rth ern direction. The characteristic of th e geotherm al g ra d ie n t after th e ave rag ed d ata of 46 d ee p bo reh o les drilled alo n g S aksagan iron-ore stripe w as given. The given d ata allow us to state ab o u t hig h variations of GTG values. T he o b tain ed results will b e greatly ap p reciated in in terp re tin g the features of the overall structure of the Kryvbas, especially its w estern side an d th e cen tral p a rt tectonics, b y h y d rogeological and m ining services in view of fu rth e r d e e p e n in g of th e m ines. T he tem p eratu re d ata will h elp in d eterm in in g areas an d zones of n eo tec to n ic an d seism ic activation.
Key words: u n d e rg ro u n d te m p e ra tu re b ac k g ro u n d , g eo th erm al g rad ien t, S aksagan iron-ore stripe. Gordienko, I.V. (2005). Anomalous thermal flow and its geothermal interpretation in the cent ral part of the Ukrainian Shield. Extended ab stract of candidate's thesis. Kiev (in Ukrainian).