RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES6002, doi:10.2205/2007ES000285, 2008

6. Kimberlites

[20]  The kimberlitic section of the information database dealt with comparative analysis of spatio-temporal laws of development of carbonatites and kimberlites of the world (age analysis of scale and productivity of magmatism, regional-tectonic laws of localization of fields and massifs, structural-morphological parameters of massifs). Thus a statistical analysis was carried out in order determine coefficients of pair and partial correlation and factor analysis. Correlations for average values of tectonic and magmatic characteristics of kimberlitic fields were also established. Average ratios of a number of diamondiferous kimberlites to their total number in separate fields is directly linked to a cumulative index of a number of systems of controlling deep faults. This criterion has the significant partial correlation coefficient (0.48), obtained by excluding the influence of other interrelated factors. Moreover, a significant reverse correlation is mentioned for diamondiferous rocks with the manifestation rate in the regions of rocks, affiliated to kimberlites, and carbonatites (-0.45). The partial coefficient of correlation in this case is equal to -0.6. It's worth to be mentioned that the share of diamondiferous massifs in the fields is closely linked to the age of kimberlitic magmatism (0.53).

[21]  A positive correlation of diamondiferous parameters of kimberlites with their age, cumulative index of crustal faults, a number of a massif's phases, i.e. the life-span of their formation, fraction of autolithic breccias (reversely connected to a fraction of porphyric facies), fraction of octahedral forms and achromatic differences of diamonds was established. It is well known that kimberlitic massifs of Canada and the Republic of South Africa are confined to the zones of deep lineaments of predominantly submeridional orientation, traceable by gravity-magnetic anomalies, dike zones and linear morphological relief structures. Thus, the fraction of rhombic-dodecahedron, cubic and corroded crystals is much more directly linked (coefficient of correlation 0.4-0.5) to separate systems of lineaments and their intersection, than the diamondiferous fraction as a whole or the content of octahedrons. The last-mentioned, in their turn, correlate directly with a number of systems of crustal disturbances. Kimberlitic hearths are more exposed due to the presence of intersecting lineament structures. It entailed the increase of oxygen potential and the rapid drop of temperatures at the stage of formation of the diamond content in the mantle. At the same time the presence of differently oriented crustal faults, controlling a massif, could have led to a faster incorporation of massifs. It preserved the primary formations and ensured the high diamondiferous characteristics of kimberlites as a whole.

2007ES000285-fig12
Figure 12
2007ES000285-fig13
Figure 13
2007ES000285-fig14
Figure 14
[22]  Additionally, same as for carbonatites, a statistical analysis of the scale of manifestation of kimberlitic magmatism in the leading provinces was carried out (Figure 12), and also according to geological periods. The analysis also included the areas of outcrop of the massifs (Figure 13). These data have shown that in the geological history the manifestations of kimberlitic magmatism occurred unevenly. The most active manifestations took place in the Middle Proterozoic, Ordovician, Carbon and most of all in the Cretaceous period. The highest diamond content in kimberlites also was determined for those periods. It is also noteworthy that the combined analysis of these data with the results of the age evolution of carbonatites (that was mentioned above), confirmed the earlier discovered tendency of kimberlites being late in comparison to carbonatites [Belov et al., 1999]. As for carbonatites, the rate of kimberlitic magmatism was escalating in time, characterized by a number of magmatic outbursts during a fixed period. Over the last 100 million years 10 of such outbursts were detected, in the preceding 100 million years there were 5 such outbursts, and in the earlier epochs the number of them went down to 3-4 (Figure 14).

2007ES000285-fig15
Figure 15
[23]  As a result of the statistical processing of the entire database it was concluded that significant criteria predicting diamondiferous qualities of massifs could be reliably revealed for the separate provinces and regions, taking into account their specific geological parameters. Hence, the Yakut province serving as an example, the similar dependencies at a significant level of correlations were established. Therefore the parameters of tectonic-magmatic control of the regions (sub-provinces and fields) could be determined. Their linkage to the zones of intersections of deep faults of different orientation, the presence of a small fraction of rocks, related to kimberlites, and carbonatites, and also the Archaean age of cratons, confirming the Clifford rule, were established. However, the correlation analysis failed to reveal this rule due to the absence of mutability of this parameter. A less informative criterion of the diamond content of fields is their age. It was shown by the abovementioned analysis of histograms in the section, devoted to the age evolution of kimberlitic magmatism. The rule of temporal lagging of kimberlites behind carbonatites appears to be the general tendency (Figure 15). At that in many provinces and regions the maxima of manifestation of productive kimberlitic magmatism differ considerably in age.

[24]  We suppose that for the sake of a real predicting evaluation it would be necessary to apply a wide range of examined cumulative parameters, reliably connected to the productivity of massifs. Applying the factorial analysis in the variant, using semi-quantitative data (maximum likelihood factors), we need to assess the contribution of parameters into the change of the evaluated characteristics (the diamond content). One of the main factors of diamondiferous qualities related to the factor loadings values is the intersection node of deep faults and the crust type. The prognostic productivity of kimberlites ( Fp ) was calculated by the regression equation, well-known in the factorial analysis:

Fp = Cm + St cdot (F1 cdot Sc1 + ldots Fncdot ScN),

where Cm is the average value of diamond content of the massifs in a sampling, St is its standard deviation.

[25]  A certain shortcoming of the selected criteria is their insignificant summarized contribution into the mutability of the evaluated parameter - about 40%. The comparison of the obtained evaluations of diamond concentration of 57 massifs with the real ones (evaluated by computer) has shown satisfactory results. The coefficient of correlation was 0.57. On excluding separate parameters with relatively small factor loadings this coefficient was equal to 0.52. Thus we could come to a conclusion that factorial analysis could be applied for evaluating the diamond content of massifs, even belonging to different provinces and regions, on the basis of structural-tectonic data.

[26]  In the framework of solving this complicated prognostic information task a comparative analysis of the tectonic position data and the structure of kimberlites and carbonatite complexes was carried out, which shown the number of ore-bearing ultrabasic alkaline complexes with carbonatites, diamondiferous and non-diamondiferous kimberlites (and lamproites) and related rocks (associated in the same fields). Some of the positions show an acute difference between the two formations. First of all, it's the localization in the regions with different type of the basement. For kimberlites such comparison was made only in relation to fields. Within the borders of the Archaean cratons an absolute majority of the fields with diamondiferous massifs was localized. The reverse correlation was observed for the carbonatite complexes. The Anabarsky shield can serve as an example of the revealed tendency. The data analysis by V. V. Kovalsky and the others [Kovalsky et al., 1974] has established that moving in the north-eastern direction kimberlites tend to be replaced by carbonatites. The first ones are mainly concentrated within the Archaean basement, the second - in the area of Proterozoic development. Further east lies the Tomtorsky massif, confined to the zone of Udzhinsky paleorift, superimposed on the Proterozoic basement. The analogical pattern is noticeable in the position of the both formations related to riftogenic structures. Carbonatites tend to lie close to rift structures. Kimberlites and their fields (first of all diamondiferous) lie at a distant from them. According to the data of E. D. Cherny and the others, within the borders of the Chadobetsky uplift, and also at a part of the Maimecha-Kotuiskaya province (the region of massif Dalbykha), located in the area of the Taimyr-Angarsky aulocogen, there is a large number of kimberlitic bodies, a part of them is diamondiferous. These kimberlites are often associated with related rocks. In the Kola peninsula in close vicinity (at a distance of several kilometers) lie the Yermakovsky field of poor diamondiferous kimberlites and related rocks and the carbonatite complex Tsentralny at the edge of the Kandalaksha graben. Moving away from the rift the number of rock related to kimberlites decreases in their fields or in adjacent territories. Thus in the south-eastern field of sub-province Slave in Canada single carbonatite dykes were established. The specific index of the areal distribution of rocks related to kimberlites is understated, because for the majority of fields an exact number of massifs of alpicrites, kimpicrites and carbonatites cannot be established.

[27]  At the same time a similarity was revealed between kimberlites and carbonatites. This is a predominant control of massifs and fields by submeridional and to a less extent by north-western and north-eastern lineaments. A part of massifs and fields is controlled by two and even several lineaments. Therefore a summarized number of massifs, mentioned in the corresponding table columns, often exceeds their total number. A prevailing number of massifs of carbonatites and kimberlites is controlled by intersection nodes of crustal faults. For kimberlitic massifs there is no sufficient information related to this criterion, so the number of massifs, controlled by a three-directional intersection node is obviously understated. According to the data obtained by V. A. Milashev and Yu. V. Tretyakov [Milashev and Tretyakova, 2003; Vasilenko et al., 1997], the most productive massifs of Yakutia are localized in zones of triple junction of linear anomalies, less productive in the zones of conjugated two-directional anomalies. Anomalies are established by data of aerial photography. It confirms the information content of tectonic criteria. The majority of massifs of carbonatites are of a circular shape, that is close to the morphology of the major part of diamondiferous kimberlites, with isometric index of outcrop shape more than 0.5.

[28]  It has to be mentioned that the data analysis in the database, has confirmed the Clifford rule related to industrially diamondiferous kimberlites. Regarding lamproits, komatiits, alkaline basaltoids, not submitting to Clifford rule, they practically don't have any industrial diamondiferous deposits. Note that even the famous Argyle mine deposite related to lamproites, is located near the Archeaen craton. The statistical analysis has also shown that large submeridional and to the less extent north-eastern lineaments play a considerable part in forming highly diamondiferous kimberlites. During several tectonic-magmatic cycles they could have served as suppliers of deep energy and fluids. Regrettably, these structures are poorly represented in the upper crust and it's difficult to detect them by the traditional geological-geophysical methods.


RJES

Citation: Belov, S. V., E. O. Kedrov, A. A. Burmistrov, and A. A. Soloviev (2008), Analysis of evolution and ore-bearing factors of rare-metal carbonatites and diamondiferous kimberlites, Russ. J. Earth Sci., 10, ES6002, doi:10.2205/2007ES000285.

Copyright 2008 by the Russian Journal of Earth Sciences

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