RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 11, RE3005, doi:10.2205/2009ES000408, 2010
Power distributions in ore and oil genesis – interpretation and generating mechanismsM. V. Rodkin1, 2, I. A. Zotov3, E. M. Grayeva2, L. M. Labuntsova2, A. R. Shatakhtsyan2 1IITP RAS, Moscow, Russia 2Geophysical Center RAS, Moscow, Russia 3IGEM RAS, Moscow, Russia
As it is well known, the Gutenberg-Richter recurrence law plays a tremendous role in seismology. This law is a first empirical power-series distribution with a heavy tail that has come to prominence. Later on analogical correlations were discovered in many other natural processes. It is assumed that it points out at unbalanced dynamic character of these processes. In particular, it was discovered that power distribution was implemented for values of reserves of large mineral deposits.A power distribution of a number of hydrocarbon deposits for their size value is implemented so well, that it is used for predicting a number of not yet discovered different types of mineral deposits in a certain region [Kontorovich et al., 1985; Burstein, 2006]. In ore geology the character of distribution of deposits' number from the volume of reserves remained unclear [Rundkvist et al., 2006]. There is an evidence of a long-normal distribution of values of concentration of ore components in deposits [Tutcotte et al., 1997]. Study of distribution laws of size values and concentration values is expected to be rather useful (by analogy with seismology) for understanding the processes of ore and oil genesis. Indeed, implementation of power dependence, analogical to the Gutenberg-Richter law, between a number of mineral deposits and deposits' size values would have provided an additional argument for considering the processes of ore and oil genesis as a result of unbalanced systems with developed positive recurrent network, existing in the lithosphere. Regarding the long-normal law of distribution of concentration values, this result can be explained by formation of mineral deposits as a result of a line of consecutive episodes (stages) of concentration of an ore component. Hence the long-normal distribution law emerges as a result of multiplication of independent ore enrichment coefficients at each of these stages. It's worth to be mentioned that the phased character of processes of large deposits' formation was described in the summarizing monograph [Rundkvist et al., 2006] as a characteristic feature of processes of ore genesis. For the statistical analysis of ore deposits we used the data of GIS (GIS KSKM) titled "Largest Deposits" [Largest Mineral Deposits ..., 2006]. The data on various types of ore from a sufficient for our analysis number of largest mineral deposits were statistically evaluated. At representing data on deposits' volumes in linear coordinates the points, related to largest deposits, turned out to be isolated. However at a distribution function' representation in logarithmic coordinates the diagram obtained turned out to be continuous and practically rectilinear in the area of largest deposits, with values of distribution rate β changing for different ore components from 0.7 to 1.4. Such character of diagrams confirms the similarity of processes, leading to formation of various types of deposits, including the largest ones. Realization of power distribution law supposes implementation of rather specific conditions. The works [Rodkin, 2006; Rodkin et al., 2008], related to hydrocarbon deposits, described a model analogical to a great extent to the model, applied for interpretation of a diagram of earthquakes' recurrence [Rodkin, 2001]. Hence a model of power distribution law resulting from a range of incidental by duration processes of filling in potential hydrocarbon traps in quasi-avalanche- like regime of their filling was developed. The possibility of such regime of hydrocarbon deposits' formation was confirmed by the parameters of their modern enrichment. Average rate of this replenishment was proportional to the volume of a given deposit [Muslimov et al., 2004; Rodkin, 2006]. It correlates with development of positive recurrence, essential for realization of power distribution law. In the case of ore deposits the evidence of their modern replenishment are also available [Krasnyi, 2008]. However, in general, the mechanism of rapid (in geological sense) and avalanche-like formation of ore deposits isn't justified; in any case, such a mechanism can hardly be considered typical. For the purpose of interpretation of the obtained power distribution another model was suggested. In this model a power distribution of ore deposits was formed on the basis of the log-normal distribution law, supplemented by a relatively weak chain of positive recurrence.
The figure shows the values of difference of average concentration of various ore components in upper and lower crust reservoirs (according to the data [Taylor and McLennan, 1988]) compared to characteristic values of different kinds of large ore deposits ([Rundkvist et al., 2006], V.1, Table 1.2). A close correlation of these values was obvious, more close than at comparing the volumes of deposits with concentrations of corresponding components in upper or lower crust, taken separately. Hence it can be supposed that the process of large ore deposits' formation could be a product of transformation of large volumes of the Earth's crust from one crust (or mantle) reservoir into another. The physical mechanism of separation and concentration of ore components could be the process of separation and outcrop of relatively less compatible components in the processes of metamorphic transformations and melting of rock [Frank et al, 1989; Urusov et al., 1997]. The model of formation of deposits as a by-product of transformation of lithospheric blocks with arbitrary values of blocks' sizes gives birth to a distribution law of deposits' values, close to a long-normal one. A transformation of this distribution law into an empirically observed power distribution law assumes also the existence of a positive recurrence, ensuring the primary accumulation of raw material in deposits, largest by their volumes. Such mechanism could be the outcome of correlation of linear volumes of transformed parts of the Earth's crust. In application to a private case of transformation of a part of the upper crust in the pace of development of deep thrusts such correlation answers the famous empirical law, according to which powerful thrust zones are characterized by averagely large amplitudes of shifting movements. Another possible mechanism of positive recurrence could be realized in the model of ore deposits' formation by flows of transmagmatic fluids, when an ore-bearing fluid, spreading over a slowly hardening magma, heats it therefore creating prerequisites for improving transportation qualities of the fluid-spreading channel. Such mechanism of deposits' formation was convincingly confirmed by the data of detailed study of the unique Norilsk cluster of ore-bearing deposits. The suggested model clarifies both the possible mechanism of realization of empirically observed power distribution law of a number of ore deposits from sizes of their volumes and the nature of exposed close correlation between volumes of different components of and the difference of concentrations of the corresponding component in reservoirs of the upper and lower crust.
Received 26 November 2009; accepted 11 December 2009; published 22 January 2010.
Keywords: size of ore deposits, power distribution, processes of formation of ore deposits
Citation: (2010), Power distributions in ore and oil genesis – interpretation and generating mechanisms, Russ. J. Earth Sci., 11, RE3005, doi:10.2205/2009ES000408.Copyright 2010 by the Russian Journal of Earth Sciences