GEOLOGICAL-MINERALOGICAL MODELS IN CONNECTION WITH PROSPECTING OF DIAMONDIFEROUS KIMBERLITES

Abstract

The conducted research of the geological structure and material composition of kimberlites of the Siberian platform demonstrated the complexity and diversity of geological-mineralogical and paleogeographic environments that should be considered during the organization of forecasting and prospecting works for diamonds in each specific region. We have created a generalized model of the diamond-bearing pipe of the Yakut kimberlite province; it traced the nature of the conjugation system of bodies “main pipe–satellite–incurrent and pre-pipe dykes”. The host rocks (at the explored depth of the pipes) are terrigenous-carbonate formations of the Lower–Upper Cambrian and Lower Silurian – limestones, dolomites, marls, aleurolites and intermediate species of these rocks. Layers and lenses of rock salt, and interlayers and lenses of gypsum and anhydrite are significantly developed among the host rocks in the southern part of the Siberian platform. Some kimberlite bodies are covered with sedimentary strata of J1, the present capacity of which varies from a few meters to 20 m, and some – with ter-rigenous rocks of P–C (up to 100 m and more). Upper Palaeozoic complex of sedimentary and volcanogenic-sedimentary rocks is intruded by numerous bed and cross-cutting trap bodies. Such parts one can distinguish in diatreme kimberlite structures (top to bottom): a bell, which in non-eroded apparatus crowns a ring bank; volcanic (vertical) channel; the root part – incurrent channel in the form of the dike body. Each of these parts of the pipe is composed of rocks with certain mineralogical and textural-structural features. Certain regularities in the change of rocks create a unique vertical zoning of primary diamond deposits at each of the ancient platform. The kimberlite xenoliths from the Siberian platform are divided into four groups: 1) sedimen-tary rocks of the platform cover, which are host for diatremes; 2) igneous rocks – traps, intruded the sedimentary cover before the formation of diatremes; 3) metamorphic rocks that form the platform basement; 4) hypogene rocks, mainly of mantle origin. The content of indicator minerals-satellites of diamond – pyrope, picroilmenite and chrome spinel – on deep levels is higher than in the upper parts of the section, although in different pipes it differs. The degree of secondary minerals development in general, and carbonatization of kim-berlites, in particular, greatly decreases with the depth. The composition and nature of the host rocks are of great importance for the direction and intensity of secondary mineralization devel-opment in kimberlites. Prospecting of kimberlitic diatremes is the difficult task in the case of rock disintegration products absence, which reduces the effectiveness of heavy-concentrate-mineralogical method of prospecting. The effectiveness of geophysical methods of prospecting is significantly reduced due to the low magnetization of the rocks composing the diatreme, and because of their overlapping by thick series of magmatic or sedimentary rocks. The reliable criterion for finding such dia-tremes are structural-textural characteristics and material composition of host substrate rocks, of diatremes and overlying sediments, especially in the case of diamondiferous placers formation. Close attention should be paid to the diagnosis and definition of typomorphic features of primary and secondary kimberlite minerals, especially considering the fact that the main part of the sec-ondary minerals is unstable during transport in the aquatic environment, however, even in case of substantial changes it points the close distance of migration from erosion areas. The most important prospecting information one can obtain by the study of typomorphic fea-tures of diamonds themselves and their paragenetic associations, characteristic for specific kim-berlite fields and diatremes. Usually the distribution of certain mineral associations in ancient and modern sedimentary formations is confined within specific diamondiferous areas. Analyzing the distribution of typomorphic features of minerals and their paragenetic associations in the area and in the section, you can define the sources of their removal in mixed-age Upper Palaeozoic and Mesozoic sediments. Local prediction of the diamond primary sources is possible due to larger-scale studies using electronic databases. It is necessary to pay attention to the possibility of the ancient crusts of weathering development in the upper horizons of the diatremes, because they significantly alter the petrophysical properties of source rocks, and this greatly hampers their prospecting with the use of geophysical methods.