Searches for diamond-bearing rocks which are not associated with kimberlites in the Nuratau mountains, Uzbekistan

Dadaev D.M.

Institute of Geology and Geophysics of Academy of Sciences of Republic of Uzbekistan, Uzbekistan

Dimman82@mail.ru

Up to middle seventieth years of the last century searches of diamondlike bed-rocks were limited, according to «rule of Clifford», in central parts of precambrian cratons . But after the diamonds which have been not connected with kimberlites have been discovered in the folded Paleozoic areas in different regions - in the peridotites of Armenia and the Koryak highlands, in the olivine basalts of Kamchatka, in the kamptonites of Middle Timan; in alkali olivine basalts in Kasteksky range of Kazakhstan, in the metamorphic rocks of the North Kazakhstan, the region of search of diamond bed-rocks has been expanded.

At present the facts of the presence of accessory diamond in the rocks of alkaline-ultramafic and alkaline basaltic composition [1]. By petrographic composition they are lamprophyres (kamptonites) alneites, limburgites, picrites and picritic porphyries. Morphology of the body is close to the kimberlite - the pipe explosion, dikes, vents, stocks, sills. The origin of the diamond due to similar reasons for kimberlites - the birth of mantle conditions at high temperatures and pressures, followed by the removal of xenogenic. These ideas, along with data on direct findings of diamond in Hissar Ridge, served as a basis for raising the subject in 1987, works to identify diamondiferous volcanic pipes and dikes of alkaline basalts and ultramafic of Hissar range and Nuratau mountains. They culminated in 1990, signs of a potential development of a set of diamond-bearing basalts of the Southern Tien Shan and the establishment of small diamonds in several dikes and explosion pipes, as well as individual bodies of picrites of Asmansay complex in North Nuratau. At subsequent stages of search operations and specialized mapping primary concentrate mineralogical searches in Hissar region, Bukantau, North and South Nuratau have been obtained about the presence of small diamonds in the picrites related to subvolcanic formations of Tubabergen Formation (Bukantau) and installed a group of potentially diamond-bearing volcanic pipes alkali basalts and dikes (Bukantau, Hissar, North and South Nuratau) and Chagatai carbonatite complex (South Nuratau). In South Nuratau in the basin Aktepasay in pipes and dikes of the explosion-trachytes of Chagatai carbonatite complex were found small diamonds.

Despite the establishment of diamonds in loose deposits and favorable geological and tectonic conditions, the search of geological routes can not always identify the location of new volcanic pipes. Therefore, in search of potentially diamond-bearing formations, delineate them in the plan, as well as determining the depth of occurrence of elements using geophysical methods of searches in the magnetic, gravimetric and electrical prospecting. None of these methods, taken separately, can uniquely solve the search problems. Magnetic prospecting is used to detect magnetic anomalies and direct detection of volcanic pipes. Methods of electrical and gravimetric included in geophysical complex, are additional and used to determine the nature of magnetic anomalies, contour tracing of tubes with their root parts, localization of the tubes under the cover of sediments.

In Uzbekistan, the effectiveness of geophysical methods in the magnetic prospecting, gravimetric and electrical prospecting in searching and detailed evaluation of primary diamond deposits not associated with kimberlites, was established by geophysical surveys conducted in the mountains Bukantau and Nuratau [2].

Approbation of a rational complex of micro-geophysical researches on diatreme Tusun was carried out in scale 1:1000 - with use of the combined survey of magnetic field and its gradient, gravity surveys and electrical profiling.

The pipe of explosion Tusun is located on the right side of the same named ravine and is limited by fault zone Darinsay, South Nurata. Diatreme is elongated in northwest direction, the size of 350 × 20-70 m. It break through bearing sandstone-shale thickness of Naukatsay suite (S₁v nkt), often crumpling, turning it into the fold and mylonitized. Dip of contacts of pipe is steep (80-90^o).

On section Tusun the combined survey of a magnetic field and scale gradients 1:1000 is executed on a grid 10×5m. The magnetic susceptibility potentially of diamondlike rocks composing pipe Tusun, changes in a wide range from 43 to 8151×10^-5 a unit of SI-system depending on a petrographic types, mineralogical structure, degree of supergene and hydrothermal-metasomatic alterations and is caused primarily by the presence of magnetite and its contents. The magnetic susceptibility of olivine-pyroxene trachybasalts depending on the content of magnetite varies between 5499-8151 × 10^-5 units of SI-systems lava-breccia - 1540-3910 × 10^-5 units of SI-system tuff-breccias - 220-502 × 10^-5 units of SI-systems. Strongly changed differences are characterised by low values of a magnetic susceptibility 43×10^-5 a unit of SI. Considerable distinction on magnetic properties diamondlike ores and containing rocks creates favorable conditions for effective application magnetic exploration for their searches, and the differentiation of pipe rocks on magnetic properties depending on a petrographic varieties allows to use data magnetic exploration for lithological differentiation of pipes of a difficult structure and to predict material structure of the buried.

The presence of rocks with a reverse magnetization characteristic of the volcanic pipes (for example, kimberlite pipes of Yakutia, diatreme of Uchkuduk in the mountains Bukantau). For pipes of Yakutia revealed that brecciated kimberlites are magnetized on the current field and, accordingly, are marked by positive anomalies. Magmatic kimberlite possess a vector of natural residual magnetisation opposite to a modern field and, accordingly, are marked by negative anomalies of the return magnetized. By what or communications between diamondlikes and magnetisation kimberlite it is not established [Savrasov, 1962].

The reasons for the reverse magnetization of rocks are interpreted by different researchers in different ways. One is associated with a reverse magnetization self-inverse of vector of natural remanent magnetization, the other with the paleomagnetic age of rock formation, when earth's magnetic field was opposite in sign to the modern, and third with magnetic reversal of taking root magnetic rocks by a field of containing magnetic rocks.

Hence, according to magnetic prospecting can be concluded that the rods trachybasalts, notes the positive anomalies are brecciform type with a small number of fragments of host rocks, and basalts, fixed negative anomalies - magmatic.

Over the diatreme recorded a positive magnetic anomaly north-trending linear morphology. From the north-eastern flank of the anomalies observed zone of negative values, of the magnetic field due to an oblique magnetization. Transverse dimensions of the anomalies of 20m in the north-west to the 70-100m to the southwest along the strike of 400 m. According to the type curve anomaly is identical vertical component of magnetic field Z on the vertical seam endless stretch to a depth of oblique magnetization. The morphology of the anomaly in terms of overall linear. In the south-eastern circuit anomaly curve is replaced by the Z-type curve of the horizontal component of the magnetic field H is isometric morphology, which is, perhaps, with a change in trend on the tube or with a sub-latitudinal self-inverse natural remanent magnetization vector, which in principle is characteristic of igneous rocks, or the location tube with respect to forms of relief.

The overall contour of the magnetic anomaly is much broader magnetized body diatreme. In this connection, the boundaries of the pipe on a map of the magnetic field of Ta is determined not confidently with a large error. Causes difficulties and oblique magnetization, in which changes frequently and type curves, respectively, the principles of mapping. In general it may be noted that geologic contour of the pipe covers the epicentral magnetic anomalies of Ta.

On the map of local anomalies of the magnetic field boundaries are defined by the pipe with more confidence, in connection with the suppression of the slowly decaying component of the magnetic field. But in this case, as with any transformation, there are fictitious local anomalies related to the mathematical properties of the transformation is not related to the geological structure and distort the appearance of abnormal curves. Geological outline tube covers virtually all local anomalies Ta, both positive and negative. Error in determining the contour of the tube (with the inclusion of local anomalies of both signs) on the map of local anomalies of ± 10-15 m, depending on the parameters of the applied transformation. For other options, the transformation error can reach up to 20m or more.

References:

1. Geology and genesis of diamond deposits. TSNIGRI Moscow, Vol. 1, 1989. 242 p., Vol. 2, 1989. 424 p.

2. Golovko A. Characteristics of diamonds and pipes of the mountains Bukantau / / Proceedings of the Conference on "The igneous, metasomatic formations and associated mineralization", Tashkent, 2005, 76-79p.

3. Divaev F.K, Shumilova TG, Yushkin NP etc. The first discovery of diamonds in the shonkinite-porphyry North Tamdytau (Central Kyzylkum, West Uzbekistan) / / DAN, 2009, Volume 424, № 6,. 785-787.

4. Divaev F.K, Chagatai trachyte-carbonatite complex South Nuratau. Abstract diss.kand.g.m.n.. Tashkent, WRI, 2000, 25 p.