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

[e-mail]

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.

Comparison of the epithermal gold deposits in the Mesozoic tectonic and magmatic activation zones of Transbaikalia and North Caucasus (as example Baley and Radujnoe deposits)

Kaigorodova E.N.

The Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry RAS, Moscow, Russia

[e-mail]

Baley-type epithermal deposits are known in all fold belts, which were subjected to activation. In 1976, a similar Baley-type object was discovered in the North Caucasus at Hulam-Bezengi Gorge in Kabardino-Balkaria. Au-Ag deposit Radujnoe is localized in the North Jurassic depression related to the Mesozoic tectonic-magmatic activation (TMA) and processes of riftogenesis. A number of autors suppose that there is a direct relationship between the riftogenesis and formation of Au-ore deposits. For example, the Baley deposit is localized in Undine-Dain depression. However, scale of expression of TMA in the North Caucasus and Trans-Baikal is differed. In the North Caucasus TMA has led to the formation of Chegem-Uruh volcano-plutonic belt, extending through the mountainous part of the republics of Kabardino-Balkaria and North Ossetia, while in the Transbaikal TMA covered huge territory (virtually all of Transbaikalia, North-East Mongolia and North-West China). Therefore, the scale of the Mesozoic productive mineralization in the Caucasus is much more less.

North Jurassic depression was formed on the basement of PR-PZ rocks. In the Radujnoe deposit area the basement is composed of Paleozoic granites (belorechensk granites). Jurassic sedimentary rocks are presented by sandstones, siltstones and mudstones. Here is widely distributed the formations of volcanic necks, extrusive domes, lava flows (two phase of volcanism - consistently differentiated basalt-rhyolite formation and contrast-differentiated trachybasalt-trachyrhyolite one). Baley-type does not characterized by such intense development of volcanic processes. In this respect the deposit Radujnoe is similar to the Au-Ag deposits of northeastern Russia. It should be also noted that in the Baley ore field are marked deep facies of igneous rocks associated with later mezozoic TMA (micro/middle-grainy leucocratic granites). At the Radujnoe deposit area were described only the volcanic facies. There are widespread dissemination of the injection-explosive breccias with quartz cement (chalcedony often).

We note other classification aspects of Radujnoe field to Baley-type: the role of intersection of steeply dipping faults in the formation of ore pillars; near-surface typical mineralization is disseminated (epithermal typomorphic minerals) - calcite, adularia, clay minerals; intensive argillization of rocks of the crystallic basement; the intensive silicification of host sedimentary rocks until to form monomineralic quartz metasomatic rocks; Au is in native form and dispersed in sulfides. However, there are features that distinguish Radujnoe from Baley types: quartz-sulfide thin veins form a network of "injection", their thickness is less than 5 cm; in the apical parts of the volcanic bodies barite and gypsum are developed; Au-Ag mineralization is combined spatially with silver-barite-polymetallic mineralization, which is also associated with volcanic processes and is localized in the sedimentary rocks, forming stratum body. We can concluded that the Radujnoe deposit represents the first stage of Baley-type mineralization - adularia-carbonate-quartz with Au (second stage of Au-pyrargyrite-miargyrite-carbonate-quartz-mineralization is not developed at the Radujnoe deposit) with the imposition of the silver-barite-polymetallic mineralization Sadon-type.

References:

1. Krivosheev V.G. (2006) Score prospects of Au-Ag ore-mineralization and associated barite-polymetallic mineralization in Chegem Uruh-volcanic-plutonic belt. Nalchik.

2. Spiridonov A.M., Zorina L.D., Kitaev N.A. (2006) Gold-bearing ore-magmatic system of Transbaikalia, Novosibirsk Academic Publishing House "Geo".

The position of the antimony mineralization in the ore-forming process at the Suzdal gold deposit (Eastern Kazakhstan)

Kolesnikova M.K.

V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia

[e-mail]

The problem of the position of antimony mineralization in the ore-forming process is actual for many gold deposits occurring in the black shale sequences of orogenic belts. This mineralization is gold-bearing or superimposed on the gold-bearing ore with redistributing of gold and forming complex mineral assemblages. Antimony mineralization is typical for gold deposits of Yenisei Ridge, Verkhoyansk-Kolyma gold-bearing province [1,2]. It is also occurred on a number of gold deposits of the Western Kalba gold-bearing belt in Eastern Kazakhstan. Antimony mineralization is superimposed on the early productive gold-sulfide ores (Suzdal, Bakyrchik, Bolshevik), gold-bearing granites (Zherek) and metabasalts (Zanan, Alimbet) [3]. The relationship of antimony mineralization with gold sulfide ores is studied on example of Suzdal deposit.

The Suzdal deposit is situated in Semipalatinsk region of Eastern Kazakhstan in the northwestern part of the Western Kalba gold belt. It lies in the southern periphery of the Semeitau volcano–plutonic complex and is located at the juncture of the NW-trending Char-Gornostai-Zimunai and NE-trending Suzdal fault zones. The set of basic and acidic dikes, small intrusive stock-like bodies of monzonite-porphyry and granosyenite-porphyry is widely occurred in the Suzdal deposit. The dikes of diabase are most common. Suzdal deposit refers to the type of sedement-hosted gold deposits in the mineralized zones, and hosted in the carbonaceous limestone and terridgenous sequences of the Early Carboniferous age. The content of sulfides range from 0.5 to 10-15%. The mineralization is represented by disseminated, nest-veinlet and stratiform mineralization. The gold-sulphide mineralization is accompanied by low-temperature processes of silicification, carbonation, sericitization and chloritization. The ore bodies are distinguished mostly by the results of assay. The gold content in ores ranges from 1.5 to 106 ppm (average 6.4-16.2 ppm). The process of gold mineralization at the Suzdal deposit had a multistage and polygenic character. On the deposit are the following stages of ore deposition: 1. Syngenetic pyrite mineralization with low gold content in the turbiditic carbonaceous sandstones and siltstones; 2. Early highly productive gold-bearing pyrite-arsenopyrite finely disseminated mineralization with "invisible" gold in sericitized rocks; 3. Late productive gold-polysulfide mineralization with native gold in zones of brecciation and silicification of rocks; 4. Veins of quartz-carbonate-stibnite composition. The main stages 2 and 3 according to ⁴⁰Ar/³⁹Ar dating (sericite from the ore association) has age 281.9 ± 3.3 Ma and 248.3 ± 3.4 Ma. The main ore minerals of those stages are gold, arsenopyrite, pyrite and pyrrhotite [4].

Fig. a. Radial stibnite in the nest with the aggregate of quartz. SEM image. b. Association of Ag-containing gold (1), ulmanite (2), arsenopyrite (3), calcite (5) with micaceous aggregate (4). SEM image. c. Replacement of pyrrhotite (1) by stibnite (2). SEM image. d. Berthierite (2) develops along the boundary between the crystals of pyrite (3) and arsenopyrite (1). SEM back-scattered image.

Quartz-carbonate-antimony mineralization is wide spread at the Suzdal deposit, and occurs both in the near-surface and at deeper horizons (500-600m). It is occurred as nests and veinlets with thickness up to several centimeters, and is superimposed on all types of previously mineralized rocks and consists mainly of stibnite, native antimony and rare cinnabar. The gangue minerals are quartz, carbonate and sericite in the selvages of veins. Stibnite is the most abundant mineral. It forms a thin veinlets, spotted or in the form of radiating discharges in quartz-carbonate clods (Fig. a). It occurs in close association with berthierite and native antimony. Berthierite forms tabular crystals (Fig. c). Native antimony is represented by small roundly inclusions in carbonate or stibnite. Stibnite and berthierite often cement and substitute crushed crystals of arsenopyrite and pyrite (Fig. c, d). There are cases of replacement of pyrrhotite by stibnite. As a result superimposed on the antimony mineralization on the gold-sulfide ores formed association of gold with aurostibite, berthierite, villiaumite, ulmanite, breithauptite (Fig. b). A later generation of arsenopyrite is characterized by the presence of antimony from 0.2 to 1.8 wt.%.

According to fluid inclusions study the values of homogenization temperature of the gas-liquid inclusions in quartz and carbonate from this stage are within the range of 230-290º C. Age of antimony mineralization according to the analysis of sericite by ⁴⁰Ar/³⁹Ar method is 241.9 ± 2.7 Ma. The age gap between antimony mineralization and the late gold-polymetallic association is of about 6 million years.

Quartz-carbonate-stibnite mineralization probably ends the process of mineralization and superimposed on the early gold-bearing ore. Sb mineralization is not a gold-bearing, but it regenerates and redistributes gold of earlier stages.

References:

1. Bortnikov N.S., Gamyanin G.N., Vikenteva O.V., Prokof'ev V.Y., Prokopiev A. V. Sarıl and Sentachan gold-antimony deposits (Sakha-Yakutia): Example of combining mesothermal gold-quartz and epithermal antimonite ore / / Geology of Ore Deposits, 2010, V.52, № 5. P. 381-417. (Published in Russian).

2. Genkin F.D., Lopatin V.A., Savel'ev R.A. and other. Gold ore of the Olympic deposits (Yenisei Ridge, Siberia) // Geology of ore deposits. 1994. V.36. № 2. P. 111-136. (Published in Russian).

3. Metallogeny of Kazakhstan. The ore formation. Deposits of gold / Red. college: Abdulin A.A., Kayupov A.K. etc. - Alma-Ata, "Science" of the Kazakh SSR, 1980. 224 p. (Published in Kazakhstan).

4. Kovalev K.R., Kalinin Y.A., Naumov E.A., Pirajno F., Borisenco A.S. A mineralogical study of the Suzdal sediment-hosted gold deposit, Eastern Kazakhstan: Implication for ore genesis // Ore Geology Reviews. 2009. V. 35. #2 P.185-205.

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