From North-east of Siberian plate

Yakovlev V.G., Pavlushin A.D.

Diamond and Precious Metal Geology Institute SB RAS, Yakutsk, Russia

[e-mail]

At present, the problem of the nature of native sources of alluvial diamonds, common in the north-eastern part of the Yakut diamond-bearing province, is still actual. On the territory of nijnelensk district a new industrial type of diamond source presented with volcano-sedimentary rocks of Upper Triassic Carnic stage – tuffites, recently has been discovered [1].

The aim of this study is to determine the characteristic structural and impurity defects in diamond crystals selected from the tuffites of the Lensky region by the method of IR-spectroscopy (A collection of diamonds has been provided by "Nizhne-Lenskoye" public corporation). Diamonds are geo-referenced to the stretched outputs of diamond deposits of Carnia located on the right and left banks of the Lena River, in a region of Bulkur anticline.

The studies were conducted on the infrared Fourier spectrometer "FT-801" in the shooting mode - absorption in the frequency range 450 - 3900 cm^-1. The characteristics of the IR-spectra of diamond crystals of I, III and V of mineralogical varieties according to Yu.L. Orlov have been obtained.

All systems of absorption bands have general peaks at wavelengths 1100, 1215, 1282 cm^-1 corresponding to defect A, have same peaks at wavelengths 1010, 1100, 1175, 1332 cm^-1 corresponding to defect B₁ and a general single peak at 1375 cm^-1which is responsible for defect C. Enumerated defects are related with nitrogen in various coordination positions in a diamond structure. Most of spectra include one peak at 3107cm^-1 and 1405cm^{-1, in} few cases, they accompanying with satellite at 3235cm^-1. Their presence is interlinked with fluctuations of C-H groups in the structure. In most cases, spectra peaks at wavelengths 1525 and 1550cm^-1 are also fixed. They haven’t visible associations with absorption bands related with A, B₁ - defects.

At single instances, peaks at 1451cm^-1 and 2785cm^{-1 in} the frequency range of spectra are detected.

On the basis of individual characteristics of the spectra two groups of diamonds have been revealed.

For the first group of diamonds, besides absorption bands corresponding to A, B₁ defects, presence of couple of peaks at wavelengths 1525 and 1550 cm^-1 are typical. According (V.G Vince, A.P. Eliseev) these two peaks are fixing B₂ – defect. In some cases, a insignificant offset to 1-1,5 cm^-1 from mean value in the sides of higher or low-frequency range of a spectrum may occur. This set of defects characterizes I and V mineralogical varieties of researched diamonds.

The second group of diamonds yields spectrum lines at frequencies 1100, 1215, 1282 cm^-1 and corresponds to defect A, lines at 1010, 1100, 1175, 1332 cm^-1 corresponding to defect B_1. Other peaks characterizing defects-impurities in this group of diamonds were not found. The defect’s set of this group determine I, III and V diamond varieties.

Thus, each diamond group is characterized with specific set of defects in the structure and individual morphologic characteristics. It gives some reason to identify two genetically insulated groups of diamonds with individual history of growth and the subsequent existence.

References:

1. Pavlushin A.D., Grakhanov S.A., Smelov A.P. Paragenetic association of minerals on the surface of diamond crystals from deposits of the Carnic layer of the North-east Siberian Platform // National geology, p.3-12, 2010, No,5

2. Vince V.G., Eliseev A.P., Sarin V.A. The physical basis of modern methods of refinig natural and cut diamonds // Precious metals. Precious stones, p. 127-148, 2009, No, 3

Native gold from alluvial deposits in the mid-Amga river (Eastern Siberian platform)

Zhuravlev A.I., Okrugin A.V.

Diamond and Precious Metal Geology Institute, SB RAS, Yakutsk, Russia

[e-mail]

It has long been known that in the eastern Siberian platform within the Vilyui syneclise there are extensive bar placers of the “Vilyuian-type” small-sized flake gold [1] closely associated with Rh-rich platinum and rutheniridosmine [2]. Description of alluvial gold from the areas of junction of the Vilyui syneclise with the Aldan shield and the Baikal-Patom foldbelt is given in [3]. In our paper we describe native gold from alluvial deposits of the Amga r. for comparison with the metal from the neighboring areas. In panning alluvial deposits from the mid-Amga r. area we found small particles of gold. Along with the dominant ilmenite, Ti-magnetite, garnet and staurolite, the heavy concentrates include small grains (normally <1mm) of Cr-spinel, pyroxene, disthen, tourmaline, zircon, rutile etc. They are characteristic of various magmatic and metamorphic formations that enter into the composition of the crystalline basement and the platform cover in the transition zone between the Aldan shield and the Vilyui syneclise complicated by the structure of the buried Yakut uplift. The Amga r. has its origin in the area of Archean and Proterozoic rocks of the Aldan shield, then running over the Cambrian carbonate units and further, in its middle and lower reaches, washing out Mesozoic terrigenous rocks. In its upper course, rare dolerite dikes of the Middle Paleozoic Chara-Sinsk dike belt are exposed. From geophysical data, numerous pipe-type (Khompu-Maya kimberlite field) and ring-type (alkaline-ultrabasic complexes with carbonatites) anomalies of Middle Paleozoic age are delineated within the buried Yakut uplift. Also outlined are anomalies of large plutons and a NS linear abyssolith (Great Dike of Yakutia) of basic-ultrabasic rocks with potential PGE-chromite mineralization [4]. Within the Yakut uplift area, the basement rocks were subject to erosion, i.e., indicator minerals of the above-mentioned potentially metalliferous anomalies might have got into the alluvium in the Amga r. through the Jurassic or earlier intermediate reservoir rocks.

Native gold is sporadically observed throughout the studied Amga r. area. It occurs as single (sometimes as many as a few tens) well-rounded fine platelets in heavy concentrate samples 0.1 to 1 m³ in volume. The smallest dust-like gold particles were also found in interstream areas in the Lena-Amga interfluve in the samples of quarried Lower Jurassic sands and conglomerates along the road leading from Yakutsk to Tommot. The largest gold grains from the Amga r. alluvium are 0.5 to 0.8 mm across. Their thickness does not exceed 0.05 mm, and so they often have torn and turned down edges. Microprobe analysis showed that most of the small gold grains have a very high fineness ranging from 950 to 995‰. The central parts of larger and thicker grains often include relics of the primary low- and medium-grade gold (590 to 880‰) with a fringe of high-grade gold. In one such grain, a tabular inclusion of impurity-free galena 30x7 microns in size was found in its central medium-grade part (Fig.). In low-grade gold, Hg content is 0.8%, while in medium- and high-grade gold it is less than 0.1-0.2 %. The Amga r. gold is most close in fineness to that from the Chara r. where, along with the high-grade metal, medium-grade gold grains are often found [2]. Gold from the Kenkeme r. alluvium has high fineness.

Fig. Inner structure of fine-plate gold (sample OB 57-6/180), Amga r.

a – image in back scattered electrons: white – high-grade (970-990‰) fringe, grey – medium-grade (800-850‰) inner part of a gold particle, dark-grey – a narrow strip of low-grade (590-680‰) gold, dark-grey to the right – a platy inclusion of galena surrounded by a thin rim of medium-grade gold, black – small voids, b-d – images of Au, Ag and Pb, respectively, in characteristic X-rays. The photo was taken by analyst N.V. Leskova on a scanning microscope JSM-6480LV.

All the grains contain a lot of tiny round voids up to 1 micron in size, less frequently up to 3-5 microns. The voids in the central low- and medium-grade part of gold grains comprise no more than 5-10% of the total volume, while in the high-grade fringe they range up to 20-30% and, locally, to 50%. This may be explained by the removal of Ag from the metal matrix in exogenic conditions. Samusikov and co-authors [5] showed experimentally the process of Ag removal from Au-Ag alloys by etching in nitric acid with the formation of spongy-porous fine gold. Accordingly, one may assume that in various exogenic conditions, refining of low- and medium-grade gold particles could gradually occur as a result of Ag removal from their near-surface portions and the formation of a microporous fringe of fine gold on them. Around the periphery of such a fringe there is a discontinuous, extremely thin rim of the massive metal. This compaction of microporous gold at the surface is likely to be the result of mechanical microforging of the metal during subsequent transportation of gold particles in the alluvium.

The conducted studies showed that native gold from the mid-Amga r. deposits had long stayed in exogenic conditions of platformal placer formation, which is recorded in its internal structure as a microporous high-grade fringe. The gold accompanying indicator minerals are suggestive of the presence in the region of a wide range of potentially metalliferous bedrocks buried under the Mesozoic-Cenozoic sedimentary cover [4].

The work was completed with the support of the Russian Foundation of Basic Research Grant № 11-05-00747.

References:

1. Trushkov, Yu. N., Izbekov. E.D., Tomskaya, A.I. and Timofeev, V.I. Gold deposits of the Vilyui syneclise and its framing. 1975. M: Nauka, 148 p. (in Russian).

2. Okrugin, A.V. Platinum placer deposits of the Siberian platform. Yakutsk. Yakutian branch of the SB RAS Publishing House. 2000.184 p. (in Russian).

3. Glushkova, E.G. and Nikiforova, Z.S. Comparative characteristics of typomorphic features of native gold from alluvial deposits in the mid-Lena r. basin (southern Siberian platform) // Tikhookeanskaya Geologiya, 2010. N3. p. 34-44 (in Russian).

4. Okrugin, A.V., Mishnin, V.M., Andreev, A.P. and Bekrenev, K.A. Topomineralogic characteristics of potentially metalliferous units of the buried Yakut uplift (eastern Siberian platform) // Otechestvennaya Geologiya. 2010. N5. p. 13-22 (in Russian).

5. Samusikov, V.P., Leskova, N.V. and Zayakina, N.V. Submicrostructural characteristics of gold-silver alloys. Transactions of the 3d Eurasian Symposium on the Problems of the Material and Machine Strength in the regions with cold climate. Part 2. Yakutsk. 2006. p. 94-104.

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