Mineralogical and Geochemical Features of Native Gold From Placers of the Evota Gold-Bearing Region (Russia, Aldan Shield)

Evota gold-bearing region is located in south of Republic Sakha (Yakutia) within the Nimnyr terrane of Aldan shield. A large numbers of gold-bearing placers are known on studied territory, but the primary sources for them have not been established. In this work, based on the study of the mineralogical and geochemical features of gold from alluvial deposits of the Evota gold-bearing region, possible genetic types of primary sources are considered. Obtained data showed that native gold in the studied objects has a very high, high and medium fineness. The roundness of gold is different. Almost ore crystals with sharp edges and well-rounded individuals with polished faces were found. The fineness of the studied gold grains was determined by microprobe analyzer Cameca Camebax-micro and varies in the range from 812 to 1000 %⁰. A thin (up to 20 μm) high-grade rim was found in two grains (cr. Zolotoy). The central part of one of them has a fineness of 865 %o, and in the edge part it reaches 1000 %⁰, which indicates that this gold was in the hypergenesis zone. Admixture elements determined as traces and are presented Cu, Pd, Fe, Ni. Minerals-microinclusions - quartz, potassium feldspar, pyroxene, staurolite, maldonite, bismuthite revealed in gold grains. In some watercourses, for example, in the creek Sukhoi, only very high fineness gold (993-1000 %⁰) was found. The presence of high-grade gold (cr. Sukhoi), intergrowths of gold with bismuthite (cr. Zolotoy), as well as inclusions of maldonite (cr. Yagodny) gives opportunity for assuming that primary sources could be basic ores such presented in the P. Pinigin deposit. At the sites (cr. Elovyi, r. Evota), both medium-grade and very high-grade gold were found, the fineness range varied from 827 to 998 %⁰. The presence of gold with medium fineness and good roundness in studied watercourses probably indicates an additional supply of gold from primary sources formed as a result of the alkaline magmatism development of the Mesozoic age. Thus, for gold with high and very high fineness by admixture-elements, chemical composition and microinclusions, a genetic relationship with primary sources similar to the P. Pinigin deposit was found; for gold grains with medium fineness, formation in gold-ore mineralization characterized for the Mesozoic stage ore development such in deposits of Central-Aldan ore region is assumed.

Types of Tellurium Mineralization of Gold Deposits of the Aldan Shield (Southern Yakutia, Russia)

The published and original data on the tellurium mineralization of gold ore deposits of the Aldan Shield are systematized and generalized. The gold content is related to hydrothermal-metasomatic processes caused by Mesozoic igneous activity of the region. The formation of tellurides occurred at the very late stages of the generation of gold mineralization of all existing types of metasomatic formations. 29 tellurium minerals, including 16 tellurides, 5 sulfotellurides and 8 tellurates have been identified. Tellurium minerals of two systems predominate: Au-Bi-Te and Au-Ag-Te. Gold is not only in an invisible state in sulfides and in the form of native gold of different fineness, but also is part of a variety of compounds: montbrayite, calaverite, sylvanite, krennerite and petzite. In the gold deposits of the Aldan Shield, three mineral types are distinguished: Au-Ag-Te, Au-Bi-Te, and also a mixed one, which combines the mineralization of both systems. The decrease in the fineness of native gold is consistent with the sequence and temperatures of the formation of Te minerals and associated mineral paragenesis from the epithermal–mesothermal Au-Bi-Te to epithermal Au-Ag-Te. The conducted studies allowed us to determine a wide variety of mineral species and significantly expand the area of distribution of Au-Te mineralization that indicates its large-scale regional occurrence in the Aldan Shield.

Trace element geochemistry and isotopy (Sm, Nd) of lamproites of the Aldan Shield

<p>Lamproites of the Aldan Shield were found (Vladykin 1985) at the beginning of 80-es (for the first time in the USSR), being mainly the intrusive varieties of lamproites, though there occur among them some dyke and volcanic varieties. The general geological and geochemical features of lamproites of the Aldan shield were reported at the VI International Kimberlite Conference at Novosibirsk in 1995 (Vladykin 19971).</p><p>In  Aldan Shield there are known 14 locations of lamproites mostly referred to the Mesozoic rifting. This zone stretches out over all Aldan Shield, from the Murun massif in the Western part of the shield up to the Konder massif in the Eastern part of the shield. These occurrences of lamproites are of Jurassic age (120-150 m.a.). Only lamproites of Khani massif in the SW part of the Aldan Shield are more ancient. At first (according to the data of V.V.Arkhangelskaya) the Khani massif was considered to be Paleozoic, then using K-Ar method (VSEGEI) it was established Proterozoic age of biotite pyroxenites of the massif 1800 m.a. We found the dyke of olivine lamproites of the massif that crosses the biotite pyroxenites. We obtained even more ancient age – 2700 m.a. by zircons from these lamproites with a device SHRIMP (VSEGEI) (Vladykin, Lepekhina - 2005).</p><p>New data on Sr-Nd – systematization of the lamproites of the Aldan Shield have been obtained. The ratios 87Sr/86Sr in lamproites of Aldan vary from 0.703 to 0.708, whereas έ Nd – from -6 to -25. The source of Aldan Shield lamproites is enriched mantle ЕМ-1 (рис.1), that is consistent with their geological position (Vladykin -1997). They are situated between the Aldan Shield and the Siberian platform, where did not occur subduction. The North American lamproites (Leucite Hills, Smoky Bewt, Prery Creak ecc) have a similar position between the Canadian shield and the North-American platform and the same mantle source.</p><p>Compared to the Australian lamproites, the lamproites of the Aldan Shield have lower concentrations of rare-earth elements. The TR spectra for the Aldan lamproites (fig. 2) are rather uniform. A slight slope of the spectrum curves and slight Eu-anomaly are typical. For the earlier olivine lamproites lower TR concentrations are typical as compared with more differentiated leucite and sanidine lamproites.</p><p>The lamproites of the Aldan Shield originated from the enriched mantle source ЕМ-1, the age of that, according to Pb isotopic data, obtained for the rocks of the Murun massif (Vladykin 19972) is estimated as 3200 m.a. The dykes of the olivine lamproites of the Khani massif are the oldest lamproites in the world (2700 m.a.). The TR spectrum of the same type is indicative of similar genesis of the lamproites from various massifs of the Aldan Shield. In spite of the deep mantle source of the Aldan lamproites, they don’t bear diamonds actually, since the diamonds were likely burnt during their crystallization (at t- 1200-1000o C).</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.68d9d1da710063179070161/sdaolpUECMynit/12UGE&app=m&a=0&c=833cbfbac13f62c9a0cf1376ced7de51&ct=x&pn=gepj.elif&d=1" alt=""></p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.52e855ea710063379070161/sdaolpUECMynit/12UGE&app=m&a=0&c=1874a7ee7bd883faf28c1a71fad727e1&ct=x&pn=gnp.elif&d=1" alt=""></p><p>RFBR 09-05-00116, 08-05-9000.</p><p>References:</p><p>Vladykin N.V. First occurence of lamproites in the USSR.//Doklady Academii Nauk SSSR, 1985, Vol..208, N 3, p.718-722. (in Russia).</p>

The Yamalakh Horst (Southern Yakutia): features of geology, geomorphology and history

The paper presents geological and geomorphological details for the Yamalakh horst (Central Aldan region, Yakutia), which hosts the Chompolo field of barren lamprophyre in its southern part and a gold deposit in the north. The rocks resulted from Mesozoic alkaline magmatism in the Aldan shield. The horst origin relates to large-scale alkaline intrusions in post-Early Jurassic time.