Tokur gold ore–placer cluster in Amur Province: Geological–structural features and perspective of its development

The geological–structural features and gold-potential perspective of the Tokur ore–placer cluster in the Amur Province have been studied. This ore–placer cluster is a syncline composed of Paleozoic blackshale sequence, which is cut through by Late Paleozoic and Late Mesozoic intrusions. A granitoid massif is suggested at the depth on the basis of geophysical data. The cluster is distinguished by high productivity. Approximately 37 t gold from ore deposits, primarily from the Tokur deposit, and 60 t from placers have been mined out. The Tokur deposit is compared with the Natalka and Degdekan deposits from the Yana–Kolyma Province. Perspectives of the cluster area for large gold–quartz deposits and technogenic gold placers have been estimated.     

Residual-eluvial gold placers in northeastern Russia (Dal’nii placer)

We consider the formation of the Dal’nii (Dal’nyaya) eluvial gold placer (Bol’shoi Anyui ore–placer district, western Chukchi Peninsula), related to the Dal’nii (Dal’nee) gold-bearing porphyry Mo–Cu occurrence. The Dal’nii placer is located within the transition between the Kur’ya Ridge and Anyui basin, which has been relatively stable at the recent (Pliocene–Quaternary) tectonic stage. Minor recent uplift has determined the slight denudation of interfluves, the leading role of eluvial processes in the formation of a loose cover on them, and the preservation of the relict matter of pre-Pliocene chemical-weathering crusts (including the oxidized zones of orebodies) in present-day eluvium. The Dal’nii placer consists of relict weathering-crust placers altered by recent eluvial processes in different degrees. Therefore, it is relatively rich in metal, whereas the primary lode contains mainly fine-sized gold, which is almost not released from ore under periglacial lithogenesis in present-day interfluves. We suggest calling this genetic type of placers “residual-eluvial.” The primary lodes being highly eroded (during the formation of residual concentrations, which serve as an intermediate reservoir for these placers), residual-eluvial placers or their parts might not be directly related to specific orebodies at the present-day level of erosional truncation.     

Native gold in complex Ti–Zr placers of the southern West Siberian Plain

Typomorphic features of native gold and its contents in complex Ti–Zr placers in the southern West Siberian Plain are reported. Three of the placers are of littoral-marine genesis, and two formed under conditions of an alluvial piedmont plain. Native gold from the studied Ti–Zr placers occurs mainly as flattened thin (?0.1 mm) particles which underwent mechanical action. It is marked by wide fineness variation and the abundance of a very fine (990–1000‰) variety. Most likely, this is chemically transformed clastogenic metal. The gold content of the productive bed (5–30 mg/m³ native gold and 8–140 ppb bulk gold) is consistent with the dispersion of heavy ore and accessory minerals during mechanical migration in water flows simultaneously with their concentration on geochemical barriers. The native-gold content of complex Ti–Zr placers shows a higher negative correlation with the primary source–placer distance than those of Ti and Zr minerals and a positive correlation with the degree of hydrodynamic reworking (gravity concentration) of transit terrigenous material. On the southern framing of the West Siberian Plain, some regions of northern Kazakhstan are promising for gold of complex Ti–Zr placers as well as fine- and thin-gold placers, gold-bearing weathering crusts, and primary gold deposits.     

青海省东昆仑地区晚古生代—早中生代火山活动与区域构造演化

从沉积建造分析入手,通过对区内晚古生代—早中生代火山岩组合的构造属性识别,认为区内晚古生代—早中生代构造演化是一个连续的过程,它奠定了该区的基本构造格局,也是金与铜多金属矿产的主要成矿时期;晚古生代—早中生代发育的双岩浆弧是在同一动力学机制下不同阶段形成的造山岩浆弧：陆缘的钙碱性岩浆弧和陆内的高钾钙碱性岩浆弧。晚古生代—早中生代构造岩浆旋回可以划分为２个阶段,早期的俯冲造山阶段形成了与蛇绿岩有关的火山岩类和弧火成岩类,晚期的大洋闭合和碰撞造山阶段则形成了钾玄岩系列火山岩。     

Yasny lode-placer cluster: Geological and structural features and gold potential

The geological and structural features and gold potential of the Yasny lode-placer cluster in Amur province have been investigated. The lode-placer cluster is an intrusive domal uplift elongated in the nearmeridional direction and surrounded by Neogene loose sediments. The cluster comprises placers that yielded 15 t gold mined from there and small occurrences of gold-quartz and gold-base-metal lodes. Association of native gold with cinnabar in the Yasny Creek placer allows us to forecast a new source of gold-mercury mineralization in the basin of this creek, which could be compared with the Kyuchyus deposit in Yakutia. Gold nuggets 79 kg in total weight were mined from Gar-2 River placer. They are comparable in weight and association with quartz to the world’s largest Holtermann Plate nugget from Australia. Gold-quartz lodes have been forecasted in the basin of the Gar-2 Creek.     

Early Paleozoic gabbro-granitoid associations in eastern segment of the Mongolian-Okhotsk foldbelt (Amur River basin): Age and tectonic position

Magmatic rocks of the Pikan and Un’ya massifs situated in eastern segment of the Mongolian-Okhotsk foldbelt are studied using isotopic-geochronological (U-Pb zircon dating) and geochemical methods. Two rock complexes different in age are recognized in the Pikan massif: the high-Al gabbro-tonalite association of the Middle Ordovician (468 ± Ma) and granodiorite-granite association of the Late Silurian-Early Devonian (415 ± 7 Ma). The Late Ordovician age (454 ± 5 Ma) is established for leucocratic granites of the Un’ya massif. As is suggested, the Pikan and Un’ya massifs are “allogenic blocks” detached from continental framework of the Mongolian-Okhotsk foldbelt and tectonically emplaced into the foldbelt structure at the last stage of its development.     

Timing of closure of the eastern Mongol–Okhotsk Ocean: Constraints from U–Pb and Hf isotopic data of detrital zircons from metasediments along the Dzhagdy Transect

The Mongol–Okhotsk Belt, a major structural element of East Asia, is probably the youngest orogenic segment within the Central Asian Orogenic Belt. However, the timing of final closure of the Mongol–Okhotsk Ocean remains unresolved. Here, we present detrital zircon U–Pb–Hf isotopic data and whole-rock geochemical data (major and trace elements and Sm-Nd isotopes) for the metasedimentary rocks from the Un'ya–Bom Terrane, Dzhagdy Terrane, and the eastern part of the Tukuringra Terrane. Our new zircon U-Pb ages suggest that all sedimentary formations along the Dzhagdy Transect are early Mesozoic in age, rather than Paleozoic as previously thought. The detrital zircons from the metasedimentary rocks in the Un'ya–Bom Terrane, the Dzhagdy Terrane, and the eastern part of the Tukuringra Terrane yielded the youngest concordant ages of 194 ± 4, 193 ± 2, and 171 ± 2 Ma, respectively. Moreover, we note that the so-called sedimentary formations of these terranes are not single sedimentary sequences as previously suggested, but a set of an olistostrome or tectonic mélanges composed of rocks of different ages and origins. These sedimentary formations are probably relics of the Mongol–Okhotsk remnant basin that formed in the “gaps” between the southern margin of the North Asian Craton and the Amur Block during their collision. The absence of detrital zircons younger than 171 Ma in the sedimentary rocks of the Mongol–Okhotsk basin implies that the final closure of this basin could have taken place at the boundary of the Early and Middle Jurassic as a result of the collision or the development of the Mongol–Okhotsk orogenic belt in this region. After that, the Mongol–Okhotsk Belt underwent intense deformation related to within-plate strike-slip faulting, which could be attributed to the late Mesozoic rotation of the North Asian Craton relative to the continental massifs of East Asia.     

Typomorphism and bedrock sources of placer-forming minerals of the Umyt’ya rare metal–titanium placer (Khanty-Mansi Autonomous District)

The paper reports the results of study of the main placer-forming minerals (ilmenite and zircon) from the Umyt’ya coastal–marine placer in the western part of the Khanty-Mansi Autonomous District, Yugra. Microprobe study of 1450 zircon grains taken from the upper and lower ore bodies of the Umyt’ya placer (29 samples) and their comparison with zircon in magmatic rocks from the adjacent folded framing of the North and Nether-Polar Urals (40 analyses) allowed us to recognize five geochemical types of the mineral: Aurbakh (prevailing in the Umyt’ya placer), Severorudnichnyi, Tagil-Kytlym, Shemur, and unknown source. Crystallomorphological characteristics of the identified geochemical types of zircon are given. According to the microprobe data (300 analyses), ilmenite is subdivided into six groups. Comparison of these groups with the composition of ilmenite from magmatic complexes of the North and Nether-Polar Urals showed that they correspond to the same complexes as zircon. Based on peculiarities of the geological structure of the eastern slope of the North and Nether-Polar Urals, four separate areas were identified, each comprising spatially close magmatic complexes regarded as possible sources for ore minerals of the placers: Shchekur’ya–Khorasyur, Yalpigner–Chistop, Vizhai–Ivdel, and Denezhkin Kamen. Based on additional criteria, the Yalpigner–Chistop area was the main source for the Umyt’ya placer.     

Native gold in Ti-bearing placers of the Ariadna cluster and its bedrock sources (Primorye region)

Centuries of intense mining of noble metals from placers of the southern Far East has exhausted their reserves. The prospects for increasing the raw material base of the region are connected with complex exogenic deposits, where noble metals play a subordinate role. The study of native gold from Ti-bearing placers spatially and genetically associated with the Ariadna mafic-ultramafic massif located on the right side of the Bol’shaya Ussurka River reveals the placer-forming role of polygenic and polychronous bedrock sources. Mafic-ultramafic rocks yielded cupriferous and mercuric gold; quartz veins with Au-As mineralization likely serve as another source of placers.     

Gold resource potential of the eastern Siberian Platform: Placers and their feeding sources

The data on typomorphism of placer gold and its localization in the eastern Siberian Platform have been integrated for the first time. The alluvial, pseudolode, and eolian morphology of gold determines the origin of placers, which is explained not only by hydrodynamic but also by eolian processes. The recent and older alluvial (Au-bearing reservoir rocks in the second case), as well as eolian (unconventional type), genetic types of placers are recognized in the given territory. Precambrian and Mesozoic stages of ore formation are distinguished by the set of typomorphic attributes of placer gold (chemical composition, microinclusions, internal structure, etc.). On the basis of distinguishing features of placer gold, Precambrian lode deposits of great depth spatially related to outcrops of the basement have been forecasted for the first time and classified into low-sulfide gold-quartz, Au-bearing banded iron formation, Au-Cu porphyry, and Au-PGE types. The inferred shallow-seated gold-silver and gold-sulfide-quartz disseminated deposits occur locally and are related to the Mesozoic tectonomagmatic reactivation. The elaborated methods and approaches to the study of placer gold typomorphism in the eastern Siberian Platform also could be helpful in providing insights into placer origin, and forecasting and prospecting gold deposits on the East European Platform, which is close to the Siberian Platform in geological evolution.     

PRINCIPLES IN DIFFERENTIATING STRUCTURAL-TECTONIC ZONES IN ORE-PETROGRAPHIC PROVINCES AND ORE DISTRICTS

The Zeya-Bureya plain and its mountain fringe are components of the Mongolo-Okhotsk folded belt, a northern segment of the East Asia Hercynian structure located between the ancient Siberian and Chinese platforms. The basement is a complex combination of Proterozoic, Sinian-lower Paleozoic and middle Paleozoic folded structures. In the Tomesk synclinorial zone, what appear to be parageosynclinal middle to upper Paleozoic structures, rest directly on Proterozoic crystalline foundation, and are correlative with similar formations in the western slope of the ancient Bureya-Girin massif. Structural trends in the southern half are north-northeasterly and north of latitude 52°; the trend is sublatitudinal. Deep rifts have been particularly important in the tectonics of the region. Regional Mesozoic structure shows inherent features from earlier stages, expressed in Paleozoic structural trends and faults. The largest and deepest of the Mesozoic troughs are associated with Paleozoic synclinoria and uplifts; swells are associated with pre-Sinian basement highs and Paleozoic anticlinoria. Maps showing geophysical anomalies and fields and tectonic features are included.—C. E. Sears.     

Material composition and geochemical characteristics of ores of the Malinovskoe gold-ore deposit (Primorskii Krai,Russia)

The mineral composition and geochemical characteristics of the ores of the Malinovskoe gold-ore deposit are studied by the data from mining works (ditches, cleanings, and boreholes). It is found that the ore–magma system of the deposit was formed in several stages of mineralization characterized by two phases of magmatism differing in age. In terms of the set of features (the geological–structural position of the deposit, as well as the material composition and geochemical characteristics of the ores), the deposit is attributed to the gold–tourmaline type of mineralization associated spatially and genetically with the “raremetal” granitoid magmatism. This type has not previously been found in Primorskii Krai. The studies of the material composition and geochemical characteristics of the ores allow us to ascertain the correlations between the elements along with the reasons of their origination. By analogy with other gold-ore formations of the Russian Far East, the mineralogical and geochemical model of the deposit is developed (Be–Sn–Cr–Ba–Au–Cu–Mo–Pb–V–Ti–Co–W–Ag–Bi–Ni–Mn–Sr–Zn–Sb–As modeling element series of vertical zoning), which enables us to estimate the levels of the erosion section of the ore bodies and to evaluate their prospects. It is found that the most productive associations in the deposit are the gold–bismuth geochemical association (Au–Ag–Bi–Cu–As–Co) and, to a lesser degree, the gold–tungsten association (W–Au–Ag–Cu–Bi–As).     

Early Mesozoic age of protolites of metasedimentary rocks from the Tukuringra terrane in the Mongolian–Okhotsk fold belt: Results of LA–ICP–MS U–Pb geochronological studies

This report presents the results of U–Pb geochronological LA–ICP–MS studies of detrital zircons from metasedimentary rocks of the western part of the Tukuringra terrane. The obtained data show that the Algainskaya, Garmakanskaya, and Teploklyuchevskaya series in the western part of the Tukuringra terrane are not of Middle Paleozoic but of Early Mesozoic age. It is shown that the main sources of clastic matter for these formations were Late Precambrian, Paleozoic, and Late Triassic–Early Jurassic complexes of continental massifs of the Amur superterrane.     

Microinclusions of ore minerals in gold from the Miass placer zone (South Urals): Evidence for source rocks

Microinclusions of ore minerals were found and studied in grains of native gold for characterization of the mineralogy of placer gold of the South Urals. One hundred ten unrounded and poorly rounded grains with a size of 1–2 mm from eight placer zones were studied. Microinclusions of ore minerals were detected in six placers of the Miass zone. The list of minerals includes sulfides, arsenides, Cu-bearing Au, and PGEs. All microinclusions show links to certain deposits of ore gold and chromite occurrences. It is suggested that the northern flanges of the Talovsky and Nurali massifs containing gold placers with PGEs require a search for PGE mineralization.

     

Genesis of gold-bearing placers and their possible sources (Eastern Siberian Platform)

Typomorphic features of placer gold in the eastern Siberian Platform were studied to determine its genesis. It was established that this region contains abundant alluvial bar placers, which were formed mainly by the erosion of gold reservoirs of different ages. Discovery of gold with the points of eolian reworking in the Quaternary sediments allows us to predict the eolian gold placers in individual areas of the northeastern and central parts of the eastern Siberian Platform. Identification of two types of gold on the basis of typomorphic features in the Cenozoic deposits indicates that placer gold was mainly derived from the Precambrian sources spatially confined to the exposure of ancient basement, as well as from local ore occurrences related to the Mesozoic tectonomagmatic activation. Thus, two main stages of ore formation (Precambrian and Mesozoic), were substantiated for the first time in the eastern Siberian Platform.     

内蒙古虎拉林地区砂金与岩金的关系

虎拉林地区砂、岩金矿的空间位置关系十分密切。在虎拉林河上中下游及虎拉林岩金矿分别采取样品，结果表明虎拉林河砂金粒度大，分选较差，磨圆度差；砂金形态总体上以粒状为主；溶蚀系数均值小于1。微量元素含量特征表明砂金的主要物质来源是近源，与岩金矿关系密切且具有继承性。虎拉林岩金矿床的矿石中自然金颗粒粗大，矿体的上部遭到剥蚀，虎拉林河砂金主要来源于虎拉林岩金矿床，砂金可作为岩金的找矿标志。自然金化学-结晶温度图表明砂金的原生Au源为中低温热液矿床。     

Composition and conditions of formation of gold-telluride mineralization in the Tissa-Sarkhoi gold-bearing province (East Sayan)

The structure and petrologic composition of new gold-ore provinces in southeastern East Sayan (Tissa-Sarkhoi cluster) are considered. Several morphogenetic types of gold mineralization have been established: quartz veins with beresitization zones, veinlet-disseminated ores in granitoids, and listwaenitization and sulfidation zones in effusions of the Sarkhoi Group and intrusive rocks of the Late Riphean Khorin-Gol complex. According to geochronological dates and some mineralogical and geochemical features, the gold mineralization is close in age to these Precambrian island-arc complexes. Parageneses of two stages of ore formation have been recognized: early high-temperature (250–460 °C) gold-pyrite and late low-temperature (110–280 °C) gold-telluride. The latter mineralization is widespread and is represented by tellurides of Au, Ag, Pb, Bi, and Ni — petzite, calaverite, hessite, tellurobismuthite, altaite, and melonite. Native gold associated with these tellurides is characterized by a fineness of 750–900‰. The intimate temporal and spatial relationships of the gold mineralization with island-arc volcanoplutonic complexes and the wide occurrence of its veinlet-disseminated type suggest that this is porphyry gold mineralization related to the Late Riphean-Vendian island-arc magmatism.     

Native gold in Cu- and Ni-bearing pyroxenite-cortlandite intrusions and placers of the Dambuki ore cluster,the Amur region

The Dambuki ore cluster of the Upper Amur region is characterized by numerous and high-grade gold placers. A few small primary deposits and occurrences of gold-quartz type were formerly considered to be the main source of placers. However, the study of widespread Early Cretaceous Cu- and Ni-bearing ultramafic intrusions has shown that gold occurring in primary Cu-Ni ores and products of their weathering practically does not differ from placer gold in morphology, fineness, or geochemical features. In both cases, elements typical of Cu-Ni ore (Cu, Fe, Pd, Ni, etc.) are detected as impurities of gold particles. These data indicate that the Au-and Pt-bearing Cu-Ni ore mineralization may be the main source of placer gold. This ore mineralization is genetically related to small pyroxenite-cortlandite sills and dikes, abundant in the Dambuki ore cluster. The primary gold-quartz deposits and occurrences also contributed to the placers, but their part was rather small.     

MAGMATISM OF THE EASTERN SAYAN

New geological and petrological data on the range of magmatic complexes and formations of the Eastern Sayan show two primary magmas: basic and granitoid. These magmas were formed through melting hard deep-seated layers of the earth crust: basaltic and sialic. During the geosynclinal stage the development of magmas belonging to the Archean, Proterozoic, and Salair [Cambrian] volcanic cycles proceeded consecutively from ultrabasic and basic formations formed in a pre-orogenic or earlier-orogenic geosynclinal development stage to granitoids set up in a synorogenic or later-synorogenic development stage. During the platform stage middle Paleozoic (Lower Devonian) and Mesozoic-Cenozoic cycles of magmatism proceeded directly, without the geosynclinal preparatory stage. Their development, accompanied by faulting, proceeded in reverse order from acidic and alkalic intrusions to predominantly basic eruptives. A further development of deep-seated basic and granitoid magmas was determined first by magmatic differentiation and later by assimilation phenomena which took place during the magma's passage into upper structural layers. The granitoids of geosynclinal magmatic complexes correspond petrochemically to the intermediate types of calc-alkalic rocks of the Pacific Ocean belt. The granitoids and alkalic rocks of the Lower Devonian platform magmatic complex resemble those of the Cenozoic East-Asia alkalic province. The composition of the granitoid magma belonging to the volcanic cycle is conditioned initially chiefly by the sial environment and geosynclinal strata. Magmatic complexes and formations are characterized by definite endogenic mineralizations. Chromium, nickel, cobalt, platinum, diamond, asbestos and other deposits are genetically connected with Proterozoic basic and ultrabasic rocks; gold, muscovite and tin-rare metal pegmatite with upper Proterozoic granitoids. Copper, galenaite and gold-ore occurrences are related to the postmagmatic manifestations of Salair granitoids. Deposits of pyrochlore carbonatites, molybdenite, graphite and others belong to Lower Devonian acidic and alkalic granitoids. — Auth. English summ.