Basites of Vilyui-Markhinsky dike belt (Vilyui paleorift) and their relations with Nakyn field kimberlites

The Vilyui-Markhinsky dike belt (VMDB), which was formed as a result of Devonian rifting on the eastern margin of the Siberian Platform, is the marginal part (area of scattered rifting) of the Vilyui paleorift structure. The Nakyn field is located in the central part of the belt, but is controlled by an independent system of NNE-trending tectonic faults. The belt dyke intrudes the Nyurbinskaya kimberlite pipe. On their contact, specific breccias were formed resulting from the interaction between degassing products of basic magma with kimberlites. The typical zonality of the dyke endocontact indicates a later dyke introduction. Dolerite dikes thermally metamorphosed breccias in which high-temperature neoplasms of andradite, Al-lizardite, and clinochrysotile were generated. VMDB basites represent a single association, in which two series of rocks are distinguished: moderate-titanium (TiO2 ~ 2.5 wt. %) with normal alkalinity and low P2O5 content, and high-titanium (TiO2 ~ 4.4 wt. %), occasionally with moderate alkalinity. The differences in the dike composition are insignificant and are the result of natural variations in the composition of individual bodies. 40Ar/39Ar dating of the VMDB basites, the method characterized by the best results convergence, shows that they formed in a narrow timeframe corresponding to the Upper Frasnian – Famenian stage of the Upper Devonian (368.5 to 376.3 Ma). The location of the Nakyn field basites and kimberlites is controlled by faults of various types, orientation and age. Kimberlites formed first, and VMDB intrusions followed.

THE MECHANISMS OF THE DISTRIBUTION OF HYDROTHERMAL-METASOMATIC GOLD- BEARING FORMATIONS (EAST OF THE SIBERIAN PLATFORM)

Hydrothermal-metasomatic formations with Au content from 0.5 to 3.0 g/t, sometimes up to 24-32 g/t, were found in the east of the Siberian platform in the brecciated zones. They form, along ancient faults activated in the Mesozoic, extended narrow formations in terrigenous-carbonate and sandy layers of PZ-MZ age. The comparison of metasomatites with gold-ore metasomatites of the Kuranakh and Tabornoye deposits makes it possible for the first time to predict ore sources formed as a result of hydrothermal-metasomatic processes. Perhaps these sources represent a huge metallogenic potential.

THE UPPER ORDOVICIAN BIOSTRATIGRAPHY OF THE CHUNYA RIVER BASIN (RIGHT TRIBUTARY OF THE PODKAMENNAYA TUNGUSKA RIVER, SIBERIAN PLATFORM)

The article deals with presented results of studying Upper Ordovician sections of the Mangazeya Formation in the Chuya River basin (right tributary of the Podkamennaya Tunguska River) of the Siberian Platform. The given layer by layer section description and new data on leading groups of fauna (ostracods, trilobites, brachiopods, conodonts) allowed for significantly expanding paleontological characteristics of enclosing sediments. The description of the Baksan Horizon deposits is given for the first time, it updates the earlier description of underlying formations in the stratotypic section of the Dolbor Horizon 2 km above the mouth of the Upper Chunku R. (left bank). As a result of biostratigraphic analysis for different groups of fauna, both from published sources and newly obtained data, it has become possible to clarify the stratigraphic position of the section located 0.4 km below the mouth of the Amutkan River. The correlation of studied sections with the Gaindinskaya-3 test well was carried out.

New Findings of Lower Cambrian Trilobites in the Southern Hövsgöl Area (Mongolia) and Their Significance for Stratigraphy and Interregional Correlation

—Based on the materials collected by L.N. Repina in the southern Hövsgöl area (Mongolia) in 1986–1988, we describe three sections and identify lower Cambrian trilobites. The sections of Mt. Hurtel Harna and along the Ujigin Gol River were studied earlier. We consider trilobites that have been found in these sections for the first time: Redlihia zharkovi, Lermontoviella shanganica, Erbia granulosa, Kootenia siberica, and Parapoulsenia lata. The section along the ravine of the Zuun-Shuvuutyn-Sair Brook and the findings of trilobites in it are described for the first time. The found new trilobites make it possible to correlate the coeval lower Cambrian strata in the southern Hövsgöl area, Altai–Sayan folded area, and western Siberian Platform and refine the age of the certain parts of the Egyin Gol, Ukhaa Tolgoi, and Ujigin Gol formations.

Baikal–Vilyui Paleoproterozoic Belt of The Siberian Platform: Regional Gold-Controlling Structure

The article demonstrates that the placer gold content of the Vilyui Syneclise is governed by the regional structure of the crystalline basement of the Siberian Platform—the Baikal–Vilyui Paleoproterozoic belt, the boundaries and tectonics of which are substantiated by analysis of the gravitational field structure. The belt includes a system of basement blocks with a common northeastern strike, which form horsts (of the Suntar type) and grabens (of the Kempendyai type). The gold-bearing placers of the Vilyui Syneclise are mainly confined to the Suntar, Tyukyan, and Chybyda blocks of the belt, the metamorphic and igneous rocks of which were the primary gold sources in the sedimentary cover. The suture zone of the Baikal–Vilyui belt was very permeable to deep fluids responsible for gold migration. The types of possible primary gold sources (gold–platinoid, low-sulfidation gold-quartz, and gold–silver) reflect the peculiarities in the evolution of Early Precambrian gold during sedimentation in the Vilyui Syneclise. The areas controlled by the Suntar, Kempendyai, Tyukyan, Chybyda, and Khapchagai blocks of Early Precambrian rocks, which may contain gold objects, should be considered promising for buried gold placers in the Vilyui Syneclise.

Vilyui-Markha interfluve of Yakutia: a region prospective for Cu-Ni-PGM mineralization

Materials are discussed on geologically similar structure of Norilsk ore district (NOD) and Vilyui-Markha interstream area (VMIA), which implies the latter prospects for Cu-Ni-PGM mineralization. Data is provided on Cu-Ni-Co-Zn mineral occurrences within Ygyattinsky diamondiferous region centered on VMIA. Geological similarities of NOD and VMIA are summarized below. Both regions are located in Mesozoic tectonomagmatic activation domain at the edge of large troughs: NOD is localized in the Siberian platform foredeep, and VMIA lies at Vilyui syneclise margin. NOD deposits are hosted by centroclines of large brachysynclines at slopes of Pyasinskoye dome. Within VMIA, prospects are located at Syuldyukarskoye dome slopes in margibal parts of large troughs. NOD and VMIA comprise Paleozoic marl-carbonate strata overlain by carbonaceous Carboniferous-Permian overburden, which could be assimilated by basaltic magma in Mesozoic contributing to its ore differentiation. Norilsk-Kharaelakh ore-bearing intrusion in VMIA appears to be similar to Kholomolokh sulfide-rich intrusion with its inferred extensive ore-bearing sills. They have similar composition and structure including presence of pegmatites. Ore-controlling Norilsk-Kharaelakh fault is believed to be simi- lar to Khatyryk-Kholomolokh fault, which was traced for 24 km by drilling and is one of sutures within deep-seated kimberlite-controlling Vilyui-Markha zone. This fault’s impact zone host concentrated geochemical anomalies and Cu-Ni-Co-Zn-Pt-Au-Ag Khomustakh occurrence. Based on this, Norilsk-type Cu-Ni prospects ranked as a potential ore cluster and three ore fields were identified

Diamond study features in forecasting and prospecting (Siberian platform)

Based on the completed research and data summary on diamonds specific features (an example from the Siberian platform), conclusions were made that indications exist at all levels of their location: from kimberlite-related provinces, subprovinces, fields, diatremes to ore shoots. Population of diamond varieties in kimberlite rocks forms parasteric, rather than paragenetic, associations related to their coexistence in kimberlites. Morphological criteria of kimberlite diamond potential were defined depending on rounded diamonds content in them. Significant differences in kimberlites diamond potential were not revealed to the explored depth of diatremes. A conclusion about the absence of horizontal concentric zonality in the change of diamond typomorphic features was made.

Diamond study features in forecasting and prospecting (Siberian platform)

Based on the completed research and data summary on diamonds specific features (an example from the Siberian platform), conclusions were made that indications exist at all levels of their location: from kimberlite-related provinces, subprovinces, fields, diatremes to ore shoots. Population of diamond varieties in kimberlite rocks forms parasteric, rather than paragenetic, associations related to their coexistence in kimberlites. Morphological criteria of kimberlite diamond potential were defined depending on rounded diamonds content in them. Significant differences in kimberlites diamond potential were not revealed to the explored depth of diatremes. A conclusion about the absence of horizontal concentric zonality in the change of diamond typomorphic features was made.

Acritarch Elektoriskos? williereae (G. & M. Deflandre, 1965) Vanguestaine, 1979 emend. nov. from the Upper Ordovician of the Siberian Platform: New Morphological and Stratigraphic Data

The morphology of acritarch species Elektoriskos? williereae (G. & M. Deflandre, 1965) Vanguestaine, 1979, previously considered as a Silurian index of the Llandovery, has been clarified, the diagnosis emended, and the stratigraphic distribution expanded. According to new data, the first appearance of E.? williereae was confined to the upper part of the Baksan Horizon near the boundary of the Sandbian and Katian stages of the Upper Ordovician. Co-occurrence of E.? williereae with representatives of the genera Gordonirundum, Nirundella, Peteinosphaeridium, and Sacculidium is a distinct, well-recognizable palynological characteristic of the Katian deposits of the Siberian Platform which can serve for identification, dating, and correlation of the acritarch-bearing strata. Morphological variations in E.? williereae are a stable diagnostic feature of the species that distinguishes it from other taxa. It is possible that the species was sensitive to paleoenvironments and, probably, highly adaptive, which allowed it, remaining almost unchanged, to overcome the global Late Ordovician cooling, which became fatal for many other groups of organisms.