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Über dieses Buch

The basis of this investigation is the petrographic and geochemical understanding of principal igneous rock types of the Noril’sk region, in order to demonstrate that these data provide unique and self-evident solutions to the problems of petrogenesis and mineralization. The results of the investigations are presented in two volumes: the first includes mainly text and the second contains illustrations.

In the first volume, the state of the main problems of the genesis of igneous rocks with reference to Traps and related ore deposits is discussed, as well as short petrological descriptions of igneous complexes in the region, the mineral and geochemical diversity of the rocks, and aspects of the differentiation of basaltic melts and mineralization are described. Taking into account the vast number of publications on the petrology of Traps of the Noril’sk region, primary attention in the monograph is given to earlier unknown phenomena, as well as other aspects that are of great importance for solving genetic problems. Some exotic geologic targets such as the Mikchandinsky differentiated cover, the magnetite lava flow of the Putorana Plateau, the magmatogenic breccia of Kharaelakh and others are described in detail.

The second volume contains an atlas of Rock Indications of igneous rock-types; formally identified reference rocks from all igneous complexes of the region, as well as photographs of thin sections of typical rocks and analytical tables of rocks and minerals from the key sections of sedimentary units and intrusions. Each rock type has been geochemically and petrographically analysed thereby providing a formal identity, complete with a photograph of the thin section.

Photomicrographs of the rocks in this book will be a useful aid in visualizing the diversity of rock types in the Traps; each photograph reflecting a unique combination of minerals.

Inhaltsverzeichnis

Frontmatter

1. Principal Issues Surrounding Trap Magmatism of the Siberian Platform

The Noril’sk region has a special place in the basalt field of the Siberian Platform. It is characterized by a complex geodynamic environment, the most entire sequence of the volcanogenic succession, great variety of hypabyssal intrusions of varying composition, and with different degree of differentiation. In this region, the Noril’sky-type layered intrusions and related largest sulfide Pt–Cu–Ni and Pt low-sulfide deposits such as the Oktyabrsky, Talnakh, and Noril’sk-I are located. Geological investigations in the region are carried out over many decades, but in spite of this, the main problems related to the origin of the volcanogenic succession, layered intrusions, and ore deposits are still relevant. In Sect. 1.1, features of geological setting and magmatism in the northwest of the Siberian Platform are described. A great deal of attention is drawn to the problems of Trap magmatism and ore formation. In Sect. 1.2, the present notions of geologists about the composition of parental magma of Siberian Traps, their chamber differentiation, and the origin of high-magnesian Traps, pegmatoids, and sulfide ores of the Noril’sk deposits are considered. In connection with these matters, a short review of the published experimental data in the field of differentiation and crystallization of silicate and ore liquids, as well as petrogenesis and ore formation that form the basis for genetic models, is given.
V. V. Ryabov, A. Ya. Shevko, M. P. Gora

2. Effusive and Explosive Complexes of the Noril’sk Region

Among the “large igneous provinces” of the world, the Siberian Traps province had been, from the geological standpoint, one of the most extensively studied regions as long ago as by the 1970s of the last century. The principle of tectonomagmatic recurrence laid the foundation for the unified stratigraphic scheme of partioning the volcanogenic series. Such recurrence is represented by rhythmic alternation of pyroclastic and effusive rocks in the volcanogenic sequence. Five tectonomagmatic cycles (phases) are distinguished, all rocks derived from these cycles being of regional occurrence. The most complete volcanogenic sequence is in the Noril’sk Region. In Sects. 2.1, 2.2 and 2.13, the general characteristics of flood basalts, the structure of lava flows, the mineralogical and petrographic characteristic of flood basalts, and the mineralogical, petrographic, and petrochemical description of rocks from 11 suits of the volcanogenic sequence are given. Section 2.14 is concerned with the description of nine marker horizons of basalt flows, which are characterized by great thickness (up to 120 m) and can be traced over the entire basalt field of the Platform. Section 2.15 is aimed to description of the complex of “anomalous formations” in volcanogenic rocks. The formations include local manifestations of high-magnesium and subalkaline basalt lavas among flood basalts, exotic manifestations of flows of komatiite-like rocks and magnetite lavas and tuff paleovolcanoes, the belt of numerous dykes of varying composition, diatremes with sulfide and magnetite ores, calderas with beds of limestone and anhydrites, as well as manifestations of native copper and bitumen in volcanogenic rocks. In Sect. 2.16, the large-scale flood basalt eruptions of the Siberian Platform are discussed.
V. V. Ryabov, A. Ya. Shevko, M. P. Gora

3. Intrusive Complexes of the Noril’sk Region

Trap intrusions of the Siberian Platform are subdivided into intrusive complexes, which were formed during six tectonomagmatic cycles. Ten intrusive complexes are recognized in the Noril’sk Region. The key parameters for subdivision of intrusive complexes are as follows: situation of intrusions in a geological structure of the region, shapes of intrusions, the degree of differentiation, mineralogical and chemical composition of rocks (normal, high magnesian, subalkaline, and silicate), features of the internal structure of intrusions, association of intrusions with deposits or manifestations of certain ore mineral resources, types of deposits or ore manifestations, age relationships to other intrusions, as well as features of aureoles of altered rocks associated with these intrusions. The most of intrusive complexes combine several types of intrusions, which possess some individual features apart from their common similarity.
Trap intrusions are subdivided into undifferentiated, weakly differentiated, and differentiated (layered). In Sects 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9 and 3.10, the petrology of intrusions is described in which their own specific features are most clearly demonstrated. The most detailed petrology is given for the intrusions of Noril’sk Complex such as Upper Talnakh and Noril’sk-I, which are associated with sulfide Pt–Cu–Ni and Pt low-sulfide deposits.
V. V. Ryabov, A. Ya. Shevko, M. P. Gora

4. Mineralogy and Geochemistry of Traps

Variations in mineralogy and compositions of principal rock-forming minerals serve as an indicator of basaltic magma differentiation. Features of the composition of olivine, pyroxenes, plagioclase, spinel group minerals, micas, and ilmenite are described in Sect. 4.1. The compositions of olivine and plagioclase in layered intrusions vary from forsterite to fayalite and from albite to anorthite, respectively. Pyroxene is present as augite of the varying composition; pigeonite and orthopyroxene are less abundant. Minerals of the spinel group are represented by a series of spinel, chromite, and magnetite. Chromespinellides form accessory impregnations and accumulations in gabbrodolerites and pegmatoids. Micas occur as phlogopite–annite with the Fe/(Fe + Mg) ratio ranging from 1 to 90 at%.
In Sect. 4.2, features of the geochemistry of Cr, PGE, Ni, Ti, and S in basalts, barren, and ore-bearing intrusions with the different degree of differentiation are described. Features of the PGE and Cr distribution in sequences of ore-bearing horizons and in various rock types are considered. In the geochemistry of nickel, the main attention is drawn to its distribution in rocks and minerals, as well as to behavior of nickel in the process of sulfide formation. The Ti content in traps and its variations in the sequences of layered intrusions depend on the initial composition of a basaltic melt and the extent of its differentiation. In the geochemistry of S, the main attention is drawn to the sulfur isotopic composition in sulfides and sulfates of the Talnakhsky ore junction.
V. V. Ryabov, A. Ya. Shevko, M. P. Gora

5. Basalt Magma Differentiation as a Source of Variety in Traps

The origin of layered intrusions and related ore formation is one of the main problems of the magmatic and ore geology of traps of the Siberian Platform. This chapter gives the critical review of available genetic conceptions. The model of the fluid–magmatic differentiation of basalt melt and ore formation in traps is proposed as an alternative to these conceptions. The leading role in these processes is assigned to three mutually related factors: tectonics, magmatism, and fluid regime of ore–magmatic systems. For validation of the model, a cause-and-effect relationship between these factors is established, the role of each is identified, and a system of natural evidences is formulated. The tectonics of the Noril’sk Region is dominated by faults that govern the placement of layered intrusions, ore zones, ore accumulations, and deposits. The long-continued tectonic activity of deep faults resulted in formation of thick zones of disintegrated rocks. Formation fluids from sedimentary rocks of the Platform cover were attracted into these zones due to decompression with fluidized zones formed along the faults. The formation fluids served as a source of S, F, Cl, CH4, H2, H2O, and other volatile components, whereas the basaltic magma was a source of ore-forming metals. Formation fluids activated by a melt intruded through the fault zones extracted metals from a basaltic magma, and so became ore-forming fluids. The fluid–magmatic interaction of a melt with volatiles facilitated prechamber differentiation of the melt into coexisting immiscible silicate–silicate and ore–silicate melts of variable composition, and crystallization of the melt completed the process of petrogenesis and ore formation.
V. V. Ryabov, A. Ya. Shevko, M. P. Gora

Backmatter

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