By Academician Dmitri RUNDKVIST, Head of Research of the Vernadsky State Geological Museum, RAS,
Yuri GATINSKY, Dr. Sc. (Geol. & Mineral.), chief research worker of the same museum,
Andrei TKACHEV, Cand. Sc. (Geol. & Mineral.), senior research worker of the same museum
Drilling of an oil well (Tyumen Region).
Since the 1980s the interest in the formation and regularities of distribution of large and superlarge mineral deposits has been continuously growing. The development of the metallogenic theory, which started in the 1960s-1970s, made it possible to twin from the forecast of specific perspective territories to the assessment of their probable potential resources in quantitative terms. On the other hand, the currently adopted methods allow to improve the criteria of evaluation of the areas we are interested in, while the global correlation of information on mineral resources, reserves, output of minerals carried out in the USA, Canada, Australia and France provides convincing evidence that practically all raw material reserves are based just on such giant deposits.
In Russia, about 70 percent of the mineral resources are concentrated in the giant and large deposits making up 5 percent of their overall quantity, but totally covering over 50 percent of the output of mineral raw materials*.
Among them we can name iron deposits of the Kursk magnetic anomaly, chromites and apatites on the Kola Peninsula, mica (common mica) in Karelia, nickel, copper, platinum and palladium in the region of Norilsk (north of the Krasnoyarsk Territory), gold in the north of the Irkutsk Region and the upper reaches of the Kolyma*, diamond-bearing kimberlite pipes in Central Yakutia**, algam in the northeast of Yakutia and Chukotka, rare metals in Tuva and the East Sayan Mountains.
As for foreign deposits, mention may be made of mercury deposits in the south of Spain, copper and polymetal deposits in Poland, algam deposits in the southwest of
* See: Yu. Safonov, "Future of Russian Gold". Science in Russia, No. 1, 2000. -Ed.
** See: O. Bazanova, "Riches of the North", Science in Russia, No. 4, 2004. -Ed.
* See: D. Rundkvist et al., "Geodynamics of the 21st Century", Science in Russia, No. 6, 1998. -Ed.
Main large and superlarge deposits and metallogenic belts.
England, in South China, Malaysia and Indonesia, rare earth elements in the north of China, gold and diamonds in the Republic of South Africa, bauxites in Guinea, phosphates in Egypt and Morocco, copper, hard lead, zinc, rare metals and uranium in Australia, etc.
Since 2003 the Russian Academy of Sciences has carried out research program "Large and Superlarge Deposits of Strategic Mineral Raw Materials: Geological Profiles, Conditions of Formation, Fundamental Problems of Comprehensive Development and In-depth Processing". Twenty-two institutes of the Department of Geosciences of the Russian Academy of Sciences are engaged in the accomplishment of the mission. Within the framework of the program the electronic "Metallogenic Map of Large and Super-large Deposits" (scale 1:2,500,000) has been compiled in the Vernadsky State Geological Museum. The analysis of this map and accompanying database shows that the deposits under consideration are confined to five global metallogenic belts (Pacific, Peri-Atlantic, Central-Asian, Mediterranean, Afro-Asian) and to similar large metallogenic provinces on the shields of paleoplatforms. The distribution of such mineral resources within the mantles of ancient and new platforms is rather irregular; in this case we can most likely speak, about different types of mineralization connected with the nature of sediments, paleogeographical and paleo-climatic conditions.
GLOBAL METALLOGENIC BELTS
These belts developed basically in the Phanerozoic (the last 540 mln years) and in some cases-from the end of the Mesoproterozoic-beginning of the Neoproterozoic (1,200 - 850 mln years ago). The most intensive ore accumulation took place at the junctions of oceanic and continental plates with the predominance of geodynamic conditions of passive and active edges of the latter, island arc, zones of collision (of continents) and riftogenesis. Within these belts the active edges of lithospheric plates are the most widespread and efficient (except for the Afro-Asian belt).
Thus, the Pacific belt frames from the East Siberian, Chinese and Australian platforms as well as a number of middle massifs in the northeast of Russia and on the Indo-Chinese Peninsula, from the west-both American platforms, from the south-the Antarctic platform. The repeated manifestations of different geodynamic processes within this belt led to an exceptional variety of the types of mineralization increasing the probability of discovery of new large and superlarge deposits there. Let us note the clear zoning of the belt under consideration first established by Academician Sergei Smirnov in 1946: pyritic and copper-porphyritic mineralization is confined to its inner zones and rare metals-to the fringe ones.
At the same time, the results of geophysical studies point at the essential discontinuity of the crustal structure at a number of nodes of the Pacific belt, which, to all appearance, increases its permeability for fluids (heated anabatic gas flows of different chemical composition) and ore solutions. In our opinion, the most perspective complexes in this respect are complexes of the Cainozoic (the last 70 mln years) active edges in the central and north segments of the Andes in South America, East Indonesia, Papua New Guinea, in the western part of Philippines (copper-porphyritic and copper-molybdenum-porphyritic mineralization, often with gold and silver) and the Late Mesozoic (140 - 70 mln years ago) Okhotsk-Chukchee and Katasiatic types in the northeast of Russia and in the South East China (deposits of gold, silver, tungsten, molybdenum, etc.).
The zones of crust extension, riftogenesis and intra-plate magmatism in the Late Mesozoic in the northeast of Russia
(algam) and in the Cainozoic in the west of the USA, in South China, North Vietnam and East Australia (polymetals, algam, tungsten, boron, beryllium, rare earth elements, etc.) are highly promising as well. A number of such nodes of concentration of large and superlarge deposits comply with the results of cluster analysis* of the density of their distribution.
The periatlantic belt is confined to the Paleozoic (570 - 250 mln years ago) folded structures of the Appalachy, Newfoundland and Western Europe. From the east it frames the North-American platform and from the west-the East European platform and was formed mainly during the development and closing of Yapetus, the Early Paleozoic ocean (570 - 400 mln years ago).
We spatially unite with this belt (to a certain extent conventionally) the Late Paleozoic folded zones of Western and Central Europe which came into existence during the closing of another ancient ocean Paleotethis about 250 mln years ago. By and large, the integrity of the Periatlantic belt was disturbed at the end of the Mesozoic-Cainozoic during the opening of the Northern Atlantic.
The zonality incidental to this belt makes it similar to the Pacific belt: pyritic deposits are confined to the inner parts and rare metals-to the fringe parts. However, in the east of North America the position of these zones relative to the Atlantic is opposite to what is mentioned above for the second belt, which is caused by younger age of the oceanic depression in the first one. However, on the whole, the metallogeny here can also be represented by a rather simple scheme, in which the Paleozoic island arc and orogenic (collision) complexes in the east of the USA, south-east of Canada, in Spain, Portugal, France and in the south-west of England (complex massive sulphides, algam, tungsten, lithium, muscovite, fluorite, etc.) should be considered to be the most perspective in respect of discovery of new large manifestations of mineralization. The paleorift structures of the Paleozoic and Mesozoic in Spain, France, Germany (mercury, fluorite, baryte, oolitic (with round granular structure) iron ore) are of interest too.
The Central Asian belt is located between the East European, Siberian, Tarim (North-West China) and North Chinese platforms with a branch between the latter and Yangtze platform (South China). It was formed from the Mesoproterozoic until the mid-Mesozoic and corresponds to the period of development of the paleo-Asiatic ocean from the opening until the final closing. Its zonality is less intense in comparison with the previous ones which is explained by the complexity of structure of the folded complexes and their partial overriding by mantles of depressions and young platforms.
Deposit of the tin ore.
As to Ural, the predominating deposits there are deposits of the inner and interim zones (copper-sulphide, magmatic and metasomatic** iron-ore, gold), in Central Asia and Transbaikalia-the deposits of the inner and fringe zones (copper-porphyritic, rare metals). Evaluating the possibility of opening of new large and superlarge deposits in the Central Asian belt, it is necessary first of all to turn attention to the complexes of the Paleozoic and early Mesozoic active edges (copper-porphyritic mineralization, molybdenum, etc.), collision zones, especially to the faults development sites in terrigenous rocks*** (gold), as well as to the areas of manifestation of basic magmatism within the plates (tantalum, niobium, beryllium, etc.).
The Mediterranean belt, in many ways, is inherently similar to the previous one, but it is more young (mainly, Mesozoic-Cainozoic) and with more clear zonality. It is confined to the folded orogenic belt of the same name and is located between the ancient and adjoining Late Paleozoic East European, Tarim and South-Chinese platforms in the north, African, Arabian and Indian platforms in the south. As to the belt zonality, it should be noted that the majority of deposits which come into existence in the passive edges (phosphorites, manganese, complex deposits) and manifestations connected with the encroachment and subsequent laterization (occurring in the conditions of hot climate) of tectonically embedded ultrabasic rocks implement ultrabasic rocks (Nickel) are confined to its south part, and in the east-to its west (Gondwana) part.
At the same time the north and east parts of the belts are rich mainly with copper-porphyritic, rare metals and other deposits connected with the subduction and collision volcanism as well as intrusions. It is these last types of mineralization which are potential within the limits of the entire belt from the viewpoint of discovery of new large and superlarge deposits.
The Afro-Asian belt extends from the East-African rift system (with a tap to the Red Sea) along the present-day passive Arabian edge, fault systems of Pakistan, Pamir and Central Asia to the Baikal-Stan zone in the east of Asia. The most unique feature of the Cainozoic geodynamics here is the prevalence of expansion and thinning of the continental
* Cluster analysis is a procedure of grouping objects into classes (clusters) distinguished for more uniformity than the entire set of objects. -Ed.
** Metasomatism is the process of replacement of rock with a change of the chemical composition. -Ed.
*** Terrigenous rocks form at the bottom of seas and oceans from the fragmentary material drifted from the land; they have sand or clay composition. -Ed.
Density of the distribution of large deposits.
crust to the point of its full intracrustal discontinuity in the Red Sea rift, vast shear displacements of adjoining blocks and variety of manifestations of magnetism within the platforms-from acid and alkali to primary one. In the south of the belt the riftogenesis started already in the mid-Mesozoic during disintegration of this part of Gondwanaland and was accompanied with the intrusion of carbonatites with mineralization of rare earth elements and strontium in Malawi. North-ward, in Tanzania and Kenya, the early stages of the Cainozoic riftogenesis are characterized with the formation of diamond kimberlite pipes in Eocene (50 - 34 mln years ago) and hydrothermal fluorite manifestations in the line of faults in the Miocene (23 - 5 mln years ago). Atlantis, a large exhalation-sedimentary* polymetal deposit, is fixed in the axis of the Red-Sea rift.
In view of the aforesaid, let's outline the most probable zonality of deposits within this belt. Manifestations of complex ores of the Red Sea type connected with the extrusive rocks of contrast series can be confined to the rift axes. In the board of sections structures at the early stages of riftogenesis, the stratified, often differential, gabbronorite intrusive rocks of the similar composition with sulfide copper-nickel mineralization and platinum-group minerals are usually associated with alkali-olivine-basalts and basic continental lavas. At the more ephebic stages the intrusion of superalkaline granitoids, alkaline and alkaline ultrabasic intrusions with carbonatites is confined to the rift boards. They bear a wide mineralization complex from the tin-tungsten to niobium one and rare earth elements. So, by analogy with other riftogenic structures it is possible to expect discovery of new deposits within the Afro-Asian belt as well.
METALLOGENIC PROVINCES ON THE SHIELDS OF ANCIENT PLATFORMS
The structure of platform shields is rather heterogeneous with Archean kernels of ancient cratons** (3.6 - 2.5 bin yeas) and accretion-collision belts of the Paleoproterozoic (2.0- 1.6 bin years), Meso- and Neoproterozoic (1.4 - 0.6 bin years) age. The folded complexes of the Archean and Paleoproterozoic are covered with protoplatform mantles in some segments. The igneous complexes of intraplate activation are widely developed on the shields. Each of these elements has its own metallogenic characteristics. Because of differences in age, structure, manifestation of intraplate activation and characteristics of the cratons of the Northern (Lavrasia) and Southern (Gondwana) chains they are examined separately.
The Northern chain includes structures of the Baltic, Ukrainian, Aldan and Canadian shields. They are characterized by the more neanic age (in comparison with the southern ones) of the greenstone (composed of altered volcanic rocks of the basic composition) belts (3.1 - 2.7 bin years), high degree of metamorphism (up to granulites) of the volcanic complexes of the ultrabasic composition of these belts and other Archean rocks, widespread development of the solid masses of anorthosites (igneous rocks of the basic composition). Among the Proterozoic orogenic structures the Paleoproterozoic ones occupy larger areas than those in the south.
Giving general estimation of the metallogeny of the Archean cratons and Proterozoic orogens of this chain, we would like to note that for the purpose of new discoveries the deposits of jaspilites, pyritic polymetals with gold in the greenstone Archean belts and on the active edges of the Paleoproterozoic, lithium, tantalum and mica in pegmatites, titanium, iron, vanadium with anorthosites (mafic intrusive rocks of the basic composition) and nickel, copper, cobalt, chrome and rare earth elements with stratified intrusions are rather important and perspective. Among the activation (intraplate) manifestations of different age, the alkaline and alkaline-carbonatite intrusions with beryllium, zirconium, tantalum, yttrium, niobium, rare earth elements, uranium, apatite and fluorite are the most important ones.
Cratons and accretion-collision belts of the Southern chain confined to the Precambrian shields of South America, Africa, Hindustan, China and Australia began to develop 3.6 bin years ago and 700 mln years later some of
* Exhalation is a gradual ejection of volcanic gases and vapors. -Ed.
** Cratons are the most ancient and stable tectonic crustal blocks. -Ed.
them had an accumulated protoplatform mantle. This suggests a considerably more ancient age of the continental lithosphere of this chain. The metamorphism of the greenstone belt rocks here is less intensive than in the North owing to which the charged comatiites (ancient lavas of the ultra-basic composition) are widespread there. The basic solid anorthosite masses are rather rare there and, at the same time, these belts are characterized by the presence of very old diamantiferous kimberlites (up to the Early Mesoproterozoic). The metallogenic provinces of the shields of this chain are more numerous.
Comparing the metallogeny of large and superlarge deposits of both chains of the ancient shields, it is not difficult to note that in the structures of the Gondwana chain, concurrent with jaspilites, rare metal pegmatites, stratified basic intrusions with chrome, platinum-group minerals, copper, nickel, common with the Lavrasian ones, there occur new types of mineralization, unknown or underdeveloped in the North. These are, first of all, giant deposits of gold and uranium in the protomantles (Witwatersrand, Republic of South Africa), polymetalliferous deposits among sedimentary rocks widespread on the south shields, superlarge gold-antimonous deposits. The wider age range is typical there for diamantiferous kimberlites and alkali-carbonatite intrusions with phosphorus, copper, niobium, rare earth elements, etc. Deposits of lateritic* bauxites, sedimentary phosphates and iron ores are also widely presented. It is possible to expect a discovery of new large deposits in the examined provinces in respect of all of the above-mentioned types.
According to the data of cluster analysis, the largest centers of accumulation of large and superlarge deposits are located in the east of Canadian, in the north of Baltic and on the Aldanian shields as well as on the shields of South America, Southern Africa and Western Australia.
DEPOSITS IN THE MANTLES OF PLATFORMS AND MEDIAN MASSIFS
Among the manifestations of mineral resources, confined to the Neoproterozoic and Phanerozoic mantles of platforms and median massifs, there are two separate groups. The first group-sedimentary, chemogenic (formed during sedimentation from the solution), infiltration and residual deposits located directly in stratified mantle masses, the second-connected with the intraplate igneous activity.
The sedimentary and sedimentary-volcanogenic complexes and weathering crusts contain deposits which almost never form extensive metallogenic belts or large provinces. The regularities of their location are determined not so much by tectonic and geodynamic characteristics, as by the sedimentation environment, paleogeographical and paleo-climatic conditions. These are mainly oolitic manifestations of iron ores of the Paleozoic, Mesozoic and Cainozoic manganese deposits with oxidation zones, infiltration manifestations of uranium.
Classical telethermal lead and zinc deposits of the Mississippi type in the Paleozoic mantle of the North American Platform (USA) and their analogs on other platforms appeared under the effect of deep mineralized waters. We should also note sedimentary phosphorites and lateritic weathering crusts basically of the Cainozoic age, formed mainly in the tropical and subtropical climate, with huge deposits of bauxites, iron, nickel and cobalt. Among large and superlarge chemogenic deposits we should name the Paleozoic and Mesozoic salts. Fluvial and littoral placers on the platform shields, mainly Cainozoic diamond placers, are widespread there. It is probable that practically each of the above-mentioned types is promising with regard to discovery of new large and superlarge deposits.
The complexes of intraplate magmatism within the mantles are considerably less common. First of all, we should single out stratified basic and ultrabasic intrusions often confined to the marginal zones of continental lava fields of the basic composition. They are associated with the deposits of iron, titanium, tungsten, copper, nickel, platinum-group elements and cobalt. The manifestations of phosphorus, rhyobium, rare earth elements, yttrium, scandium, phlogo-pite (mica rich in magnesium) are confined to the rocks of basic composition with high concentration of alkali and carbonates. Large hydrothermal copper and arcenic deposits are associated with the zones of Mesozoic riftogenesis. Diamantiferous kimberlites rupture the mantle rocks on the Russian and Siberian platforms (Devonian), in the Republic of South Africa (Cambrian and Cretaceous), in Angola and the Congo (Cretaceous), in Tanzania (Paleogene). The formation of diamonds in the Popigay structure in the north of the Siberian platform (near the coast of the Laptev Sea) is confined to the Paleogene. The majority of the mentioned types of manifestations of intraplate magmatism in the platform mantles are promising with regard to discovery of new large and superlarge deposits.
The established regularities of confinement of belts and provinces of large and superlarge deposits distribution to the tectonic and geodynamic structures are only the first approximation in the analysis carried out by us. The connection of these deposits with deep anomalies in the structure of the continental Earth's crust and underlying upper mantle revealed in accordance with the findings of geophysical research is of prime importance. Involvement of geophysical data in the tectonic and metallogenic analysis in which at present a group of authors of the project under consideration are engaged will make it possible to obtain substantiated forecast of discovery of new large and superlarge deposits.
Illustrations supplied by the authors.
* Laterite is a product of weathering in the conditions of tropical climate. - Ed.
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