Geological & Metallogenic Framework of Georgia by
Alexander G. Tvalchrelidze
Figure below represents a three-dimensional geological map of Georgia. From the beginning of our century and, namely, in the thirties-sixties the Caucasian region in general, and Georgia in particular, were subject to numerous geological, geotectonic, petrologic and investigations. It seems to be almost impossible to quote even the most important investigations on the geology of the Caucasus that have been thoroughly systematized by Milanovskiy and Khain (1963) and in a collective monograph (Geology..., 1964). The above-mentioned two important works generalized all data and theories on the geological structure of Caucasus, including Georgia, based on the older ideas about the geosynclinal development of mobile belts. Introduction of the plate tectonics approach made local geologists reconsider the existing theories. Among a great number of publications investigations by Adamia, Buadze and Shavishvili (1977) and Adamia et al. (1981) must be mentioned where the Phanerozoic history of the Caucasus is interpreted as gradational development of an active continental margin of the ocean Paleo- and Mezotethys.
An interesting approach have been developed in the recent article by Adamia et al. (1997) and a number of publications by Gamkrelidze (1997, etc.) who used a terrane analysis for explaining the complex structure of the Caucasus. According to the classical definition (Howell, 1989), a terrane (or a "tectono-stratigraphic exotic bloc or complex") represents a geological body of the regional extent, limited by deep faults, that differs from neighboring bodies by its geological structure and, therefore, by geological history. Large terranes of the first order (or superterranes) may survive large displacement including substantial drift. These "composite" or "displacement" terranes consist of two or several subterranes that have been amalgamated before their accretion to the continental margin.
According to Gamkrelidze (1997), the Eastern Mediterranean mobile belt consist of five first order terranes (see Figure below). During the Paleozoic, Mesozoic and Early Cenozoic time these terranes underwent horizontal displacement in different directions within the oceans, consequently, Proto-, Paleo-, Mezo- and Neotethys. In Middle Cenozoic time these terranes were step-by-step glued to the Euro-Asian Plate. Later, during the Late Alpine orogenesis (Neogene-Pliocene), the above terranes survived intense deformations. Thus, in course of the whole geological history of the province, its northern margin was active whereas the southern - passive.
Figure below represents a three-dimensional map of tectonic & metallogenic zoning of Georgia. Three first order terranes are fixed here. The Greater Caucasus terrane accreted to the Euro-Asian Plate within the Hercinian time (Gamkrelidze, 1997). The Baibut-Sevanian terrane that has its ophiolitic suture southward from Georgia, is presented by the Somkhito-Karabakh subterrane - a typical andesitic island arc which continues eastward and westward along thousand kilometers and has a global extent (Tvalchrelidze, 1977). Different subterranes of the Black-Sea-Transcaucasian terrane represent different parts of the oceans Mezo- and Neotethys. Two main tectonic elements played a decisive role in lithogenesis here: pelagic oceanic Chkhalta-Tphani subterrane and the Transcaucasian Median Mass different parts of which have underwent relatively independent development (Tvalchrelidze, 1984) in form of particular subterranes. Exposures of the Paleozoic crustal basement of this median mass are fixed within the Dzirula, the Khrami and the Loki massifs (see the Geologic map).
The Greater Caucasus terrane is mainly composed of pre-Cambrian and Paleozoic rocks. According to numerous investigations (see, for instance, Gamkrelidze, 1957; Milanovskiy and Khain, 1963; Geology..., 1964; Adamia, 1974, 1977; Borsuk, 1977 and many others), within the Greater Caucasus a granitic-metamorphic complex presented by different crystalline schists and paragneisses have been intruded by Paleozoic granites. During the Hercinian tectogenesis this terrane was accreted to the Euro-Asian Plate (Adamia et al., 1997; Gamkrelidze, 1997). Lower Jurassic rocks here are presented by shallow sea sandy-argylleous rhythmical sediments with lenses of limestones.
The Black-Sea-Transcaucasian terrane is separated from the Greater Caucasus terrane by the Greater Caucasus Main Fault that played a role of a subduction zone in Early Jurassic (Adamia, Buadze and Shavishvili, 1977) and after the Bathonian (Middle Jurassic) orogenesis has been modified into a typical deep thrust. The latter is saturated by numerous dikes of Bajocian gabbro-diabases. The central part of the Black-sea-Transcaucasian terrane is occupied by the Dzirula outcrop of the Transcaucasian Median mass. Presence of pre-Cambrian (?) ophiolites have been announced here (Tectonics..., 1984). This massif consists of Paleozoic schists intruded by numerous Paleozoic and Jurassic gabbros, quartz diorites, granites accompanied by pegmatite veins. Paleozoic rocks are overlapped by shallow water Jurassic clays and Cretaceous limestones. By that time Dzirula Massif losted it independance, and Mezozoic rocks concordantly continue into the Dzirula and the Middle Mtkvari subterranes.
Main sequence of the Chkhalta-Tphani subterrane consists of a thick (up to 3-10 km in different places) Lower Jurassic series of black schists (argyllitic deep sea sediments with organic material metamorphosed up to green schists) enriched at different levels by arcose sandstones and metamorphosed limestone strata as well as by fissure flows of tholeiitic bimodal basalt-ryolitic volcanism (Beridze, 1973; Borsuk, 1977; Tvalchrelidze, 1984). This sequence concordantly continue the Dizi series of Permo-Triassic quartz sandstones and marbles (Gamkrelidze, 1957). Thus, the ocean Mezotethys have been originated in Late Paleozoic but had a local distribution. Early Jurassic deep-sea sedimentation ended in Aalene time, and the Chkhalta-Tphani subterrane took no part in Alpine tectonogenesis.
Middle Jurassic (Bajocian) basalt-andesite-dacite-ryolite formation is typical of the Gagra-Java subterrane. In the beginning of Middle Jurassic this structure have been broken out the Transcaucasian median mass and involved into the Mesotethyan events (Gamkrelidze, 1969). The lower part of the thick (up to 3 km) volcanic sequence is composed of tholeiitic fissure pillow lavas. In Middle Bajocian calc-alkaline andesitic central-type volcanoes have been originated (Dzotsenidze, 1948; Janelidze et al., 1984, etc.), and the Gagra-Java trough have consequently been transformed into an island arc (Lordkipanidze, 1980). In Bathonian time carboniferrous molassic sediments have been deposited here. Simultaneously, granitic stocks have been intruded just in the marginal suture between the Gagra-Java and the Chkhalta-Tphani subterranes. Jurassic limestones (including reef) lie at the top of a stratigraphic column.
The Mestia-Tianeti subterrane has originated in the Middle Jurassic time, when calcaceous sandstones and limestones were deposited here. These sediments are followed by a flysh sequence with rare lenses of limestones.
In the Dzirula and the Middle Mtkvari subterranes Jurassic rocks are presented by a carbonate sequence. In the Dzirula subterrane thickness of Upper Jurassic limestones gradationally decreases towards the Dzirula massif forming several subsequent reefs near the Kutaisi city.
Before Cretaceous all described subterranes of the Black-Sea-Transcaucasian terrane were already amalgamated and Cretaceous carbonate sedimentation took place in centripetal succession from the Dzirula Massif. Shallow-water organogenic limestones are typical of the southern part of the Gagra-Java and Mestia-Tianeti subterranes. Within the Dzirula and the Middle Mtkvari subterranes carbonate sediments of this age are relatively thicker (up to 500-700 m) and often interlayered with andesitic lavas and pyroclastics.
Cenozoic sediments are practically absent in the Gagra-Java and the Mestia-Tianeti subterranes. Only thin (50-150 m) Paleogene shallow-sea carbonate and flyshoid sequences are presented here. In the Dzirula and the Middle Mtkvari subterranes Paleogene sediments lie in the basement of, correspondingly, the Rioni and the Mtkvari river troughs, and their thickness gradationally increases towards the Black and the Caspian seas. Paleogene sediments are followed here by the Oligocene Maikop series - an intermountain molasse formation with clayey, often rhythmical rocks and few limestones. This series underwent intense dia- and autigenesis (Khamkhadze, 1984). Thickness of this series increases westward and eastward from the Dzirula Massif. In central parts of the Rioni and the Mtkvari troughs the Maikop series host oil and gas occurrences whereas in vicinity of the Dzirula massif rich and large manganese deposits were formed (the Chiatura Mine). Neogene sandy- clayey sediments in the Dzirula and the Middle Mtkvari subterranes form a thick (up to 2 km) sequence. Near the Black Sea cost Neogene rocks are superimposed by sallow marine Quaternary sandy-clayey sediments of the consequent littoral terraces.
In Early Cenozoic in the southern part of the Black-Sea-Transcaucasian terrane a rift trough of the Adjara-Trialeti subterrane starts to develop directly upon carbonate Cretaceous sediments (Adamia et al., 1987; Lordkipanidze, 1980; Adamia et al., 1981, etc.). Paleogene and Early Neogene trachibasalt-trachiandesite volcanism enlarges from the eastern edge of this rift (where tuffaceous sandstones and clays form a rhythmical series) towards the West (where typical volcanic flows from volcanic cones interlayered with tuffites). In Late Neogene a number of quartz-monzonite-diorite massifs have been intruded predominantly in the western, tectonically more active, part of the mentioned rift. Orogenesis occurred here by the end of Neogene, just before the Quaternary period.
In the Somkhito-Karabakh terrane (in reality this unit represents a subterrane of the Baibut-Sevanian terrane. At the Georgian territory this subterrane is the single on. Due to this fact we will use later the term "Somkhito-Karabakh terrane" for distinguishing it from subterranes of the Black-Sea-Transcaucasian terrane) Middle Jurassic rocks lie directly upon the crystalline basement of the Transcaucasian median Mass. The sequence starts with a basal conglomerate that overlaps Paleozoic granites, for instance, near the Khrami and the Loki massifs, followed by arcose sandstones. Outside Georgia, in Armenia and Karabakh, a thick calc-alkaline subaquaous andesitic volcanic-sedimentary series is developed (Ore..., 1982). But northward, towards Georgia, this island arc affinity decreases its thickness, and manifestations of Middle Jurassic volcanism became rare (Kekeliya et al., 1984). Here a thick ore-bearing (Madneuli Mine) Cretaceous andesitic (minor basalt-andesite-dacite-ryolite) calc-alkaline formation created numerous volcanoes and vast fields of acid subaqual pyroclastics in intervolcanic areas. Especially thick (up to 1-1.5 km) this sequence is within the Bolnisi Mining District (Kekeliya et al., 1984, etc.). Paleogene and Neogene strata continue the Cretaceous volcanic sequence forming andesitic subaeral affinity of the subsequent calc-alkaline series interlayered with fresh-water bog and lacustrine rocks. The most interesting event is presented by thick (several hundred m) flows of massive andesitic aeral lavas, which form a volcanic plateau of the Southern Georgia. The same phenomena (but much less intense) have been encountered on the southern slope of the Greater Caucasus (Ore..., 1982).
The modern premier case metallogenic investigation has been effectuated by my father, Academician George A. Tvalchrelidze who published in 1961 an important monograph (Tvalchrelidze, 1961). This publication explained metallogenic features of Georgia from the eugeosynclinal point of view. Later he distributed his approach entirely on the Mediterranean mobile belt (Tvalchrelidze, 1972). Introduction of the New Plate Tectonics obliged the author to change his metallogenic method. First of all, he paid attention to typomorphic metallogeny of global volcanic belts (Tvalchrelidze, 1977) and in early eighties published the first "mobilistic" metallogenic schemes of the Caucasus (Tvalchrelidze, 1980; 1984).
Nowadays there is a possibility to interpret main mineralization features of Georgia based on above mentioned terrane analysis.
George Tvalchrelidze (1966) distinguished the Proterozoic, Paleozoic, Caledonian, Hercinian, Cimmerian (early-Middle Jurassic), and Alpine metallogenic epochs of the Caucasus. The first two of them have been manifested within the Northern Caucasus pre-Cambrian-Paleozoic ophiolite belt and the related volcanic island arc: the Tyrny-Auz-Pshekysh suture and the Caucasus Foreridge (Adamia et al., 1977; 1997). Within boundaries of Georgia a single example of pre-Cambrian - Early Paleozoic mineral deposits is observed on the Dzirula massif (Rikoti deposit of facing gabbros) where the exact age of mineralization is almost impossible to evaluate. We were obliged, therefore, to distinguish a single pre-Hercinian epoch without its exact dating.
Cimmerian (Early Jurassic) mineralization in different subterranes manifested in deep-ocean (Cu-pyrrhotite ores, roof shales in the Chkhalta-Tphani subterrane) and shallow-water marine (refractory clays in the Dzirula subterrane) environments.
George Tvalchrelidze (1961; 1966) considered a single Alpine epoch that occurred in different metallogenic zones in slightly different time. But thorough observations on ages of almost all mineralization clearly indicate presence of, at last, three different epochs in Cenozoic. The first of them occurred in Late Cretaceous, the second one took place in Oligocene time, and the third epoch manifested in Neogene. All of them are characterized by different mineral formations in different terranes and subterranes.
Early Alpine epoch produced two typical island arcs at the northern and southern peripheries of the Transcaucasian Median Mass. The former (Gagra-Java subterrane) is characterized by vein and strata-bound Pb-Zn and barite deposits as well as iron-bearing coal basins. The latter (Somkhito-Karabakh terrane) produced auriferous polymetallic base metals, facing tuffs, lithographic stones. The central part of the Black-Sea-Transcaucasian terrane by that time was characterized by orogenic environment producing facing materials and shallow-water Al-bearing sandstones, which have been deposited in a closed basin of the Dzirula subterrane. In the shallow-water basin (Mestia-Tianeti subterrane), superimposed upon a newly consolidated continental margin (Chkhalta-Tphani subterrane), auriferous realgar-orepigment mineralization has been formed.
Middle Alpine epoch has manifested only in the central part of the Transcaucasian Median Mass. During the Oligocene transgression westward and eastward from the Dzirula massif shallow-water molassic basins have originated. Molassic sediments, inclined towards, consequently, the Black (Dzirula subterrane) and the Caspian (Middle Mtkvari subterrane) seas, were subject to specific sedimentation with typomorphic dyagenetic and autigene mineral formations. Manganese deposits, manganese-related spongolite and chalcedony mineralization are typical to this epoch. Simultaneously, in lowland areas, where molassic sediments were thicker, first accumulations of hydrocarbons occurred, locally associating with halogens.
Late Alpine epoch was complex. Within the Greater Caucasus terrane and along its Main Fault typical orogenic both granite-related and telethermal ore formations originated. Southward, in shallow water conditions (Gagra-Java subterrane) phosphorite ores were deposited. In the central part of the practically entirely consolidated Black-Sea-Transcaucasian terrane shallow-water quartz sands have been deposited. In the Adjara-Trialeti rift subterrane a typical porphyry copper system as well as iron skarn deposits were related to quartz-monzonite-diorite intrusive bodies. Simultaneous postvolcanic processes resulted in agate formation. In shallow subsequent fresh-water lakes and bogs, correspondingly, diatomites and carboniferous strata have been deposited. In the Somkhito-Karabakh terrane last volcanic paroxysms and a postvolcanic activity produced perlite, obsidian and agate deposits.
Quaternary epoch is governed by climatic conditions of the Greater and the Lesser Caucasus as well as by features of their relief. Different alluvial (including Au placers), deluvial and proluvial sediments, weathering core and modern bog environment determined origination of corresponding modern mineral deposits.
|Main Ore/Mineral Formations of Georgia
||Pre-Cambrian and Early Paleozoic
||Cimmerian (Early Juras-sic)
||Mo, As, W, Sb
||Cu, roof shales, facing marmo-rized limesto-nes
||coal, Fe, Pb-Zn, barite, facing, building and flux limestones, anhydrite, gypsum, lime
||cement and building lime-stones, anhyd-rite, gypsum, foundry sands
||foundry sands, brick earth, mi-neral paints
||talc, pottery pegmatites, facing granites and quartz dio-rites
||facing marmo-rized limesto-nes, refractory clays
||Al, gumbrine, porcelain kaoli-nite, facing te-schenites, fa-cing marmori-zed and buil-ding limesto-nes, lime, dolo-mites, chalk
||oil, thermal wa-ters, Mn, chal-cedony
||gumbrine, qu-artz sands, lime, cement limestones
||peat, mineral paints, cement clays, brick earth
||Au-As, facing marmorized, cement, buil-ding limesto-nes, lime
||foundry sands, brick earth
||thermal waters, Fe, quartz sands
||coal, Au-Pb-Zn-Cu, zeolites, di-atomites, facing syenites, mar-morized lime-stones, gypsum, refractory clays, agate, opal
||peat, acid-resis-ting andesites, facing basalts, cement clays
||oil, gas, hallo-ides
||Glauber salt, gravel, foundry sand, cement clays
||Fe, Au-Ag-Cu-Pb-Zn-barite, lithographic stones, flux li-mestones, facing tuffs
||perlite, agate, obsidian
||Au placer, fo-undry sands, fa-cing basalts, cement scoriae, anhydrite
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