EVOLUTION OF GRANITOID COMPOSITION AND ORE MINERALIZATION ON EXAMPLE OF FAR EAST REGION

Kopylov M.I.

FGUP “Dal'geophysics”, Khabarovsk, Russia, kopylov@dalgeoph.ru

Granitoid magmatism of Far East region

The granitoids of all the age epochs, i.e. Archean, Proterozoic, Paleozoic, Mesozoic and Cenozoic ones are widespread within the Far East region. The Archean granitoid magmatism is characteristic only of the Aldan shield. The Proterozoic granitoids have maximal concentrations within the Stanovoi Ridge, and within the Bureinskiy median mass to a lesser extent. The Paleozoic granitoids of mainly granite composition are most widely developed in the Khankaiskiy and Bureinskiy median masses. The Mesozoic magmatism within the Far East region occupies an absolutely leading position and is practically spread through the whole Pacific mobile belt (PMB), from Chukotka in the north to sia Minor in the south. The Cenozoic granitoid magmatism is developed as a narrow stripe within the East Sikhote-Alin volcanic belt. Several epochs are recognized in development of PMB granite magmatism. The main ones of them are Triassic-Jurassic, Jurassic (150 Ma) – Early Carboniferous (100 Ma), Late Carboniferous (70 Ma) – Paleogenic (55-15 Ma) and Neogenic (13-8 Ma). The study of intrusive magmatism in depth has been carried out by using the data of deep geophysical investigations (GSZ, MOVZ, MTZ, gravimetry, magnitometry). In the studied sections, granites are ubiquitously observed in depth 10 km, and they are followed by complex combination of granite and basalt fields in depth of 20 km. Granites disappear in fact in depth 30-40 km, and ultrabasites dominate in the 50-60 km sections.

Evolution of granitoids in changing the geochemical compositions of elements

When considering the distribution of geochemical elements in granitoids in coordinates of geological time, the evolution of their changes in quantitative and spatial respects. Contents of petrogenic and REEs in granitoids invariably depend on the occurring geological processes (Byhover, 1984). At the same time, an inversible evolution of the matter composition of the Earth's crust, lithosphere and upper mantle occurred. The rift structures in the primary and oceanic crust are represented by ophiolites with dunite-harzburgite inrusions that contain magnesial and rich chromites with platinoids, and continental ophiolites being rich in iron and titanium with ultrabasites without rhombic pyroxene with ilmenito-titanomagnetite ores.

During evolution and continentalization of the Earth's crust, an increase in its thickness occurs, and development of acid and acid-alkaline magmatism results in formation of rock magmatic sequence of complex composition, assemblage of geochemical elements and ore-bearing systems and deposits respectively.

Of particular interest is geochemistry of magmatic rock elements that can throw light on the occurring thermodynamic processes during formation of geological structures, ore systems and deposits (Ryabchikov, 2000). The data of spectral, silicate and chemical analyses on acid, middle and basic magmatic formations of Pre-Amur region have been used for comparative characteristics. A coefficient of content variability of elements and oxides that is calculated by use of clark values for each epoch of magmatic rock formation has been employed in systematizing the data. A quantitative increase in rare elements from ancient formations to the younger ones is observed in the various age granites, granodiorites, diorites and gabbros, by the available data of average concentrations of some petrogenic and rare elements. Thus, Sn content in young granites increase twice compared to the Proterozoic granites. An increase in some rare elements (Li, Nb, Rb, Yb) and petrogenic ones (MgO, K₂O, FeO and Fe₂O₃) is also noted. In ancient gabbroids, increase in contents of petrogenic elements (MgO, CaO, K₂O) as well as those of femic profile elements (Ni, Co, Cr and Cu) and contents of rare elements (Nb, Zr, Rb, Sr, Sn, Be, Yb) in young gabbroids is fixed. By variability coefficient, the highest gradients are noted for Fe₂O₃ and MnO. On the whole, in granites of Pre-Amur, the variability coefficient is more than 1, i.e. more than clark values (by Deli), except Na₂O and K₂Othat likely indicates the significant metasomatic process taken place there. In contrast, an excess of petrogenic oxide contents (Fe₂O₃, FeO, Na₂O, K₂O, TiO₂) over clark ones, except MgO and CaO is specific of gabbroids. The higher clark contents of rare elements (Yb, Sr, Mo, Sn (only for Mesozoic granites) as well as higher contents of Ni and Cu are characterized for the Pre-Amur granites of all ages. An excess of Cr, Co, Ni, Cu, Pb and Nb clark values is specific of the Proterozoic gabbros.

Discussion of results

The productive tin, gold, Mo, Cu mineralization in all volcano-plutonic belts is mainly associated with the Mesozoic and Mesozoic-Cenozoic metallogenic epochs. In addition, conjugated “rejuvenation” of both magmatism and mineralization occurs in direction from inner parts of the continent to its margins. An association of hydrothermal mineralization with definite metallogenic epochs, its spatial conjugation with aureoles of magmatism, element coincidence of structural-petrological and metallogenic zonation are all the factors that are of great importance in predicting the new ore-bearing systems.

References

Bykhover N.A. (1984) Distribution of world mineral resources on epochs of ore formation. M., Nauka, 576p.

Ryabchikov I.D. (2000) Sulfide phases in Earth's mantle and behaviour of chalcophile elements. Geology of ore deposits. V.42, N2, P.141-146.