Home – Home – Jornals – Russian Geology and Geophysics 2006 number 12

Platiniferous ultramafic-mafic magmatic assemblages of different types of mobile belt structures from the folded framing of the Siberian and South Chinese cratons are considered in a wide span of time, from Precambrian to Mesozoic. Attention is given to platinum mineralization linked to diverse complexes formed under different geodynamic settings: ultrabasic and basic intrusions of Precambrian greenstone belts, layered ultramafic-mafic intrusions of marginal near-platform rift-related structures, Alpine-type ultrabasic rocks of Riphean-Early Paleozoic ophiolite belts, volcanic-plutonic complexes of Paleozoic collision orogens, picrite-dolerite and alkali-basite volcanic-plutonic associations of Hercynian and Permian-Triassic systems.

We investigate cratonic terranes in northeastern Russia which have very high mineral potential but remain poorly explored or not explored at all. The suggested synthesis of the available data from an underexplored but exceptionally rich province provides evidence of high metallogenic prospects of cratonic terranes and their surroundings in northeastern Russia as well as in other regions. Especially good prospects are expected for gold mineralization, which shows continuous development from Precambrian to Cenozoic structures of northeastern Russia.

A general metallogenic analysis and metallogenic zoning of the Altai-Sayan orogenic area (ASOA) were carried out in terms of the modern plate tectonics and mantle geodynamics concepts. The Altai-Sayan folded area is an example of a polyaccretionary orogenic system that resulted from the long evolution of the Paleoasian ocean. The main metallogenic belts have been recognized, in which typical ore associations (model types of mineral deposits) and their ages and geodynamic settings of formation have been established. A total of 48 metallogenic belts including 450 mineral deposits of 70 model types were studied. These belts are related to four time spans (metallogeny epochs) corresponding to the cycles of geodynamic processes that led to the formation of the polyaccretionary orogenic area: Riphean-Vendian (1200-620 Ma); Vendian-Silurian (620-410 Ma); Devonian-Early Carboniferous (410-320 Ma); and Late Permian-Triassic (260-205 Ma). Study was also given to typical geodynamic settings in which ore-forming productive systems originated. It is shown that the metallogenic evolution of the ASOA was determined mainly by the multistage formation of active continental margins and island-arc systems in the Riphean-Vendian and Early (V-S) and Middle (D-C₁) Paleozoic.
At the postorogenic stage, the ASOA evolution was the most productive in the Triassic. The geodynamic and metallogenic events in this period were determined by the tectonothermal activity on the periphery of the Permo-Triassic Siberian superplume in the interblock zones of the orogenic collage, which led to serious shifts along the plate and block boundaries, the formation of near-fault troughs and grabens, appearance of rift structures, and development of anorogenic granitoid magmatism (manifested as alkali and subalkalic rare-metal granites) and alkali-basaltoid magmatism.
Transpression settings (oblique subduction) and plume magmatism are shown to have played a key role in the formation of mantle and mantle-crustal ore-forming systems. For the Middle Paleozoic and Mesozoic stages, new geochronological evidence has been obtained, and spatial and temporal correlations for the formation of the main types of mercury, gold, and rare-metal deposits have been made.

In the Orhon-Selenge trough (OST) (northern Mongolia), which is part of the extended Permo-Triassic Selenge volcanoplutonic belt, the formation of ore-bearing porphyry complex and large-scale stockwork Cu-Mo mineralization (Erdenetiyn-Ovoo deposit) was preceded by multiphase magmatism, which was accompanied by ore mineralization of different scales and types. The evolution of the Permo-Triassic magmatism successively resulted in ore occurrences, which form a single metallogenic series (predominance of Cu and permanent presence of Mo): native Cu (P; differentiated basalt-andesite-rhyolite series)-Cu-Ni-sulfide, Cu-skarn, and Cu-vein (P₂-T₁; basites and granitoids of the Selenge complex)-native Cu (P₂-T₁; trachyandesite-basalt series)-porphyry Cu-Mo (T; Erdenet ore-bearing porphyry complex). In this series, the intensity of fluid flow grows and the redox potential of the endogenous system shifts to that of more oxidizing conditions, which favors the transfer of ore-forming elements from depth and increases the Cu-bearing capacity of magmatogene fluids. The development of magmatism and accompanying mineralization in the OST is assumed to be related to one of the lower-mantle plumes that existed in the vast Asian area in the Permo-Triassic.

The internal structure and proportions of the main rocks of the Kuvalorog gabbro-cortlandite massif, the largest intrusion on the Kamchatka Peninsula, are considered. The intrusive rocks are shown to abound in pargasitic amphibolite and biotite. Ultrabasic orthopyroxene contains melt inclusions with porphyritic pargasite phenocrysts, which point to a high water content in the parental melt of this intrusion. In trace-element patterns of rocks and minerals the massif is similar to Ni-bearing traps of the Siberian Platform. The behavior of lanthanides in the cortlandites evidences that their parental magma was melted out of a garnet-bearing mantle source and then crystallized under high-pressure conditions. Like the productive intrusions in the Noril'sk district, the rocks and all rock-forming minerals of the Kuvalorog massif are depleted in Ni relative to chondrite C1, which indicates a high Ni-sulfide ore potential of the massif.

After the Earth had melted in the conditions of mostly reduced fluids, its chemistry included two distinct groups of light and heavy elements with high and low oxygen affinity, respectively. Light elements, with their density lower than in Fe but oxygen affinity higher than in FeO, accumulated in the essentially oxygenic sphere composed of silicates and oxides. Heavy elements with low oxygen affinity and native Fe sank to the Earth's center and formed the iron core. Thus the Earth's protomaterial partitioned to make an oxygen-free core where liquid iron stores enormous amounts of H₂, CO, CH₄, S, H₂S, and other reduced gases surrounded by an almost 3000 km thick mantle in which the constituent minerals contain 75-80% oxygen. This separation of elements at the beginning of the Earth's history predetermined the specific behavior of fluids in all deep processes, including metallogeny, for the billions of years which followed.
Self-organization of the Earth's upper layers by means of granite formation produced the crust. Having lost its granite-forming components, the mantle graded into solid depleted mantle underlain by non-depleted asthenosphere impregnated with fluids. The mineralizer capacity of asthenospheric fluid systems correlates with their T and P conditions and the related maturity of lithospheric blocks. Therefore, the deep-seated origin of mineral deposits stems from two feeding fluid super-systems, the core and the asthenosphere, each with its typical chemistry. The two sources produced the respective metallogenic provinces with the chemistry of deposits controlled by the causative mineralizer systems.

Fluid and melt inclusions in minerals from magmatic rocks and associated ores and metasomatites were studied. Based on the results obtained as well as on experimental data, possible evolution trends are considered for the phase composition of magmatogene fluids separating from granitoid and basic melts at different depths. Three types of fluids have been recognized according to their composition and phase state, which differ in metal contents: (1) homogeneous supercritical, (2) heterophase

We present a synthesis of numerical modeling data for the evolution of mantle-crust systems in oceanic and continental spreading zones from decompression melting with the associated generation of mafic magmas and fluid release in their crystallization to mineral deposition in the crust. Model parameters were chosen to match those appropriate for natural magmatic-fluid systems in slow-spreading mid-ocean ridges (MOR) and the Siberian trap province. The evolution of a melting region was modeled for two cases: (i) a hot spot beneath a mid-ocean ridge, with 7-10 km thick oceanic crust underlain by metasomatized lithosphere, and (ii) a melting region beneath anomalously thick crust. Magmatic systems beneath thick crust were found out to be more compact and symmetrical and undergo a longer evolution.
Numerical modeling for continental melting zones with regard to the lithospheric structure and the size of the juxtaposed cratons and plates allowed the following inferences: (1) the extent of the predicted lithospheric melting region slightly exceeds the length of the respective lava field, (2) the melting zone has a layered structure (therefore, melts derived from a relatively homogeneous substrate should be homogeneous and of the same type), (3) magma chambers are relatively independent, which provides a qualitative explanation for the known cyclicity of lava compositions and the spatial distribution of major-element compositions of rocks in igneous provinces.
The behavior of the compositions of fluids outgassing at the solidus boundary from the crystallizing basaltic melt were computed using the Selektor software in a flow reactor and a step source modifications. Modeling shows that a quasi-steady temperature profile of a fluid-magmatic system related to a 30-40 km deep magma source sets up for 0.5 to 1 Myr. We infer that uncondensed reduced fluids vent on the seafloor and produce graphite and Fe, Ti, and Mn ferrite deposits found in the crest of the Mid-Atlantic Ridge. The numerical results were supported by physical modeling of carbon precipitation during the interaction of synthetic gas with mafic and ultramafic minerals. Carbon-related mineralization associated with gas condensation is controlled by the relative contents of C, H, Cl, F, and S in magmatic fluids. The composition of outgassing fluids changes notably within the liquidus-solidus range in crystallizing magma. During retrograde boiling, fluid separates into a low-density fraction and a brine.
Retrograde boiling in magma chambers and auto-metasomatism of igneous rocks are similar in mid-ocean ridges and in the Siberian trap province in the case of very low wall-rock assimilation and contamination of mafic melts. Assimilation of crustal material, especially carbonates and salt-bearing rocks, coal beds, hydrocarbons, or oil water, by mafic melts produces anomalous magmatic fluids with up to 60 rel.% total hydrocarbons, including 45-50% CH4, and total H₂O, H₂S, N₂, and with H₂ two orders of magnitude higher than CO₂ and CO. These very fluids promote the formation of mineral deposits hosted by igneous rocks.

We report results of computer modeling of physicochemical ore formation processes at mercury deposits accumulated during the development of secondary-hydrothermal and mixed-fluid ore-forming systems. Exogenous chloride brines, oil waters of artesian basins, and petroleum pools are shown to serve as secondary mercury reservoirs and geochemical barriers.
Modeling of possible mechanisms of mercury transfer and deposition in the form of cinnabar (α - HgS) was performed for ore-forming solutions of different compositions. Four main thermodynamic models have been constructed using the Chiller program: (1) simple cooling (cooling only), for recent thermal springs, (2) mixing of high-chloride hydrothermal solutions with cold hydrosulfuric waters (mixing model), for telethermal deposits, (3) isoenthalpic boiling (P = f (T)), and (4) solution-rock interaction (rock titration model).

As inferred from melt inclusions in minerals of volcanogenic rocks, the ore-magmatic pyrite systems of Rudny Altai and Tuva had quite a specific history. Volcanism manifestations in these regions have both similarities and differences.
The chemical composition of melt inclusions in quartz of acid volcanic rocks from the pyrite deposits of Rudny Altai is close to the chemical composition of the rocks, and in REE content they correspond to island-arc magmas. At early stages of development of a magmatic system, acid melts formed, which had high temperature (1230-1250