Home – Home – Jornals – Russian Geology and Geophysics 2009 number 10

The Erdenetiyn-Ovoo magmatic center (EMC) with a porphyry Cu-Mo deposit includes the following intrusive complexes: Selenga, Shivota, ore-bearing porphyry, and post-ore dike. The EMC formed at 260-200 Ma. The geologic evolution of northern Mongolia in that period was much determined by the effect of a mantle plume, which showed two periods of activity: Late Paleozoic and Early Mesozoic. The long multistage evolution of the EMC was due to its localization on the periphery of the Late Paleozoic and Early Mesozoic areas of the plume's influence. The Shivota and post-ore basites are considered to be comagmatic to the Late Permian-Early Triassic trachyandesite-basalt and Late Triassic-Early Jurassic trachyandesite series, respectively, which are similar to the products of Late Paleozoic and Early Mesozoic within-plate magmatism in northern Mongolia. The Selenga complex, which formed before the Shivota one, and the porphyry complex, which formed before the post-ore dike one, are differentiated gabbro-granite series. Gabbro-granitoid magmatism was initiated by the melting of rocks of continental lithosphere under the action of a plume. Later on, as the plume ascended to the surface and the lithosphere became thinner, the conditions were created favoring the lithosphere breakthrough and within-plate basaltoid magmatism.
In geochemical features (high contents of LILE and LREE, low contents of HFSE and HREE) the studied basites are similar to the products of subduction magmatism. But this contradicts the geologic position of basites formed after the completion of subduction during the transition of the region to the rifting stage and during the rifting. The mantle metasomatized during the preceding subduction is regarded as the main source of basites. The high contents of alkalies and LREE in the volcanics of the post-ore dike complex and the REE patterns similar to the OIB ones evidence the influence of the plume on the magma formation. The high contents of incompatible trace elements and the Nd isotope composition corresponding to the weakly depleted mantle do not exclude a possible plume effect during the formation of the Selenga complex gabbroids. The geochemical features of the Shivota gabbros, comagmatic to volcanics produced during the Late Paleozoic within-plate activity, are partly transformed during the melt evolution in crustal chambers.
The REE patterns of the EMC basites evidence that the evolution of ascending magma was accompanied by the fractionation of amphibole. During this process, ore elements were redistributed into mineral and concentrated in amphibole-containing rocks, from which metals were later mobilized by late melts and fluids. The evolution of basaltoid magmatism of the Selenga, Shivota, and porphyry complexes is regarded as a preliminary stage of ore formation, which was considerably responsible for the EMC productivity.

A representative sample of microdiamonds in calc-silicate and garnet-pyroxene-quartz rocks and gneisses from the cross section of an adit driven at the Kumdy-Kol' deposit (Northern Kazakhstan) has been analyzed. Microdiamonds from these rocks were studied by Fourier-transform infrared spectroscopy for the first time. It has been established that nitrogen impurity content (300-3000 ppm) and nitrogen aggregation degree (14-75%) vary widely and do not correlate with each other. The variation is probably due to the uneven distribution of nitrogen in crystals and to their specific internal structures.
The results of the study show that in most diamondiferous rocks, diamonds crystallized from a fluid/melt of composition varying between aqueous-carbonate and aqueous-silicate end-members. Spectroscopy studies partly disagree with literature data on individual nanoinclusions in diamonds. The cause of this discrepancy may be the evolution of the fluid/melt during diamond crystallization.

The criteria for the evaluation of the REE composition of phosphorites and sedimentary rocks have been determined. These data are required to reconstruct depositional environments. Literature data on the geochemistry of some phosphorite deposits of Eurasia are considered. The REE patterns of Mesozoic phosphorites of the East European Platform are studied. On the basis of REE contents, the ratios of lanthanides and fields on the La-(Nd + Sm)-(Y + Dy), La-(Ce + Nd + Sm)-(Y + Dy), and LREE-MREE-(HREE × 10) diagrams have been determined as indicators of climate and the depth and facies conditions of sedimentation.

Platinum group element (PGE) mineral assemblage has been discovered in the gold placers along the Burgastain Gol and Iljgen Gol (Western Mongolia). It includes isoferroplatinum (Pt₃Fe) grains with inclusions of cooperite (PtS), laurite-erlichmanite (RuS₂-OsS₂), cuprorhodsite-malanite (CuRh₂S₄-CuPt₂S₄), irarsite-hollingworthite (IrAsS-RhAsS), and bowieite (Rh₂S₃).
It has been established that the isoferroplatinum assemblage was generated from a volcanoplutonic picrite complex in the Ureg Nuur area, which is widespread in the central part of the Harhiraa accretionary terrane. According to composition, the PGE mineral-Cr-spinel assemblage was referred to as the Ural-Alaskan type.

The study presents results of a hydrogeological and hydrogeological research conducted on the Mesozoic-Cenozoic sedimentary cover and Precambrian-Paleozoic platform deposits of the Pre-Yenisei petroleum subprovince. The hydrogeological structure of the study area is found to be of a transition type from the West Siberian to Tunguska artesian basins, with its own set of pertinent parameters, such as groundwater depths, rock permeabilities, water chemistry and gas compositions, gas saturation, vertical zoning, etc. The upper part of the sedimentary section is known to be flushed with the infiltration waters to depths of 2-2.5 km. The deeper intervals contain the older sedimentary waters with the moderate metamorphic grade. The results of the study provide implications for the petroleum potential of the area of interest.

Sedimentary processes in the paleocoastal part of the Bengal basin that occured in the Tertiary and Quaternary have been addressed. Three indicators were used: sedimentary bedding forms, microstructure of the sediment, and trace fossils. Various forms of sedimentary structures developed under the influence of dynamic geomorphic processes in the study area in the Quaternary. The microstructure analysis of the sedimentary materials was made by two methods: microphotography and Digital Color Analysis (DCA). The microstructure analysis shows that the geomorphic process remained very dynamic in the Quaternary, influencing the form, thickness, and mineral composition of the sediment strata. The enrichment of the sediments in heavy minerals evidences either oscillating or combined flow sedimentation, while under stable conditions light-mineral deposition took place. The digital data of microfabric study by the DCA method also show that larger particles deposited in the oscillating or high-flow environment and evidence a greater amount of heavy minerals like ferruginous materials. Trace fossils found in the sediments of this area also strongly support the concept that the environment remained dynamic during the Tertiary and Quaternary. The Late Tertiary deposition shows that, during these periods, the sediments were transported from tide-dominated marine coast with low flow energy, which is typical of hot and humid conditions. From Late Tertiary to Early Quaternary, the macrotidal coast became mesotidal (wave-dominated). The second phase is the Middle Pleistocene, when the environment was stable, favoring the continuous deposition of finer particles under low- to medium-flow energy conditions. The third phase, the Recent, is marked by the shoreline shift and modification of the environment. In the Early-Middle Holocene, the shoreline started to shift, which modified the geomorphic conditions of this place from coastal to estuarine and, finally, inland fluvial.

We have simulated ungrounded horizontal loop transient responses of a two-layer earth consisting of a magnetically viscous layer above (model 1) or below (model 2) a nonmagnetic layer. The transient responses of a two-layer magnetically viscous earth can be computed using the superposition principle because magnetic relaxation and eddy current responses are independent at electrical conductivities typical of the real subsurface. The transients are presented and analyzed in the form of Y = f(h₁) functions, where h₁ is the upper layer thickness and Y is the response (at some fixed time) of a two-layer ground normalized to that of a uniform ground with its magnetic viscosity as in the upper (model 1) or lower (model 2) layer. In model 1, the Y function increases as magnetic viscosity grows in the upper layer while the latter is thinner than the loop size, but the magnetic relaxation responses of a thicker upper layer are almost identical to that of a uniform magnetically viscous ground. In model 2, the Y responses are likewise almost identical to that of a uniform magnetically viscous ground (h₁ = 0) as far as the thickness of the upper layer remains small, but they decrease, first slowly and then ever more rapidly, after the layer becomes 15-20% thicker than the transmitter size. The effective sounditng depth in a magnetically viscous ground being controlled by the size of the transmitter, it is reasonable to use geometrical sounding to resolve the vertical distribution of magnetic viscosity.

We explore the possibility to identify a nonconducting object of a known size and geometry buried in sand-clay conducting sediments lying over frozen ground. The anomaly, which is located on the surface of the low-conducting layer, is detected with a multi-electrode dc array. Its 3D electric field is simulated as a boundary-value problem using specially designed software. The new algorithm is tested with synthetic and field data and the results are used to develop the procedure of object identification and location.

The paper presents objections against assigning picritic and some other igneous rocks to lamproite as suggested by N.V. Vladykin, both in general and in particular for the case of the large Tomtor pluton of alkaline and carbonatite rocks in northwestern Sakha Republic (Yakutia). Classifying the Tomtor picrites (picritic porphyry and its breccia-like varieties) as lamproite appears to be inconsistent with their chemistry, mineralogy, and geology. Discussed are errors and poorly proven postulates in the related publications by Vladykin, which contradict the available field data.