Russian Journal of Pacific Geology最新文献

Abstract

The relationship between strong earthquakes and powerful volcanism in subduction zones are discussed. It is shown that abnormally strong earthquakes or a group of strong earthquakes and powerful volcanic events or volcanic activation of large areas can be considered as an interconnected geodynamic pair. At high level of horizontal compression, magma-conducting faults are clamped and volcanism in the upper part of the crust is hindered. Mega-earthquakes or a series of strong earthquakes reduce the level of the horizontal compression in the crust of the island arc (continent active margin), which creates favorable conditions for the resumption of volcanic activity. For major faults, located along the strike of subduction zones, upward movement of magma during volcanic eruptions or intracrustal magmatism leads to an increase in the stresses of horizontal compression in the surrounding rocks up to the pressure level of rising magma. As a result, the stress state of horizontal compression is restored in the crust and, thus, the cycle is closed. The state in which earthquakes are an effective mechanism for reducing stresses occurs once again.

Abstract

The peculiarities of the distribution of rare-earth elements (REEs) in the surface bottom sediments in the Gulf of Tonkin (South China Sea) are considered. According to the results of granulometric analysis, the sediments are mainly represented by pelitic silt, silty pelite, and miktites. The REE contents normalized to the chondrite composition show the absence of cerium anomaly, the presence of negative europium anomaly and significant fractionation of light and heavy REEs. The values of indicator ratios La_N/Yb_N, Cr/Th, Th/Co, La/Sc, and Th/Sc indicate the predominance of rocks of intermediate and acidic composition in the erosion areas, due to which sediments were formed. The values of the Ce/La and (Fe + Mn)/Ti ratios characterize the surface sediments as typically terrigenous without the admixture of endogenous exhalative material. The Eu/Sm ratio values indicate low permeability of the sediments for upward fluid flows, which is consistent with the measured methane concentrations. The analysis of correlations indicates the dominance of the terrigenous source of REEs and that clay minerals and iron hydroxides play a significant role in the migration and accumulation of REEs.

Abstract

The structure and chemical composition of ferromanganese crusts formed on the sedimentary cover of the underwater volcanoes of the Peter the Great Seamount and the Vasilkovsky Ridge, located near the continental slope of Primorye (Sea of Japan), are studied. The crusts were formed as a result of cementation and precipitation of manganese hydroxides on the sediment surface. The signs of the hydrothermal nature of these formations are very high titanium and low aluminum modules, low contents of nonferrous and rare-earth elements (REEs), and an excess of heavy REE contents over the light ones. An ore substance was probably deposited in the Pleistocene–Holocene from postvolcanic hydrothermal solutions. Unlike the crusts deposited on basalts in the central parts of the Sea of Japan, these crusts do not contain inclusions of fine grains of medium- and high-temperature mineral phases of nonferrous metals.

Abstract

The African–Antarctic sector of the Southern Ocean is the least studied part of the World Ocean; its structure and evolution of the tectonosphere remains debatable. The complex development history of the region under study, accompanied by manifestations of intense magmatic and tectonic activity, has contributed to the formation of a number of large underwater ridges and rises. Identifying the features of the deep structure of the tectonosphere on the basis of the analysis of geophysical information and understanding the geodynamic nature of the morphostructures of the region under study is an urgent problem of marine geophysics and geodynamics. The structural analysis of the anomalous gravity and magnetic fields of the studied region was carried out. Zoning scheme by the anomalous fields as well as a generalized scheme have been constructed. The results of these studies made it possible to identify heterogeneous blocks of the lithosphere, which were formed on different spreading ridges and are separated by submarine rises.

Abstract

The study of diatoms from the sediments of core Gz-1 (Gryazevoe Lake, Magadan oblast) has made it possible to identify the response of diatom communities to environmental changes in the Late Pleistocene and Holocene and to establish the main stages of the transformation of the lake ecosystem. Three diatom zones and four subzones, reflecting the evolution trend of the lake diatom flora during the transition period from the last glacial maximum to the Holocene, are distinguished based on the changes in the concentration of diatom valves in sediments, the ratio of representatives of ecological groups and dominants. This trend is similar to the changes in the lacustrine diatom communities of the Arctic regions noted in the last millennium in terms of the change in taxonomic composition and dominants. The data of diatom studies, lithological and petrophysical characteristics of sediments indicate that the most significant changes in the natural environment occurred at the Pleistocene–Holocene boundary from 12.6 to 11.1 ka.

Abstract

Geological observations and U–Pb dating of zircons from intrusions of the Yana–Kolyma gold province constrain the age of orogenic gold deposits to the time interval between 150 ± 3Ma and 108 ± 1 Ma. The Natalka deposit is the largest key deposit in the gold province. We determined the age of mineralization as the Valanzhinian (136 ± 1 to 132 ± 2 Ma) by ⁴⁰Ar/³⁹Ar dating of muscovite and by Re–Os dating of arsenopyrite and native gold. There are no known manifestations of magmatism of this age in the province, therefore, we conclude that the formation of gold–quartz mineralization is very likely related to metamorphism and metasomatism triggered by regional-scale shear deformations.

Abstract

The microthermometric and bulk analysis data on fluid inclusions in vein quartz of 16 gold–quartz deposits located in the different sectors of the Yana-Kolyma belt are presented. The deposits were found to have formed in the temperature range of 350–220°C by weakly mineralized (<5 wt % NaCl-eq.) carbon dioxide-aqueous fluids that are typical for orogenic ore-forming systems. The composition of the salts is dominated by sodium bicarbonate; the trace elements are K, B, As, Sb, Rb, and Cs. The mole fraction of CO₂ varies from 0.19 to 0.03 depending on the RTX conditions. The mole fraction of methane in carbon dioxide does not exceed 0.1. Intense degassing of the solutions in the range of 320°–270°C could be one of the reasons for ore deposition. Gold-bearing quartz veins and veinlets were formed at depths of 6–3 km at a decrease in pressure from 1.5–1.0 to 0.9–0.8 kbar while the dynamic regime of the orogeny evolved. At the final stage, the opening of fractures led to a change of the regime from lithostatic to hydrostatic with a pressure drop to 0.5–0.1 kbar. At the final stages of mineral formation, the fluid systems have increased salinity and a gas phase comprising essentially nitrogen and methane, which indicates their uptake from another source. Thus, the parameters of fluid inclusions in quartz reflect a long history of evolutionary ore-forming processes that are related to development of orogen.

Abstract

This article considers the mineralogical and geochemical features of the Early Cretaceous gold ore deposits in the Northern Okhotsk region, that is, the southeastern flank of the Yana-Kolyma orogenic belt and the Okhotsk sector of the Okhotsk–Koryak orogenic belt. Dating results are reviewed and the general and distinctive geological, mineralogical, and geochemical features of different mineralization types are shown. A close genetic link of ore mineralization with Early Cretaceous magmatism and formation of the Uda-Murgal magmatic arc is assumed.

Abstract

Copper mineralization in the Magadan oblast (Northeast of Russia) has been established in Middle–Upper Riphean quartz–chloritoid and chlorite shales of the of the Prikolymsky terrane. Compared to the upper crust, copper shales of the Oroek ore occurrence are noticeably enriched in a narrow range of trace elements: Cu, Ag, Au, Rh, and Se, with enrichment factors ranging from 10-fold (Se, Rh) to 100-fold (Ag, Au) and even 1000-fold (Cu) suggesting, apparently, their synchronous involvement in ore formation. In addition, the studied samples are slightly enriched in Cd, Li, Co, Zn, V, U, Sc, Y, and REE. Copper shales show relatively flat rare earth element (REE) patterns similar to the chondrite-normalized pattern and have no distinct positive or negative Eu anomalies and are dominated by light lanthanides. The Ce/Ce* and Eu/Eu* ratios indicate that ore formation proceeded under oxidizing conditions. The geochemical data we obtained show that the Riphean host rocks could serve as a source of trace elements and REE ore-forming fluids. In the ores, covelline and chalcocite are the main copper minerals; less common are roxbyite, idaite, nukundamite, spioncopite, chalcopyrite, silver-bearing (Ag about 1 wt %) bornite, and yarrovite, as well as native gold. Fluid inclusions in quartz suggest that copper mineralization was formed at temperatures of 212–190°C and a pressure of 1 kbar from hydrothermal solutions of medium salinity (from 13.8 to 12.4 wt % eq NaCl) and high density (0.95–0.98 g/cm³) saturated with chlorides of Ca, Mg, and K. The copper shales of the Prikolymsky terrane differ from sedimentary copper and copper shales of the African Copper Belt in low Co and Ni contents and from the Kupfershifer in low Pb and Zn contents. The results we obtained can be used for predicting new deposits.

The Talnikovoye ore field, where previous academic research and exploration works revealed porphyry copper mineralization, is described. Our data show that the mineralization is confined to the Turonian granodiorite and quartz diorite intrusions (91 Ma, U–Pb metohod) and accompanying hydrothermal–explosive breccias. Biotite–epidote–chlorite propylites are widespread within the ore field; in the southern part, quartz–sericite phyllic alteration is superimposed on the K-feldspar alteration halo. The ore occurs in zones of intense quartz, chlorite–epidote–quartz (with chalcopyrite and molybdenite), sulfide-potassium feldspar–quartz (with chalcopyrite and bornite), and sulfide–quartz–sericite (with chalcopyrite) stockwork veining with copper–molybdenum mineralization. They have moderate concentrations of Cu (0.1–0.3%, reaching 1.1%) and Mo (up to 0.1%), low concentrations of Au (up to 0.1 g/t) and Ag (up to 2.6 g/t), and have a Cu–Mo–(Au, Ag, Pb, Zn, Sb, and As) geochemical signature. Fluid inclusion data suggest the formation of quartz in mineralized veinlet during the magmatic–hydrothermal transition (430–150°С) with solutions at high (50 wt % NaCl equiv.), medium and low concentrations (5–18.9 wt % NaCl equiv.) involved under the cooling–dilution scenario. The values of the main geochemical indicators, such as Cu/Mo (30–60) and Cu/Au (>1 × 10⁵) ratios allowed us to assign the Talnikovoye ore field to the porphyry copper–molybdenum type characteristic of continental-margin volcano-plutonic belts formed on the mafic island-arc basement. The geochemical parameters of ore-bearing porphyry granitoids indicate their formation in a setting of a subduction-to-transform plate boundary transition.