Modeling the Seismicity of the Altai–Sayan–Baikal Region

Doklady Earth Sciences - Numerical modeling of the dynamics of the block structure and the resulting seismicity of the Altai–Sayan–Baikal region has been carried out. The earthquake...     

Metallogeny of the northeastern Gangdese Pb–Zn–Ag–Fe–Mo–W polymetallic belt in the Lhasa terrane,southern Tibet

The northeastern Gangdese Pb–Zn–Ag–Fe–Mo–W polymetallic belt (NGPB), characterized by skarn and porphyry deposits, is one of the most important metallogenic belts in the Himalaya–Tibetan continental orogenic system. This belt extends for nearly four hundred kilometers along the Luobadui–Milashan Fault in the central Lhasa subterrane, and contains more than 10 large ore deposits with high potential for development. Three major types of mineralization system have been identified: skarn Fe systems, skarn/breccia Pb–Zn–Ag systems, and porphyry/skarn Mo–Cu–W systems. In this study, we conducted a whole-rock geochemical, U–Pb zircon geochronological, and in situ zircon Hf isotopic study of ore-forming rocks in the NGPB, specifically the Jiangga, Jiaduopule, and Rema skarn Fe deposits, and the Yaguila Pb–Zn–Ag deposit. Although some of these deposits (porphyry Mo systems) formed during the post-collisional stage (21–14 Ma), the majority (these three systems) developed during the main (‘soft collision’) stage of the India–Asia continental collision (65–50 Ma). The skarn Fe deposits are commonly associated with granodiorites, monzogranites, and granites, and formed between 65 and 50 Ma. The ore-forming intrusions of the Pb–Zn–Ag deposits are characterized by granite, quartz porphyry, and granite porphyry, which developed in the interval of 65–55 Ma. The ore-forming porphyries in the Sharang Mo deposit, formed at 53 Ma. The rocks from Fe deposits are metaluminous, and have relatively lower SiO₂, and higher CaO, MgO, FeO contents than the intrusions associated with Mo and Pb–Zn–Ag mineralization, while the Pb–Zn–Ag deposits are peraluminous, and have high SiO₂ and high total alkali concentrations. They all exhibit moderately fractionated REE patterns characterized by lower contents of heavy REE relative to light REE, and they are enriched in large-ion lithophile elements and relatively depleted in high-field-strength elements. Ore-forming granites from Fe deposits display ⁸⁷Sr/⁸⁶Sr_(i) = 0.7054–0.7074 and ε_Nd(t) = − 4.7 to + 1.3, whereas rocks from the Yaguila Pb–Zn–Ag deposit have ⁸⁷Sr/⁸⁶Sr_(i) = 0.7266–0.7281 and ε_Nd(t) = − 13.5 to − 13.3. In situ Lu–Hf isotopic analyses of zircons from Fe deposits show that ε_Hf(t) values range from − 7.3 to + 6.6, with T_DM(Hf)^C model ages of 712 to 1589 Ma, and Yaguila Pb–Zn–Ag deposit has ε_Hf(t) values from − 13.9 to − 1.3 with T_DM(Hf)^C model ages of 1216 to 2016 Ma. Combined with existing data from the Sharang Mo deposit, we conclude that the ore-forming intrusions associated with the skarn Fe and porphyry Mo deposits were derived from partial melting of metasomatized lithospheric mantle and rejuvenated lower crust beneath the central Lhasa subterrane, respectively. Melting of the ancient continental material was critical for the development of the Pb–Zn–Ag system. Therefore, it is likely that the source rocks play an important role in determining the metal endowment of intrusions formed during the initial stage of the India–Asia continental collision.     

Crustal Structure of the Baltic Shield Along the Pechenga–Kostomuksha–Lovisa Geotraverse

Results of geologic and geophysical modeling are presented, based on detailed seismic studies along two profiles—Pechenga-Kostomuksha and Lieksa-Lovisa. Density, geothermal, magnetic, and geoelectric models were obtained from the interpretations of various geophysical fields and correlated with the reference seismic sections. All the models were combined in order to compile a geologic-geophysical crustal section. The crustal thickness along the Pechenga-Kostomuksha-Lovisa geotraverse varies from 38 to 65 km. Two anomalous structures have been observed that are referred to as the Belomorian-Karelian and Ladoga-Bothnian zones. These zones are characterized by enhanced values of magnetic fields, presence of seismic foci and wave attenuation, and variation of the depth and magnitude of modern crustal movements. These zones are distinguished by the discontinuity M reconstruction, an increase in transitional layer thickness (to 25 km) at the base of the crust, and an increase in depth down to the discontinuity M (50 to 65 km). On average, the crust is thinner (40 km) in the ancient part of the shield than in the younger Svecofennian province (45 km). The velocity differences also are important: for example, the crust of the ancient shield is characterized by lower velocities and the transitional high-velocity layer is absent or thinner. The Karelian granite-greenstone area (a fragment of the Archean craton) has the most simple and balanced deep structure. Within the Karelian area, the layers are nearly horizontal and their thickness is rather constant. The northeastern part of a fragment of the Murmansk block has similar crustal characteristics within the Kola area, where it has undergone Early Proterozoic deformation. Geological and geophysical data for the Pechenga-Varzuga zone suggests that there was intracontinental rifting and a subsequent construction regime during the Svecofennian orogeny that involved a considerable part of the shield. The deep-crustal structure is more complicated to the south. An increase in volume of material with the properties of granulites and basic rocks is observed in the upper crust. The rocks form an inclined alternation of high-density and high-velocity plates and lenses. The packet of tectonic clustering of supracrustal rocks is most conspicuous in the Lapland-Kolvitsa granulite belt. The packet thickness does not exceed 13 km.     

Native Alloys of the Pd–Pt and Ni–Cu–Al Systems from the AS Luna-24

The close intergrowth of two native alloys of the compositions Ni_0.59Cu_0.24Al_0.15Fe_0.01Mn_0.01 and Pd_0.55Pt_0.36Rh_0.09 with a size of 10 μm has been discovered in the regolith from the Mare Crisium. A conclusion on its exhalative origin is made.     

Application of the region–time–length algorithm to study of earthquake precursors in the Thailand–Laos–Myanmar borders

In order to examine the precursory seismic quiescence of upcoming hazardous earthquakes, the seismicity data available in the vicinity of the Thailand–Laos–Myanmar borders was analyzed using the Region–Time–Length (RTL) algorithm based statistical technique. The utilized earthquake data were obtained from the International Seismological Centre. Thereafter, the homogeneity and completeness of the catalogue were improved. After performing iterative tests with different values of the \(r_{0}\) and \(t_{0}\) parameters, those of \(r_{0}\) = 120 km and \(t_{0}\) = 2 yr yielded reasonable estimates of the anomalous RTL scores, in both temporal variation and spatial distribution, of a few years prior to five out of eight strong-to-major recognized earthquakes. Statistical evaluation of both the correlation coefficient and stochastic process for the RTL were checked and revealed that the RTL score obtained here excluded artificial or random phenomena. Therefore, the prospective earthquake sources mentioned here should be recognized and effective mitigation plans should be provided.     

Heavy metal contamination in the vicinity of the Daduk Au–Ag–Pb–Zn mine in Korea

The heavy metal contamination and seasonal variation of the metals in soils, plants and waters in the vicinity of an abandoned metalliferous mine in Korea were studied. Elevated levels of Cd, Cu, Pb and Zn were found in tailings with averages of 8.57, 481, 4,450 and 753 mg/kg, respectively. These metals are continuously dispersed downstream and downslope from the tailings by clastic movement through wind and water. Thus, significant levels of the elements in waters and sediments were found up to 3.3 km downstream from the mining site, especially for Cd and Zn. Enriched concentrations of heavy metals were also found in various plants grown in the vicinity of the mining area, and the metal concentrations in plants increased with those in soils. In a study of seasonal variation on the heavy metals in paddy fields, relatively high concentrations of heavy metals were found in rice leaves and stalks grown under oxidizing conditions rather than a reducing environment (P<0.05).     

Silica Distribution in the System Quartz–Water–Vapor Depending on the Temperature Gradient

Careful measurements of temperature on the surface of autoclaves indicate that small temperature gradients (TG) occur in the standard electric furnaces. These gradients can affect the results of long-lasting (up to 775 days) experiments in the system quartz–water–vapor at 300°C. From the bottom of the autoclave to its top, the temperature decreased in the upper parts of the electric furnaces and increased in their lower parts (TG =–0.08 and 0.15°C/cm, respectively). In the upper parts of the electric furnaces, the concentration of dissolved silica (m) was close to the quartz solubility (10 mmol/kg), and no other changes took place, which is consistent with the currently conventionally admitted notion that quartz is stable under these conditions. In the lower parts of the electric furnaces, m decreased to 0.03 mmol/kg, and opal was precipitated on the walls of the capsules above the solution (the opal was transformed into secondary quartz with time). These data suggest that no equilibrium silica distribution between liquid and vapor water phases was reached. We have suggested and analyzed as wide as possible circle of hypotheses conceivably able to explain this unequilibrated state. The most realistic explanation of the phenomenon seems to be that distillation is initiated by preferable evaporation of the solution in its thin (<100 nm) layer at the meniscus edge. A mathematic model of the process is suggested. The model is consistent with experimental data. The phenomenon in question can be detected in various experimental and technological systems and hampers the attainment of complete equilibrium. In natural systems, this phenomenon can lead to the migration of cavities partly filled with solution at an inversion of the geothermal gradient (beneath sills and lava flows).     

An Event-Based Depositional Model for the Paleocene–Eocene Sandy–Clayey–Siliceous Sequence of the Russian Platform

An event-based depositional model for the Paleocene—Eocene sandy–clayey–siliceous deposits of the Russian Platform was proposed. The model was based on pulsational input of pyroclastic material and intrusion of sandy injectites. These processes should be taken into account to identify the stratigraphic position of the Paleocene–Eocene lithostratigraphic units in the eastern, southeastern, and southern parts of the Russian Platform.     

Some new data on the genesis of the Linghou Cu–Pb–Zn polymetallic deposit—Based on the study of fluid inclusions and C–H–O–S–Pb isotopes

The Linghou deposit, located near Hangzhou City of Zhejiang Province, eastern China, is a medium-sized polymetallic sulfide deposit associated with granitic intrusion. This deposit is structurally and lithologically controlled and commonly characterized by ore veins or irregular ore lenses. In this deposit, two mineralization events were identified, of which the former produced the Cu–Au–Ag orebodies, while the latter formed Pb–Zn–Cu orebodies. Silicification and calc-silicate (skarn type), phyllic, and carbonate alternation are four principal types of hydrothermal alteration. The early Cu–Au–Ag and late Pb–Zn–Cu mineralizations are characterized by quartz ± sericite + pyrite + chalcopyrite + bornite ± Au–Ag minerals ± magnetite ± molybdenite and calcite + dolomite + sphalerite + pyrite + chalcopyrite + galena, respectively. Calcite clusters and calcite ± quartz vein are formed during the late hydrothermal stage.The NaCl–H₂O–CO₂ system fluid, coexisting with NaCl–H₂O system fluid and showing the similar homogenization temperatures (385 °C and 356 °C, respectively) and different salinities (16.89–21.68 wt.% NaCl eqv. and 7.70–15.53 wt.% NaCl eqv.), suggests that fluid immiscibility occurred during the Cu–Au–Ag mineralization stage and might have given rise to the ore-metal precipitation. The ore-forming fluid of the Pb–Zn–Cu mineralization mainly belongs to the NaCl–H₂O–CO₂ system of high temperature (~ 401 °C) and mid-high salinity (10.79 wt.% NaCl eqv.).Fluids trapped in the quartz-chalcopyrite vein, Cu–Au–Ag ores, Pb–Zn–Cu ores and calcite clusters yielded δ¹⁸O_H2O and δD values varying from 5.54‰ to 13.11‰ and from − 71.8‰ to − 105.1‰, respectively, indicating that magmatic fluids may have played an important role in two mineralization events. The δ¹³C_PDB values of the calcite change from − 2.78‰ to − 4.63‰, indicating that the CO₃^2 − or CO₂ in the ore-forming fluid of the Pb–Zn–Cu mineralization was mainly sourced from the magmatic system, although dissolution of minor marine carbonate may have also occurred during the ore-forming processes. The sulfide minerals have homogeneous lead isotopic compositions with ²⁰⁶Pb/²⁰⁴Pb ranging from 17.958 to 18.587, ²⁰⁷Pb/²⁰⁴Pb ranging from 15.549 to 15.701, and ²⁰⁸Pb/²⁰⁴Pb ranging from 37.976 to 39.052, indicating that metallic elements of the Linghou deposit came from a mixed source involving mantle and crustal components.Based on geological evidence, fluid inclusions, and H–O–C–S–Pb isotopic data, the Linghou polymetallic deposit is interpreted as a high-temperature, skarn-carbonate replacement type. Two types of mineralization are both related to the magmatic–hydrothermal system, with the Cu–Au–Ag mineralization having a close relationship with granodiorite.     

Kysylga gold–silver deposit in the terrigenous sequences of the Verkhoyansk–Kolyma mesozoides

The paper reports the mineralogical and geochemical features of the Kysylga gold deposit located in the hornfelsed Norian sedimentary rocks and classified with low-sulfide gold–quartz type of deposits typical of the Verkhoyansk–Kolyma metallogenic province. Detailed typomorphic study of the major minerals (quartz, arsenopyrite, and gold) of the ore veins shows that the deposit is assigned to the gold–silver type. Mineralogical and geochemical data substantiate this conclusion.     

Geological and C–O–S–Pb–Sr isotopic constraints on the origin of the Qingshan carbonate-hosted Pb–Zn deposit,Southwest China

Located in the western Yangtze Block, the Qingshan Pb–Zn deposit, part of the Sichuan–Yunnan–Guizhou Pb–Zn metallogenic province, contains 0.3 million tonnes of 9.86 wt.% Pb and 22.27 wt.% Zn. Ore bodies are hosted in Carboniferous and Permian carbonate rocks, structurally controlled by the Weining–Shuicheng anticline and its intraformational faults. Ores composed of sphalerite, galena, pyrite, dolomite, and calcite occur as massive, brecciated, veinlets, and disseminations in dolomitic limestones.

The C–O isotope compositions of hydrothermal calcite and S–Pb–Sr isotope compositions of Qingshan sulphide minerals were analysed in order to trace the sources of reduced sulphur and metals for the Pb–Zn deposit. δ¹³C_PDB and δ¹⁸O_SMOW values of calcite range from –5.0‰ to –3.4‰ and +18.9‰ to +19.6‰, respectively, and fall in the field between mantle and marine carbonate rocks. They display a negative correlation, suggesting that CO₂ in the hydrothermal fluid had a mixed origin of mantle, marine carbonate rocks, and sedimentary organic matter. δ³⁴S values of sulphide minerals range from +10.7‰ to +19.6‰, similar to Devonian-to-Permian seawater sulphate (+20‰ to +35‰) and evaporite rocks (+23‰ to +28‰) in Carboniferous-to-Permian strata, suggesting that the reduced sulphur in hydrothermal fluids was derived from host-strata evaporites. Ores and sulphide minerals have homogeneous and low radiogenic Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.561 to 18.768, ²⁰⁷Pb/²⁰⁴Pb = 15.701 to 15.920, and ²⁰⁸Pb/²⁰⁴Pb = 38.831 to 39.641) that plot in the upper crust Pb evolution curve, and are similar to those of Devonian-to-Permian carbonate rocks. Pb isotope compositions suggest derivation of Pb metal from the host rocks. ⁸⁷Sr/⁸⁶Sr ratios of sphalerite range from 0.7107 to 0.7136 and (⁸⁷Sr/⁸⁶Sr)_200Ma ratios range from 0.7099 to 0.7126, higher than Sinian-to-Permian sedimentary rocks and Permian Emeishan flood basalts, but lower than Proterozoic basement rocks. This indicates that the ore strontium has a mixture source of the older basement rocks and the younger cover sequence. C–O–S–Pb–Sr isotope compositions of the Qingshan Pb–Zn deposit indicate a mixed origin of the ore-forming fluids and metals.     

Reappraising the P–T evolution of the Rogaland–Vest Agder Sector,southwestern Norway

The Rogaland-Vest Agder Sector of southwestern Norway comprises high-grade metamorphic rocks intruded by voluminous plutonic bodies that include the ~1000 km~2 Rogaland Igneous Complex(RIC).New petrographic observations and thermodynamic phase equilibria modelling of three metapelitic samples collected at various distances(30 km,10 km and ~ 10 m) from one of the main bodies of RIC anorthosite were undertaken to assess two alternative P-T-t models for the metamorphic evolution of the area.The results are consistent with a revised two-phase evolution.Regional metamorphism followed a clockwise P-T path reaching peak conditions of ~ 850-950 ℃ and ~7-8 kbar at ~1035 Ma followed by high-temperature decompression to ~5 kbar at ~950 Ma,and resulted in extensive anatexis and melt loss to produce highly residual rocks.Subsequent emplacement of the RIC at ~930 Ma caused regional-scale contact metamorphism that affected country rocks 10 km or more from their contact with the anorthosite.This thermal overprint is expressed in the sample proximal to the anorthosite by replacement of sillimanite by coarse intergrowths of cordierite plus spinel and growth of a second generation of garnet,and in the intermediate(10 km) sample by replacement of sapphirine by coarse intergrowths of cordierite,spinel and biotite.The formation of late biotite in the intermediate sample may suggest the rocks retained small quantities of melt produced by regional metamorphism and remained at temperatures above the solidus for up to 100 Ma.Our results are more consistent with an accretionary rather than a collisional model for the Sveconorwegian Orogen.     

Natural polycomponent solid solutions of the Ru–Os–Ir–Pt–Fe system from the eastern Witwatersrand (South Africa)

This study presents compositional features of platinum-group element mineralization derived from the Late Archaean placers in the eastern part of the Witwatersrand basin. The significant presence of platinum-group minerals, formed by polycomponent solid-solution series in the system Ru–Os–Ir–Pt(±Fe), was determined using an electron microprobe analysis. Compositional data indicate that the source for polycomponent solid solutions of the Ru–Os–Ir–Pt–Fe system was the Archaean mantle of the Earth, slightly differentiated with respect to platinum-group elements.     

Glacial–interglacial records of the reflectance of sediments from the Norwegian–Greenland–Iceland Sea (Nordic seas)

 The reflectance of sediments (gray level) were measured on 11 sediment cores from the Norwegian–Greenland–Iceland Sea (Nordic seas). The analyzed time interval covers the past five glacial–interglacial cycles. Although the results demonstrate that the gray-level method has a potential for stratigraphic purposes, it is indicated that gray-level changes in the Nordic seas are not necessarily driven by variations in the content of biogenic calcite. A detailed comparison of gray-level values with contents of total CaCO₃ (carbonate) and total organic carbon (TOC) reveals no overall causal link between these proxies. However, specific glacial core sections with layers containing organic-rich sediment clasts as a consequence of iceberg-rafting seem to correlate well with law gray-level values. Of those cores which show relatively high and comparable carbonate values in the last three main interglacial intervals (stages 11, 5.5, and 1), stage 11 is always marked by the highest gray-level values. A close inspection of the surface structure of the foraminiferal tests as well as the conduction of reflectance measurements on these tests leads to the conclusion that enhanced carbonate corrosion occurred during stage 11. The test corrosion not only affected the reflectance of the tests by making them appear whiter, it also seems responsible for the comparatively high gray-level values of the total sediment in stage 11. In contrast, the relatively low gray-level values found in stages 5.5, and 1 are not associated with enhanced test corrosion. This observation implies that variable degrees of carbonate corrosion can have a profound effect on total sediment reflectance. Received: 6 September 1998 / Accepted: 4 April 1999     

Fusulinoids from the Bashkirian–Moscovian transition beds of the Shahreza region in the Sanandaj–Sirjan Zone,Iran

International Journal of Earth Sciences - The presence of the Bashkirian–Moscovian (lower Pennsylvanian) sequence with mixed siliciclastics and fossil-rich carbonates has long been known from...     

U–Pb LA–ICP–MS Age of Detrital Zircons from the Lower Riphean and Upper Vendian Deposits of the Luga–Ladoga Monocline

The results of LA–ICP–MS U–Pb analyses of detrital zircons from the Precambrian deposits of Luga–Ladoga monocline are discussed. The age spectra of the zircons separated from the Riphean to Upper Vendian sandstones from the Shotkusa-1 well demonstrate dominance of the Paleo- and Mesoproterozoic grains while the Archaean zircons are subordinate. The Riphean debris sources were local swells of the Northern Ladoga basement. The sequence interval presumably corresponding to the Vasilieostrov Formation (Upper Vendian) has yielded not only Paleo- and Mesoproterozoic zircon ages, but Neoproterozoic as well, implying a Timanide provenance: these zircons (527 ± 9 and 516 ± 13 Ma) allow deposition of a significant part of the Shotkusa-1 sequence at the very beginning of the Cambrian.