Ore-forming mechanism for the Xiaoxinancha Au-rich Cu deposit in Yanbian,Jilin Province,China: Evidence from noble gas isotope geochemistry of fluid inclusions in minerals   总被引：2，自引：0，他引：2  

KEISUKE Nagao  HIROCHIKA Sumino 《中国科学D辑(英文版)》2008,51(2):216-228

The Xiaoxinancha Au-rich copper deposit is one of important Au-Cu deposits along the continental margin in Eastern China. The deposit consists of two sections: the Beishan mine (North), composed of altered rocks with veinlet-dissemination sulfides and melnicovite-dominated sulfide-quartz veins, and the Nanshan mine (South), composed of pyrrhotite-dominated sulfide-quartz veins and pure sulfide veins. The isotope compositions of noble gases extracted from fluid inclusions in ore minerals, i.e. ratios of 3He/4He, 20Ne/22Ne and40Ar/36Ar are in the ranges of 4.45―0.08 Ra, 10.2―8.8 and 306―430, respectively. Fluid inclusions in minerals from the Nanshan mine have higher 3He/4He and 20Ne/22Ne ratios whereas those from the Beishan mine have lower 3He/4He ratios. The analysis of origin, and evolution of the ore fluids and its relations with the ore-forming stages and the ages of mineralization suggests that the initial hydrothermal fluids probably come from the melts generated by partial melting of oceanic crust with the participation of fluids from the mantle (mantle-plume type)/aesthenosphere. This also corresponds to the continental margin settings during the subduction of Izanagi ocaneic plate towards the palaeo-Asian continent (123―102 Ma). The veinlet-dissemination ore bodies of the Beishan mine were formed through replacement and crystallization of the mixed fluids generated by mixing of the ascending high-temperature boiling fluid with young crustal fluid whereas the melnicovite-dominated sulfide-quartz veins were formed subsequently by filling of the high-temperature ore fluid in fissures. Pyrrhotite-dominated sulfide-quartz veins in the Nanshan mine were formed by filling-deposition-crystallization of the moderate-temperature ore fluids and the pure sulfide veins were formed later by filling-deposition-crystallization of ore substance-rich fluids after boiling of the moderate-temperature ore fluids. The metallogenic dynamic processes can be summarized as: (1) formation of fluidand ore substance-bearing Adakitic magma by degassing, dewatering and partial melting during subduction of the Izanagi plate; (2) separation and formation of ore fluids from the Adakitic magma; and (3) success-sive ascending of the ore fluids and final formation of the Au-rich Cu deposit of veinlet-dissemination and vein types by secondary boiling.  相似文献   

Cosmic‐ray exposure age and heliocentric distance of the parent body of the Rumuruti chondrite PRE 95410     

Tomoya Obase  Daisuke Nakashima  Tomoki Nakamura  Keisuke Nagao 《Meteoritics & planetary science》2020,55(5):1031-1047

We measured concentrations and isotopic ratios of noble gases in the Rumuruti (R) chondrite Mount Prestrud (PRE) 95410, a regolith breccia exhibiting dark/light structures. The meteorite contains solar and cosmogenic noble gases. Based on the solar and cosmogenic noble gas compositions, we calculated a heliocentric distance of its parent body, a cosmic‐ray exposure age on the parent body regolith (parent body exposure age), and a cosmic‐ray exposure age in interplanetary space (space exposure age) of the meteorite. Assuming a constant solar wind flux, the estimated heliocentric distance was smaller than 1.4 ± 0.3 au, suggesting inward migration from the asteroid belt regions where the parent body formed. The largest known Mars Trojan 5261 Eureka is a potential parent body of PRE 95410. Alternatively, it is possible that the solar wind flux at the time of the parent body exposure was higher by a factor of 2–3 compared to the lunar regolith exposure. In this case, the estimated heliocentric distance is within the asteroid belt region. The parent body exposure age is longer than 19.1 Ma. This result indicates frequent impact events on the parent body like that recorded for other solar‐gas‐rich meteorites. Assuming single‐stage exposure after an ejection event from the parent body, the space exposure age is 11.0 ± 1.1 Ma, which is close to the peak of ~10 Ma in the exposure age distribution for the solar‐gas‐free R chondrites.  相似文献   

Particle size effect on the saturation of methane hydrate in sediments – Constrained from experimental results     

Hailong Lu  Tatsuji Kawasaki  Toshiyasu Ukita  Igor Moudrakovski  Tetsuya Fujii  Satoshi Noguchi  Tadaaki Shimada  Masaru Nakamizu  John Ripmeester  Chris Ratcliffe 《Marine and Petroleum Geology》2011,28(10):1801-1805

Except for those occurring at seafloor, most of natural gas hydrate form in sediments and are subject to the influence of sediment. Among these factors, the particle size effect on hydrate saturation level in sediment have been studied with a series of silica sands with various sizes, and the results obtained clearly indicate that particle size does play an important role in affecting the saturation level of hydrate in sediments. The proton relaxation times of water confined in the same series of silica sands, which were determined with NMR measurement, show logarithmic relationship with particle size. By comprehensive consideration of the results of hydrate saturation and water proton relaxation times, the particle size effect observed is tentatively explained by the water availability for hydrate formation in sediments.  相似文献   

Cosmogenic nuclides in the Brenham pallasite     

M. Honda  M. W. Caffee  Y. N. Miura  H. Nagai  K. Nagao  K. Nishiizumi 《Meteoritics & planetary science》2002,37(12):1711-1728

Abstract— Cosmic‐ray‐produced (cosmogenic) nuclides were studied in fragments of the Brenham pallasite, a large stony iron meteorite. The contents of light noble gases (He, Ne, and Ar) and long‐lived radionuclides (¹⁰Be, ²⁶Al, ³⁶Cl, and ⁵³Mn), produced by nuclear reactions with cosmic rays, were measured in the separated metal and olivine phases from numerous samples representing a wide range of shielding conditions in the meteoroid. The distribution of cosmogenic nuclide concentrations in the metal follows patterns similar to that observed in large iron meteorites. Shielding effects were estimated from the relative proportions of low‐ and high‐energy reaction products. The production rates varied, from surface to interior, by a factor of more than several hundred. The ³⁶Cl‐³⁶Ar cosmic‐ray exposure age of Brenham is 156 ± 8 Myr. This determination is based on a multiple nuclide approach that utilizes cosmogenic nuclide pairs. This approach not only yields a “shielding independent” exposure age but also demonstrates that the production of cosmogenic nuclides occurred in a single stage. The depth profiles of ¹⁰Be in the stone phase and ⁵³Mn in the metal phase are shown superimposed on corresponding profiles from the Apollo 15 long drill core. Surprisingly low abundances of lithophile elements, such as K, U, and Th, provided a unique opportunity to examine the production systematics of those nuclides whose inventories typically have significant contributions from non‐cosmogenic sources, particularly radiogenic contributions. The U and Th contents of the olivine samples are extremely low, allowing detection of cosmogenic ⁴He production from oxygen, magnesium, silicon, and iron.  相似文献   

Juxtaposition of adakite, boninite, high-TiO₂ and low-TiO₂ basalts in the Devonian southern Altay, Xinjiang, NW China   总被引：1，自引：0，他引：1  

Hecai Niu  Hiroaki Sato  Haixiang Zhang  Jun'ichi Ito  Xueyuan Yu  Takashi Nagao  Kentaro Terada  Qi Zhang 《Journal of Asian Earth Sciences》2006,28(4-6):439-456

We present petrographic and geochemical data on representative samples of the Devonian adakite, boninite, low-TiO₂ and high-TiO₂ basalts and associated rocks in the southern Altay areas, Xinjiang, NW China. These volcanic rocks mostly occur as tectonic blocks within suture zones between the Siberian and Junggar plates. Adakite occurs in the Suoerkuduke area ca. 40 km south of Fuyun, and actually represents a poorly-sorted massive volcaniclastic deposit, mostly consisting of a suite of hornblende andesite to pyroxene andesite. The geochemical features of the adakite suggest its generation by melting of subducted oceanic crust. Boninite occurs in the Saerbulake area ca. 20 km southwest of Fuyun, as pillowed lava or pillowed breccia. It is associated with high-TiO₂ basalt/gabbro and low-TiO₂ basalt. The boninites are metamorphosed, but contain relict clinopyroxene with Mg# (=100*Mg/(Mg+Fe)) of 90–92, and Cr₂O₃ contents of 0.5–0.7 wt% and chromian spinel with Cr/(Cr+Al) ratio of 0.84. The bulk rock compositions of the boninites are characterized by low and U-shaped REE with variable La/Yb ratios. They are classified as high-Ca boninite. The Cr-rich cpx phenocryst and Chromian spinel suggests that the boninites were formed by melting of mildly refractory mantle peridotite fluxed by a slab-derived fluid component under normal mantle potential temperature conditions. Basaltic rocks occur as massive flows, pillowed lavas, tuff breccia, lapilli tuff and blocks in tectonic mélanges. Together with gabbros, the basaltic rocks are classified into high-TiO₂ (>1.7 wt%) and low-TiO₂ (<1.5 wt%) types. They show variable trace element compositions, from MORB-type through transitional back-arc basin basalt to arc tholeiite, or within plate alkalic basalt. A notable feature of the Devonian formations in the southern Altay is the juxtaposition of volcanic rocks of various origins even within a limited area; i.e. the adakite and the boninites are associated with high-TiO₂ and low-TiO₂ basalts and/or gabbros, respectively. This is most likely produced by complex accretion and tectonic processes during the convergence in the Devonian–Carboniferous paleo-Asian Ocean between the Siberian and Junggar plates.  相似文献   

Late Paleozoic adakites and Nb-enriched basalts from northern Xinjiang, northwest China: Evidence for the southward subduction of the Paleo-Asian Oceanic Plate   总被引：9，自引：0，他引：9  

Haixiang  Zhang  Hecai  Niu  Hiroaki  Sato  Xueyuan  Yu  Qiang  Shan  Boyou  Zhang  Jun'ichi  Ito  Takashi  Nagao 《Island Arc》2005,14(1):55-68

Abstract   Volcanic rocks consisting of adakite and Nb-enriched basalt are found in the early Devonian Tuoranggekuduke Group in the northern margin of the Kazakhstan-Junggar Plate, northern Xinjiang, northwest China. The geochemical characteristics of the andesitic and dacitic rocks in this area resemble that of adakites. The relatively high Al₂O₃, Na₂O and MgO content and Mg^♯ values indicate that the adakites were generated in relation to oceanic slab subduction rather than the partial melting of basaltic crust. A slightly higher SrI and a lower ɛ _Nd( t  = 375 Ma) compared to adakites of mid-oceanic ridge basalt (MORB) imply that slab sediments were incorporated into these adakites during slab melting. The Nb-enriched basalt lavas, which are intercalated in adakite lava suite, are silica saturated and are distinguished from the typical arc basalts by their higher Nb and Ti content (high field strength element enrichment). They are derived from the partial melting of the slab melt-metasomatized mantle wedge peridotite. Apparently, positive Sr anomalies and a slightly higher heavy rare earth element content in these adakites compared to their Cenozoic counterparts indicate that the geothermal gradient in the Paleo-Asian Oceanic subduction zone and the depth of the Paleo-Asian Oceanic slab melting are between those of their Archean and Cenozoic counterparts. The distribution of the adakites and Nb-enriched basalts in the northern margin of the Kazakhstan-Junggar Plate, northern Xinjiang, indicates that the Paleo-Asian Oceanic Plate subducted southward beneath the Kazakhstan-Junggar Plate in the early Devonian period.  相似文献   

Effective elastic thickness of island arc lithosphere under Japan   总被引：1，自引：0，他引：1  

Takeshi Kudo  Atsushi Yamaji  Muneyoshi Furumoto  Toshiyasu Nagao 《Island Arc》2001,10(2):135-144

Abstract Using topography and observed gravity anomalies, we have estimated the effective elastic thickness as a measure of strength of Japanese island arc lithosphere. The thickness is found to range from about 3 km to >20 km. The thickness seems to be controlled primarily by the thermal state of the lithosphere. The higher the heat flow, the thinner is the elastic plate. However, several areas show significant deviations. The smaller effective elastic thickness in the northern Ryukyu arc than that inferred from heat flow may be attributed to the stress regime. In Japan, extensional tectonics are going on only in the Ryukyu arc region. Shallow subducting slab under the south-western Japan frontal arc probably increases the effective thickness by several kilometers. The determined effective elastic thickness suggests that when we consider vertical movements in the volcanic arc, we should take account of topographic and subsurface loading over a few hundred kilometers. However, if the dip of the slab is shallow, the flexural responses of the underlying slab, not only that of the island arc lithosphere, should be taken into account for the compensation, as is the case of the south-western Japan frontal arc.  相似文献   

A possible case of SES selectivity at Kozu-shima Island, Japan     

Y. Orihara  Y. Noda  T. Nagao  S. Uyeda 《Journal of Geodynamics》2002,33(4-5)

Geoelectric potential difference monitoring has been conducted in Kozu-shima Island about 170 km south of Tokyo. During the monitoring period (14 May 1997–30 November 1999), there were 21 anomalous geoelectric changes that could have been VAN-type seismic electric signals (SES) and 26 M>3 earthquakes within about 20 km. Possible correspondence between the geoelectric changes and earthquakes has been examined. There seems to be some systematic tendency regarding the dependence of appearance/non-appearance of changes and their polarity on epicentral location. This observation, however, is preliminary and admittedly inconclusive because of small number of cases and non-uniqueness in signal-earthquake correlation.  相似文献   

Distribution and sources of organic matter in surficial sediments on the shelf and slope off Tokachi, western North Pacific, inferred from C and N stable isotopes and C / N ratios     

Toshihiro Usui  Seiya Nagao  Masanobu Yamamoto  Koji Suzuki  Isao Kudo  Shigeru Montani  Atsushi Noda  Masao Minagawa 《Marine Chemistry》2006,98(2-4):241-259

Organic carbon (C) and total nitrogen (N) contents and corresponding isotope ratios were determined in surficial sediment (0–3 cm) at 94 stations ranging from 21 to 1995 m water depth off Tokachi, Hokkaido, Japan, to elucidate the distribution and source of sedimentary organic matter. Suspended particulate organic matter (POM) in the seawater and suspended POM and sediment in the Tokachi River were also examined. δ¹³C, δ¹⁵N and C / N ratios of the samples in the Tokachi River suggest that the spring snowmelt is an important process for the transport of terrestrial organic matter to the coastal waters. δ¹³C values of suspended POM in the surface seawater were higher in May and November than in August, while δ¹⁵N values of the POM were higher in May and August than in November. These changes are attributed to seasonal changes in phytoplankton growth rate and nitrate availability. δ¹³C and δ¹⁵N values in the sediments off Tokachi were lowest near the Tokachi River mouth, and increased offshore to constant values that persisted from 134 to 1995 m water depth. The spatial variation in C / N ratios in the sediment mirrored those of δ¹³C and δ¹⁵N. Comparison of δ¹³C, δ¹⁵N and C / N ratios in the sediments off Tokachi with those in the Tokachi River and seawater indicates that about half of the organic matter in the sediment was of terrestrial origin near the Tokachi River mouth, and the sedimentary organic matter from 134 to 1995 m water depth was of marine origin. The organic C content in the sediment was high near the Tokachi River mouth, and also around 1000 m water depth. The C content was significantly correlated with silt plus clay content, with different regression lines for those stations shallower and deeper than 134 m, owing to several stations of higher C content with the elevated C / N ratio on the inner shelf. These results suggest that transport and deposition of organic-rich fine sediment particles by hydrodynamic processes were major factors controlling C content off Tokachi. In addition, the supply of a fraction of terrestrial organic matter with high C / N probably also affected C content on the inner shelf.  相似文献   

Combined electromagnetic, geochemical and thermal surveys of Taal volcano (Philippines) during the period 2005–2006     

J. Zlotnicki  Y. Sasai  J. P. Toutain  E. U. Villacorte  A. Bernard  Julio P. Sabit  Juan M. Gordon Jr  Ernesto G. Corpuz  M. Harada  J. T. Punongbayan  H. Hase  T. Nagao 《Bulletin of Volcanology》2009,71(1):29-47

Since 1572, 33 phreatic to phreatomagmatic eruptions have occurred on Taal volcano (Philippines), some of them causing several hundred casualties. Considering the time delay between two consecutive eruptions, there is an 88% probability that Taal volcano should have already erupted. Since 1992, several phases of seismic activity have been recorded accompanied by ground deformation, opening of fissures, and surface activity. The volcanic activity of Taal appears to be controlled by dike injections and magma supply, buffered by a hydrothermal system that releases fluids and heat through boiling and subsequent steaming. In early 2005, a multidisciplinary project was launched for studying the hydrothermal activity. To map the hydrothermal system, combined surveys were carried out to investigate self-potential, total magnetic field, ground temperature and carbon dioxide soil degassing, along with satellite thermal imaging of the Main Crater Lake. The elevated temperatures and high concentrations of carbon dioxide, as well as electromagnetic anomalies, indicate large-scale hydrothermal degassing. This process is enhanced along the tectonic features (e.g., crater rim and faults) of the volcano, while active fissures opened along the E–W northern flank during the 1992–1994 seismic activity. Heat and fluids from the hydrothermal system are essentially released in the northern part of the crater, which is bounded to the South by a suspected NW–SE fault along which seismicity seems to take place, and dikes are thought to be intruded. During the January 2005 surveys, a new seismic crisis started, and the felt earthquakes prompted spontaneous evacuation of hundreds of inhabitants living on the volcano. Repeated surveys show changes of self-potential, total magnetic field, and ground temperature with time, without any noticeable spatial enlargement. These observations suggest that the northern flank located between the crater rim and the 1992–1994 fissures is connected with a deep thermal source in Main crater and is reactivated during seismic crises. This sector could be subjected to flank failure.  相似文献