Characteristics of Porphyry Cu Mineralization at Waisoi (Namosi District), Viti Levu, Fiji     

Akira Imai    Yukiko Ohbuchi    Takayuki Tanaka    Seiya Morita  Kentaro Yasunaga 《Resource Geology》2007,57(4):374-385

The porphyry Cu deposits at Waisoi in Namosi district, Viti Levu are separated into two deposits: the Waisoi East deposit and the Waisoi West deposit. In the Waisoi East deposit, quartz porphyry is exposed and in the Waisoi West deposit, diorite porphyry is sporadically exposed in addition to a small body of quartz porphyry. The mineralization in the Waisoi East deposit is characterized by the bornite–chalcopyrite–pyrite assemblage associated with traces of molybdenite and native gold. Polyphase fluid inclusions in stockwork quartz veinlets show homogenization temperatures ranging from 210 to >500°C. The high‐grade Cu mineralization in the Waisoi West deposit is characterized by the bornite–chalcopyrite–pyrite assemblage accompanied with sheeted and stockwork quartz veinlets. Polyphase fluid inclusions occasionally containing hematite flakes in quartz veinlets in the center of the Waisoi West deposit homogenize at temperatures ranging from 450°C to >500°C. However, fluid inclusions in stockwork quartz veinlets in the periphery, homogenize at lower temperatures around 210°C. Both in the Waisoi East and Waisoi West deposits, primary bornite–chalcopyrite–pyrite assemblage in the high Cu‐grade zone was deposited at the upper stability limit of chalcopyrite with respect to sulfur fugacity. Thus, the principal Cu mineralization at the Waisoi deposits occurred at a relatively high sulfur fugacity, that is, in a high‐sulfidation environment.  相似文献   

Dredge petrology of the boninite‐ and adakite‐bearing Hahajima Seamount of the Ogasawara (Bonin) forearc: An ophiolite or a serpentinite seamount?§     

Akira Ishiwatari  Yuki Yanagida  Yi‐Bing Li  Teruaki Ishii  Satoru Haraguchi  Kazuto Koizumi  Yuji Ichiyama  Masaru Umeka 《Island Arc》2006,15(1):102-118

Abstract During the Hakuho‐Maru KH03‐3 cruise and the Tansei‐Maru KT04‐28 cruise, more than 1000 rock samples were dredged from several localities over the Hahajima Seamount, a northwest–southeast elongated, rectangular massif, 60 km × 30 km in size, with a flat top approximately 1100 m deep. The rocks included almost every lithology commonly observed among the on‐land ophiolite outcrops. Volcanic rocks included mid‐oceanic ridge basalt (MORB)‐like tholeiitic basalt and dolerite, calc‐alkaline basalt and andesite, boninite, high‐Mg adakitic andesite, dacite, and minor rhyolite. Gabbroic rocks included troctolite, olivine gabbro, olivine gabbronorite (with inverted pigeonite), gabbro, gabbronorite, norite, and hornblende gabbro, and showed both MORB‐type and island arc‐type mineralogies. Ultramafic rocks were mainly depleted mantle harzburgite (spinel Cr? 50–80) and its serpentinized varieties, with some cumulate dunite, wehrlite and pyroxenites. This rock assemblage suggests a supra‐subduction zone origin for the Hahajima Seamount. Compilation of the available dredge data indicated that the ultramafic rocks occur in the two northeast–southwest‐oriented belts on the seamount, where serpentinite breccia and gabbro breccia have also developed, but the other areas are free from ultramafic rocks. Although many conical serpentinite seamounts 10 km in size are aligned along the Izu–Ogasawara (Bonin)–Mariana forearc, the Hahajima Seamount may be better interpreted as a fault‐bounded, uplifted massif composed of ophiolitic thrust sheets, resembling the Izki block of the Oman ophiolite in its shape and size. The ubiquitous roundness of the dredged rocks and their thin Mn coating (<2 mm) suggest that the Hahajima Seamount was uplifted above sealevel and wave‐eroded, like the present Macquarie Is., a rare example of ophiolite exposure in an oceanic setting. The Ogasawara Plateau on the Pacific Plate is adjacent to the east of the Hahajima Seamount, and collision and subduction of the plateau may have caused uplift of the forearc ophiolite body.  相似文献   

In situ hydraulic tests in the active fault survey tunnel, Kamioka Mine, excavated through the active Mozumi-Sukenobu Fault zone and their hydrogeological significance     

Tsuyoshi  Nohara  Hidemi  Tanaka  Kunio  Watanabe  Noboru  Furukawa  Akira  Takami 《Island Arc》2006,15(4):537-545

Abstract   The spatial hydrogeological and structural character of the active Mozumi-Sukenobu Fault (MSF) was investigated along a survey tunnel excavated through the MSF in the Kamioka Mine, central Japan. Major groundwater conduits on both sides of the MSF are recognized. One is considered to be a subvertical conduit between the tunnel and the surface, and the other is estimated to be a major reservoir of old meteoric water alongside the MSF. Our studies indicate that part of the MSF is a sub-vertical continuous barrier that obstructs younger meteoric water observed in the south-eastern part of the Active Fault Survey Tunnel (AFST) and water recharge to the rock mass intersected by the north-western part of the AFST. The MSF might be a continuous barrier resulting in the storage of a large quantity of older groundwater to the northwest. The observations and results of in situ hydraulic tests indicate that the major reservoir is not the fault breccia associated with the northeast–southwest trending faults of the MSF, but the zone in which blocks of fractured rocks occur beside high angle faults corresponding to X shears whose tectonic stress field coincides with the present regional stress field and antithetic Riedel shears of the MSF. The results from borehole investigations in the AFST indicate that secondary porosity is developed in the major reservoir due to the destruction of filling minerals and fracture development beside these shears. The increase in hydraulic conductivity is not directly related to increased density of fractures around the MSF. Development of secondary porosity could cause the increase in hydraulic conductivity around the MSF. Our results suggest that minor conduits of the fracture network are sporadically distributed in the sedimentary rocks around the MSF in the AFST.  相似文献   

NEW TRENDS IN THE NUMERICAL STUDY OF COASTAL SEDIMENTARY PROCESSES     

Xiping YU  Akira WATANABE 《国际泥沙研究》2003,(2)

1 INTRODUCTION The conventional method for numerical study of coastal sedimentary processes has been based on the solution of a phase-averaged wave equation, the steady nearshore current equations, the continuity equation for sediment, and an empirical net sediment transport equation. This strategy has advantages in meso-scale problems but it loses accuracy in the vicinity of a structure, where local scour usually occurs. To have a good representation of both meso-scale sediment transport …  相似文献   

Paleomagnetism of Pleistocene widespread tephra deposits and its implication for tectonic rotation in central Japan     

Hiromi Iwaki  Akira Hayashida 《Island Arc》2003,12(1):46-60

Abstract   A single layer of widespread tephra deposits possibly can provide an instantaneous record of the past geomagnetic field and potentially can indicate even a small-scale tectonic rotation compared to a range of geomagnetic secular variations. We report paleomagnetic data of the Ebisutoge–Fukuda tephra, which is dated at approximately 1.8 Ma and is distributed in central Japan between the Osaka–Kyoto area and the Boso Peninsula. The Fukuda volcanic ash layer and its correlative ash deposits in the Osaka–Kyoto area, near Lake Biwa and in the Mie and Niigata areas yield identical site mean declinations of approximately −170° after tilt correction, whereas moderate inclination shallowing is observed in the upper unit at several localities. Anisotropy measurements both of low-field magnetic susceptibility and of anhysteretic remanent magnetization suggest that the inclination shallowing results from the biased alignment of magnetic grains, which were deposited in the fluvial environment. The source volcanic unit, Ebisutoge pyroclastic deposits in the Takayama area, yields a mean declination of approximately −155°, showing clockwise deflection from the magnetic directions of the correlative tephra deposits. These results suggest that no significant rotation occurred between the Osaka–Kyoto, Mie and Niigata areas, but that the Takayama area suffered a clockwise rotation in respect to the other areas during the Quaternary. This rotation might have been caused under an east–west stress field associated with the collision of the Okhotsk Plate with the Eurasia Plate.  相似文献   

Paleozoic ophiolites and blueschists in Japan and Russian Primorye in the tectonic framework of East Asia: A synthesis   总被引：20，自引：1，他引：20  

Akira Ishiwatari  Tatsuki Tsujimori 《Island Arc》2003,12(2):190-206

Abstract   Ophiolites and high-pressure (HP) metamorphic rocks are studied to test continuation of Paleozoic and early Mesozoic geological units from Japan to Primorye over the Japan Sea. The early Paleozoic ophiolites are present on both sides, and the late Paleozoic ophiolite of south-western Japan may also have its counterpart in Primorye. The Shaiginskiy HP schist and the associated Avdakimov gneiss in Primorye, both tectonically underlying the early Paleozoic ophiolitic complex, yield a 250-Ma phengite and hornblende K–Ar age, which is intermediate between those of the Renge (280–330 Ma) and Suo (170–220 Ma) blueschists in south-western Japan. This age also coincides with that of the coesite-bearing eclogites in the Sulu–Dabie suture in China and several medium-pressure metamorphic rocks in East Asia. On the basis of these results and other geological data, the authors propose the 'Yaeyama promontory' model for an eastward extension of the Sulu–Dabie suture. The collision suture warps southward into the Yellow Sea and detours around Korea, turns to the north at Ishigaki Island in the Yaeyama Archipelago of Ryukyu, where it changes into a subduction zone and further continues toward south-western Japan and Primorye. Most ophiolites from this area represent crust–mantle fragments of an island arc–back-arc basin system, and the repeated formation of ophiolite–blueschist associations may be due to the repetition of the Mariana-type non-accreting subduction and Nankai-type accreting subduction.  相似文献   

Concentrations of 222 Rn, Its Short-Lived Daughters And 212 Pb And Their Ratios Under Complex Atmospheric Conditions And Topography     

Toshio Kataoka  Eiji Yunoki  Mitsuo Shimizu  Tadashige Mori  Osamu Tsukamoto  Satoshi Takahashi  Hironori Fudeyasu  Yukitaka Ohashi  Ken Sahashi  Toshihiko Maitani  Koh'ichi Miyashita  Toru Iwata  Takayuki Sasaki  Yoko Fujikawa  Akira Kudo  Roger H. Shaw 《Boundary-Layer Meteorology》2003,107(1):219-249

Atmospheric activity concentrations of ²¹²Pb and short-lived ²²²Rndaughters, together with meteorological elements, have been observed continuously atthree sites at Kamisaibara Village in Japan. In addition, atmospheric activity concentrationof ²²²Rn, equilibrium-equivalent concentration of ²²²Rn and conditionsof the lower atmosphere were observed for three intensive observation periods at Akawase,one of the three sites in Kamisaibara Village. The equilibrium-equivalent concentration of²²²Rn is almost the same as the atmospheric activity concentration of short-lived²²²Rn daughters.The activity concentrations of ²¹²Pb and the short-lived ²²²Rn daughtersand their ratio were low in the daytime owing to convective mixing, and high at nightowing to the surface-based inversion during periods of no precipitation. Their variationshave several patterns corresponding to the scale of the drainage wind or weak mixing.Mechanical mixing due to strong winds through both day and night during the first andsecond observation periods made the atmospheric activity concentrations of ²¹²Pb and the short-lived ²²²Rn daughters continuously low. However, their ratios werecontinuously high during the first period yet continuously low during the second period.This difference can be explained by the effect ofextraction of ²²⁰Rn and ²²²Rndue to strong winds and snow cover. There were also cases in which the ratio of theatmospheric activity concentration of ²¹²Pb to that of the short-lived ²²²Rndaughters at night was equal to or less than the ratio in the daytime. Thisinverse trend, asin the periods of no precipitation mentioned above, is considered to be due to near-neutralconditions on these nights.We find a difference in the ratio of the equilibrium-equivalent concentration of²²²Rn (the activity concentration of short-lived ²²²Rn daughters) tothe activity concentration of ²²²Rn during the first observation period and thatduring the second. The difference can be explained by snow cover on the ground. Wealso find differences among the ratios of the activity concentration of the short-lived²²²Rn daughters to that of ²²²Rn during the three observation periods.These differences can be explained by the submergence of paddy fields.  相似文献   

Source mechanisms of deep and intermediate-depth low-frequency earthquakes beneath Iwate volcano, northeastern Japan   总被引：2，自引：0，他引：2  

Haruhisa Nakamichi  Hiroyuki Hamaguchi  Satoru Tanaka  Sadato Ueki  Takeshi Nishimura  Akira Hasegawa 《Geophysical Journal International》2003,154(3):811-828

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"Brown Ore" from the Fukasawa Kuroko Deposits, Northeast Japan: Its Characteristics and Formation Process     

Osamu ISHIZUKA  Akira IMAI 《Resource Geology》1998,48(2):53-73

Abstract: Brown–colored sulfide ore (brown ore) occurs in the easternmost part of the Tsunokakezawa No. 1 orebody of the Fukasawa kuroko-type deposits, northern Honshu, Japan. As this type of ores also occur in the marginal or uppermost part of several other kuroko deposits in Japan, the formation of brown ore appears to be repeated in the process of kuroko formation. The brown ore is characterized by its higher Ag concentration (up to around 2000 g/t) than ordinary black ore (Zn–Pb ore) of volcanogenic massive sulfide deposits. The brown ore from the Fukasawa deposits can be divided into following three ore types based on its texture and mineral composition: pyritic brown ore, principal brown ore and “diseased” brown ore. Primary precipitation textures such as framboidal– and colloform-textures and compositional zoning within sulfide grains are significant in the brown ores. This seems to be due to lack of overprinting high temperature mineralization resulting in preservation of primary features. The Ag-Au mineralization is widely observed within the brown ores. Silver and gold are especially concentrated in the barite veinlets in the principal brown ore, which are supposed to be fillings of conduit of hydrothermal solution precipitated in the latest stage of hydrothermal activity. This mineralization seems to occur at waning stage of brown ore formation by ore solution at a lower temperature (around 250°C) than that of main part of brown ore (around 270°C). Relatively low fluid temperature and contribution of oxic ambient seawater may be responsible for the development of the Ag-Au mineralization in the brown ore. The occurrence of framboidal-rich pyritic brown ore having negative δ³⁴S values (less than –10%) and filamentous texture of sphalerite, seeming remnant of bacteria, indicate the presence of intensive microbial activity in the hydrothermal area for brown ore formation. Formation environment of each ore type of the brown ore is supposed to be as follows: Pyritic brown ore is likely to have formed on the sea-floor around redox boundary at temperature (around 240°C) lower than ordinary black ore. Principal brown ore seems to have been formed beneath the shell of the pyritic brown ore at temperature around 270°C. Footwall of the brown ore is disseminated tuff breccia corresponding to feeder zone of hydrothermal fluid. Overprinting chalcopyrite mineralization is not observed in the brown ore except in limited part of “diseased” ore, which occurs just above the disseminated tuff breccia. Based on the features distinct from the ordinary black ore, the brown ore can be regarded as a product in the marginal part of submarine hydrothermal system, where temperature and flow rate of hydrothermal solution was relatively low and microbial activity was intensive. The brown ore seems to well preserve its primary features after its deposition and might show the initial feature of some part of the ordinary stratiform black ore.  相似文献   

Monte Carlo simulation of normal grain growth in 2- and 3-dimensions: the lattice-model-independent grain size distribution     

Akira Miyake 《Contributions to Mineralogy and Petrology》1998,130(2):121-133

The phenomenon of normal grain growth in pure single phase systems is modeled with the Monte Carlo technique and a series of simulations are performed in 2- and 3-dimensions. The results are compared with natural and experimental monomineralic rock samples. In these simulations various lattice models with different anisotropic features in grain boundary energy are examined in order to check the universality of the simulation results. The obtained microstructure varies with the artificial parameter T ^′ in each lattice model, where T ^′ means scaled temperature and controls thermal fluctuation on grain boundary motion. As T ^′ (thermal fluctuation) increases, the boundary energy anisotropy characterizing each lattice model becomes less important for the evolution of the microstructure. As a result the difference in the grain size distribution among the lattice models, which is significantly large for low T ^′, is reduced with increasing T ^′. The distribution independent of both the lattice model and the dimension is obtained at sufficiently high T ^′ and is very close to the normal distribution when carrying out the weighting procedure with a weight of the square of each grain radius. A comparison of the planar grain size distribution of the natural and experimental rock samples with the 3-D simulation results reveals that the simulations reproduce very well the distributions observed in the real rock samples. Although various factors such as the presence of secondary minerals and a fluid phase, which are not included in the simulation modeling, are generally considered to influence the real grain growth behavior, the good agreement of the distribution indicates that the overall grain growth behavior in real rocks may still be described by the simplified model used in the present simulations. Thus the grain size distribution obtained from the present simulations is possessed of the universal form characterizing real normal grain growth of which the driving force is essentially grain boundary energy reduction through grain boundary migration. Received: 7 January 1997 / Accepted: 25 August 1997  相似文献