Concentration mechanisms of iron oxides and alumina in deep weathering crusts (Goshikidai, Kagawa, Western Japan)

Concentration of Fe-oxides and alumina in weathering processes are main geological reactions for lateritization and bauxitization, respectively. In western Japan, red-coloured soil formed by weathering processes developed in many places. This soil is composed of hydrous Fe-oxide minerals, hydrous alumina minerals and other minerals. It was formed in the upper part of deep weathering crust by weathering processes under some kind of sub-tropical climate, probably in the Pliocene. One of these occurrences is observed in the upper part of Goshikidai and Konodai, west part of Takamatsu city, northeast Shikoku Island, west Japan. A deep weathering crust is distributed on wide hilly plains ranging from 250 to 400 m a.m.s.l. in the northwestern region of Takamatsu city. Original rock of the weathering crust is bronzite andesite and glassy bronzite andesite, so-called ‘sanukite’. The andesites had been weathered under some special climate, and the geological age of the weathering is the same as above. The mineral assemblage and formation mechanism are similar to those of laterite and bauxite. The weathering crust developing in this region are subdivided into the three following zones: (1) A zone, composed of hydrous Fe-oxides and metahalloysite with small amounts of gibbsite and it is associated with white veins of metahalloysite; (2) B zone, composed of hydrous Fe-oxides and metahalloysite (some material is associated with -cristobalite); and C zone, composed of metahalloysite or halloysite and -cristobalite with relict crystals of feldspar and quartz, and some material is associated with montmorillonite. Chemical analyses of the materials of the three zones show the formation mechanisms of the weathering crust.     

Statistical Features of Vein Systems in the Hishikari Epithermal Gold Deposit, Japan

Abstract. The Hishikari epithermal gold deposit consists of parallel vertical veins hosted by the Quaternary volcanic rocks (QVR) and Cretaceous sedimentary rocks (CSR) at the shallow and deep levels, respectively. In order to reveal the litholog‐ical influence on the evolution of vein systems in the Hishikari field, vein density, width, spacing and position were statistically examined using cores sampled from three boreholes. The statistical features of the veins identified were summarized as follows, (i) The QVR have lower mean vein densities than the CSR. (ii) The former has smaller deformation degrees (ratios of total vein width per length) than the latter, (iii) Cumulative frequencies of vein density obey negative‐exponential distributions in both the types of rocks, (iv) Cumulative frequencies of vein width obey power‐law distributions with exponents of about ‐1 in both the types of rocks, but consist of two segments in the CSR. (v) Cumulative frequencies of vein spacing obey lognormal distributions with coefficients of variation between 1.8 and 3.5 in both the types of rocks. And (vi) fractal dimensions of vein position are between 0.45 and 0.52 at measure lengths between 0.1 and 10 m without distinctive difference between the two types of rocks. The feature (i) indicates higher ductility of the QVR against fracturing than the CSR. The feature (ii) implies that large deformation was accumulated in rocks themselves (e.g., alteration zones) in addition to the veins examined in the QVR compared to only the veins in the CSR. The feature (iv) suggests that the vein systems in the CSR reached a stage at which the several selected large veins grew as main conduit, that potentially provided pathways for ore‐forming fluids to ascend from deep levels. The coefficients of variation of vein spacing and fractal dimensions of vein position (features v and vi) indicate clustered distribution of the veins in the field. This study might be helpful for a better understanding of lithological influence on the evolution of vein systems not only in the Hishikari field but also in other fields.     

Occurrences of Ore Minerals and Fluid Inclusion Study on the Kingking Porphyry Copper–Gold Deposit, Eastern Mindanao, Philippines

The Kingking deposit is a gold‐rich porphyry copper deposit and the southernmost deposit at the eastern Mindanao mineralized belt, Philippines. It is underlain by Cretaceous–Paleogene sedimentary and volcanic rocks that are intruded by mineralized Miocene diorite porphyries and by barren Miocene–Pliocene dacite and diorite porphyries. The main alteration zones in the deposit are the inner potassic zone and the outer propylitic zone. The biotite‐bearing diorite and hornblende diorite porphyries are the primary host rocks of mineralization. Two dominant copper minerals, bornite and chalcopyrite, which usually occur as fracture fillings, are associated with fine crystalline quartz veinlet stockworks in the mineralized diorites. Minor secondary covellite, chalcocite and digenite are also observed. The primary Cu‐Fe sulfide phases initially deposited from ore fluids consisted of bornite solid solution (bnss) and intermediate solid solution (iss), which decomposed to form the bornite and chalcopyrite. Peculiar bornite pods that are different from dissemination and are associated with volcanic rock xenoliths in biotite‐bearing diorite porphyry are noted in a drill hole. These pods of bornite are not associated with quartz veinlet stockworks. Fluid inclusion analyses show three types of inclusions contained in Kingking samples: two‐phase fluid‐rich and vapor‐rich inclusions and polyphase hypersaline inclusions from porphyry‐type quartz veinlet stockworks. The liquid–vapor homogenization temperatures (T_H) and the dissolution temperature of halite daughter crystals (T_M) from the polyphase hypersaline inclusions predominantly range from 400°C up to >500°C. The wide range of T_H and T_M may be due to heterogeneous trapping of variable ratios of vapor and brine. For some inclusions, T_H > T_M and in some cases, T_H < T_M, indicating that some of the brine was supersaturated or saturated with NaCl at the time of entrapment. Calculated salinity of the polyphase hypersaline inclusions ranges from 40 to 60% NaCl equivalent. Temperature and vapor pressure of mineralized fluid were estimated to be 400°C and 16 MPa.     

Pressure dependence of electrical conductivity of (Mg,Fe)SiO<Subscript>3</Subscript> ilmenite

The electrical conductivity of (Mg_0.93Fe_0.07)SiO₃ ilmenite was measured at temperatures of 500–1,200 K and pressures of 25–35 GPa in a Kawai-type multi-anvil apparatus equipped with sintered diamond anvils. In order to verify the reliability of this study, the electrical conductivity of (Mg_0.93Fe_0.07)SiO₃ perovskite was also measured at temperatures of 500–1,400 K and pressures of 30–35 GPa. The pressure calibration was carried out using in situ X-ray diffraction of MgO as pressure marker. The oxidation conditions of the samples were controlled by the Fe disk. The activation energy at zero pressure and activation volume for ilmenite are 0.82(6) eV and −1.5(2) cm³/mol, respectively. Those for perovskite were 0.5(1) eV and −0.4(4) cm³/mol, respectively, which are in agreement with the experimental results reported previously. It is concluded that ilmenite conductivity has a large pressure dependence in the investigated P–T range.     

Relationship Between Solidification Depth of Granitic Rocks and Formation of Hydrothermal Ore Deposits

Chemical analysis of biotite in representative granitic rocks in Japan shows that the total Al (^TAl) content changes with the metal type of the accompanying hydrothermal ore deposits and increases in the following order: Pb‐Zn and Mo deposits < Cu‐Fe and Sn deposits < W deposits < non‐mineralized granitic rocks. The ^TAl content of biotite in granitic rocks may be a useful indicator for distinguishing between mineralized and non‐mineralized granitic rocks. A good positive correlation is seen between the ^TAl content of biotite and the solidification pressure of the granitic rocks estimated by sphalerite and hornblende geobarometers and the mineral assemblages of the surrounding rocks. These facts suggest that the ^TAl content of biotite can be used to estimate the solidification pressure (P) of the granitic rocks. The following empirical equation was obtained: where ^TAl designates the total Al content in biotite on the basis O = 22. According to the obtained biotite geobarometer, it is estimated that Pb‐Zn and Mo deposits were formed at pressures below 1 kb, Cu‐Fe and Sn deposits at 1–2 kb, W deposits at 2–3 kb and non‐mineralized granitic rocks were solidified at pressures above 3 kb.     

Geochronological and petrological investigations of Miocene felsic igneous rocks in the Amakusa Islands,southwest Japan: Possible extension of the Setouchi Volcanic Belt

The opening of the Japan Sea led to the separation of southwest Japan from the Eurasian continent. Subsequent to this event, a diverse range of igneous activities occurred in southwest Japan. On the back-arc side of the region, igneous activity commenced at approximately 22 Ma and persisted for an extended period. In the trench-proximal region of southwest Japan, magmatism initiated around 15.6 Ma, immediately following the cessation of the Japan Sea opening, in correlation with the subduction of the Philippine Sea plate beneath southwest Japan. The Amakusa Islands in western Kyushu host felsic to intermediate igneous rocks with Miocene radiometric ages. There has been a debate regarding the attribution of the igneous rocks in Amakusa Island among the Miocene igneous rocks in southwest Japan. To address this issue, we conducted zircon U–Pb dating and analyzed the major- and trace-element compositions of felsic igneous rocks in the Amakusa Islands to elucidate their characteristics. The obtained U–Pb ages range from 14.5 to 14.8 Ma, suggesting contemporaneity between magmatism in the Amakusa Islands and the Setouchi Volcanic Rocks in the trench-proximal region of southwest Japan. The major and trace element compositions of the felsic igneous rocks exhibit similarities to the dacites of the Setouchi Volcanic Rocks. These findings support previous suggestions that the magmatism in the Amakusa Islands can be correlated with the Setouchi Volcanic Rocks, based on the discovery of a high-Mg andesite dike and paleo-stress analysis utilizing the direction of dikes and sills. Therefore, the Setouchi Volcanic Belt is proposed to extend further west than the previously identified Ohno volcanic rocks in eastern Kyushu. The subduction of the Shikoku Basin of the Philippine Sea plate toward western Kyushu supports the hypothesis that the Kyushu-Palau Ridge was positioned west of Kyushu at ~15 Ma.     

Determination of cation distribution in (Co,Ni,Zn)2SiO4 olivine by synchrotron X-ray diffraction

 The cation distribution of Co, Ni, and Zn between the M1 and M2 sites of a synthetic olivine was determined with a single-crystal diffraction method. The crystal data are (Co_0.377Ni_0.396Zn_0.227)₂SiO₄, M _r  = 212.692, orthorhombic, Pbnm, a = 475.64(3), b = 1022.83(8), and c = 596.96(6) pm, V = 0.2904(1) nm³, Z = 4, D _x  = 4.864 g cm⁻³, and F(0 0 0) = 408.62. Lattice, positional, and thermal parameters were determined with MoKα radiation; R = 0.025 for 1487 symmetry-independent reflections with F > 4σ(F). The site occupancies of Co, Ni, and Zn were determined with synchrotron radiation employing the anomalous dispersion effect of Co and Ni. The synchrotron radiation data include two sets of intensity data collected at 161.57 and 149.81 pm, which are about 1 pm longer than Co and Ni absorption edges, respectively. The R value was 0.022 for Co K edge data with 174 independent reflections, and 0.034 for Ni K edge data with 169 reflections. The occupancies are 0.334Co + 0.539Ni + 0.127Zn in the M1 sites, and 0.420Co + 0.253Ni + 0.327Zn in the M2 sites. The compilation of the cation distributions in olivines shows that the distributions depend on ionic radii and electronegativities of constituent cations, and that the partition coefficient can be estimated from the equation: ln [(A/B)_M1/(A/B)_M2] = −0.272 (IR _A -IR _B ) + 3.65 (EN _A −EN _B ), where IR (pm) and EN are ionic radius and electronegativity, respectively. Received: 8 April 1999 / Revised, accepted: 7 September 1999     

U–Pb and fission‐track dating of a submarine pyroclastic rock from southwest Japan

The Okinoshima Formation crops out on Okinoshima Island and comprises a thick sequence (> 200 m) of pyroclastic rocks and alternating beds of sandstone and mudstone. Because Okinoshima Island is located between Honshu and Tsushima Island, the Okinoshima Formation potentially provides an important record of volcanism during the opening of the Japan Sea in northwest Kyushu, as well as a record of the formation of the present Genkai Sea region. In consideration of the lack of previous geochronological work, dating (fission‐track and U–Pb) of igneous zircons extracted from the Okinoshima Formation were undertaken and studied the clay mineral alteration in the pyroclastic material in order to reveal its thermal history. These data are used to constrain the age of the Okinoshima Formation and the present Genkai Sea region. Our results show that no thermal event has reset the fission‐track age after deposition of the pyroclastic rocks, and that the Okinoshima Formation was deposited at 16.2 Ma. The present Genkai Sea region is a deep‐sea basin, and its formation at 16.2 Ma was accompanied by submarine volcanism and rapid subsidence that marked the climactic stage of Japan Sea formation. After 16 Ma, the tectonic setting of the present Genkai Sea region changed from one of extension (related to the formation of the Japan Sea) to one of compression, with uplift occurring under the influence of the clockwise rotation of southwest Japan. Consequently, after 16 Ma the present Genkai Sea region became isolated from the forming processes of the Japan Sea.     

Regionality and seasonality of submesoscale and mesoscale turbulence in the North Pacific Ocean

The kinetic energy (KE) seasonality has been revealed by satellite altimeters in many oceanic regions. Question about the mechanisms that trigger this seasonality is still challenging. We address this question through the comparison of two numerical simulations. The first one, with a 1/10° horizontal grid spacing, 54 vertical levels, represents dynamics of physical scales larger than 50 km. The second one, with a 1/30° grid spacing, 100 vertical levels, takes into account the dynamics of physical scales down to 16 km. Comparison clearly emphasizes in the whole North Pacific Ocean, not only a significant KE increase by a factor up to three, but also the emergence of seasonal variability when the scale range 16–50 km (called submesoscales in this study) is taken into account. But the mechanisms explaining these KE changes display strong regional contrasts. In high KE regions, such the Kuroshio Extension and the western and eastern subtropics, frontal mixed-layer instabilities appear to be the main mechanism for the emergence of submesoscales in winter. Subsequent inverse kinetic energy cascade leads to the KE seasonality of larger scales. In other regions, in particular in subarctic regions, results suggest that the KE seasonality is principally produced by larger-scale instabilities with typical scales of 100 km and not so much by smaller-scale mixed-layer instabilities. Using arguments from geostrophic turbulence, the submesoscale impact in these regions is assumed to strengthen mesoscale eddies that become more coherent and not quickly dissipated, leading to a KE increase.     

Vertical coupling and dynamical source for the intraseasonal variability in the deep Kuroshio Extension

Ocean Dynamics - In the power spectrum, the upper and deep parts of the Kuroshio Extension have distinctly different peaks. The former peaks around 200 days, while the latter is mainly at the...