Glacial–interglacial productivity and environmental changes in Lake Biwa,Japan: A sediment core study of organic carbon,chlorins and biomarkers

Temporal changes in paleoproductivity of Lake Biwa (Japan) over the past 32 kyr have been studied by analyzing bulk organic carbon and photosynthetic pigments (chlorins) in the BIW95-5 core. Primary productivity was estimated on the assumption of C/N_org values of 8 for autochthonous organic matter (OM) and 25 for allochthonous OM and using an equation developed for the marine environment. The estimate indicates that primary productivity ranges from 50 to 90 g C m^?2 yr^?1 in the Holocene, while it is ～60 g C m^?2 yr^?1 on average in the last glacial. Pheophytin a and pheophorbide a are the major chlorins. A downcore profile of chlorin concentration normalized to autochthonous organic carbon (OC) shows a decreasing trend. Chlorin productivity was corrected by removal of the effect of post-burial chlorin degradation. The temporal profile of chlorin productivity thereby obtained resembles that from autochthonous OC.The difference in primary productivity between the Holocene and the glacial for the lake is markedly smaller than that for Lake Baikal situated in the boreal zone. This difference between the two lakes is probably caused by the difference in their climatic conditions, such as temperature and precipitation. Precipitation at Lake Biwa is relatively large during the glacial and the Holocene because of the continuous influence of the East Asian monsoon. Lake Baikal precipitation is generally small as a result of control by the continental (Siberia) climate regime. In addition, a significant difference in productivity between the glacial and the Holocene for Lake Baikal may be essentially controlled by the hydrodynamic systems in the lake.Lake Biwa terrigenous OM input events occurred at least five times over the period 11–32 kyr BP, suggesting enhanced monsoon activity. Molecular examination of the layer with a large input of terrigenous OM during the Younger Dryas indicates that concentrations of terrigenous biomarkers such as n-C₂₇–C₃₁ alkanes, lignin phenols, cutin acids, ω-hydroxy acids and C₂₉ sterols are high, suggesting that soil OM with peat-like material entered the lake as a result of flooding. An enhanced sedimentation rate in the last 3000 years might have been partially caused by agricultural activity around the lake.     

Inferred Indium Resources of the Bolivian Tin‐Polymetallic Deposits

Tin‐polymetallic base metal deposits of Miocene age in the Eastern Cordillera in Bolivia were studied by ICP/MS and EPMA for major and minor elements, paying an attention to indium concentration of the ore deposits. The highest indium content and 1000 In/Zn ratio of individual ore deposits are 5,740 ppm and 22.2 for the Potosi deposits, 2,730 ppm and 7.4 for Bolivar deposit, 2,510 ppm and 17.5 for Siete Suyos–Animas deposits, and 1,290 ppm and 3.3 for San Vicente deposit. The same content and ratio of composite samples of the studied deposits are up to 292 ppm and 4.0 for Potosi deposits, 3,080 ppm and 11.3 for Huari Huari deposit, 100 ppm and 0.3 for Tuntoco deposit, 152 ppm and 1.8 for Porco deposit, 103 ppm and 59.2 for Animas deposit, and 1,160 ppm and 3.7 for Pirquitas deposit. Those of zinc concentrates are as follows: 1,080 ppm and 2.1 at San Lorenzo; 584 ppm and 1.7 at Bolivar; 499 ppm and 1.23 at Porco; 449 ppm and 1.21 at Reserva, and 213 ppm and 0.61 at Colquiri deposit. Indium occurs mostly in dark colored sphalerite and that of the Potosi deposit was found to have one of the highest concentrations, containing up to 1.27 wt% In. Petrukite was discovered in the Potosi deposit, and indium minerals are expected to occur in the Huari Huari deposit and others with the high 1000 In/Zn ratios. The indium contents of the zinc concentrates and composite samples were applied to the produced and remaining ores, then the total amounts of indium in the Bolivian tin‐polymetallic base metal deposits are speculated to be more than 12,000 tons In, which is bigger than that of South China (11,000 tons) and the Japanese Islands (9,000 tons). Sphalerites of the Potosi deposit have one of the highest ranges of indium, similarly to those of the San Vicente deposit. Both the San Vicente and Potosi deposits are rich in silver, implying significance of both silver‐polymetallic and tin‐polymetallic environments for the concentration of trace amounts of indium.     

Baroclinic circulation and its high frequency variability in Otsuchi Bay on the Sanriku ria coast,Japan

Hydrographic observations were made in Otsuchi Bay on the Sanriku ria coast, Japan, to provide clear images of the baroclinic circulation extending over the bay together with the associated intrusion of lower-layer water (bottom water) from outside the bay. In summer, a prominent baroclinic circulation with flow speeds \({>} 0.1\ \text{ m }\ \text{ s }^{-1} \) extends over the greater part of the bay. A main pycnocline (thermocline), which separates the upper and lower layers, is located at a depth of 15–40 m in and around the bay. The direction of the lower-layer flow (inflow into and outflow from the bay) is opposite to that of the upper-layer flow, which are baroclinically coupled to each other. Moreover, with regard to the lower-layer flow, the inflow tends to occur mainly through the northwestern part of the bay mouth, whereas the outflow tends to occur mainly through the southeastern part. The inflow and outflow alternate on time scales of several to a few tens of hours, and the flow directions are sometimes related to the tidal ones, although the relationship is not applied persistently. In winter, the baroclinic circulation is considerably weaker than in summer, because the stratification breaks down.     

Constituent material properties and clast-size distribution of volcanic breccia

Volcanic breccia is a representative heterogeneous rock. As a preliminary study for determining its mechanical properties in laboratory and field experiments, its material properties and structural features were described. Three-dimensional clast-size distribution was also estimated for the study site using the theory of stereology. With this distribution determined, a numerical simulation was performed to investigate the relationship between the volumetric proportion of clasts in a sample and the areal proportion and maximum length of clasts on a lateral surface of the sample. The following observations were made: (1) coefficients of variation for volumetric proportion are dependent on the size of the sample, and became large when the size of the sample is small; (2) samples with a small coefficient of variation can be selected at any size if a restriction is placed on the ratio, D_max/D₀, of the maximum length of clasts to the size of the sample; and (3) the volumetric proportion of clasts in each sample can be estimated from the observed areal proportions within an error range that depends on the whole volumetric proportion.     

Granitoid Types Related to Cretaceous Plutonic Au-Quartz Vein and Cu-Fe Skarn Deposits, Kitakami Mountains, Japan

Abstract. Early Cretaceous granitic intrusions are associated with Au‐quartz veins and Cu‐Fe skarns in the the Kitakami Mountains, which are underlain by the late Paleozoic of continental margin‐type sedimentary rocks and Mesozoic accretionary complexes. The plutonic rocks are divided into potassic, high‐Sr/Y calc‐alkaline and low‐Sr/Y calc‐alkaline series. All the metallic mineral deposits are spatially associated with small stocks and plugs; they show no consistent association with the larger plutonic bodies. The plutonic rocks generally belong to the magnetite series but less oxidized in the southwestern part of the Kitakami Mountains where Au‐quartz veins occur. The gold deposits are classified into high and low sulfide types. The high sulfide type contains a high volume of sulfide minerals mostly of chalcopyrite, arsenopyrite and pyrrhotite with low bulk Au/Ag ratios. This type occurs almost exclusively in and surrounding the Orikabe pluton, including two most important gold deposits (Oya and Kohoku) of the Kitakami Mountains. The pluton is composed of potassic gabbroids, potassic granitoids of the shoshonite ‐ high‐K calc‐alkaline series (Orikabe type), and less potassic Sasamori‐type granodiorite. All these rocks belong to a moderately oxidized magnetite series. The Orikabe pluton has one of the lowest initial Sr ratio (0.70392) in the Kitakami Mountains, and the Au‐Cu‐dominant ore components of the high sulfide type Au deposits are considered magmatic in origin carried by the juvenile magmas from the upper mantle. The low sulfide type is generally plain quartz vein with a low volume of sulfides and a high bulk Au/Ag ratio. The associated minerals are often scheelite and/or arsenopyrite and pyrrhotite. The ore deposits include historically famed Au‐quartz veins at Shishiori and Ogayu. They are widespread in the southwestern Kitakami Mountains and may be later than the high sulfide type in age, and are hosted most commonly in the sedimentary rocks, which surround small weakly oxidized magnetite‐series plutons of low to intermediate Sr/Y series. These less differentiated intrusions typically include quartz dior‐ite and granodiorite. Some ore components of this type may have derived from the host sedimentary rocks. Among other mineral deposit types in the region, the largest ore deposit is Kamaishi Cu‐Fe skarn (magnetite ores of 58 MT, Fe 50–64 %; Cu 143 KT). It is related to the high‐Sr/Y series Ganidake granodiorite stock, which is a strongly oxidized magnetite‐series body. In contrast, the second largest deposit in the mountains, Akagane deposit, is a similar‐type skarn but associated with an intrusion classified as less oxidized, ilmenite to intermediate series, and that is intermediate in Sr/Y of calc‐alkaline series granodiorite. Degree of magmatic differentiation appears to be not critical factor in the formation of Au‐quartz vein and Cu‐Fe skarn deposits in the region, but is definitely significant for controlling the distribution of the Mo‐mineralization to the east.     

Geology, Wall-rock Alteration and Vein Paragenesis of the Bilimoia Gold Deposit, Kainantu Metallogenic Region, Papua New Guinea

The Bilimoia deposit (2.23 Mt, 24 g/t Au), located in the eastern Central Mobile Belt of mainland Papua New Guinea, is composed of fault‐hosted, NW–NNW‐trending Irumafimpa–Kora and Judd–Upper Kora Au‐quartz veins hosted by Middle–Late Triassic basement that was metamorphosed to medium‐grade greenschist facies between Middle–Late Triassic and Early–Middle Jurassic. Mineralizing fluids were introduced during crustal thickening, rapid uplift, change of plate motions from oblique to orthogonal compression, active faulting and S₃ and S₄ events in an S₁–S₄ deformation sequence. The Bilimoia deposit is spatially and temporally related to I‐type, early intermediate to felsic and late mafic intrusions emplaced in Late Miocene (9–7 Ma). Hydrothermal alteration and associated mineralization is divided into 10 main paragenetic stages: (1) chlorite–epidote‐selvaged quartz–calcite–specularite vein; (2) local quartz–illite–pyrite alteration; (3) quartz–sericite–mariposite–fuchsite–pyrite wall‐rock alteration that delimits the bounding shears; (4) finely banded, colloform‐, crustiform‐ and cockade‐textured and drusy quartz ± early wolframite ± late adularia; (5) hematite; (6) pyrite; (7) quartz ± amethyst‐base metal sulfides; (8) quartz–chalcopyrite–bornite–Sn and Cu sulfides–Au tellurides and Te ± Bi ± Ag ± Cu ± Pb phases; (9) Fe ± Mn carbonates; and (10) supergene overprint. Fluid inclusions in stage 4 are characterized by low salinity (0.9–5.4 wt% NaCl equivalent), aqueous–carbonic fluids with total homogenization temperatures ranging from 210 to 330°C. Some of the inclusions that homogenized between 285 and 330°C host coexisting liquid‐ and vapor‐rich (including carbonic) phases, suggesting phase separation. Fluid inclusions in quartz intergrown with wolframite have low salinity (0.9–1.2 wt% NaCl equivalent), aqueous–carbonic fluids at 240–260°C, defining the latter’s depositional conditions. The ore fluids were derived from oxidized magmatic source initially contaminated by reduced basement rocks. Wall‐rock alteration and involvement of circulating meteoric waters were dominant during the first three stages and early part of stage 4. Stage 5 hematite was deposited as a result of stage 4 phase separation or entrainment of oxygenated groundwater. Gold is associated with Te‐ and Bi‐bearing minerals and mostly precipitated as gold‐tellurides during stage 8. Gold deposition occurred below 350°C due to a change in the sulfidation and oxidation state of the fluids, depressurization and decreasing temperature and activities of sulfur and tellurium. Bisulfides are considered to be the main Au‐transporting complexes. The Bilimoia deposit has affinities that are similar to many gold systems termed epizonal orogenic and intrusion‐related. The current data allow us to classify the Bilimoia deposit as a fault‐controlled, metamorphic‐hosted, intrusion‐related mesothermal to low sulfidation epithermal quartz–Au–Te–Bi vein system.     

Hydrothermal Alteration of Limestone and Mineral Exploration of Zn-Pb Skarn Deposits in the Sako-nishi Area of the Kamioka Mine, Central Japan

Abstract: Crystalline limestone of the Sako-nishi area in the Kamioka Zn-Pb mine, central Japan, is depleted in ¹⁸O and ¹³C toward the center of mineralization due to interaction with hydrothermal fluids with a dominant meteoric water component. The relationship between isotopic composition and mineral assemblage, texture, the chemical composition of the minerals, and the bulk chemical composition in the limestone was examined. A decrease in the δ¹⁸OSMOW value correlated with: (1) increase of fine-grained calcite which is enriched in Mn and exhibits a bright cathodoluminescence, (2) progressive hy-drothermal alteration of clinopyroxene in the original limestone into tremolite within the weakly-altered zone, and into chlorite and actinolite within the strongly-altered zone, (3) dominance of hydrothermal chlorite in altered limestone having δ¹⁸O values of less than 10%. This chlorite was enriched in Fe compared to mafic minerals in the unaltered limestone. The enrichment of Fe and Mn was more conspicuous in calcite and chlorite in skarn deposits. The occurrence and chemical composition of hydrothermal minerals in the limestone, skarn, and ore indicate that the ¹⁸O–depleted zones were formed in the later stage from fluids, which were responsible for mineralization and skarnization, and for Fe and Mn enrichment. The Al, Mn, and Fe contents, and the ratios of Mg/(Mg+Mn+Fe), Al/Mg, and Mn/Sr in the hydrochloric acid leachate of limestone varied with decreasing δ¹⁸O and δ¹³C values, reflecting increases in high-Mn calcite and high-Fe chlorite. These indexes were useful for the identification of hydrothermally altered limestone. Furthermore, the potential score weighted by each index was more effective and accurate means of detecting promising mineralization zones. An anomalous potential score due to the presence of hydrothermal minerals in the outcropping limestone occurred along the Atotsu–1GO fault. This structure indicates that the skarn deposits of the Sako-nishi area belong to Mozumi-type Zn–Pb skarn deposits, in which fissures and faults served as major passages for the hydrothermal fluid. High-Mn carbonate and high-Fe chlorite widely occur in base-metal vein deposits and Zn-Pb type skarn deposits. Leaching of altered rock with hydrochloric acid in addition to stable isotope composition and cathodoluminescence imaging is effective for geochemical exploration for hydrothermal deposits because it makes possible the detection of the elemental composition of hydrothermal minerals such as chlorite and carbonate and because of the rapidity and convenience of analysis.     

Fractionated Ilmenite-series Granites in Southwest Japan: Source Magma for REE-Sn-W Mineralizations

Abstract. Various leucocratic biotite granites, low-temperature I-type, from the middle zone of the Sanyo ilmenite-series granitic terrane were studied chemically. These granites are locally associated with REE-Sn-W mineralizations, and were compared with unmineralized granites and batholithic Ryoke granites in three areas of the Chubu, Kinki and Chugoku Districts. They are unique in the region because they have extremely low ferromagnesian components but high Rb/Sr and 10000Ga/Al ratios. These granites are divided petrographically into the main phase, finer-grained marginal phase and younger sheets and dikelets. These rocks have increasing of HREE+Y and Nb+Ta contents in this order, which is also followed by decreasing zircon saturation temperature from 780 to 725C. Together with the mode of occurrence of these granites, the leucogranitic magmas are considered to have formed by in-situ fractionation of the host granitic magmas near the top of the magma chambers. The concentration of HREE, Y, Nb and Ta in these Sanyo Belt leucogranites is principally controlled by magmatic fractionation.     

Fluid Inclusion Study of Skarns in the Maruyama Deposit, the Kamioka Mine, Central Japan

Abstract: Fluid inclusions in skarn minerals in the Maruyama deposit, the Kamioka mine, central Japan were studied. Homogenization temperatures (Th) of fluid inclusions in 48 skarn minerals (hedenbergite, andradite, epidote and quartz) were measured, and gas composition of fluid inclusions in 12 skarn minerals was measured with a quadrupole mass spectrometer. The maximum Th value of primary inclusions in hedenbergite is 380C with peaks around 360C. Primary inclusions in hedenbergite near contact between skarn and limestone have slightly lower Th values and their distribution has a tendency of long trail skirt toward low temperature, which indicates ceasing of skarnization coincides with temperature decrease. Fluid inclusions in andradite and quartz in the hedenbergite skarn have lower Th values, in this order, than those in hedenbergite. CH4–detected fluid inclusions are localized around the Maruyama fault. Gas composition of the fluid inclusions indicates that fluid trapped in the hedenbergite has CO2 content less than 1 mole % and is not in equilibrium with graphite.