Abstract Among Kuroko deposits in the Hokuroku district, some differences are observed in the modes of occurrence and the characteristics of ore which are due mainly to the different environments of ore formations. The Furutobe deposit, a typical Kuroko deposit, is composed of the siliceous ore zone and the stratiform ore zone and occurs in and on dacitic lavas and pyroclastics. The Furutobe deposit may have formed in the area where active volcanic action occurred before ore formation. The sea bottom consisted mainly of unconsolidated dacitic pyroclastics and well-fissured dacitic lavas which gave a favorable condition for upwelling of hydrothermal solutions and circulation of seawater. The siliceous ore zone is composed of fissure and interstice-filling ore which formed in dacitic lavas and pyroclastics. The stratiform ore zone is divided into two types by modes of occurrence: (1) the primary ore deposits which seem to have been formed on the sea floor and in the unconsolidated tuffs; (1) ore deposits which are believed to have been transported and redeposited in a trough by slumping from the primary ore deposits. The Matsuki ore deposit, a somewhat peculiar Kuroko deposit, contains a group of unit orebodies consisting mainly of high-grade yellow ore and occurs in mudstone and dacitic pyroclastics. The Matsuki ore deposit may have formed in the area where the preceding volcanism was minimal and the sea bottom consisted mainly of mudstone. The mudstone, overlying the permeable dacitic pyroclastics, behaved as an aquiclude layer against the rising hydrothermal solution and circulating seawater. The ore deposit is considered to have been formed in unconsolidated mudstone and dacitic pyroclastics from the modes of occurrence of the deposits and the existence of benthonic foraminifera in the mineralized zone.

Abstract The textures of ores collected from two localities in the Ezuri mine are described. Basing our study on observation of modes of occurrence, on cut and polished faces of hand specimens, and on microscopic examination of polished sections and polished thin sections, we interpret that yellow (chalcopyrite-pyrite) ore was lithified before the slumping of barite-rich black ore. In contrast, sphalerite-rich ore was partially consolidated prior to slumping. Recrystallization of sphalerite-rich ore is common, with sphalerite recrystallizing first, followed by chalcopyrite and other sulfides, and finally by chalcedonic quartz, quartz, and barite.

Abstract Although Besshi-type ore deposits have been generally metamorphosed, some of the weakly metamorphosed ore has relict textures which are common in Kuroko ores. Therefore, textures of Besshi-type ores should be significant in the interpretation of the diagenetic recrystallization features of Kuroko ores. Graded bedding of pyrite observed in the Motoyasu mine is neither size grading nor density grading but is interpreted to reflect rhythmic changes in the degree of supersaturation due to intermittent discharge of hydrothermal solutions on the sea floor. Chalcopyrite or sphalerite fills interstices between pyrite cubes and also occurs as inclusions within the pyrite. This texture seems to be a recrystallization product. Rounded pyrite grains with relict colloform texture and interstitial chalcopyrite from the Kune mine recrystallized into pyrite cubes with inclusions and embayments of chalcopyrite.

Abstract The isotopic composition of strontium in sulfate minerals from the Fukazawa and Kosaka ore deposits has been measured in order to evaluate the importance of seawater in the development of the Kuroko deposits. The 87 Sr/ 86 Sr values in samples of anhydrite and gypsum from the sekkoko (gypsum) units in both deposits fall in a narrow range (0.7082-0.7087) whose upper limit approaches that estimated for Miocene seawater. The 87 Sr/ 86 Sr values of the analyzed barites are generally slightly lower than those of the anhydrite and gypsum from the sekkoko and cover a wider range (0.7069-0.7079). None of the ratios are higher than that estimated for Miocene seawater. Coarsely crystalline barite specimens from the siliceous ore zones have 87 Sr/ 86 Sr values which are indistinguishable from those of fine-grained barites in the strata-bound sulfide ores. Anhydrite was probably deposited in shallow convection cells established in the immediate vicinity of rhyolite intrusives. The mixing of a heated, seawater-dominant hydrothermal solution which had acquired a limited amount of isotopically nonradiogenic strontium from the Miocene volcanics with relatively fresh seawater within the porous, poorly consolidated tuff-aceous sediments near the seawater-sediment interface appears to be the most reasonable mechanism of sekkoko formation. The quantity of barite in individual orebodies greatly exceeds that which could be formed from the barium contained in the white rhyolite domes associated with the ore deposits. The measured depletion of strontium and cations in the volcanics underlying the ore horizon, and the alteration of the Paleozoic basement 450 m below the Kuroko orebodies in the Kosaka area strongly suggest that the ore solutions penetrated beneath the Miocene volcanic cover to leach these cations from the basement metamorphics. The 87 Sr/ 86 Sr data for barite are consistent with a model in which seawater is progressively modified through interaction with the Miocene volcanics, the Paleozoic basement, deep granitic plutons, and possibly through the addition of magmatic fluids associated with the granitic plutons. The most appealing model for the formation of the Kuroko strata-bound ores would seem to entail precipitation of the minerals from a hydrothermal solution within the discharge vent or in the interior of a hydrothermal plume formed immediately above the vent exit in the overlying seawater. However, before a full understanding of the mechanism-of sulfide and barite precipitation is possible, much more must be learned about the nature of the vent, the flow rate of hydrothermal solutions, the mixing properties of the ore solution with seawater, and the thermal gradients in the upper part of the feeder channel.

Abstract In the Kuroko deposits of Japan anhydrite is very abundant in sekko ore, which underlies the strata-bound sulfide ores. Anhydrite in sekko ore is usually nodular. The diameters of the anhydrite nodules vary with stratigraphic position and range from a few millimeters to several meters. The pyroclastic rocks in sekko ore horizons have undergone intense hydrothermal alteration. Pyroclastic rocks containing large amounts of anhydrite are altered to Mg chlorite. Pyroclastic rocks containing sericite and sericite-montmorillonite mixed layer clay minerals generally contain small amounts of anhydrite. The strontium content of the sekko anhydrite ranges from about 200 to 2,000 ppm and tends to increase both stratigraphically upward and according to nodule size. The filling temperature of fluid inclusions in the anhydrites ranges from ca. 240° to 340°C. The mode of occurrence, texture, Sr content, nature of the contained fluid inclusions, and isotopic composition of Sr, S, and O in anhydrite together with the mineralogy of the sekko ore suggest that anhydrite deposition was probably related to subsurface mixing of fluids.

Abstract In the first part of this paper, the following three aspects of wall-rock alteration around the Fukazawa deposits in the Hokuroku district of Japan are discussed: (1) the existence of an Na 2 O-depleted dacite mass with a lateral dimension of 1.5 X 3.0 km immediately below the ore horizon; (2) the spatial zoning of alteration minerals in the hanging-wall and footwall rocks; and (3) the relative mobilities of the major elements during alteration of the footwall dacite. In the second part of this paper, an example of the application of computers in the statistical treatment of physical and chemical data of wall rocks around the Ezuri deposits is presented, and the potential of the method in exploration of the Kuroko deposits is discussed.

Abstract The variation of Na 2 O in the acid lava underlying Kuroko deposits has been examined as a convenient exploration guide. First, the precise spatial distribution of the low Na 2 O anomaly around certain Kuroko deposits was established using a large number of samples collected systematically from the footwall acid lava. Then the applicability of using the anomaly for delimiting promising areas around the Kosaka and the Fukazawa deposits was examined. The following results have been obtained. 1. The most economical threshold can be set at 0.36 percent Na 2 O in the Kosaka area and 0.32 percent Na 2 O in the Fukazawa area. The small difference in the Na 2 O value of the thresholds is compatible with previous results. 2. The areal distribution of the low Na 2 O anomaly, however, is somewhat different in extent and shape from that presented previously. The difference in distribution is probably due to a different interpretation of high Na 2 O masses near the footwall of orebodies; namely, they are. regarded as either unaltered remnants or as intrusions. 3. The general areal extent of the anomaly in this study is two or three times as large as the presently known extent of ore and has a high correlation with small offsets of ore distribution. The variation of Na 2 O in the footwall is particularly useful for detailed exploration, but the extent may be too small for regional explorations.

Abstract Tetsusekiei beds or chert-hematite layers stratigraphically overlie massive sulfide ores in many Kuroko deposits. Because of the small size of individual grains (less than 1 µm) in these samples, fluid inclusion studies or mechanical separation of individual minerals for isotopic or chemical analysis is difficult. A chemical method, which involves treatment of the powdered samples with an HCl-HI-H 3 PO 2 mixture and with HF, was developed to attain quartz- and hematite-enriched fractions. Oxygen isotope analyses of these fractions, together with the determination of Fe and Si contents of each fraction, allow estimates to be made of the oxygen isotope compositions of "pure quartz" and hematite in the samples. The δ 18 O values of these "pure" minerals were estimated for eight tetsusekiei samples from four Kuroko mines in the Hokuroku district of Japan: a range of 6 to 21 per mil for the quartz and —19 to +2 per mil for the hematite. The application of the oxygen isotope fractionation factors of quartz-water and magnetite-water of Becker and Clayton (1976) to the δ 18 O values of coexisting quartz and hematite in these samples yields the following values for the temperature and δ 18 O values of hydrothermal fluids responsible for the formation of the tetsusekiei ores: Fukazawa mine: T = 110° ± 30°C, δ 18 O water = -9 ± 5 per mil, and Furutobe mine: T = 90° ± 30°C, δ 18 O water = —3 ± 5 per mil. These data, together with the occurrence of the tetsusekiei bed and the temperature and δ 18 O history of hydrothermal fluids during sulfide mineralization obtained by Pisutha-Arnond and Ohmoto (1983), suggest that the tetsusekiei ores formed during the waning stage of a hydrothermal activity and that a positive correlation exists between the temperature and the δ 18 O values of fluids: negative δ 18 O values at T < 200°C and near zero to positive δ 18 O values at higher temperatures.

Abstract Detailed petrographic and fluid inclusion studies of the stockwork siliceous ores from five major Kuroko deposits (Kosaka, Fukazawa, Furutobe, Shakanai, and Matsumine) have revealed an essentially identical sequence of mineralization and thermal history for each deposit: formation of the majority of black ore minerals (sphalerite, galena, pyrite, and minor barite and quartz) during intensifying stages of hydrothermal activity at T = ~200° to 330°C, followed by formation of the majority of yellow ore minerals (chalcopyrite and quartz) during a thermal maximum at T = 330° ± 50°C and minor sphalerite mineralization while the temperatures again decreased to 260° ± 50°C. Despite the change in temperature, salinities of the ore-forming fluids remained fairly constant at about 3.5 to 6 equivalent weight percent NaCl except for occasional values up to around 8 weight percent. Concentrations of elements in the ore-forming fluids, in moles/kg H 2 O, as determined by the chemical analyses of fluids extracted from sulfides and quartz, were: Na = 0.60 ± 0.16, K = 0.08 ± 0.05, Ca = 006 ± 0.05, Mg = 0.013 ± 0.008, Cl = 0.82 ± 0.32, and C (as CO 2 ) = 0.20 ± 0.15; and less than 6 ppm each for Cu, Pb, Zn, and Fe. The range of δD values of the Kuroko fluids, estimated from the analyses of fluids extracted from sulfides and quartz, is between —30 and +15 per mil, a range much larger than the range of —26 to —10 per mil reported by previous investigators. The range of δ 18 O values of the Kuroko fluids (—6 to +4‰), esfmated from the δ 18 O analyses of quartz and the temperature data (this study) and from the δ 18 O values of quartz and hematite in the tetsusekiei ores (Tsutsumi and Ohmoto, 1983), is also much larger than that suggested by previous investigators (0 ± 1‰). There is a distinct correlation between the δ 18 O values and the temperatures of the Kuroko fluids: fluids of pre-and post-main-stage sulfide mineralization with temperatures less than about 200°C had much lower δ 18 O values than those of the main sulfide mineralization at T 250°C (-2 to +4‰). Comparison of the isotopic and chemical compositions of the Kuroko fluids with those of pore fluids in the Deep Sea Drilling Project core samples and thermochemical consideration of the stability relationships among alteration minerals have led to a model of continuous interactions during the diagenetic through hydrothermal stages between pore fluids (seawater) and volcanic rocks within a thermally intensifying system. The model explains the following observed characteristics: (1) the ranges of δD, δ 18 O, and salinity values of the Kuroko. fluids; (2) the high H 2 O contents and δ 18 O values of country rocks around the Kuroko deposits; (3) the correlation between the δ 18 O values and fluid temperatures; (4) the concentrations of Na, K, Ca, Mg, and Cl in the fluids; (5) the spatial and temporal relationships among the alteration minerals (zeolite, montmorillonite, transition, and sericite + chlorite assemblages) around the Kuroko deposits; and (6) the Na, K, Ca, Mg, and Cl geochemical halos in the rocks around the Kuroko deposits. Some of the important processes that have caused alteration of the isotopic and chemical compositions of fluids and rocks are thought to be: formation of hydrous minerals(zeolites and smectites) in the country rock and cation exchange reactions between fluids and these minerals during diagenetic stages; formation of calcite in the country rock through reactions between fluids and organic matter during the diagenetic and low-temperature hydrothermal stages; dehydration reactions involving diagenetic minerals; and oxygen isotope exchange reactions between fluids and plagioclase, hydrogen isotope exchange reactions between fluids and smectite, dissolution of plagioclase, and cation exchange reactions between fluids and minerals during high-temperature hydrothermal stages.

Abstract The isotopic composition of lead was investigated in and around Kuroko deposits of the Hokuroku district, Japan. Although the ore leads of these deposits were found to occupy a narrow isotopic range, each ore deposit has a characteristic isotopic composition. Within a given ore deposit, black ore has a uniform isotopic composition but is significantly higher in radiogenic lead than yellow ore. The differences between ore types are, however, smaller than those between ore deposits. The volcanic host rocks are in general lower in radiogenic lead than the ores, whereas the deeper, older formations, in particular the Sasahata Formation and the Paleozoic basement, have more radiogenic lead than the ores. On the basis of the isotopic distribution we conclude that a major part of the lead in the Kuroko deposits was derived from igneous, probably volcanic rocks with an uncertain but significant contribution coming from the underlying pre-Nishikurozawa formations. The ore fluids reached the Sasahata Formation and most likely also the Paleozoic basement. Each ore deposit within the district was formed by a local hydrothermal system. The difference in isotopic composition between the yellow and black ores reflects a shift in the proportions coming from the two major sources due to the temperature evolution of the hydrothermal system. The yellow ore seems to have a greater igneous rock lead component than does the black ore.