Kuroko Mineralization Research Articles

Environment of Kuroko mineralization in the Aketooshi deposits, Hanawa Mine

Environment of Kuroko mineralization in the Aketooshi deposits, Hanawa Mine

Relationships between the evolution of the Neogene volcanism and the Kuroko mineralization in the vicinity of the Hokuroku District

Relationships between the evolution of the Neogene volcanism and the Kuroko mineralization in the vicinity of the Hokuroku District

Development of Submerged Sedimentary Basin and Environment of Kuroko Mineralization in the Hokuroku District

Development of Submerged Sedimentary Basin and Environment of Kuroko Mineralization in the Hokuroku District

Relationship between Kuroko Mineralization and Paleostress Inferred from Vein Deposits and Tertiary Granitic Rocks In and Around the Hokuroku District, Northeast Japan

In the inner zone of the northeastern Japan arc, kuroko-type massive sulfide deposits of copper, lead, and zinc are related to submarine dacitic volcanism. There are four horizons that host such deposits in and around the Hokuroku district, the main deposits occurring at 14.32 Ma and coinciding with the end of an extensional regime followed by a weakly compressive regime. The change in tectonic style is indicated by the fracturing patterns in the vein deposits and Tertiary granitic rocks that were emplaced at 13.9 to 6.0 Ma and show similar alignment. They are also associated with explosive dacitic volcanism that occurred after the main kuroko mineralization. During the middle of the extensional regime, which was characterized by intensive submarine bimodal volcanism, no kuroko mineralization took place. Therefore, the observed change in tectonic regime might have been the trigger for kuroko mineralization. In addition to the main kuroko deposits, small-scale kuroko deposits can be found at different horizons. Two deposits are hosted within the first dacitic lava after the beginning of a marine transgression that coincided with the beginning of the extensional regime earlier than the main kuroko mineralization, and three deposits are hosted within dacitic rocks emplaced in the weakly compressive regime after the main kuroko mineralization. However, large-scale kuroko deposition appears to have been inhibited by explosive volcanism during this stage.

Volcanic Sequences Related to Kuroko Mineralization in the Hokuroku District, Northeast Japan

Abstract. Kuroko deposits are a representative volcanic‐hosted massive sulfide deposit and the Hokuroku district is economically the most important Kuroko containing province in Japan. There are two cycles of the bimodal volcanic sequence in the Hokuroku district. The pre‐ore volcanism started with basaltic activity and was followed by intensive felsic hyaloclas‐tic activity under bathyal conditions. The post‐ore sequence also began with basaltic activity intercalated with mudstone and was followed by alternating beds of pumice tuff with several lava flows and mudstone. Kuroko deposits are situated in the final period of the pre‐ore felsic volcanic sequence of the first bimodal volcanic cycle.Based on a detailed investigation of existing age data, it was concluded that the felsic volcanic sequences in the pre‐and post‐Kuroko formation can be divided into a pre‐ore dacite group (16–13.5 Ma), a D2 dacite group (lower unit of the post‐ore volcanic sequence, 12.7±0.6∼ ll Ma) and a Dl dacite group (upper unit of the post‐ore sequence including quartz‐porphyry and granitoid, 11sim;10 Ma) in ascending order. Field and microscopic observations show that the pre‐ore dacite is characterized by aphyric to plagioclase‐phyric lava and the post‐ore dacitic rocks are characterized by quartz‐plagioclase‐phyric aphanitic lava and dome. These three dacite groups are petrochemically discriminated by SiO2‐Al2O3 and CaO‐TiO2 diagrams, excluding altered specimens. The distribution of the normative compositions on the Q‐An‐Ab‐Or diagram suggests that the pre‐ore dacites trend on the 5 kb cotectic line (equilibrated to 10—15 km deep) and those of the post‐ore trend along the 1 kb line (a few km deep). The secular variation of the major elements indicates that the rhyolitic members genetically related to the Kuroko formation could be the most differentiated products in the pre‐ore felsic volcanism.The distribution of Nb against SiO2 content in the pre‐ and post‐ore bimodal volcanic cycles indicates that these two volcanisms could have been generated by different magmatic origins. The difference would have been caused by the tectonic conversion from a back‐arc to an island‐arc setting.

Kuroko and Hydrocarbon Deposits from Northern Honshu, Japan: A Possible Common Hydrothermal/Magmatic Origin?

Abstract. Northern Honshu is the most important area for mineral and oil resources in Japan. Many kuroko deposits and oil and gas fields are distributed in two belts along the northeast Japan arc, the kuroko metal‐belt on the Pacific side and the oil‐belt on the Sea of Japan side. The kuroko deposits are located mainly in the Green Tuff strata which formed as a result of submarine vol‐canism during the late Miocene and Pliocene. Most of the source rocks of the oil and gas deposits formed at the same time as the kuroko deposits and some of them are located in reservoirs of hydrothermally‐altered volcanic rocks in the Green Tuff region. There is general agreement that the kuroko deposits formed as a result of submarine hydrothermal and magmatic activity whereas almost all petroleum geologists and geochemists consider that hydrocarbon deposits were generated independently of such activity. Since the discovery of hydrothermally‐generated petroleum in the Guaymas Basin, Gulf of California, however, it is clear that petroleum can be formed almost instantaneously in terrestrial and submarine hydrothermal areas. The paleo‐northeastern Sea of Japan is therefore considered to be a potential area for hydrothermal petroleum generation because thick organic‐rich sediments overlie an active submarine volcanic area. Several lines of geological and geochemical evidence suggest the possibility of hydrothermally‐enhanced maturation of organic matter and the contribution of magmatic activity to the formation of these deposits. Although most of the oil and gas in northern Honshu has been generated conventionally as a consequence of the high geothermal gradients there, it appears that some of the oil and gas fields may have formed as a result of extensive hydrothermal and magmatic activity during the late Miocene to Pliocene. Because of the much steeper angle of the faults in the vicinity of the Hokuroku basin than in the Akita basin, the magmatic contribution to the kuroko mineralization would have been far greater than to the oil and gas deposits of the Niigata and Akita basins. We therefore propose a strong relationship between metal and oil and gas generation in northern Honshu based on the structure and tectonics of the northern Honshu arc‐backarc system.

Relationships between the evolution of the Neogene volcanism and the Kuroko mineralization in the vicinity of the Hokuroku district

Relationships between the evolution of the Neogene volcanism and the Kuroko mineralization in the vicinity of the Hokuroku district

Lead isotope measurements on ores, igneous and sedimentary rocks from the kuroko mineralization area.

Lead isotope measurements on ores, igneous and sedimentary rocks from the kuroko mineralization area.

A chloride complex model for Kuroko mineralization

A chloride complex model for Kuroko mineralization at the Uchinotai-nishi deposit of the Kosaka mine is constructed. Possible chemical composition of the ore solution and possible physicochemical properties of the ore-forming environments calculated on the basis of the thermochemical data of HELGESON (1969) can be limited to narrow ranges, if more than 1 ppm Cu is assumed to be dissolved in the ascending Kuroko-forming solution. The model ore-forming environments derived here are consistent with the data of temperature and sulfur fugacity estimated from studies of fluid inclusions, mineral equilibria and sulfur isotopes of the Kuroko deposits. The characteristic mineral zoning of the deposits are explicable by differential precipitation of ore-constituent minerals from the model solutions moving along the gradients of the model ore-forming environments. In order to explain the distribution of sulfate minerals in the deposits, it is necessary to assume mixing of the ascending solution and the coeval seawater. In comparison with the chemistry of the natural metal-rich brines, the model solutions are low in Na/K ratio, while high in Na/Ca and reduced sulfur to metal ratios. It is suggested that these features might be characteristic of volcanogenic ore-forming solutions, which are likely to retain equilibrium conditions at higher temperatures as a result of their rapid ascent from the source reservoir.

Oxygen and hydrogen isotopic study on the water of crystallization of gypsum from the Kuroko type mineralization

The δ18O and δD values of water of crystallization of gypsum from the Kuroko type mineralization in Japan mostly fall in narrow ranges from -6 to -8 and from -75 to -90‰, respectively. While the mineralization has generally been accepted as of submarine hydrothermal origin, the present results indicate that the gypsum ores are in isotopic equilibrium with meteoric waters instead of sea water. The most plausible interpretation for this observation is that the primary submarine anhydrite or gypsum was hydrated or re-equilibrated with meteoric waters after the uplifting of the deposits. However, there is some evidence that meteoric hydrothermal solution might have been involved in certain stages of Kuroko mineralization at Shakanai. The δ18O and δD values of gypsum of the Tsutsumizawa deposit in Hanaoka, Northeast Honshu, fall on a line with a slope (δD/δ18O) of -5.3. This suggests that the deposit was formed by fractional hydration of an anhydrite deposit under a limited supply of water, of which isotopic ratios were similar to present-day meteoric waters. Gypsum ores of Noto and Osaka-Matsushiro, Southwest Honshu, are isotopically quite different from most of other Kuroko deposits. These gypsum ores seem to have been hydrated by hydrothermal waters which isotopically are significantly shifted from meteoric waters.