Neogen

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Subsequent geomorphic studies and work related to bauxite exploration have produced better dating and descriptions of relationships among erosion surfaces in South America (Aleva, 1979, 1984 Krook, 1979 McConnell, 1968 Menendez and Sarmentearo, 1984 Zorme-veld, 1969). The most economic bauxite deposits occur on the Neogene (55 Myr) surfaces. 

Keigwin, L. D. (1982). Isotope paleoceanography of the Caribbean and east Pacific role of Panama uplift in late Neogene time. Science 217,350-353. 

Besides, without addictive AICI3 as a crystal conversion agent, phase composition of most neogenic Ti02 particles was anatase in our experiment. Conversions active energy finm anatase to rutile was 460 kJ/mol [5], with temperature arose, crystal conversion rate as well as mass fraction of rutile would increase [6,7]. Hence, after a lot of heat accumulated, phase composition of particle-sintered layer was rutile. 

General overview and classification of hydrothermal ore deposits of Neogene age... 

Main hydrothermal ore deposit types of Neogene age that formed in and around the Japanese Islands are Kuroko deposits and epithermal vein-type deposits. This classification is based on the form of the deposits. 

Figure 1.18. Variation of Fe " /(Fe + Mg) and tetrahedral AI of chlorite from hydrothermal ore deposits Japanese Neogene Cu-Pb-Zn vein-type (open circle) and Kuroko deposits (solid circle). Localities 1 Ashio, 2 Yatani, 3 Toyoha, 4 Kishu, 5 Sayama, 6 Mikawa, 7 Furutobe, 8 Hanaoka, 9 Wanibuchi, 10 western Bergslagen (Shikazono and Kawahata, 1987).

Figure 1.83. Variation of Fe /(Fe + + Mg) and tetrahedral Al of chlorite from hydrothermal ore deposits Japanese Neogene Cu-Pb-Zn vein-type (open circle) and Kuroko deposits (solid circle). Localities 1 Ashio (Nakamura, 1960, 1963) 2 Yatani (Hattori, 1974) 3 Toyoha (Shikazono 1974a, Sawai, 1984) 4 Kishu (Shirozu, 1958) 5 Sayama (Shirozu, 1958) 6 Mikawa (Nagasawa, 1961) 7 Furutobe (Shirozu et al., 1975) 8 Hanaoka (Hayashi 1961, Hayashi and Oinuma, 1965 Tsuzuki and Honda, 1977 Shirozu et al., 1975) 9 Wanibuchi (Sakamoto and Sudo 1956, Iwao and Minato 1959, Katsumoto and Shirozu, 1973) 10 western Bergslagen (Baker et al., 1983) (Shikazono and Kawahata, 1987).

The Fe +/Mg and Fe- /Fe values of chlorite from Kuroko deposits and Neogene Cu-Pb-Zn vein-type deposits differ greatly (Fig. 1.83). Chlorite from Kuroko deposits contains lower Fe +/Mg and higher Fe /Fe " " values than this from the Neogene vein-type deposits in Japan. The most likely explanation for these differences is that these two types of deposit formed at different states of oxidation, although other... 

Consequently, the composition of chlorite in the discharge zone depends largely on the chemical nature of fluids (factors such as Fe "/Mg, SO /H2S, pH, aj 2+) and temperature. Movement of fluids may also be an important cause for the variability in the ratio of Fe " to Mg in hydrothermal chlorite. Wide compositional variations in chlorite from the hydrothermal ore deposits in Japan, including Kuroko and Neogene Cu-Pb-Zn vein-type deposits, are considered to reflect the variable chemical nature of ascending ore fluids and fluids that mix with ascending ore fluids at discharge zone. 

Substantial amounts of homogenization temperature data on the Neogene vein-type deposits in Japan are available (e.g., Enjoji and Takenouchi, 1976 Shikazono, 1985b) and they are summarized in Fig. 1.87. 

Figure 1.87. Summary of filling temperatures of fluid inclusions from Neogene vein-type deposits in Japan. Solid circle represents average filling temperatures of fluid inclusions for individual deposits (Shikazono, 1985b).

Shikazono (1985b) summarized the assemblage and mode of occurrence of common gangue minerals from more than 70 Neogene epithermal vein-type deposits in Japan. 

Figure 1.106. S 0-8 C of carbonates from Neogene vein-type deposits in Japan (open circle = calcite solid circle = rhodochrosite and Mn-calcite solid triangle = dolomite cross = siderite) (Shikazono, 1989).

Figure 1.109. Sulfur isotopic compositions of Neogene Au-Ag vein-type and disseminated-type deposits. Sulfur isotopic compositions on the samples from the Yatani deposits (Sample No. YT26 from Zn-Pb vein S S = -)-3.3%o), and HS72050305-YT1, YT24 and NS-3 from Au-Ag vein (average S S = +3.3%c)) by Shikazono and Shimazaki (1985) are also plotted. Base-metal rich implies the sample containing abundant sulfide minerals but no Ag-Au minerals from base-metal rich deposits and also from Ginguro-type deposits (Shikazono, 1987b).

Figure 1.116. Lead isotopic variation in Japanese Neogene ores. The majority of data fall in a relatively narrow range which is no more than twice the experimental uncertainty indicated by the replicate analyses of NBS-SRM-981 standard (Sasaki et al., 1982).

However, in contrast to these geologic and tectonic studies, very few studies on the relationship between tectonics and hydrothermal system in Neogene age have been carried out. Therefore, these studies are briefly summarized and then the relationship between geologic and tectonic evolution and evolution of hydrothermal system associated with the mineralizations (Kuroko deposits, epithermal veins) are considered below. 

Unfortunately tectonic situations of the regions other than Northeast Honshu of Neogene age are not well understood. However, it seems evident that even in the regions other than Northeast Honshu epithermal Au-Ag vein-type deposits formed when the uplift started and the area of land expanded. In addition to the paleontologic data, the country rocks of epithermal Au—Ag mine districts also suggest that epithermal Au-Ag vein-type deposits have formed under the subaeiial condition welded tuff occasionally occurs in the mine area (e.g., Sado, Nebazawa, Northeast Hokkaido) and in general submarine sedimentary rocks and volcanic rocks are poor or absent in the Au-Ag mine districts (e.g., epithermal An-Ag vein-type deposits in Kyushu). 

The ages of Neogene mineralization and hydrothermal alteration in and around the Northeast Honshu and Hokkaido have been determined by K-Ar data on K-minerals (K-feldspar, sericite). These data are summarized in Fig. 1.147 and Table 1.26. 

Hattori, K. and Sakai, H. (1979) D/H ratios, origins, and evolution of the ore forming fluids for the Neogene veins and Kuroko deposits of Japan. Econ. GeoL, 74, 535-555. 

Horikoshi, E. (1975b) Statistical distribution of strikes of the Neogene vein deposits in the NE Japan arc. In Horikoshi, E. (ed.). Island Arcs, Marginal Seas, and Kuroko Deposits. Mining Geology Special Issue, 11, 117-124 (in Japanese). 

Ikebe, N. (1973) Neogene biostratigraphy and radiometric time-scale. J. Geosci., Osaka City U., 16, 51-67. 

Ikebe, N. (1978) Bio- and chronostratigraphy of Japanese Neogene with remarks in paleogeography. Cenozoic Geology of Japan. Prof, Ikebe, Y. Mem. Vol., Osaka U., 13-34 (in Japanese). 

Inoue, T. (1969) On the time of mineralization in Neogene Tertiary and the Kuroko Belt in the Green Tuff Region of Northeast Japan. Rept. Research Inst. Underground Resources. Mining Coll. Akita U., 38, 17-30 (in Japanese). 

Nakamura, T. and Hunahashi, M. (1970) Ore veins of Neogene volcanic affinity in Japan. In Tatsumi, T. (ed.), Volcanism and Ore Genesis. Tokyo U. Tokyo Press, pp. 215-230. 

Niitsuma, N. and Akiba, F. (1984) Subduction of plate and tectonics of the Japanese Islands of Neogene age. Kaiyokagaku (Ocean Science), 163, 4-9 (in Japanese). 

Nishiwaki, C., Matsukuma, T. and Urashima, Y. (1971) Neogene gold-silver ores in Japan. Mining Geology Special Issue, The Society of Mining Geologists of Japan, 3, 409-417. 

Otsuki, K. (1989) Reconstruction of Neogene tectonic stress fields of northeast Honshu Arc from metalliferous veins. Geology Soc. Japan, Mem., 32, 281-304 (in Japanese with English abst.). 

Ozawa, A. (1963) Neogene orogenesis igneous activity and mineralization in the central part of northeast Japan. I. On the Neogene igneous activity. Japan. Assoc. Mineral. Petrol. Econ. GeoL, 50, 167-184 (in Japanese). 

Shikazono, N. (1985b) Gangue minerals from Neogene vein-type deposits in Japan and an estimate of their CO2 fugacity. Econ. Geol., 80, 754-768. 

Shikazono, N. (1987b) Isotopic composition and origin of sulfide sulfur of Neogene Au-Ag and base metal vein-type deposits in Japan. U. Tokyo Fac. Sci. J. Sec., 21, 239-255. 

Shikazono, N. (1989) Oxygen and carbon isotopic compositions of carbonates from the Neogene epithermal vein-type deposits of Japan Implication for evolution of terrestrial geothermal activity. Chem. Geol, 76, 239-247. 

Tsukada, Y. and Uno, T. (1980) On the hydrothermal alteration halo associated with the Ohe Mn-Pb-Zn mineralization, west Hokkaido. Mining Geology, 30, 63-72 (in Japanese with English abst.). Tsunakawa, H. (1986) Neogene stress field of the Japanese Arcs and its relation to igneous activity. Tectonophysics, 124, 1-22. 

Utada, M. (1980) Hydrothermal alterations related to igneous activities in Cretaceous and Neogene formations of Japan. Mining Geology Special Issue, 8, 67-83. 

Watanabe, M. (1979) Fluid inclusions in some Neogene ore deposits in the Green tuff region, Japan. Mining Geology, 29, 307-322. 

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