日本列島の鉱床鉛同位体比
1982 年 32 巻 176 号 p. 457-474
詳細
1982 年 32 巻 176 号 p. 457-474
The metallic mineralization in the Japanese islands can be divided into three groups in view of the ore lead isotope systematics.
(1) The Paleozoic to Mesozoic stratiform (Besshi-type) mineralization is composed of cupriferous iron sulfide ores in which lead is usually a minor constituent (to the level of 10 1-10 2 ppm). The leads are isotopically variable, being consistently low in both 207Pb/ 204Pb and 208Pb/ 208Pb ratios as compared to the lead in the "major (conformable) lead orebodies" of similar ages.
(2) The pre-Neogene mineralization excluding the Besshi-type deposits is mostly related to Cretaceous to Paleogene granitoids. Galena is fairly common and forms sizable orebodies in a few skarn-type deposits. Lead isotopic composition is more or less variable regionally, and even locally in some cases, all being, however, low in 206Pb/ 204Pb and high in 208Pb/ 204Pb ratio in comparison with the conformable ore leads.
(3) The Neogene mineralization is represented by the kuroko-and related vein-type mineralization of middle Miocene age, carrying abundant lead as one of the major ore metals. The leads are isotopically rather uniform with consistently low 206Pb/ 204Pb and high 208Pb/ 204Pb ratio as compared to the major lead ore system; 43 samples from 30 localities give average 208Pb/ 204Pb 207Pb/ 204Pb and 208Pb/ 204Pb ratios of 18.471±. 108 (2σ), 15.609±.036, and 38.677±.220, respectively. A remarkably consistent isotopic composition close to this average is observed in the kuroko ores from an extensive area of northeast Japan.
The Besshi-type ore leads except for those of the Mesozoic Taro deposit show an isotopic pattern similar to that of the ocean volcanic leads, strongly suggesting that the major evolutionary environment of these leads has been in the oceanic mantle. The Taro leads show isotopic features close to the major lead ore system and, in this respect, are unique among Japanese ore leads.
The leads of the pre-Neogene and Neogene mineralizations may be identified as crustal lead with respect to their main evolutionary environments. The 207Pb/ 204Pb and 208Pb/ 204Pb ratios of the pre-Neogene leads exhibit a trend in which the relatively high values of these ratios occur in the area where the related granitoids have relatively high initial 87Sr/86Srr ratios; some Neogene leads seem to follow the same trend. The presence of a relatively old and thick continental crust in the source region of these leads can be deduced.
The available isotopic data for the oceanic sediment lead occurring closer to the Japanese islands (samples near the Mariana volcanic arc and those from the Ryukyu Trench) apparently deviate from the major spectrum of the Pacific sediments data and plot closer to the data for the Japanese Neogene and pre-Neogene ore leads which may approximate the average of Japanese crustal leads. A relatively 208Pb-impoverished and 208Pb-enriched character indicates that the crustal lead around the Japanese islands has been strongly influenced by the lead evolved in the lower continental crust.
