中国地方中央地区における後期中生代の金属鉱化作用

In the central part of the Chūgoku district, various types of the ore deposits including the pegmatite and pyrometasomatic ones together with some kinds of the lodes are found produced inferably in relation to activity of the late Mesozoic granites. The paragenetic relations of the ore and gangue minerals comprised in these deposits, identification of the minor-grained ore minerals and their microscopic textures displaying exsolution or some others have been disputed with respect to the Cretaceous granitic rocks on one hand, and the relationships between the deposits concerned and their wall rocks composing of the pre-intrusives have also been dealt with to a certain extent on the other.
As the pre-Cretaceous igneous activities, those in the Ryôké plutonic metamorphism, those of the intrusives appeared either prior or subsequent to the former, of so-called Yakuno intrusives, of the ultrabasic rocks and of others are considered worthy mentioning. Nevertheless, it seems common that these intrusives are not used to accompanying the noticeable deposits of metallic ores exclusive of scarce occurrence of the chromite deposits in the ultrabasic intrusives, of the pegmatite deposits containing the rare-earth minerals in the Ryôké intrusives and of those in the wall rocks consisting of the Yakuno intrusives.
Igneous activities in Cretaceous are represented by those of volcanics such as rhyolite and andesite etc. in the Arita to Miyako epoch of the lower Cretaceous and of granitic intrusives occurring as stocks and batholiths.
Rhyolitic rocks seem to have not related to genesis of the metallic ores other than the Rôseki deposits through their own activities but to have played a role in structural control regarding certain ore deposits.
The Hiroshima granitic complex are observed containing the pegmatite deposits and the quartz-wolframite lodes of pneumatolytic to hydrothermal origin within their own masses together with the pyrometasomatic deposits and some other kinds of the lodes included mainly in the Paleozoic formations and partly in the rhyolites in their surroundings. Mineralizations recognized in these deposits are characterized by contents of Bi, Sn, W and F, high-temperature exsolution textures of the ore minerals, and more content of Fe in the earlier stage and of Pb in the later stage, lacking in low-temperature minerals bearing Sb and so forth.
In the granitic rocks of the central plutonic group are found the parts comprising the ore deposits resembling to those contained in the surroundings of the Hiroshima granite as well as the parts composing the wall rocks for some kinds of the deposits. Within the granitic masses of the San'in type, there are many deposits of molybdenite, muscovite, and Cu-Pb-Zn-Fe sulfides and several sorts of the lodes, all of which have been produced at lower temperature and in shallower depth than in the case of those appeared in the Hiroshima granite. Along the southern margin of the San'in granitic complexes are there some mesothermal lodes as many as wolframite-and molybdenite-bearing ones classified into pneumatolytic to hydrothermal deposits with characteristics resembling to those produced in the Hiroshima granite.
Between the areas composing of two types of the granitic rocks mentioned above, certain formations or rock masses accompanying various kinds of magnetite deposits are found exposed.
Into the bargain, the paragensis of Bi- and Sn-bearing minerals with other sulfide ores, that of cubanite with sphalerite and exsolution textures of Cu-Fe sulfides in the latter are to be considered in more details.