A Contribution to the Knowledge of the Cassiterite Veins of Pneumato-Hydatogenetic or Hydrothermal Origin. A Study of the Copper-Tin Veins of the Akenobe District in the Province of Tajima, Japan
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概要
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At the present time, it is generally accepted that tin veins can be deposited from pneumatolytic as well as hydrothermal solutions, although those of pneumatolytic origin, containing characteristic pneumatolytic minerals such as fluorite, topaz, tourmaline, apatite and other fluorine and boron-bearing minerals, and accompanied by typical pneumatolytic alterations of the country rock, are of commoner occurrence. That the veins under consideration were not formed under pure pneumatolytic conditions is indicated by the absence of characteristic pneumatolytic alterations of the wall-rocks. The slaty rocks in which the veins are enclosed are often intensely chloritized, silicified, epidotized and sideritized, while no characteristic pneumatolytic minerals have been observed in them even under the microscope. These alterations are regarded to be characteristic of hydrothermal processes. Moreover, the veins themselves contain usually more or less siderite and chlorite, as can easily be revealed under the microscope. These minerals occur, through rather sparingly, in ordinary cassiterite veins, and are most abundantly associated with hydrothermal tin veins, siderite, particularly, being of deposition, in most tin veins, in the last stage of pneumatolysis, or in pneumato-hydatogenetic or hydrothermal stages. The occurrence of chalcedony, evidently transformed from an opaline silica, as an important gangue-mineral is most instructive. This mineral is an extremely rare associate of ten-stone. Only a few examples of tin veins containing chalcedony have hitherto been reported, and they all belong to the category formed under hydrothermal or allied conditions. As already fully discussed under the heading of "the deposition of the main cassiterite ore," the presence of chalcedony in the veins under consideration indicates that they were formed under hydrothermal conditions at a temperature below 360℃. On the other hand, it must be noted that some minerals regarded as common associates of pneumatolytic deposits, such as fluorspar, wolframite and bismuth ores with sporadic microscopic topaz, are present in the veins. There minerals are, of course, of very common occurrence in the deposits formed at high temperatures, but may be deposited either from gaseous solutions or from superheated aqueous solutions containing soluble or gaseous compounds of fluorine, tungsten and bismuth. Typical hydrothermal veins containing scheelite, wolframite, fluorspar, bismuth ores, etc. are numerous even in this country. The veins of this district are unique in the fact that the orebringer for them is a dioritic magma. Considering, however, that acid rocks can be derived from a basic magma by processes of differentiation chiefly due to fractional crystallization and setting or by the expulsion of the residual fluid magma, and that an acid differentiation-product with a composition or quartz-monzonite-pegmatite or aplite, akenobeite, is actually found as a small boss in this district, the genetic connection of the veins with the dioritic rocks is highly probable. Summarizing all that has been stated, the copper tin veins of the Akenobe district were deposited from hydrothermal solutions, still containing fair quantities of mineralizers, at gradually decreasing temperatures, chiefly considerably below 360℃. The solutions had naturally a temperature far above the critical point of water (364℃) and were gaseous in character, after emanation from the consolidating diorite magma. As they ascended through the surrounding slate complex, the rate of the fall of temperature was very rapid, and they soon changed to superheated hydrothermal solutions. Lastly, the writer frankly states that he is quite in the dark as to whether the stannic oxide first separated from the solutions in the colloid state and subsequently became crystalline, or whether it crystallized directly as cassiterite by chemical reactions between stannic fluoride and other compounds.
- Inperial University of Tokyo,東京帝國大學理學部の論文
- 1920-05-10
Inperial University of Tokyo,東京帝國大學理學部 | 論文
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