The writers studied fluid inclusions in minerals from the Ohtani and Kaneuchi mines, which belong to the hypothermal or pegmatitic tungsten vein-type deposit, by means of the heating stage- and cooling stage-microscope and decrepitation methods.
(1) By the heating-stage microscope, the filling temperatures of inclusions are measured as follows (Figs. 3, 4);
Ohtani mine: quartz 375°-225°C, scheelite 337°-262°C, cassiterite 345°-297°C.The maximum filling temperature of quartz is 375°C, which is exceptional as shown in Fig. 3. Most of the filling temperature of quartz are below 300°C.
Kaneuchi mine : quartz 308°-231°C, scheelite 318°-276°C, wolframite 337°-286°C.The filling temperatures of the inclusions in scheelite, cassiterite and wolframite are somewhat higher than those in quartz. This would be due to the fact that the quartz crystals studied were deposited at the later stage of mineralization. The earlier stage quartz is white and milky in colour, so it is difficult to observe the inclusions of the earlier stage quartz under the microscope.
(2) By the cooling-stage microscope, the NaCl equivalent concentrations in inclusions are determined as follows (Figs. 5, 6);
Ohtani mine : quartz 7.4-4.0 wt. %, scheelite 8.2-6.1 wt. %, cassiterite 8.7-6.1 wt. %.
Kaneuchi mine : quartz 8.2-3.7wt. %, scheelite 8.6-6.4wt. %, wolframite 8.4-8.1 wt. %. The NaCl equivalent concentrations in scheelite, cassiterite and wolframite are higher than those of quartz. This would be due to the same reason as of the filling temperatures stated above.
(3) The filling temperature increases with the increase of the NaCl equivalent concentration (Figs. 7, 8). This would be due to the dilution of the ascending ore-forming fluid by the underground water.
(4) In the Ohtani mine, the fluid inclusions in quartz from greisen and granodiorite adjacent to the greisen envelope are nearly the same as those in the vein materials. The ore-forming fluid would be concentrated in the granitic magma existed in the deeper part underneath the present country rock, i.e., granodiorite.
(5) The decrepitation temperatures of quartz samples in both deposits are as follows :
Ohtani mine 382°-280°C, Kaneuchi mine 334°-255°C.
It is inferred that the formation temperatures of the Ohtani mine are higher than those of the Kaneuchi mine.
(6) It is generally said that the decrepitation temperatures are higher than the filling temperatures because of the overshoot effect. The samples of quartz which were used in the decrepitation method of the present study belong to the early stage of mineralization than those in the heating-stage microscope method. Therefore, it is probable that the temperature differences between the heating-stage microscope method and the decrepitation method are due to both overshoot effect and difference of the stage of quartz deposition. But, it is also probable that there exist other factors which make the difference between the filling temperatures and decrepitation temperatures.