Reaction between rhyolitic welded tuff and geothermal solution cooled or heated from the temperature in equilibrium with alteration minerals is numerically examined by the geochemical simulation code. Six varieties of the initial solutions are calculated assuming that total chloride concentrations are 0.01, 0.05 and 0.1 mole/kg and that they are in equilibrium with quartz, muscovite, calcite, albite, K-feldspar, clinochlore and laumontite at 225°C or wairakite instead of laumontite at 275°C in the H ⁺ -Na ⁺-K ⁺-CaK ⁺²-Mg ⁺²-Al ⁺³-H ₄SiO₄-H₂O-Cl ^--CO₃ ^-2 system. The initial solutons are cooled or heated by 5°C to remove saturated minerals, and subsequently reacted with rlyolitic welded tuff. Quartz and muscovite precipitated in the processes of 5°C cooling and the subsequent rock-solution reaction at low rock/water ratio, which results in enrichment of K₂O and SiO₂ in the reaction product. Calcite always precipitates in the process of 5°C heating, and laumontite (or wairakite), albite and clinochlore precipitated in the process of the subsequent rock-solution reaction at low rock/water ratio, which results in enrichment of Na₂O, CaO and MgO in the reaction product. These cooling and heating experiments represent respectively thermal condition of discharge area and recharge area in the geothermal system, and this study shows possibility of the hydrological interpretation of geothermal system from the rock alteration.

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Silician magnetite, recently found in chalcopyrite-pyrrhotite-cubanite ore from the Tengumori ore deposit of the Kamaishi mine, northeastern Japan (SHIGA, 1988), was further studied. Microprobe analyses of the mineral give 4.72 to 6.19 weight percent SiO₂ and a probable ideal formula Fe²⁺(Fe3+/1.6 Si_0.2Fe2+/0.2)O₄. The mineral is frequently in direct contact with normal magnetite having SiO₂ content less than 0.1 weight percent, suggesting a com-positional gap between them. The mineral was formed under reducing condition at a temperature approximate to 300°C, simultaneously with the associated normal magnetite and sulfides.

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High anomalies of F, Li and Rb in bedrocks have been reported around the Takatori Sn-W deposit (M.I.T.I., 1987). Among them, F is the most effective pathfinder, because it shows the highest concentrations and the highest geochemical contrast.
The purpose of the present study is to test the effectiveness of stream water as a geochemical exploration medium, with special attention to fluoride. Samples of stream water were collected from 116 sites and were subsequently analyzed. In addition, leaching experiments on bedrocks taken from this district were carried out.
The result are summarized as follows.
(1) From the distribution pattern of F ^- in stream waters, it is clear that the high anomalies of F ^- form the halo which surrounds the Takatori Sn-W deposit.
(2) The halo formed by the high anomalies of F ^- in stream water, ranging over 2×4 km, coincides with the known halo formed by the high anomalies of F in the bed rock, ranging over 1.5×3.5 km around the Takatori Sn-W deposit.
(3) Therefore, it is considered that the anomalies of F ^- in stream water are the secondary geochemical dispersion due to fluoride which dissolved from the bedrocks with high F content around the Takatori Sn-W deposit.
(4) As a result of the 6 days' leaching experiments, the high concentrations of dissolved fluoride, which is close to the fluorite saturation value (8.7mg/l·F ^-), was obtained from fluorite-bearing bedrocks. On the other hand, relatively much lower concentrations of F ^- were obtained from muscovite-bearing but fluorite-lacking bedrocks.
(5) Major F-bearing minerals around the Takatori Sn-W deposit are fluorite and muscovite. Fluorite exists in the bedrocks of which F content exceeds about 1000 ppm, nevertheless muscovite occurs also in the bedrocks with F content less than 1000 ppm. On the other hand, in the area where F content of bedrocks is higher than 500-1000 ppm, F ^- concentration in stream waters is much higher than in other area. These facts are in good agreement with the result of the leaching experiments.
(6) In case of programming the geochmical exploation of mineral deposit types associating the fluorite as alteration minerals, it is recommended that F ^- in stream waters, as well as F in bedrocks, should be used as a usefull pathfinder.

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Deep-sea Manganese nodules from the northern Central Pacific are characterized by todorokite veinlets. The veinlets develop as a network in concentric layers around a nodule core. The layers display varying degrees of reflectivities which are classified into four optically different zones (internal reflection, and low, moderate, and high reflectivity zones based on an arbitrary scale) under the ore microscope. The internal reflection zone is significantly enriched in Si and Fe, and is mainly composed of X-ray amorphous opal-A, probably of biogenic origin. The difference in reflectivity is attributed to varying amounts of todorokite crystallites and associated chemical elements across the layer. These differneces develop during nodule diagenesis.

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The uraniferous polymetallic veins in Gebel El Missikat area, Central Eastern Desert, Egypt, are mainly composed of chalcedony with subsidiary amount of fluorite and ore minerals containing uranium, molybdenium, lead, arsenic, gold, and silver. They are genetically related to late orogenic pink or red granite of Upper Proterozoic age.
The veins occur in steeply dipping tectonic fractures trending east or northeast. The polymetallic minerals formed for at least two closely related episodes.
These hydrothermal veins formed under low to moderate temperature condition and are enclosed in green alteration zones.
The contents of molybdenium, lead, and fluorine in veins increase with an increase in uranium content. The relationships of As, Sr, Rb and Zr with silica are not distinct.

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