The Hiraki mine, situated about 50 km northwest of Osaka, is one of the main "roseki" deposits in Japan (in Japan, "roseki" deposits include hydrothermal kaolinite, sericite and pyrophyllite). The ore deposit is a bedded, hydrothermal kaolinite-ore deposit, altered from dacitic tuff and tuff breccia of Cretaceous age. It is roughly concordant with the bedding plane of the host volcanic rocks. The mine produces 50, 000 tonnes per year of kaolinite concentrates. The dominant minerals are quartz and kaolinite with minor amounts of interstratified mineral and dickite.
The ore deposit was discovered in 1960 and an open-pit mining started in 1961. First, a systematic drilling survey was carried out from 1961 to 1966. The number of drilling reached 111 holes of 6, 400 m in total length. Based on the results, exploration tunnels with a 1, 700m total length were constructed from 1965 to 1972.
These exploration works have clarified the nature of hydrothermal alteration (kaolinite mineralization) and extension of the mineralized zones. On the basis of the results, it was possible to adopt large-scale underground mining methods, such as the sublevel-stoping method and mechanized cut and fill method. This is the first time that large-scale underground mining methods have been O.K. to clay deposits in Japan. The Hiraki ore is low in alkaline elements and iron and has a simple mineral assemblage (quartz+kaolinite), all of which make it suitable as raw materials for glassfibre. Because of this, the Hiraki mine is one of the leading suppliers of glass-fibre clay in Japan.
Exploration at the Hiraki mine is still continuing. To date, 149 holes have been drilled for a total of 8, 570 m of core, and its exploration tunnels total 3, 850 m in length.

View full abstract

A remarkable feature of the Toyoha deposit is occurrence of rare metals; Sn, In, W, Co, Ni, Bi, Se, Te, Ga, V etc. Two stages of the mineralization at Toyoha are recognized, and the later stage is classified into successive five substages A, B, C, D and E. Underground and microscopic observations, and EPMA analyses of the ore minerals have revealed that the rare metals precipitated from the ore solution of the substage B.
Deposition of the metals seems to have occurred due to a mixing of the hydrothermal fluid with ambient geothermal water of meteoric origin. For the rare-metal mineralization, initial hydrothermal-fluid temperature of 400°C, salinity of 5 to 7 equivalent weight percent NaCl, fo2 below the upper limit of pyrrhotite, andf _s2 at around the pyrrhotite-pyrite boundary are consistent with fluid inclusion data and mineral-assemblages. The evolution trends of the ore solutions and the occurrence of Sn and In in the later veins suggest that the later-stage mineralization was initiated by an ore solution derived from an ilmenite-series granitoid magma, while the earlier from a magnetiteseries magma. Chronological data, the zonal distribution of minerals, metal grades, and fluid inclusions, and the flow vector of the currently active geothermal solution imply that the source of the ore solution is a series of latent intrusions below the Muineyama Andesite.

View full abstract

Silicate minerals in the seven Paleogene coal seams of the Ashibetsu colliery in the Ishikari coalfield, Hokkaido, were investigated by X-ray diffraction method for low temperature ash of coal, shale and tuff.
Quartz was detected from 86.5% of the coal samples with under 40% ash and from every shaly sample or tuff. Quartz is mostly detrital. However, the fact that it exists even in the extremely low ash coal like under 1% ash suggests that there may be vegetal quartz in origin.
Plagioclase is very abundant in Torakawa seam which has many intercalated tuffs. This mineral in coal is of tuff origin.
Kaolinite exists in 96% of the coal samples from extremely low ash to 40% ash coal, whereas clay minerals other than kaolinite are present in only 27% of the same samples. Although kaolinite is one of the major mineral components of shale and tuff, which have also the other major clay minerals, there are many coal samples which contain only kaolinite as clay mineral and ash content of most of them is under 10%. According to the result of measuring Hinckley's crystallinity index of kaolinite, kaolinites with high crystallinity are concentrated in these low ash coals and a few coal samples with over 10% ash also contain kaolinites with high crystallinity but there are no other clay minerals in them. Most kaolinites in the coals without the other clay minerals are thought to be authigenic.
Illite is present in the high ash coal and detrital from muddy sediments. The greater abundance of kaolinite and the less abundance of illite suggest the coal formations generally in the fresh water environment, though partly in the brackish water environment. Both the balance between the rate of accumulation of peat and the rate of subsidence of the coal basin, and the low flow rate of water must have also made illite less abundant within coals.
Illite/smectite mixed layer mineral is found in both tuff and shale. The mixed layer mineral found in the coals without illite in Torakawa seam is just like those of tuff. Another mixed layer is accompanied with illites in coal and shaly samples. The mixed layer mineral in tuffs, coals without illite and several shales was apparently identified by applying Watanabe's identification graph. The results show that their distribution on the graph forms a pattern of illitization by diagenesis and that the mixed layers of shales tend to take the area of higher illite ratio than those of tuffs and coals without illite. The latter fact seems to show that there appeared the variety of the effect on the diagenesis depending on the difference of the original sediment.
Both smectite and K-montmorillonite often occurred in the coals of Torakawa seam. As K-montmorillonite is also present in tuff, its source rock is simply thought to be tuff. However, smectite was not observed in tuff. Either Kmontmorillonite or illite/smectite mixed layer is thought to have been transformed into smectite in coal.
Trace amount of chlorite exists near the floors of two coal seams and in the two intercalated shale samples of one coal seam. Chlorite is unstable in the acidic environment like fresh water peat. Chlorire is rare in the Ashibetsu coal seams for that reason.

View full abstract

Wurtzite and sphalerite were synthesized hydrothermally at 350°, 300° and 250°C using a temperature-gradient transporting method with sphalerite as a starting mineral. Fibrous wurtzite together with sphalerite was formed only in experiments at 300° and 250°C when a Zn-rich solution (1m ZnCL₂+1m NH₄CI) was used as the transporting media; only sphalerite was produced in experiments using a Zn-free solvent (1m NH₄Cl). Formation of wurtzite (vs. sphalerite) was not affected by the composition of the starting sphalerite nor by the addition of pyrrhotite and/or alabandite to the starting materials. These results agree with a thermodynamic prediction that the solubility of wurtzite is higher than that of sphalerite.
A comprehensive literature survey on the occurrence of wurtzite in hydrothermal ore deposits shows that wurtzite is not uncommon in shallow subaerial deposits and in submarine hydrothermal deposits of relatively young ages (mostly Tertiary or younger). For these'deposits, drastic physicochemical changes in the ore-forming solutions, such as rapid cooling by mixing with local meteoric or sea water, have been postulated during ore mineralizations. The wurtzite crystals often exhibit fibrous or radial textures, which would be produced by the above changes. Our ex-perimental data and the occurrence of wurtzite in ore deposits suggest that hydrothermal wurtzite is a metastable mineral that is formed by rapid crystal growth from highly supersaturated solutions.

View full abstract

PGM have been detected from the bottom of a twometer-thick dunite layer intercalated by a large harzburgite mass exposed along the Mukawa River, southern Kamuikotan belt, Hokkaido, Japan. The PGM occur as a complex aggregate of alloys (Unit A) and complex mixture of a pentlandite-like sulfide and Ru-Rh alloys (Unit B) included in fibrous serpentine. The unit A can be divided into the following three parts:
A1. Pt-Pd bearing native copper with domain texture
A2. an alloy of Rh and base-metals
A3. a Pt-Pd-Cu alloy including tiny grains of Rh-Ni alloy
Many small grains of Ru-Co-pentlandite are also observed in the same specimen. The occurrences, textures and chemical compositions of the PGE-bearing aggregate and Ru-pentlandite indicate that these minerals are products of the serpentinization.

View full abstract