Colombia produces numerous mineral products such as oil, coal, gold, silver, platinum, nickel, emerald etc. and is thought to have still greater potential resource supplies. On the other hand, Japan depends on other countries for most of its supply of mineral resources and it is very important to insure the supply of these resources. As for copper, our country once was an exporting country of this material, but in 1990, 99.6% of Japanese demand for copper was met through import. At that time, in 1990, the El Roble mine was developed by a joint venture as the first mechanized underground copper mine in Colombia.
The El Roble mine is located at El Carmen de Atrato which is about 80km southwest of Medellín, Colombia. The ore deposit is of submarine volcanogenic massive sulfide type and a high grade copper-gold deposit. The reserves of minable ore is about one million metric tons with copper content of 3.8%, gold content of 2.7g/t and silver content of 8.4g/t.
A lot of laboratory works and detailed field surveys were conducted during the exploration program and provided effective guides for the exploitation. In particular the lithogeochemistry of minor elements and stable isotopes of the host rocks proved of great use for determination of targets in this area.
The geology and ore deposit of the mine are described, and an example of the explorations by a Japanese company in overseas countries is reported in this paper.

View full abstract

The Sumikawa geothermal field is located on the north-east slope of the Yakeyama volcano, Akita prefecture. The Mitsubishi Materials Corporation has developed geothermal energy in this field after successful operation of the Ohnuma geothermal power plant. Subsurface geologic formations consist of dacitic pyroclastics, marine sediments, andesite lava and granodiorite intruded to these Tertiary formations. These formations are covered with the Quaternary andesite lava and lacustrine sediments. In this field, geothemal fluids are prominently found in subsurface fractures. Subsurface temperture recorded at the depth of 2, 000 m exceeds 300°C. The geothermal heat sources are probably lying beneath the YakeyamaHachimantai volcanic chain located to the south of the developing field. The geothermal resources are of the liquid dominated type accompanied by vapor dominated region at the top of the reservoir. According to the geothermal systems of the field, the production wells have carefully designed by. casing profile, drilling methods together with separating production zones have customized to prevent blowout of vapor and to produce large flow rate of vapor. Also well spacing has designed to recover efficiently heat energy during recycling of reinjection water. Numerical reservoir simulations suggest that the potential reservoir is large enough to produce vapor for 50M We an extended period of time. The Sumikawa geothermal power plant will be fully operated in early 1995. Authors have all actively engaged in the management of the Sumikawa geothermal energy developing field.

View full abstract

Strontium isotope ratios in barites were analyzed for a better understanding of the geochemical feature of the hydrothermal solution responsible for the formation of the Minamishiraoi massive barite deposit. The ⁸⁷Sr/⁸⁶Srr ratios(0.70556-0.70647) in barites were found to be between 0.70450 of andesite, the footwall rock of the deposit and 0.7088 of seawater of the Miocene age. The isotope ratios obtained showed a lower range than that of the Hokuroku Kuroko deposits. Sr isotope ratios ranged between 0.70561 and 0.70576 for barites from both the sulfide-rich ores of a stratiform orebody and the stockwork ores of a silicified zone in the Minamishiraoi deposit. On the other hand, barites from baritepredominated ores, the principal constituent of stratiform orebody had slightly higher ratios(0.70556-0.70647) than the former ones. The compositional differences of the Sr isotope among the ore types can be ascribed to temporal and spatial changes of the contribution of seawater to the barite precipitation taking place on and beneath the seafloor. It is considered that strontium isotope and oxygen isotope characteristics of the Minamishiraoi barite resulted from the isotopic compositional deviation of ore forming fluids and the temperature difference of ore formation from the typical Kuroko deposits.

View full abstract

lineament analysis and computer simulation analysis were carried out in order to understand the fracture systems around the Hishikari mine and the mechanism of the gold-quartz vein systems.
Pull-apart structure, fault gauge and drag fold, which are ovserbed in the quartz veins, indicate uplift of the basement(NAITO, 1993). Lineaments are divided into four directions, NW, NE, EW and NS directions, and four ring shaped lineaments are extracted by aerial photographic interpretation. The Hishikari mine is located in the central part of ring shaped lineaments. The distribution of ring shaped lineaments coincides with the basement structure estimated from gravity anomaly. It was understood that the ring shaped lineaments indicate the tectonic uplift of the Cretaceous basement, from the results of computer simulation analysis (Vertical Basement Displacement Method).
It is resulted that the fracture analysis provides the practical information about the buried vein systems in prospecting for epithermal gold deposits.

View full abstract

The Yanomaki halloysite mine is a massive and irregularly shaped mineral deposit, that is developed in small leucocratic granitic bodies. Because the quality of the mineral deposit deteriorates with increasing depth, the Yanomaki halloysite mine is produced due to weathering of the upper parts of the leucocratic granitic bodies.
The leucocratic granitic bodies (we temporarily call them as "quartz-plagioclase rocks") lie within a Paleogene biotite granite pluton, and are composed mainly of plagioclase (oligoclase to andesine), quartz and minor amounts of titanite. Ca contents of the plagioclase are distinctly higher than those in the host granite. Mafic minerals are rare. The quartz-plagioclase rocks are associated with greisens and pegmatites on a small scale, and field relations between the quartz-plagioclase rocks and the host granite are gradational. This evidence suggests that the quartz-plagioclase rocks are "metasomatic anorthosite" formed by alteration by a Ca-rich hydrothermal fluid. A large amount of K₂O was leached out of the host granite by the hydrothermal fluid during feldspathization. The K₂O probably remobilized into the sericite mineralization found in adjacent areas.
The quartz-plagioclase rocks also can be mined for feldspathic resources. The total quantity of the rocks is estimated to be more than 6x10⁵ tons, and approximately 2x10⁵ tons of the rocks can be mined at the present open cut.

View full abstract

Vendian-Cambrian Era of Phosphogenesis had taken place 650-530mys. with peaking in the Lower Cambrian (so-called "Pre-trilobite Cambrian"). That period of time was characterized by the broad development of firm skeleton beings. The phosphate; bearing sediments are well known through all the continents (except the Antarctid) in the sequences of that age. The phosphate deposits are usually in association with the carbonaceous formations. The sedimentary lithological types of phosphorites are: aphanic, microgranular, nodular, and organomorphic. The most typical are aphanic and microgranular. The aphanic and microgranular phosphorites are considered to be the products of chemical deposition. The phosphate-bearing mineral in phosphorites is mostly fluorine-carbonate-apatite.

View full abstract