南西太平洋地域の酸化ニッケル鉱床

抄録

The exploitation of the so-called garnieritic nickel ores was first commenced in New Caledonia in 1875. For nearly one hundred years since then, the ores of this type have been one of the most important nickel resources of the world.
In the meantime, the ores of the same type, but characterized by higher iron and lower nickel contents, became objectives of intense exploration since some twenty years in the Southwestern Pacific. More than ten are deposits of this latter type are now waiting for exploitation. This type of ores is known as lateritic nickel ores.
These nickel ores-both garnieritic and lateritic-are mainly located in the areas fulfilling the following conditions :
1) Areas where the ultrabasic rocks are emplaced in the strongly incurved parts of the Circum-Pacific Orogenic Belts,
2) Areas where the monsoon climate is prevailing and the dry and rainy seasons are regularly alternating.
The Southwestern Pacific is one of the areas of the world which satisfy eventually the above mentioned conditions, besides the Caribbean Sea area in Middle America. There, the are deposits are formed as the residual of weathering of the ultramafic rocks.
Generalized soil profile is given below in descending order : -
1 st layer iron crust
2 nd layer red laterite (upper main zone)
3 rd layer yellow laterite (transitional zone)
4 th layer decomposed rock (lower main zone)
5 th layer fresh rock (basement).
Since long time, the are bodies have been known to be emplaced in topographically particular sites ; in fact, they are located in the weakly drained sites such as wide plateaus, top of mountain ridge of which longitudinal inclination is low, and gently sloping mountain flanks.
Actually the theory on the genesis of this type of nickel ores is still not fully developed. Although extensive field and laboratory studies are needed, the writer presents some basic facts important for genetical explanation in this paper. The relation between Fe and Mg contents is graphically presented (Fig. 5 and 6). The frequency distribution of Fe to Ni ratios in each are deposit, suggesting the process of Ni concentration, is also graphically given in Fig. 7-14. The Fe-Ni ratio is not evenly distributed within soil profile, but is concentrated into two peaks, separated by a valley which corresponds to the 25% Fe domain. The upper peaks in Fig. 7-14 represent the red laterite layer which is composed mostly of iron sesquihydroxide. The lower peaks represent in turn the layer of decomposed rock (called saprolite layer), composed of various kinds of clay minerals (montmorillonite, garnierite, serpentine, chlorite etc.).
Finally actual relation among Fe-Ni ratio, layering, relative depth and pH of circulating water is demonstrated in Fig. 15, in order to discuss on the possible genetic interpretation of are emplacement.