Study of Hydrothermal Alterations on Mother Rocks to Aid Explorations for Ore Depostis

Abstract

In recent years, there has rapidly developed in Japan a strong tendency to follow the detailed yet composite American methods of geological study with the result that most mines have commenced to take up basic research and study along these lines.
The principal approaches in the study of ore genesis are (1) through "structural control" and (2) through "hydrothermal alterations on mother rocks".
In both these approaches, the ultimate objective lies in investigating the structure of the ore body, how it came to be formed in the country rock, and to determine its origin by correlation of mineralization eras through interpretations made of systematic mapping of detailed and minute observations of geological phenomena observed in rocks containing the ore body.
The only difference in the two approaches is that one is dependent upon dynamic causes, the other on petrology. In both cases, the common objective lies in the study of the character of the country rocks in relation to the passage of mineralizing solutions.
The study of rock conditions in relation to emplacements of various ore bodies, being extremely important in prospecting, now appears to be steadily heading in the direction of structural control and in general there still seems to be a lack of interest in conducting these studies from the aspect of alteration in country rock.
In most veins, unlike conceptions held in the past, it is now believed that at the time of the formtaion of the ore body the immediate alteration of the country rock is limited to the very small area along the fissures in which mineralizing solutions ascended.
The alteration of great masses of tuff and shale to the extent that it looks like liparite which is explained as having been caused by the action of large quantities of non-metalliferous hydrothermal solutions which had ascended before the introduction of minerelizing solution. Immediately after the alteration of the country rock, fissures were formed in the altered strata as they were then very susceptible to cracking. The ore bodies were then formed by the ascension of mineralizing solutions along these fissures.
Regional alteration of the country rock is a phenomenon that is the result of the action of ascending hydrothermal solutions which are the precursor of mineralizing solutions. However, as fissures are easily formed in altered rock, in a broad sense, ore deposits could be found in areas of altered rock:
Alterations in country rock assume different forms depending upon the nature of original rocks and the hydrothermal solutions.
Likely areas for prospecting can be discovered by detailed surveys of these alterations. Such studies are especially very beneficial in the explorations of mines at the initial stage.
Examples of notable discoveries in absolutely virgin lands from the above idea are illustrated by the discovery of the group of rich ore veins at Japan's representative gold mines, Takatama and Taio.