Mg-Enrichment around Some Ore-Deposits in Japan : Particularly with Reference to Hydrothermal Gypsum and Silica Deposits

Abstract

It is suggested by the present writer that Mg-enrichment is very common in the altered halos of some hydrothermal ore-deposits in Japan -with particular reference to gypsum deposits in Tertiary formations as well as to silica deposits in the bedded chert of the Chichibu Palaeozoic formation., Stoichiometric studies reveal that some of the Mg-enrichment is due to the supply of magnesium from ascending solutions, while others merely to local concentration of that component in the rocks., Magnesian-chlorite (including leuchtenbergite) is characteristic to the products of magnesium enrichment, and associated with extremely variable amount of either sericite or montmorillonite, giving rise to clayey zones in the altered aureoles., From these mineral associations, the oreforming temperatures can be estimated as having not exceeded about 450°C., The elucidation of variation in crystallinity of those minerals, however, seems to be neccesary for a more detailed temperature estimation., Attention is given to the petrochemical characteristics of the green tuff of Miocene age of regional development in Japan., These rocks are enriched in magnesium relative to other components particularly to iron, and are impoverished in SiO2, probably due to submarine alteration.,