Modes of occurrence of gangue minerals in the Shakanai kuroko deposits with special reference to silica and silicate minerals

Abstract

The purpose of this paper is to describe geological modes of occurrence of silica and silicate minerals in the Shakanai kuroko deposits and thereby to provide some additional constraints in studies of the kuroko mineralization. Some of our observations and conclusions are as follows:
(1) Silica and silicate minerals in the deposits are mostly composed of quartz and a variety of clay minerals. Their total contents in the economically valuable portions of the deposits are estimated to be 15 to 25 percents in weight.
(2) They are in part derived evidently from some altered rocks in the deposits originating from mudstone, dacitic pyroclastics, dacitic hyaloclastites and other exotic breccias. Contribution of these altered rocks is, however, comparatively insignificant, being estimated to be less than 10 percents in weight.
(3) The rest of quartz are derived from "cherty rocks" as well as from "siliceous ores". On the other hand, a majority of clay minerals are derived from certain peculiar "clayey rocks", which are interspersed among ore-lumps in the deposits. The "clayey rocks" are rather homogeneous in structure and show no distinct textures indicative of the pre-existence of mudstone, lavas and pyroclastic rocks. At least some of the silica and silicate minerals constituting the "cherty rocks", "siliceous ores" and "clayey rocks" are suggested to be closely related in origin to the kuroko mineralization.
(4) All the rocks in the foregoing are emplaced definitely within the kuroko deposits. This fact strongly suggests that the different kinds of rocks were all supplied and deposited within the period of kuroko mineralization. In other words, the duration of kuroko mineralization is suggested to have been long enough for the emplacement of all the rocks in the deposits.