In the Shakanai kuroko deposits are well developed so-called "gypsum zones" characterized by an abundance of gypsum and/or anhydrite. The purpose of this paper is to disclose their geological modes of occurrence and petrographical characteristics and thereby to provide some constraints in studies of their origin. Some of our conclusions are summarized as follows:
(1) The "gypsum zones" are always developed in close contact with or in the vicinity of the kuroko deposits, occurring generally in stratiform as a whole. Interpreting in a broad sense, it appears that they were primarily a sort of sedimentological stratum.
(2) Mineralogically, both anhydrite and gypsum of different morphological types such as alabaster, satinspar and celestite are recognized. Anhydrite and alabaster are the principal constituents and they occur usually as oval lumps of various size.
(3) The "gypsum zones" in general underlie the kuroko deposits to form a set of zonal arrangement. In some places, however, at least two sets of such zonal arrangement are known to be repeated spatially. This may be due either to repetitional mineralizations or to secondary deformation and dislocation of the pre-existing deposits formed by a single mineralization.
(4) Relative abundance of anhydrite to gypsum depends evidently on the burial depth at present, increasing signifi-cantly towards deeper levels. It is strongly suggested that gypsum was formed by hydration of anhydrite very recently, i.e., during Quaternary period.
(5) Between the "gypsum zones" and the kuroko deposits are sometimes interstratified economically workable "gypseous sulfide zones", which are similar in sulfide mineralogy to the kuroko deposits overlain. It appears that the "gypseous sulfide zones" were formed either by simultaneous deposition of sulfide minerals and a Ca-sulfate minerals or by later physical or chemical interactions between the pre-existing "gypsum zones" and kuroko deposits.
