1983 Volume 33 Issue 179 Pages 149-164
Relative abundances of a number of chemical elements have carefully been evaluated for several individual kuroko deposits from Shakanai and some other mines in Akita area. On the basis of the data obtained, an attempt has been made to verify from a geochemical point of view the recently proposed hypothesis that the kuroko deposits were originated primarily from the mean settling-flux of biogenic entities in the ocean, i.e, from the PUMOS (KAJIWARA, 1982 b, c, 1983a, b, c; KAJIWARA and TONG, 1983).
As compared with the PUMOS, the kuroko deposits are in general characterized by the enrichment of a group of metallic elements (Zn, Cu, Pb, As, Sb, Bi, Cd, Ag, In and Au) and also by the depletion of another group of metallic elements (Fe, Mn, Ba, Ni, Co, Ga, Mo, W, Ge and Hg) and other common rock-forming elements (Si, Al, Ca, Mg, Na and K). It is worth noting that the relative mode of abundance of metallic elements enriched in the kuroko deposits is fairly analogous to what is predicted for the PUMOS (see Fig. 2 in the text). Strictly to say, however, the individual deposits are never uniform in chemical composition. Giving attention to some important ore elements (Fe, Ba, Zn, Cu, Pb, Ag and Au), the deposits are found to constitute a remarkable chemical differentiation series which is characterized by the uniquely-defined trend originating from the composition of PUMOS (see Figs. 3 to 7 in the text). This "kuroko differentiation series" appears to be most likely due to fractional removal of the elements from the PUMOS, i.e., Fe>Ba>Cu>Zn>Pb = Ag = Au in the order of magnitude.
The concept of "diagenetic chemical differentiation" is proposed to account for the observed geochemical characteristics of the kuroko deposits. Our conclusions may be summarized as follows: (1) The kuroko deposits were derived primarily from the PUMOS (or at least suchlike chemically equivalent to it).
(2) Things started with the proto-kuroko formation, which can be defined as a series of diagenetic decomposition processes of the PUMOS under anoxic or euxinic submarine environments (i.e., oxidation decomposition of organic matters by seawater sulfate=seawater sulfate reduction by the organic matters) (KAJIWARA, 1982c, 1983a).
(3) Significant fractionation of elements took place during the proto-kuroko formation. Some environmental controls such as "redox control" (see Fig. 8 in the text) and "organic acid control" (see Figs. 9 and 10 in the text) are considered to have been responsible for the removal of a number of elements which are almost missing in the present kuroko deposits. Metallic elements enriched in the present kuroko deposits should be regarded as the residual ones left behind at the given organogeochemical environments.
(4) The proto-kuroko deposits thus formed have further undergone significant chemical modifications through their subsequent pre-and post-burial diagenetic processes to result the "kuroko differentiation series". A series of phase-transformations of ore minerals (especially as to Fe and Cu minerals) are suggested to have taken place during a certain stage (probably very early stage) of the sebsequent diagenetic processes. It appears that some of important ore minerals such as pyrite and chalcopyrite are essentially of diagenetic origin. Fe and Cu are assumed to have been fixed initially as their individual mono-sulfide phases.
(5) Taking into consideration the possible diagenetic phase-transformations of ore minerals, it maybe concluded that the "kuroko differentiation series" is due to a sort of "mineral solubility control" (see Fig. 12 in the text), i.e., fractional decomposition or dissociation of the minerals involved, during the diagenetic history.
