1983 Volume 33 Issue 181 Pages 327-336
The recently proposed "PUMOS" hypothesis (KAJIWARA, 1982, 1983a; KAJIWARA and HIRAYAMA, 1983) advocates that the kuroko deposits were originated primarily by the mean settling-flux of biogenic entities in the ocean, i.e., the PUMOS (primitive undifferentiated metalliferous organic sediment). The "PUMOS" hypothesis is highly successful in explaining the long-unanswered problem of the restricted time-space distribution of the kuroko deposits. The results of the present study would be also important as exploration guides for hidden kuroko deposits. Conclusions reached by this study are as follows.
(1) The time span of the formation of the kuroko deposits is correlative with that of the existence of an oxic-anoxic stratified seawater system in the proto-Japan Sea that is vitally important for accumulation of the PUMOS. The upper and lower limit of this time interval are delimited by PFSS (planktonic foraminiferal sharp surface) and FSL (foraminiferal sharp line), respectively, which are two distinct foraminiferal biostratigraphic transition lines in the late Nishikurosawa stage defined by MAIYA and INOUE (1981) (KAJIWARA, 1983b).
(2) The upper limit may also be recognizable by the regional appearance of "oxidizing red beds", which indicates the disappearance of the stratified seawater system.
(3) Recognition of the lower limit is usually difficult in volcanic areas, and age determinations of the volcanics would be the only reliable way. The fact that FSL coincides with the boundary of BLOW'S N9 and N10 suggests that 14.5±0.5 Ma should be the lower limit of the metallogenic epoch of kuroko deposits.
(4) The fact that the kuroko-bearing local basins are restricted within the general submarine topographic highs, with none being located in the general topographic lows, in the proto-Japan Sea is explained by that dense accumulation of the PUMOS can take place only in topographic highs where the supply of detrital material is minimal.
(5) The "PUMOS" hypotheis suggests that local basins that are surrounded by stratiform manganese deposits would be the most promising exploration targets for kuroko deposits.
(6) From the concept of the diagenetic chemical differentialion of PUMOS (KAJIWARA and HIRAYAMA, 1983), it is postulated that the existence of some effective "sealing rocks" which covered the proto-kuroko deposits in a short time after their formation would be necessary for preservation of the kuroko deposits.
(7) In contrast to the traditional hypotheses for the genesis of kuroko deposits, the "PUMOS" hypothesis persists that magmatic and hydrothermal activities would tend to act as destroying processes for kuroko deposits.
