An attempt to reconstruct the diagenetic evolution history of the Shakanai kuroko deposits

Studies on diagenesis of kuroko deposits(Discussion)

Abstract

The authors have demonstrated, in the preceding paired paper (SUGAWARA et al., 1982), our careful observations on the mode of occurrence of the Shakanai kuroko deposits with special reference to some sedimentological and diagenetic phenomena fossilized in them. In this paper, an attempt is made on the basis of those observations to reconstruct the possible diagenetic evolution history of the deposits and thereby to make an approach to their origin.
Our conclusions derived from this study are enumerated in the following.
(1) The ore deposits were, as a whole, primarily of sedimentary origin. In other words, the initial proto-kuroko deposits are considered to have constituted a certain exogenic "bed" or "stratum" in sedimentological sense.
(2) The proto-kuroko deposits were similar in physical dynamic nature to a "viscous liquid" or something like that. In other words, they were not of hard and dense masses as what they are at present.
(3) Since the initial deposition up to present, they have been buried in sedimentary piles for probably about 13 to 15 million years, during which the maximum burial depth has reached to about 1, 000 to 1, 500 meters and the maximum burial temperature attained has exceeded at least 150°C.
(4) Important diagenetic events are as follows.
Water-escape and compaction: Extensive development of a variety of sedimentological "water-escape" structures in the ores strongly suggest that the proto-kuroko ores have also undergone similar diagenetic compaction process-es to what are well known in other normal sedimentary rocks. Significant secondary migration or mobilization of the ore-forming materials must have taken place during these processes.
Ore-textural evolution and modification: The most primitive texture so far observed reveals that the ore minerals were initially of very minute particles of about 1 micron or less in size. The minute particles were coagulated together in the earliest stage of diagenesis to form a series of aggregates with various size and shape, which were then progressively modified, during certain subsequent diagenetic stages, by some possible repeated processes of dissolution, reprecipitation, replacement and recrystallization to complete a variety of other complicated textures.
Formation of ore-zoning: The formation of ore zoning in the deposits is also considered to have principally completed in the early stage of the above ore-textural evolution and modification processes.
Formation of "ore-lumps": The "ore-lumps" constituting the deposits are considered to have been formed on and after the completion of ore-zoning mentioned above, probably due to some consolidation process of the deposits.
(5) In some previous literatures, the fact of "size-grading of ore constituents" has been recognized to be evidence of an idea that the deposits have slided down on the submarine slopes of seafloor. However, this structure may be more likely due to the diagenetic water-escape processes rather than due to such a submarine sliding.
(6) The above conception of the diagenetic evolution history of kuroko deposits is generalized as follows.