Abstract
Unconsolidated sediments change into sedimentary rocks through their burial diagenetic processes, during which their physical and chemical properties being significantly modified or evolved. For realizing the initial nature of a given sedimentary geologic body, therefore, it is fundamentally required to date back and reconstruct its diagenetic modification or evolution history. In studies of kuroko ore genesis, however, little attention seems to have been paid to this aspect. Our careful observations and re-examinations on the mode of occurrence of the Shakanai kuroko deposits are disclosing some distinct diagenetic phenomena fosslized in them. In the present paper are described some of the results obtained, their genetic significance being discussed in the forthcoming, paired paper. Some of the important facts described here are enumerated as follows:
(1) The Shakanai deposits are composed essentially of a number of massive "ore-lumps" with various dimensions and of relatively low-grade matrices cementing them. The deposits, as a whole, constitute a bedded form to be emplaced within a single stratigraphic horizon.
(2) The ore-lumps are in most cases oval or lenticular and sometimes breccia-like in shape, the breccialike ones being restricted in occurrence to some clayey sheared zones in which "slicken-side" are well developed.
(3) Alternation of thinly bedded ores and mudstones rather commonly occurs in places, and a number of small ore-lumps are also sometimes arranged in layer conformably within mudstone beds. So-called "load-cast" structures are usually developed between the unit beds of ores or mudstone.
(4) In ores of the deposits are rather extensively developed some "water-escape" structures, which are very similar in appearance to what have been reported from normal elastic sediments by LOWE (1975).
(5) Thin ore-veinlets with rather irregular geometric patterns are commonly found not only within the massive deposits but also in both hangingwall and footwall rocks. They are generally discontinuous in occurrence and are essentially similar in mineralogical characteristics to the ores surrounding them. In the ore-lumps are also often developed some sort of small pores or openings with irregular shapes, in which idiomorphic crystals of sulfide and/or gangue minerals are sometimes observed.
(6) Ore microscopically, so-called "colloform textures" are found to exist extensively throughout the deposits, from the lower pyritic ore zone to the upper black-ore zone. The most characteristic of them is of pyrite, showing a series of textures evolving from very minute globular particles of about 1 micron or less in size, via framboidal aggregates of the minute particles, to coarser crystal aggregates including those relatively more primitive textures.
