In studies of kuroko ore genesis have to be equally taken into consideration the following two aspects, i.e., initial formation processes of ores and their modification processes during subsequent geologic history. Our careful observations on the mode of occurrence of the Shakanai deposits, which are essentially a continuation and extension of our recent works (KUMITA et al., 1980; KAMBARA et al., 1982), have disclosed some geologic key factors or conditions responsible for their formation and subsequent modifications. Important conclusions derived from the present study are as follows.
1. Environmental constraints on ore formation: The Shakanai deposits are suggested to have been formed at the "calm" local sedimentary
deeps
with no significant supply of clastic materials in space and within a certain period during which no significant volcanic activities took place in time. The assumed local sedimentary
deeps
are considered to be most likely due to the complicated submarine configuration of the pre-existing dacitic lavas.
2. Mechanical modifications of ore deposits: The ore deposits formed have undergone a variety of mechanical modifications mainly due to the intrusion of post-ore dacites, some deposits having been significantly dislocated and broken into several minor orebodies. Some of the footwall dacites, which are currently assumed to be of pre-ore and closely related with the mineralization, are also considered to have been involved in the mechanical modification processes of the deposits.
3. Chemical modifications of ore deposits: The deposits have also undergone a variety of chemical modifications both in quality and in quantity throughout their burial history, which are mainly due to possible multiple interactions with some "fluids" derived from different sources such as magmatic fluid, interstitial formation waters, and circulating seawater and/or meteoric water.
4. Preservation of ore deposits: For the good preservation of a deposit, the possible interactions with "fluids" as suggested above have to be reduced to the minimum. For its purpose, therefore, it would necessarily be concluded that some impermeable rocks against the "fluids" (i.e., compact igneous rocks or mudstone) must exist at the footwall side of the deposit as well as at its hangingwall side.
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