『Abstract
The Kuroko deposits of NE Honshu are a key type deposit for the
study of volcanogenic massive sulfide deposits. However, these
deposits have not been studied in detail since the early 1980's
and knowledge of their mode of formation is now dated. In this
study, we present the analysis of 12 samples of the Kuroko deposits,
12 samples of submarine hydrothermal minerals from the Sunrise
deposit and 6 samples from Suiyo Seamount, both of which are located
on the Izu-Ogasawara (Bonin) Arc, for 27 elements. For the Kuroko
deposit, Cd>Sb>Ag>Pb>Hg>As>Zn>Cu are highly enriched, Au>Te>Bi>Ba>Mo
are moderately enriched, In>Tl are somewhat enriched and Fe is
not significantly enriched relative to the average continental
crust. Within each of these deposits, a similar pattern of element
associations is apparent: Zn-Pb with As, Sb, Cd, Ag, Hg, Tl and
Au; Fe-Cu-Ba with As, Sb, Ag, Tl, Mo, Te and Au; Si-Ba with Ag
and Au; CaSO4. The enrichment of the chalcophilic
elements in these deposits is consistent with hydrothermal leaching
of these elements from the host rocks which are dominantly rhyolite-dacite
in the case of the Kuroko deposits, rhyolite in the case of the
Sunrise deposit and dacite-rhyolite in the case of the Suiyo Seamount
deposit. However, this pattern of element enrichment is also similar
to that observed in fumarolic gas condensates from andesitic volcanoes.
This suggests that there may be a significant magmatic contribution
to the composition of the hydrothermal fluids responsible for
the formation of the Kuroko deposits, although it is not yet possible
to quantify the relative contributions of these two sources of
elements.
The compositional data show that Sunrise and Suiyo Seamount deposits
are much closer compositionally to the Kuroko deposits from NE
Honshu than are the submarine hydrothermal deposits from the JADE
site in the Okinawa Trough which contain, on average, significantly
higher concentrations of Pb, Zn, Sb, As and Ag than each of these
deposits. In spite of the greater similarity in tectonic setting
of the Hokuroku Basin in which the Kuroko deposits formed to the
Okinawa Trough (intracontinental rifted back-arc basin) compared
to Myojin Knoll and Suiyo Seamount (active arc volcanoes), it
appears that submarine hydrothermal deposits from Myojin Knoll
and Suiyo Seamount are closer analogues of the Kuroko deposit
than are those from the Okinawa Trough. The present data are consistent
with the magmatic hydrothermal model for the formation of Kuroko-type
deposits as formulated by Urabe and Marumo [Urabe, T., Marumo,
K., 1991. A new model for Kuroko-type deposits of Japan. Episodes
14, 246-251].
Keywords: Kuroko deposits; Myojin Knoll; Suiyo Seamount; Okinawa
Trough; Volcanogenic massive sulfide deposits』
1. Introduction
2. Sample descriptions
2.1. Kuroko deposits
2.2. Sunrise deposit
2.3. Suiyo Seamount
2.4. Hakurei deposit
3. Methods of analysis
4. Mineralogy
5. Chemical composition
6. Discussion
7. Conclusions
Acknowledgements
References