THE volcanogenic massive sulphide deposits of the Hokuroko district of Japan are Miocene metal occurrences which are rich in both Zn and Pb, contain minor Cu and yield noteworthy amounts of Ag and Au (ref. 1). They formed in the Green Tuff belt of Japan in association with the felsic calc-alkaline rocks of a back-arc spreading center1. Until now no modern analogue of this 'Kuroko' type of sulphide deposit has been identified; this is in contrast to the Cyprus type of massive Cu-Zn sulphide deposit, which has such an analogue in the 'smoker' deposits of the East Pacific Rise. In 1984 and 1986, low-temperature oxide and silicate precipitates were discovered in the rift valley of the Okinawa Trough attesting to the recent occurrence of hydrothermal activity in this intracontinental back-arc basin2. On 26 June 1988, a large hydrothermal field with extensive occurrences of sulphide mineralization (the Jade hydrothermal field) was discovered and sampled by the German research vessel Sonne in a cauldron in the central part of the Okinawa Trough. Here we report geochemical and isotopic results which suggest that the Jade deposit is a modern analogue of the Kuroko-type deposits known so far only from the geological record.
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