PETROGRAPHIC AND COPPER ISOTOPIC STUDY OF EPITHERMAL BONANZA ORES IN JAPAN: EVIDENCE FROM HISHIKARI AND KORYU DEPOSITS
Preliminary Cu isotope analysis was conducted on chalcopyrite from Koryu and of composite ores of electrum, naumannite and chalcopyrite from Hishikari. The d65Cu data from Koryu and Hishikari are +1.32 to 0.61‰ and –0.59 ‰, respectively.
On the basis of the petrographic observations above, in conjunction with previous genetic study on metal precipitation, the following can be postulated on the formation of bonanza ores after the introduction of deep fluids to the epithermal system by fracture opening. Electrum and chalcopyrite precipitated as a result of flashing just above the base of boiling zone at Koryu (and possibly at Hishikari), forming deep bonanza ores mainly consisting of type 1, of which lesser amounts are also precipitated over a greater vertical distance above the base. Some electrum and chalcopyrite initially nucleated as colloids entrapping the double-terminated quartz, forming type 2 in the ascending fluid with a turbulent flow. The type 2 was carried to shallow depth, eventually precipitated near the top of boiling level, forming shallow bonanza ores at Hishikari (and possibly at Koryu, although the shallow ores were eroded).
The close associations of electrum and chalcopyrite and similarity of d65Cu values between the two deposits suggest that the origin of Au and Cu is magmatic in both deposits.