PETROGRAPHIC AND COPPER ISOTOPIC STUDY OF EPITHERMAL BONANZA ORES IN JAPAN: EVIDENCE FROM HISHIKARI AND KORYU DEPOSITS

Hishikari and Koryu are Pleistocene epithermal Au-Ag deposits near the subduction zone. They produce high-grade ores (Au> 1000g/t) that mainly consist of coexisting electrum and chalcopyrite with lesser amounts of Ag-Se-S phases (naumannite, aguilarite and Se-bearing acanthite), sphalerite and galena. Gangue minerals consist of quartz, adularia and clay minerals. The present study shows that the coexisting electrum and chalcopyrite are classified into the following two mineral assemblage by features observed in crystal forms of associated quartz in microscope. Type 1, Mosaic anhedral quartz with anhedral electrum and chalcopyrite, Type 2, Aggregates of euhedral to subhederal double-terminated quartz entrapped by anhedral electrum and chalcopyrite. Type 1 is dominant in Koryu ores whereas type 2 is dominant in Hishikari ores.

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.