17:15 〜 18:45
[HRE13-P08] Chemistry and sulfur isotope compositions of black ores of the Furutobe and Nurukawa deposits, Akita, Japan
キーワード：Kuroko deposits, Geochemistry, Sulfur isotopes, Akita
The Furutobe and Nurukawa base metal (+Au-Ag) deposits are located in the northern part of the Hokuroku district, Akita Prefecture, Japan. The deposits are hosted by a Miocene felsic-dominated volcanic succession deposited during the opening of the Sea of Japan. Exploration of Kuroko deposits took place in a short period, between 1960 and 1995, which prevented an in-depth investigation of the physicochemical conditions of their formation. The Furutobe and Nurukawa deposits consist mainly of sphalerite-rich black ores, chalcopyrite-pyrite-rich yellow ores, and quartz-pyrite-rich siliceous ores. This study presents and compares the chemical compositions and sulfur isotope signatures of black ores of the two deposits and their implications for conditions of ore-formation.
The black ores of the Furutobe deposit are composed of sphalerite, galena and barite, with minor chalcopyrite, pyrite, tennantite-tetrahedrite solid solution and electrum occurring as massive layers and fragments alternating with yellow ores. The black ores of the Nurukawa deposit are massive, occasionally contain fragments of altered host rocks, and consist mainly of sphalerite, chalcopyrite, pyrite, and quartz. The Zn, Pb and Cu contents of black ores at Furutobe vary from 10.0 to 31.4 wt% (average of 25.6 wt%), 1.0 to 15.7 wt% (av. 5.0 wt%) and 0.12 to 7.7 wt% (av. 2.4 wt%), respectively. These contents are higher than those of black ores at Nurukawa, which have Zn, Pb and Cu contents of 0.04–31.7 wt% (av. 7.32 wt%), 0.28–12.6 wt% (av. 6.4 wt%) and 0.39–1.08 wt% (av. 0.71 wt%), respectively. The bulk Au and Ag contents of black ores of the Furutobe deposit range from 0.80 to 10.5 ppm (av. 4.9 ppm) and 203 to 507 ppm (av. 314 ppm), respectively. On the other hand, the bulk Au and Ag contents of black ores of the Nurukawa deposit range from 0.08 to 3.75 ppm (av. 1.13 ppm) and 0.82 to 327 ppm (av. 109 ppm), respectively. The sulfur isotope ratios (δ34SCDT) of sphalerite from the black ores of the Furutobe and Nurukawa deposits range from +1.9 to +4.3 ‰, and from +5.3 to +7.3 ‰, respectively.
On the basis of the FeS contents of sphalerite and chemical compositions of electrum, the sulfur fugacity (logfS2 in atm) and formation temperatures of the black ores of the Furutobe deposit range from -9.0 to -6.2, and from 255 to 340 °C, respectively, while those of the black ores of the Nurukawa deposit are lower than those of the Furutobe deposit, and range from -13.0 to -9.8, and from 250 to 270 °C, respectively. The Kuroko deposits are believed to have formed through mixing between magmatic fluids and seawater. Assuming that the δ34SCDT values of magmatic fluids and seawater in the Miocene were ca. 0 ‰ and +22 ‰, respectively, the proportions of magmatic sulfur for the black ores of the Furutobe and Nurukawa deposits were estimated at ca. 85 %, and ca. 70 %, respectively. These estimates suggest a more magmatic component for the studied samples of the Furutobe deposit.
The black ores of the Furutobe deposit are composed of sphalerite, galena and barite, with minor chalcopyrite, pyrite, tennantite-tetrahedrite solid solution and electrum occurring as massive layers and fragments alternating with yellow ores. The black ores of the Nurukawa deposit are massive, occasionally contain fragments of altered host rocks, and consist mainly of sphalerite, chalcopyrite, pyrite, and quartz. The Zn, Pb and Cu contents of black ores at Furutobe vary from 10.0 to 31.4 wt% (average of 25.6 wt%), 1.0 to 15.7 wt% (av. 5.0 wt%) and 0.12 to 7.7 wt% (av. 2.4 wt%), respectively. These contents are higher than those of black ores at Nurukawa, which have Zn, Pb and Cu contents of 0.04–31.7 wt% (av. 7.32 wt%), 0.28–12.6 wt% (av. 6.4 wt%) and 0.39–1.08 wt% (av. 0.71 wt%), respectively. The bulk Au and Ag contents of black ores of the Furutobe deposit range from 0.80 to 10.5 ppm (av. 4.9 ppm) and 203 to 507 ppm (av. 314 ppm), respectively. On the other hand, the bulk Au and Ag contents of black ores of the Nurukawa deposit range from 0.08 to 3.75 ppm (av. 1.13 ppm) and 0.82 to 327 ppm (av. 109 ppm), respectively. The sulfur isotope ratios (δ34SCDT) of sphalerite from the black ores of the Furutobe and Nurukawa deposits range from +1.9 to +4.3 ‰, and from +5.3 to +7.3 ‰, respectively.
On the basis of the FeS contents of sphalerite and chemical compositions of electrum, the sulfur fugacity (logfS2 in atm) and formation temperatures of the black ores of the Furutobe deposit range from -9.0 to -6.2, and from 255 to 340 °C, respectively, while those of the black ores of the Nurukawa deposit are lower than those of the Furutobe deposit, and range from -13.0 to -9.8, and from 250 to 270 °C, respectively. The Kuroko deposits are believed to have formed through mixing between magmatic fluids and seawater. Assuming that the δ34SCDT values of magmatic fluids and seawater in the Miocene were ca. 0 ‰ and +22 ‰, respectively, the proportions of magmatic sulfur for the black ores of the Furutobe and Nurukawa deposits were estimated at ca. 85 %, and ca. 70 %, respectively. These estimates suggest a more magmatic component for the studied samples of the Furutobe deposit.