Tetrahedrite-tennantite

Tetrahedrite-tennantite composition varies widely in Kuroko deposits (Yamaoka, 1969 Yamaoka and Nedachi, 1978a Yui, 1971 Horii, 1971 Shimazaki, 1974 Kouda, 1977 Shikazono and Kouda, 1979 Ono and Sato, 1995 Ishizuka and Imai, 1998). 

Generally, tetrahedrite-tennantite composition from Kuroko deposits is characterized by high Zn content, low Fe content, high Cu content, and low Ag content compared with those from vein-tyjre deposits in Japan (Fig. 1.16). Rarely, it contains Hg up to 1 wt% (Ishizuka and Imai, 1998). 

Tetrahedrite-tennantite composition varies widely in a single orebody. For instance, Kouda (1977) analyzed tetrahedrite-tennantite from Fukazawa-Tsunokakezawa deposit and showed that the Fe and Zn contents are in a range of 0-5.5 wt% and 4.5-10 wt%, respectively. 

Wide compositional zoning and heterogeneity in a tetrahedrite-tennantite grain are common (Yamaoka, 1969 Yui, 1971 Yamaoka and Nedachi, 1978a). 

Positive correlation between Zn contents of coexisting tetrahedrite-tennantite and sphalerite exists (Shikazono and Kouda, 1979). This relation can be explained in terms of the following reaction. 

Figure 1.16. Chemical composition of tetrahedrite-tennantite (Shikazono and Kouda, 1979). A Au-Ag vein-type deposits, B Kuroko deposits, C Taishu-Shigekuma Pb-Zn vein-type deposits, D Skam deposits (Kamioka).

Chemical compositions of tetrahedrite-tennantite from epithermal base-metal vein-type deposits are characterized by (1) wide compositional variations, and (2) higher Zn and Sb contents and Ag and lower Fe, As, and Cu contents, compared with Kuroko deposits (Shikazono and Kouda, 1979). 

The differences in Zn/Fe ratio of tetrahedrite-tennantite in epithermal vein-type and Kuroko deposits and that of sphalerite in these deposits can be interpreted in terms of the following exchange reaction ... 

This equation suggests that the Fe content of tetrahedrite-tennantite positively correlates with that of sphalerite at constant temperature and pressure, indicating Fe and Zn contends of tetrahedrite-tennantite are useful to estimate physicochemical parameters (/sj, /02 etc.) as well as Fe content of sphalerite, although detailed study on thermochemical properties of tennantite-tetrahedrite solid solution is still needed. 

Shikazono, N. and Kouda, R. (1979) Chemical composition of tetrahedrite-tennantite minerals and the chemical environments of some Japanese ore deposits. Mining Geology, 29, 33-41 (in Japanese). 

The difference in mineralogy of the Kuroko and present-day back-arc deposits are (1) metastable phases such as native sulfur, wurtzite, and amorphous silica are poor in the Kuroko deposits (2) arsenic minerals such as jordanite, tetrahedrite-tennantite, native arsenic, and realgar are common in the present day back-arc deposits (Okinawa Myojinsho Knoll Caldera), but rare in Kuroko deposits except tetrahedrite-tennantite (3) secondary minerals such as cerussite and covellite are common in present day back-arc deposits (e.g., Okinawa, Myojinsho Knoll Caldera) (4) Dendritic texture is common in the present day back-arc deposits. 

Although mineralogy is different in different site, the more abundant minerals in back-arc deposits than in midoceanic ridge deposits are barite, anhydrite, electrum, As-minerals (realgar, orpiment), tetrahedrite-tennantite and galena. 

Main opaque minerals are chalcopyrite, pyrite, pyrrhotite, sphalerite and bornite (Table 2.22). These minerals commonly occur in massive, banded and disseminated ores and are usually metamorphosed. Hematite occurs in red chert which is composed of fine grained hematite and aluminosilicates (chlorite, stilpnomelane, amphibole, quartz) and carbonates. The massive sulfide ore bodies are overlain by a thin layer of red ferruginous rock in the Okuki (Watanabe et al., 1970). Minor opaque minerals are cobalt minerals (cobaltite, cobalt pentlandite, cobalt mackinawite, carrollite), tetrahedrite-tennantite, native gold, native silver, chalcocite, acanthite, hessite, silver-rich electrum, cubanite, valleriite , and mawsonite or stannoidite (Table 2.22). 

Tetrahedrite-tennantite has been reported from several deposits (e.g.. Kune, Okuki, Besshi, Ikadatsu, Sazare) (e.g., Kase, 1986). This mineral occurs in chalcopyrite-rich chalcopyrite-bornite-pyrite ores. Tellurium-bearing tetrahedrite-tennantite (Te 11.2 wt%) was found from the Sazare and Ikadatsu deposits (Kase, 1986). 

Galena, tetrahedrite-tennantite, mawsonite and native silver occur in the copper rich ores but not in ordinary pyritic ores and copper rich ores most commonly occur as offshoots, tongues and veins in the deformed deposits. This suggests that these minor minerals formed during the metamorphic deformation stage accompanied by recrystallization. 

It is noteworthy that bornite, chalcocite and tetrahedrite-tennantite which are common minerals in Kuroko deposits occur in gold bearing Besshi-type deposits. Although these minerals are considered to be secondary minerals, depositional environments of these minerals are characterized by higher /s, and foj conditions. It is also noteworthy that these deposits are rich in pyrite rather than pyrrhotite. Probably, Besshi-subtype deposits in Shikoku formed under the higher fo and /sj conditions than the deposits characterized by pyrrhotite (Maizuru, Hidaka, Kii, east Sanbagawa). Such typical Besshi-type deposits (Besshi-subtype deposits in Shikoku) are characterized by simple sulfide mineral assemblage (chalcopyrite, pyrite, small amounts of sphalerite). Inclusion of bornite in pyrite is also common in these deposits. 

Reactions (1) and (4) result in strongly acidified solutions which, in addition to dissolve large amounts of Fe(II) and SO, also leach important quantities of many other major and trace metals from the mineralizations and the host rocks, including Al, Mg, and Ca (with minor Na, K, and Ba) from the accompanying aluminosilicates, carbonates, and sulfates, in addition to Cu, Zn, Mn, As, Pb, or Cd from other sulfides (sphalerite, chalcopy-rite, galena, arsenopyrite) and sulfosalts (tetrahedrite-tennantite). Iron and aluminum are usually the most abundant metal cations of those present in AMD [5, 9, 12, 22]. 

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