Germanium-bearing Colusite in Siliceous Black Ore from the Ezuri Kuroko Deposit, Hokuroku District, Japan

Abstract. Germanium‐bearing colusite occurs with sphalerite, galena, tetrahedrite‐tennantite, chalcopyrite and pyrite in microdruses and veinlets in the siliceous black ore from the Ezuri Kuroko deposit in the Hokuroku district of Japan. X‐ray microdiffractometry of this mineral gives strongest lines at 1.60, 1.32 and 1.09 Å, which are consistent with the known powder diffraction data of colusite. On the basis of 32 S atoms per formula unit, electron microprobe analyses yield empirical chemical formulae of (Cu_{24 0}Fe_0.3Zn_1.0)_σ25.3V_1.9(A_s4.8Sb_0.2)_σ5.0Ge _1.3S₃₂ for Ge‐bearing colusite in close association with sphalerite, and (Cu_24.6Fe_0.9)_σ25.4V_1.8(A_s4.1 Sb_0.2)_σ4.3Ge_1.7S₃₂ for that coexisting with chalcopyrite, consistent with the ideal formula of Cu_24+xV₂(As, Sb)_6‐x(Sn, Ge)_xS₃₂ (x = 0 to 2) proposed by Spry et al. (1994) for this mineral species. The Ge‐bearing colusite mineralization is suggested to have occurred concurrently with consolidation of the siliceous black ore, possibly during hydrothermal modification in association with the igneous activity of the Ohtaki quartz diorite of the later Onnagawa stage. It is likely that biogenic siliceous ooze, a possible precursor of the siliceous black ore, may have served as an in situ source of Ge as well as other essential rare elements, leading to the formation of Ge‐bearing colusite during transformation or recrystallization of biogenic opal into a‐quartz.