Replacement of Ores Recorded by Textures and Chemical Compositions of Sphalerite: An Example From the Furutobe Kuroko Deposit, Akita, Japan

ABSTRACT Numerous Kuroko deposits, a type of volcanogenic massive sulphide Zn–Pb–Cu–(Au + Ag) deposits, are hosted in the Hokuroku district of northeastern Japan. In this study, samples of sphalerite‐rich black ores and chalcopyrite‐pyrite‐rich yellow ores from the Furutobe Kuroko deposit were examined in terms of their textures and chemical compositions. We integrated microtextural observations, bulk and mineral chemical analyses and sulphur isotope analysis of sulphides and sulphate to elucidate the crystallisation and replacement of massive sulphide ores of the Furutobe deposit. In the studied samples, black ores occur as sphalerite‐rich massive layers and clasts in sharp to gradational contact with yellow ores and contain 10.0–31.4 wt.% Zn, 1.0–15.7 wt.% Pb, 0.2–7.7 wt.% Cu, 0.6–10.8 wt.% Fe, 0.8–10.5 ppm Au and 203–507 ppm Ag. The chalcopyrite‐pyrite‐rich yellow ores contain 8.5–11.8 wt.% Cu, 15.7–33.9 wt.% Fe, 1.3–17.6 wt.% Zn, 0.5–4.3 wt.% Pb, 0.4–8.7 ppm Au and 224–348 ppm Ag. On the basis of mode of occurrence and textures, three types of sphalerite (I, II and III) were identified in the ores. Sphalerite I is massive and coarse‐grained and occasionally shows chromatic banding. Sphalerite II contains inclusions of pyrite and rarely shows chromatic bands. Sphalerite III occurs closer to the contact with yellow ores, texturally appears to have formed later than sphalerite I and II and is characterised by well‐developed chalcopyrite disease (minute chalcopyrite inclusions). Sphalerite I and II have similar chemical compositions, characterised by relatively low Cd (0.28–0.50 wt.%), Fe (0.05–3.18 wt.%), Ag (< 60–600 ppm) and Sb (< 800 ppm) contents compared to those of sphalerite III (0.35–0.50 wt.% Cd, 0.05–5.21 wt.% Fe, 60–1400 ppm Ag and up to 2500 ppm Sb). Electrum associated with sphalerite I and II has relatively high Ag content (with a mode of 30–32 at.% Ag) compared to that associated with sphalerite III (with a mode of 26–28 at.% Ag). The δ 34 S values of sulphides from the black and yellow ores overlap in a range from +1.9 to +4.7‰. However, the δ 34 S values of sphalerite decrease from sphalerite I to III, possibly indicating decreasing SO 4 2− /H 2 S ratio, that is, increasing amounts of reduced dissolved sulphur species in the mineralizing fluids. Based on FeS contents of sphalerite and Ag contents of associated electrum, maximum sulphur fugacity (log f S 2 in atm) and formation temperatures of sphalerite range from −9.0 to −7.6 and 255°C to 297°C for sphalerite I, from −9.0 to −7.2 and 255°C to 300°C for sphalerite II and from −7.8 to −6.2 and 275°C to 330°C for sphalerite III. The difference in texture and composition of sphalerite can be attributed to a recrystallisation process. Coupled dissolution‐reprecipitation of early formed Cd‐, Fe‐, Ag‐ and Sb‐poor sphalerite I and II through interaction with high temperature and high sulphur fugacity Cu‐rich fluids, responsible for the formation of the yellow ores, resulted in the formation of Cd‐, Fe‐, Ag‐ and Sb‐rich, heavily chalcopyrite‐diseased sphalerite III near the contact with the yellow ores and the consequent enrichment of trace elements, particularly Cd, Ag and Sb, in the black ores.