Mineralogical studies and their bearing on the genesis of massive sulphide deposits from the Semail ophiolite complex, Oman

Three massive sulphide deposits - Lasail (8 X 10⁶ tonnes), Bayda (0.75 X 10⁶ tonnes) and Aarja (3 X 10⁶ tonnes) - are found within pillow lava units of the Semail ophiolite complex of northern Oman. All comprise massive pyritic ores with some Cu and Zn, which overlie quartz-pyrite-sulphide-chlorite stringer zones in basalt. They have been interpreted as ocean-floor exhalative deposits associated with seamounts, away from spreading-axis magmatism; >500 microprobe analyses of Fe, Cu, Zn, Co, Ni, Ag, Cd, As, Sb, S for both the major and minor sulphides and oxides have been performed. The results show that although paragenetically different generations of some phases, especially sphalerite, can be distinguished geochemically, there is a large measure of geochemical uniformity for any phase, not only within single grains but also between texturally different generations and even different deposits, and that the trace-element contents of most phases are low, <0.05 wt.%. Conditions of formation of the ores have been estimated from coexisting mineral assemblages. At Lasail and Bayda, the presence of encapsulated chalcopyrite-pyrrhotite and iron-rich sphalerite within pyrite, which itself coexists with magnetite, suggests that the earliest mineralization took place at approx 350oC, aS₂ approx 10^-9 atm and aO₂ approx 10^-31 atm. At Aarja the presence of anomalous bornite suggests mineralization took place at T <150oC.-J.M.H.