Ore petrography and geochemistry of massive sulphide deposits within the Semail ophiolite, Oman.

The pillow lavas of the Semail ophiolite complex of northern Oman contain at least three exploitable massive sulphide deposits. These deposits - at Lasail (8 000 000 t), Bayda (750 000 t) and Aarja (3 000 000 t) - are dominated by pyrite, but also contain chalcopyrite and sphalerite with minor marcasite and pyrrhotite. They overlie quartz-sulphide-chlorite stockwork zones. The deposits show a number of significant mineralogical differences. At Lasail the fundamental mineralogy is augmented by the presence of primary magnetite and hematite and the earliest pyrite generation also contains chalcopyritc-pyrrhotite, chalcopyrite-cubanite and chalcopyrite-mackinawite inclusions. Bayda has a similar mineralogy. The mineralogy of these two deposits suggests an initial temperature of the ore fluids greater than 320'C. At Aarja the ore contains significant quantities of sphalerite and bornite together with minor galena and tennantite - and the most common sulphide textures are rhythmic. By analogy with mineralogically similar deposits a temperature of ore formation of less than 275'C is suggested for Aarja. Although some variation is found in the sulphide deposits of Oman, a comparison of these with other ophiolite sulphide deposits and with sulphides formed at spreading axes shows that they share many mineralogical and textural features. Sparse data from sulphides associated with seamounts on the ocean-floor hint, however, at yet closer similarities with the Oman ore deposits