The lavas of the Semail ophiolitic nappe of Oman, which formed at and very near a late Cretaceous oceanic spreading axis, are intercalated with, and overlain by a range of metalliferous sediments including cupriferous sulphides, ferruginous ochres and ferro-manganiferous umbers. The lava sequence is divisible into lower and upper informal units. The ‘lower’ lavas contain small massive sulphides and ochres which either formed by sea-floor oxidation of sulphides or were directly precipitated. In one area the basal ‘upper’ lavas contain mounds of ferruginous oxide sediments associated with umbers. The mounds may have resulted from bacterial precipitation of sulphides and/or Fe-oxy-hydroxides. Localized umbers deposited from hydrothermal solutions occur throughout and immediately above the ‘upper’ lavas, including areas where sea-floor faulting produced thick accumulations of volcaniclastic breccias. The uppermost umbers are overlain by radiolarites and foraminiferal chalk. Non-calcareous radiolarites are restricted to successions above umbers in some hollows suggesting that dissolution of calcareous pelagic sediment by acid hydrothermal solutions occurred during their ponded accumulation.

Most of the metalliferous deposits and pelagic sediments can be related to a spreading ocean ridge affected by sea-floor fault zones. Others are associated with extrusives which have island-arc affinities and built up as discrete volcanic centres on the sea-floor.

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