Origin and geochemistry of the metalliferous sediments of the Troodos Massif, Cyprus

Abstract

The Troodos ophiolite -includes a diverse assemblage of metalliferous sediments which are very similar in field relations and chemistry to those observed presently forming at modern oceanic spreading axes. Sulphide-producing hydrothermal vents were initiated at the axis of a narrow well-defined median valley, and persisted off axis, almost to the limits of the volcanically active zone. Direct oxidative erosion of the sulphide deposits led to nearby formation of iron oxide sediments locally rich in sulphide detritus (ochres). Iron in the discharging vent fluids became oxidised in the water column, and was more widely dispersed. In volcanically active regions, erupting lava disrupted the sediment, dispersing it through cooling-fractures and pillow interstices. This local rapid burial prevented total dissolution of pelagic carbonate, but generally caused chemical reduction with loss of manganese oxides. Ferromanganese sediments extremely similar to those observed at the crest of the East Pacific Rise were preserved (umbers) in fault-hollows within the lavas during longer quiescent periods. Individual source vents were of limited life, ceasing abruptly after periods of relatively stable activity. Away from the axis, non-metalliferous sediments accumulated free of admixed hydrothermal material.

Subsequent alteration of the sediment was promoted by two processes. First, ageing and gradual burial of the basal sediment, and secondly, exposure of the deeper sediments to both widespread low temperature, and localised high temperature, hydrothermal alteration. Temperatures did not normally exceed 100°C, but in the vicinity of high temperature hydrothermal conduits(up to c. 350°C), silicification has taken place (jaspers).

The chemical composition of the sediment can be described in terms of mixing of the various sediment, sources, with modification of some trace element abundances through diagenetic processes.

A great similarity to modern mid-ocean ridge sediments is observed both in terms of geometric relationship to the palaeoridge, and in chemical and mineralogical composition. Minor deviations may be explained by the different basement composition, ocean circulation, relative position of the carbonate compensation depth, and aluminosilicate accumulation rates. In Cyprus the well exposed relationships thus contribute directly to an understanding of mid-ocean ridge processes.