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Cyprus volcanogenic sulphide deposits in relation to their environment of formation

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posted on 24.03.2010, 11:59 by N.G. Adamides
The present thesis is concerned with the volcanogenic sulphide deposits associated with the Troodos ophiolite, Cyprus. Extensive petrological, chemical and field data are presented and these suggest that the ophiolite has formed on a spreading axis which was located above a subduction zone. This tectonic regime was responsible for the enrichment of the Troodos primary magma in silica and the large ion lithophile elements. Formation of the Sheeted Complex and Lower Pillow Lavas is envisaged to have taken place close to the axis of spreading. The lavas are extrusive products of subaxial chambers which exhibited both open and closed system fractionation. The Upper Pillow Lavas are considered to be derived from the same primary magma as the other members of the ophiolite but exhibit only limited fractionation. The difference between the two suites is attributed to extrusion of the Upper Pillow Lavas on the flanks of the spreading axis, from diapirically rising magma chambers. A detailed geochemical study of the hydrothermal metamorphism of the ophiolite suggests extensive mobility of components in the Sheeted Complex. All geochemical changes are in agreement with experimental and field evidence of seawater-rock interaction; they suggest enrichment of the rocks in magnesium and sodium and extensive leaching of calcium, silica, barium and zinc. Strontium is locally leached and concentrated in secondary minerals. Calculations of element fluxes during hydrothermal metamorphism suggest that the amounts leached from the Sheeted Complex can account for the amounts deposited in the sulphide deposits and metalliferous sediments (umbers). A hydrothermal model is proposed according to which seawater trapped within the Sheeted Complex is heated under the influence of the geothermal gradient, enriched in metallic and other elements by breakdown of primary silicates and finally driven off by the intrusion of magma chambers at high levels in the crust. These chambers are deduced to be those which formed the Upper Pillow Lavas, as inferred from the association of sulphide deposits with the initial stages of eruption of these lavas. Examination of the field occurrence of several sulphide deposits suggests that these are most often localised by faults which were inherited from the tensional regime of the spreading axis. Localisation at sites of structural intersections is also described. The size of the deposits is determined to a great extent by the structural disturbance in the area in which they occur, smaller deposits being favoured at strongly tectonised areas. In the absence of tectonism ore localisation may take place within the lava pile. It is concluded that the presence of a heat source is more important than structural disturbance in the formation of deposits. The results suggest, also, that there was only one period of sulphide-producing hydrothermal activity and this is temporally placed at the early stages of extrusion of the Upper Pillow Lavas. The mineralogical characteristics of deposits are described, with emphasis on the behaviour of the hydrothermal fluids at discharge sites. The main conclusions suggest that extensive interaction of the fluids with the wall rocks and seawater takes place and this determines the final mineralogical character of the deposits and the formation of copper-rich and copper-poor orebodies. The extent of interaction is determined to a large extent by the structure and results to the formation of extensive alteration zones in deposits which formed deep within the lavas. Exchange between the fluids and wall rocks is mainly characterised by enrichment of the surrounding lavas in magnesium and sodium, depletion in calcium and strong enrichment in barium and potassium. The mineralized lavas are depleted in alkalies and enriched in magnesium. The implications of the above conclusions on exploration are briefly discussed, with emphasis on the future application of geochemical methods in the search for sulphide deposits.



Le Bas, M.J.

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University of Leicester

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