Hydrothermal alteration of the basaltic lavas of the Troodos Ophiolite Complex associated with the formation of the massive sulphide deposits

Abstract

Summary The Cyprus massive sulphide deposits are restricted to and constitute an integral part of the pillow lava succession of the Troodos Ophiolite Complex. Extensive geophysical, geological and geochemical data collected in the last 20 years suggest that this complex represents an uplifted fragment of oceanic lithosphere and underlying upper mantle, which evolved in the Mesozoic at divergent plate boundaries. The massive sulphide deposits consist of the basaltophilic elements S, Fe, Cu and, less commonly, Zn. They have been formed at the original sea water—Lower Pillow Lavas or Basal Group interface and are characterized by distinct vertical zoning comprising, in downward succession, a zone of massive ore, a zone of sulphide with silica and the stockwork zone. The latter consists of veins and disseminations of pyrite in brecciated and/or pillow lavas, which have undergone extensive silicification, chloritization and argillization. The veins represent fillings of fractures in the lavas by sulphides, whereas the disseminated pyrite is believed to have been essentially produced by sulphurization of the titanomagnetite of the basaltic lavas and it often contains inclusions of euhedral leucoxene. Part of the disseminated pyrite formed from the iron present in the glassy mesostasis and the ferromagnesian minerals of the lavas. Detailed optical, chemical and X-ray diffraction studies indicate that the sulphide ore-forming solution produced extensive mineralogical and chemical changes in the stockwork lavas. These are distinctly different from the mineralogical and chemical changes in the unmineralized lavas produced by regional hydrothermal metamorphism. Mineralogical changes in the stockwork lavas include the complete pseudomorphism of the primary minerals plagioclase and pyroxene by quartz, the sulphurization of magnetite and the alteration of the glassy mesostasis to a mixture of chalcedony, chlorite, illite and, less commonly, kaolinite. The chemical changes in the stockwork lavas involved the dramatic decrease of Ca, Na and, to a lesser extent, Al and K, and considerable increase of S and Fe. Magnesium decreases considerably in the intensely mineralized lavas, whereas it increases in the weakly mineralized lavas. The stockwork lavas are invariably intensely silicified, but there was no appreciable removal or introduction of silica in these lavas. Silicification of these lavas was probably produced by the selective leaching of the bases from the primary minerals and the glass, leaving behind residual silica in the form of quartz and chalcedony. In the stockwork lavas Cu, Zn and Co increased, whereas Ni and Mn decreased in comparison with the unmineralized lavas. Field, chemical and mineralogical relationships suggest that the stockwork zones represent the roots of the massive ores and the channels through which the ore-bearing fluids have ascended to form the massive ores and the sea water-pillow lava interface. The persistent presence of illite and absence of montmorillonite as a hydrothermal alteration product in the stockwork zone, combined with experimental data and studies of the active geothermal areas, suggest that the temperature of the ore-forming fluids was 250–260°C and the pH 6.6–6.8. Recent detailed stable isotope studies suggest that these fluids were mostly heated marine waters, which leached the ore metals from the underlying Sheeted Complex.