Metallogenesis at paleo-spreading centers: Lead isotopes in sulfides, rocks and sediments from the Troodos ophiolite (Cyprus)

Abstract

New Pb isotopic data for metalliferous deposits and pelagic sediments from the Troodos ophiolite complex (Cyprus) are presented. We use these data to re-evaluate the genetic models for these deposits in the light of recent results obtained for different types of hydrothermal sites presently active on spreading ridges. In contrast to previous conclusions of earlier studies, we find that the Troodos hydrothermal deposits (sulfides as well as umbers and Mn crusts) have isotopic compositions very similar to that of most of the rocks from this ophiolite complex: they display a 207 Pb 204 Pb ratio higher than present-day typical ridges. This result has been interpreted as evidence for a subduction-related origin of the Troodos ophiolite. This different geodynamic setting of the ophiolite is also the reason for the isotopic difference between Troodos and Semail (Oman) sulfides, the latter being typical of a normal oceanic crust. When examined in more detail, the Troodos sulfide results are very homogeneous within one site, but show significant differences from one site to another. At least three different isotopic end-members are required to interpret these variations. One of these end-members is found in the pelagic sediments data analyzed here, which have a 207 Pb 204 Pb typical of continental crust signature, higher than that of the rocks and sulfides. On present ridges, the involvement of such a component has been similarly described for the Guaymas Basin and for the Gorda Rise deposits. However, some of the isotopic variations observed between sulfides cannot be interpreted only by this end-member. One possibility is that a second sedimentary component, as yet unidentified, has been involved in the hydrothermal circulation, as recently demonstrated for Red Sea brines. Finally, the determination of the exact proportion of Pb leached out of the ophiolite basaltic crust itself would require a more complete knowledge of the initial isotopic heterogeneities within the massif. Indeed, several rock results are significantly less radiogenic than those of the sulfides, which however show themselves initial compositions. This could result either from a sediment contribution to the sulfide Pb, ór from a biased sampling of the massif heterogeneities. This question remains open in the present state of the data.