Pb isotopic composition of ophiolitic volcanogenic sulphide deposits, Troodos Complex, Cyprus

Abstract

The volcanogenic sulphide ore deposits in the ophiolitic rocks of the Troodos Massif, Cyprus, are thought to be ∼80 Myr old¹ analogues of the metal sulphide accumulations actively being deposited from ∼350 °C water at a depth of ∼2.6 km on the East Pacific Rise (21°N)^2–4. In both cases, mineralization is thought to be a consequence of heat transfer by convective seawater circulation within the oceanic crust^5,6. Evidence includes fluid inclusions in quartz from ore deposits in Cyprus with modal trapping temperatures of 298–370 °C which have a bulk composition of 3.2±0.5 equiv. wt% NaCl (1σ n=273). This salinity is identical to seawater with 3.5 wt% total dissolved solids^4,7. We have examined the isotopic composition of Pb as a geochemical tracer to clarify the origin of one of the metals contained in the sulphide concentrations in the Troodos Complex. It was anticipated that the bulk of the lead would have been derived by leaching of the mafic ophiolitic rocks and, indeed, such a component is found to be present. However, the isotope ratios suggest that the lead is actually a mixture, and also contains an important component derived from sea-water.