Pb-isotope systematics of a fossil hydrothermal system from the Troodos ophiolite, Cyprus: Evidence for a polyphased alteration history

Abstract

Pb-isotopic ratios have been determined on massive sulphides from the Troodos ophiolite and their inferred source rocks: epidosites and greenschist altered dikes. The latter have distinctly higher Pb-isotope ratios than the sulphides. The high 207Pb/ 204Pb ratios for the whole rocks indicate the involvement of an old (sedimentary) Pb-component. Extensive leaching of three epidosite samples shows that the radiogenic Pb can be removed and the resulting trend for the leached samples is consistent with a Pb-Pb age of ∼95 Ma for these rocks. Inferred initial Pb isotope ratios are similar to those of the sulphides (this study) and fresh glasses from the Akaki Canyon in Cyprus. Therefore the Pb incorporated in the massive sulphides may originate from these greenschist facies hydrothermally altered rocks. The isotopic composition of the leachate Pb points toward a sedimentary origin. Calculation of the Pb-isotopic ratios for 90 Ma old “average” Troodos sediment shows that the “foreign” Pb in the epidosites may very well be derived from this source. This Pb probably does not reside within the mineral lattices of these metamorphic rocks, but rather is adsorbed on mineral surfaces or is present in microscopic particulate matter that may have been filtered from the circulating seawater. Unmodified seawater is considered an unlikely Pb source because of the extremely small dissolved Pb contents of seawater. Furthermore, no seawater or sedimentary signature is evident in the sulphides. Several models of Pb transport from different sources into these rocks may be envisaged, such as dissolution of Miocene evaporites and brine formation, equilibration of pore waters to the Pb isotope ratios of host sediments, and direct infiltration of deep sea particulate matter from the nepheloid layer. With the limited amount of data presently available it is impossible to critically determine the most likely processes of Pb transport.