Abstract

Silicification of umbers in the Troodos ophiolite complex in Cyprus provides evidence for low-temperature hydrothermal activity occurring late in the process of ocean-crust formation, after the crust had moved away from the oceanic spreading centre. Umbers, which are Mn–Fe-rich sediments, precipitate some distance away from oceanic spreading axes. The movement of the fluids partially silicifying the umbers must post-date umber formation and therefore also occurs off axis. These fluids may form part of low-temperature hydrothermal circulation similar to those identified by off-axis heat anomalies in the modern oceans. In Cyprus the silicified umbers often are located away from volcanogenic massive sulphide (VMS) deposits and are aligned along sea-floor faults. They are characterized by quartz veins as well as pervasive silicification. Recent analyses have revealed unexpectedly anomalous, 1–5 ppm, concentrations of Au in these silicified umbers.

Gold mineralization is known to occur in ophiolite complexes as part of the metal concentrations in VMS deposits. These deposits are thought to be formed by ancient hydrothermal circulation similar to that associated with black smokers observed at modern mid-ocean ridges. The research presented in this paper provides evidence for a second phase of Au mineralization in Cyprus caused by low temperature silicifying fluids resulting from oceanic hydrothermal activity which occurred away from the spreading centre. The mineralizing fluids which silicified the umbers may have been more widespread, impregnating other lithologies, and may have overprinted the first phase of Au mineralization in some VMS deposits after they had become inactive and had been moved away from the spreading centre. The mineralizing fluids passing through these VMS may have used the same structural pathways as the original base-metal-rich hydrothermal solutions which formed the VMS.

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