GOLD MINERALIZATION ASSOCIATED WITH LOW-TEMPERATURE, OFF-AXIS, FLUID ACTIVITY IN THE TROODOS OPHIOLITE, CYPRUS

Silicification of umbers in the Troodos ophiolite complex in Cyprus pr ovides evidence for low-temperature hydrothermal activity occurring la te in the process of ocean-crust formation, after the crust had moved away from the oceanic spreading centre. Umbers, which are Mn-Fe-rich s ediments, 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 t hose identified by off-axis heat anomalies in the modern oceans. In Cy prus the silicified umbers often are located away from volcanogenic ma ssive sulphide (VMS) deposits and are aligned along sea-floor faults. They are characterized by quartz veins as well as pervasive silicifica tion. Recent analyses have revealed unexpectedly anomalous, 1-5 ppm, c oncentrations of Au in these silicified umbers. Gold mineralization is known to occur in ophiolite complexes as part of the metal concentrat ions in VMS deposits. These deposits are thought to be formed by ancie nt hydrothermal circulation similar to that associated with black smok ers observed at modern mid-ocean ridges. The research presented in thi s paper provides evidence for a second phase of Au mineralization in C yprus caused by low temperature silicifying fluids resulting from ocea nic hydrothermal activity which occurred away from the spreading centr e. The mineralizing fluids which silicified the umbers may have been m ore widespread, impregnating other lithologies, and may have overprint ed the first phase of Au mineralization in some VMS deposits after the y had become inactive and had been moved away from the spreading centr e. The mineralizing fluids passing through these VMS map have used the same structural pathways as the original base-metal-rich hydrothermal solutions which formed the VMS.