Hm. Prichard et G. Maliotis, GOLD MINERALIZATION ASSOCIATED WITH LOW-TEMPERATURE, OFF-AXIS, FLUID ACTIVITY IN THE TROODOS OPHIOLITE, CYPRUS, Journal of the Geological Society, 155, 1998, pp. 223-231
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.