LA-ICPMS investigation of base and precious metal contents in rock-bearing sulfides from the Troodos Ophiolite, Cyprus: implications for the cycle of metals during hydrothermal alteration of oceanic crust
Résumé
The Troodos ophiolite in Cyprus, formed in a Cretaceous (92 Ma) supra-subduction
zone setting, is considered to be one of the best-preserved ophiolite complexes in the
world. This ophiolite hosts several dozen Volcanogenic Massive Sulfide (VMS)
deposits that are up to ~20 Mt in size and were exploited for both sulfur and base metals.
VMS deposits have been shown to originate through leaching of metals during
hydrothermal alteration of the sheeted dike complex. A recent study [1], has performed
mass balance calculations by considering the protolith composition and the secondary
mineral assemblages, and thus was able to quantitatively constrain the release of base
metals from the altered sheeted dikes. These authors report bulk metal contents of
basaltic andesite glass protoliths and variably altered facies rocks from the epidosite
zone of the Troodos ophiolite, and suggest that Cu was totally lost during the early
stages of alteration owing to complete breakdown of igneous sulfides originally present
in the protoliths, whereas the release of other base metals (Ni, Zn, Mn, Co) towards
mineralizing fluids was controlled by the secondary mineral assemblage, i.e., the
relative abundance of epidote and chlorite ± amphibole. This interpretation stresses the
role of silicate minerals in the redistribution of metals during hydrothermal alteration of
the sheeted dike complex, and contrasts with the common assumption that accessory
phases, such as sulfides, control base metal concentrations within mineralizing fluids in
VMS systems.
Here we propose to test this hypothesis by providing measurements of the metal
contents of sulfide minerals contained in various lithologies from the Troodos ophiolite.
We report laser-ablation ICPMS analyses of 14 base and precious metals (Ti, Cr, Mn,
Co, Ni, Cu, Zn, Zr, Mo, Ag, W, Pt, Au, and Pb) in sulfides present in samples
representative of different stratigraphic levels and chronological stages of the
hydrothermal alteration within the ophiolite complex. Preliminary results show a large
variability of metal concentration and behavior depending on the lithology. Igneous
sulfides present in least-altered metabasalts from the upper part of the crust are notably
enriched in Co, Ti, Cu, Pb, Mn and Ag. They likely contain most of the bulk metal
budget of the primary lavas. Importantly, numerous sulfides are still preserved in the
epidotized sheeted dike complex, and remain enriched in Co, Ti, Pb, Mn (while Cu and
Ag appear to have been remobilized). The sulfide-bearing quartz veins corresponding to
the discharge upwelling of hydrothermal fluids show very low metal concentrations,
with the exception of Ni, Pb and Cu. Finally, the massive sulfide deposits are made of
Cu-, Zn-, and Co-rich pyrite and Pb-, Mo-, Zn- (and Ti-) rich chalcopyrite. These
observations suggest that sulfides may have a significant role in controlling the
progressive release of some metals during the different steps of hydrothermal alteration,
and need to be considered in conjunction with the secondary silicate minerals.
[1] Jowitt S.M. et al. (2012). J. Geochem. Explor., 118, 47-59.