Volcanogenic Massive Sulfides (VMS) Deposits

These valuable sources of copper and zinc form in zones of submarine extensional tectonics where magma rises to the seafloor, erupting huge quantities of pillow lavas. The combination of extensional structures and focussed heat flow drives convection of seawater through the fractured oceanic crust. The water becomes heated, acidified and stripped of its oxygen by rock–water interaction, causing it to leach finely dispersed metals from the lavas. During pauses in the volcanism, focussed discharge of the hydrothermal fluid at the seafloor creates the famous "black smokers", i.e. chimneys formed by rapid precipitation of iron, copper and zinc sulfides (and sometimes gold) upon cooling of the fluid as it mixes with normal seawater.

We are investigating on-land examples of VMS deposits in the Sultanate of Oman. There, around 90 million years ago, jostling tectonic plates thrusted a slice of oceanic lithosphere onto the Arabian continental margin. Subsequent uplift and erosion exposed this slice ("the Semail ophiolite") in what is now the Oman mountains, revealing a complete cross-section through the oceanic lithosphere.

Exploration companies wondered why some of the VMS deposits in the Semail lavas contain significant gold, whereas others do not. During the PhD project of Samuel Gilgen (Gilgen et al., 2014) we addressed this question by carrying out field work and sampling in the Semail ophiolite.

Geochemical analyses of our samples showed that, contrary to earlier interpretations, VMS deposits formed not only during the early spreading-ridge volcanism (Geotimes Unit) but also during the later suprasubduction zone Alley and Boninitic Alley volcanism. This finding implies that all the lavas in the ophiolite warrent careful exploration.

Moreover, we found that the gold-bearing deposits are concentrated in the uppermost Boninitic Alley lavas, whereas the deposits richest in copper occur in the deeper Geotimes lavas. It follows that targeting the Boninitic Alley lavas should allow exploration companies to find VMS deposits that are "sweetened" in value due to their gold content.

This project was funded by the Swiss Science Foundation (SNF) and conducted in collaboration with active mining companies, with the Oman Directorate General of Minerals and with Omani geological consultancy Earth Secrets.