Volcanogenic massive sulphide (VMS) deposits have been recognized both in fossil and present-day settings (e.g. mid-ocean ridges (MORs), back-arcs, island-arcs, fore-arcs) and are associated with different lithologies leading to variable metal enrichments. More recently, a sub-type of VMS associated with ultramafic rocks has been discovered at MORs. These ultramafic-hosted VMS (UM-VMS) form in genetic relationships with detachment faults exhuming mantle rocks and are commonly enriched in base (Cu, Zn, Ni), critical (Co) and precious (Au, Ag) metals. However, they are thought to be scarce in the geological record since they are unlikely to obduct from MOR settings.
We propose, based on an extensive review of worldwide UM-VMS deposits described in ophiolites, that this scarcity is only apparent. Previously, UM-VMS have been commonly misclassified for three main reasons: i) the tectonic settings in which they form has been misinterpreted (e.g. tectonic mélanges), ii) their origin may be disputed (hydrothermal vs. magmatic) and iii) orogenic-related metamorphism and deformation locally obliterated seafloor-related mineralogical and structural features. Also, the strong focus on UM-VMS formed in MORs prevented to recognize them in other settings such as ocean-continent transition or supra-subduction zones which are more easily preserved in the geological record. Here, we discuss discriminant features applied to fossil UM-VMS worldwide which allow us to classify them as such. We show that UM-VMS are not as scarce as previously thought and, hence, represent possible undiscovered metal resources. Further genetic and exploration models are needed for new discoveries.
Clifford Patten1, Rémi Coltat2, Malte Junge3, Alexandre Peillod4, Marc Ulrich5, Gianreto Manatschal5, Jochen Kolb1
1Institute of applied geochemistry, KIT, Germany; 2Laboratoire de Géologie, CNRS-UMR 8538, Ecole Nationale Supérieure de Paris, France; 3Mineralogical State Collection Munich, Germany; 4Department of Geological Sciences, Stockholm University, Sweden; 5Institut Terre et Environnement de Strasbourg, CNRS-UMR 7063, Université de Strasbourg, France