Ultramafic-hosted volcanogenic massive sulfide deposits: an overlooked sub-class of VMS deposit forming in complex tectonic environments

Abstract

Ongoing seafloor exploration highlights that ultramafic-hosted seafloor massive sulfide deposits (UM-SMS) are ubiquitous along slow to ultra-slow spreading ridges. These deposits are closely linked to low-angle detachment faults, are hosted in variably altered ultramafic rocks and are enriched in precious (Au-Ag), critical (Co) and base metals (Cu-Zn-Ni). Ultramafic-hosted volcanogenic massive sulfide (UM-VMS) deposits, however, are commonly thought to be scarce on land but an extensive deposit review from the geological record challenges this assumption. These deposits occur in complex tectonic settings often referred to as “tectonic mélange”, and their origin is often disputed (e.g. hydrothermally overprinted magmatic Ni-Cu massive sulfide) due to a late orogenic overprint that partly erase and/or overprint earlier seafloor-related features. Because of the uncommon host rocks, ore mineralogy and tectonic environment and the limited awareness of scientific and industrial communities, these deposits are difficult to interpret. Furthermore, although seafloor exploration greatly improved our understanding of these deposits, it has also induced a strong bias by focusing almost exclusively on mid-ocean ridge (MOR) environments. Ultramafic-hosted VMS deposits in the geological record, however, mostly occur in settings more recently interpreted as ocean-continent transitions (OCT) and/or supra-subduction zones (SSZ). Ophiolites formed in these environments are more likely to be tectonically emplaced during collision favouring the preservation of the UM-VMS deposits in the geological record. In this study we review more than sixty deposits present worldwide in the geological record and discuss the key features, which enable us to unambiguously classify them as ultramafic-hosted VMS deposits. Because of their unique nature and metal endowment, the ultramafic-hosted VMS deposits should be considered as a stand-alone sub-group of VMS deposits, which possibly represent untapped mineral resource with high future potential. Their discovery, however, requires specific, robust and tailored exploration and genetic models, which need to be further developed. Finally, studies of UM-VMS deposits in the geological record highlights that ultramafic-hosted hydrothermal systems on the seafloor can occur in very diverse plate tectonic environments and should drive further seafloor exploration in less well explored tectonic environments such as ocean-continent transition and supra-subduction zone.