What processes at mid-ocean ridges tell us about volcanogenic massive sulfide deposits

Episodic seafloor spreading, ridge topography, and fault movement at ridges find (more extreme) analogs in the arc and back-arc setting where the volcanogenic massive sulfide (VMS) deposits that we mine today were formed. The factors affecting sulfide accumulation efficiency and the extent to which sulfides are concentrated spatially are the same in both settings, however. The processes occurring at mid-ocean ridges therefore provide a useful insight into those producing VMS deposits in arcs and back-arcs. The critical observation investigated here is that all the heat introduced by seafloor spreading at mid-ocean ridges is carried out of the crust within a few hundred meters of the ridge axis by ∼350°C hydrothermal fluids. The hightemperature ridge hydrothermal systems are tied to the presence of magma at the ridge axis and greatly reduce the size and control the shape of axial magma intrusions. The amount of heat introduced to each square kilometer of ocean crust during its formation can be calculated, and its removal by high-temperature convection allows calculation of the total base metal endowment of the ocean basins. Using reasonable metal deposition efficiencies, we conclude that the ocean floor is a giant VMS district with metal resources >600 times the total known VMS reserves on land and a copper resource which would last >6,000 years at current production rates. Introduction Volcanogenic massive sulfide (VMS) deposits are a particularly interesting kind of ore deposit because, viewed broadly, they are the direct product of one of the most important global processes. They form in areas of lithosphere extension where partial melts of the mantle produce new crust. Heat introduced by the melts circulates seawater through the crust, and the intimate interaction of the seawater with recently quenched and thermally fractured melt extracts metals. Over the last 50 years, we have learned a great deal about VMS deposits by observing hydrothermal systems in areas of extension at midocean ridges and back-arc spreading centers where the ancient deposits that we have long mined on land are in the process of forming and by comparing this information with the ore deposits and geothermal systems that we are mining or exploiting on land. Many disciplines (geophysics, geochemistry, petrology, heat and mass transfer, numerical methods, economic geology, and biology) and several different communities (geothermal, ore deposit, ocean, modeling) have been involved, and large literatures have developed in which many issues remain in vigorous contention and between which (and between older and recent literature) there is often incomplete exchange. Despite excellent and recent reviews (e.g., Franklin et al. 2005; Hanington et al. 2005), it remains difficult to distinguish the fundamental from the interesting details. The purpose of this paper is to articulate a perspective of what is fundamental from the hydrothermal/ ore-formation point of view. In this process, other viewpoints and interesting sidelights will be mentioned to provide entry points into other areas of interest, but this mention will be brief. The key to the analysis presented is that ∼350°C seawater convection can be seen to extract and discharge into the ocean, at the ridge axes, all the magmatic heat introduced into the Editorial handling: F. Tornos L. M. Cathles (*) Cornell University, Ithaca, NY 14853, USA e-mail: [email protected] Miner Deposita DOI 10.1007/s00126-010-0292-9