Origins of Si-Fe Cap Rocks at Extinct Seafloor Massive Sulphide (eSMS) Deposits from the TAG Hydrothermal Field (26 degrees N), Mid-Atlantic Ridge

Extinct seafloor massive sulphide (eSMS) deposits represent an understudied phenomena of modern seafloor hydrothermalism, and are thought to be a potential resource for base and precious metals if exploitation of seafloor mineral resources becomes economically viable in the future. The transition from active to inactive mounds poses important, but as of yet, unanswered questions about their preservation after hydrothermal venting ceases and oxygenated seawater circulates. This has the potential to destroy the metal tenor in SMS deposits, unless they are somehow protected. Here, we show the common occurrence of a silica-rich 'jasper' layer that forms the interface between unaltered sulphide below and oxidized metal-rich sediments above. The jasper layer is up to several m-thick and was encountered, in some form, at each of three extinct SMS deposits drilled in the TAG are of the Mid-Atlantic Ridge. The silicification events which have created these capping materials result in the decrease in permeability and is likely a common process during the waning stages of a hydrothermal cycle. As such, the Si-Fe cap could be a common product at eSMS deposits, and potentially provide an auto-preservation mechanism, restricting oxygenated seawater ingress and halmyrolysis of eSMS deposits