Hydrothermal Alteration in the Subsurface of an Oceanic Ultramafic-Hosted Hydrothermal System (as Preserved in Ligurian Ophiolites, Italy)

Abstract

Geochemistry and petrology of serpentinized peridotites from the Ligurian Ophiolites, Italy were analyzed to investigate alteration processes in the subsurface of an ocean detachment fault. Three dominant processes were identified: 1) Serpentinization of peridotites preserves a primary seafloor hydrothermal signature, with elevated light rare earth element (LREE) contents and 34S values; 2) Talc alteration + mineralization associated with a fault and high temperature fluids (~350 C) that resulted in silica metasomatism, elevated LREE + metal contents, and deposition of a massive sulfide deposit; and 3) Carbonate replacement + precipitation related to low-temperature hydrothermal fluids (20 C – 125 C) that were focused in the fault zone resulting in intense metasomatism: including enrichments of REE, metals, silica and calcium. The alteration in this on-land proxy to ocean crust constrains fluid pathways that are poorly understood on the seafloor because of difficulties sampling on the seafloor. In particular, results show that normal faults that cut the detachment surface act as conduits to transport fluids from deeper in the subsurface to the ocean floor. These results further understanding of the controls on fluid pathways and processes such as volcanogenic massive sulfide (VMS) formation and carbon sequestration in ultramafic- hosted hydrothermal systems.