Composition and Formation of Gabbro-Peridotite Hosted Seafloor Massive Sulfide Deposits from the Ashadze-1 Hydrothermal Field, Mid-Atlantic Ridge

Abstract

This paper presents mineralogical and geochemical data on seafloor massive sulfides (SMS) from the Ashadze-1 hydrothermal field at the Mid-Atlantic Ridge (MAR). The Ashadze-1 deposit is associated with the uplifted lower crust and upper mantle (oceanic core complex, OCC) of the MAR segment characterized by asymmetric mode of accretion. The OCC is represented by deep-seated gabbro-peridotite rocks exhumed on the rift valley slope along the detachment fault, during seafloor spreading. Hydrothermal processes in OCC environments result in different deposit composition and morphology compared to basalt-hosted systems. Abundant chimneys and enrichment in particular metals, including copper, zinc, gold, cobalt and tin are typical for this type of SMS deposit. The Ashadze-1 deposit is considered an example of a hydrothermal system in the initial stage of evolution marked by the young age of the sulfides (<7.2 kyr). The mineralogy of Ashadze-1 reflects primary ore-forming processes unaffected by post formation alteration. We propose a model for the primary ore-forming hydrothermal process in an ultramafic-hosted environment on the modern seafloor.