Magmatic fluids as a source of metals in seafloor hydrothermal systems

Abstract

Magmatic fluids may provide a major source of ore metals for the hydrothermal systems that produced sulfide chimneys and mounds on the modern seafloor and their ancient analogues, particularly the large volcanogenic massive sulfide deposits containing base and precious metals. This is evidenced by studies of the melt inclusions and vesicles of the volcanic rocks hosting the modern and ancient seafloor sulfide deposits, as well as recent observations of submarine volcanic eruptions and associated hydrothermal vents. Variation in the physical and chemical characteristics of magmatic fluids dominates the formation of distinct styles of volcanogenic massive sulfide deposits at convergent and divergent tectonic settings. Magmatic fluids are supercritical, mainly composed of CO 2 at the early stage of magma degassing and of H 2 O at the late stage. Various ore metals are transported as chloride and sulfide complexes in these fluids. The mixed CO 2 and H 2 O fluids, usually associated with felsic magmas, may separate into a low-salinity, low-density gaseous phase and a high-density saline liquid, the result of immiscibility as temperature decreases. Through pre-eruptive degassing, the magmatic fluids can be separated progressively into seafloor hydrothermal systems. Only a small amount (1 wt%) of metal-rich magmatic fluid would contribute over 85% of the total metals to form an ore body. Such magmatic fluids are most likely to be formed from volatile-rich felsic magmas that are prevalent at convergent plate margins and, if added to the normal circulation system, the fluids could be responsible for the formation of giant volcanogenic massive sulfide deposits, in similar manner to porphyry-type ore deposits.