SUBDUCTION-MODIFIED LITHOSPHERE AS A SOURCE FOR POST-SUBDUCTION MAGMATIC-HYDROTHERMAL ORE DEPOSITS (Invited Presentation)

Hydrous magmas are the key ore-forming agents in a number of mineral deposit types, including porphyry, skarn, greisen, pegmatite, epithermal, and some types of IOCG deposits. Arc magmas related to subduction are the most common progenitors of many of these magmatic-hydrothermal ore deposits, but an increasing number of otherwise similar deposits are now recognized to have been formed from magmas generated after subduction has ceased, in some cases many millions of years later (e.g., Bingham: Pettke et al.: Earth Planet. Sci. Lett., 2010).

Subduction magmatism is a geochemically open-system process, rooted in partial melting zones in subduction-metasomatized mantle wedge, passing through important fractionation and crustal interaction stages in MASH zones at the base of the crust (Hildreth & Moorbath: Contrib. Mineral. Petrol., 1988) and batholithic zones in the mid- to upper crust, before eventual eruption of ~20% of the total magma flux at the surface (Carmichael: Contrib. Mineral. Petrol., 2002). Although the bulk of the subduction-derived volatiles such as H₂O, S, and Cl are released in the volcanic and subvolcanic environment, where they may cause the formation of various magmatic-hydrothermal ore deposits, Davidson et al. (Geology, 2007) estimated that amphibole cumulates formed in lower crustal MASH zones may retain up to 20% of the total arc magmatic water flux. Such cumulates may also retain a significant proportion of the chalcophile and siderophile metal flux in residual sulfide phases, as well as the trace element and oxidation state characteristics of the original arc magmas.

Unlike subduction-metasomatized asthenosphere, which is rapidly convected away following the cessation of subduction, subduction-modified lithosphere remains in place, and represents a potentially fusible source for later magmas generated by other tectonic processes, such as rifting, crustal thickening, or delamination of the sub-continental mantle lithosphere. Such tectonic events may occur within a few million years following the termination of subduction due to broadly related processes, or may occur at any later time, such as during continental rifting. Magmas derived from such sources will share many characteristics of arc magmas, including their potential to form magmatic-hydrothermal ore deposits.