The Generation and Preservation of Mineral Deposits in Arc–Continent Collision Environments

Abstract

Intra-oceanic volcanic arcs are sites for major magmatic hydrothermal mineral deposits such as porphyry Cu–Mo–Au, epithermal Au–Ag and volcanic hosted massive sulfide (VMS) deposits. Arc magmas are largely generated from the mantle wedge as it reacts with components rising from the subducted slab. Variations in the tectonic framework of the arc, the nature of subduction and the components interacting with the mantle wedge will obviously have a direct effect on mineral deposit formation processes and the type of deposit resulting. Most mineral deposits found in fossil arc–continent collision zones are those that were formed in intra-oceanic subduction settings and which have been preserved as a result of the arc–continent collision event. Others appear to form either during the collision event or following it as the subduction zone changes polarity or jumps to a new position. At the loci of arc–continent collision, the first effects that may be seen are due to the ingress of continental material to the subduction channel. This may change the budget of contributed components from the slab to the mantle wedge and in some cases it appears that magmas enriched in metals such as gold could be generated. There is also some evidence that the physical effect of the ingress of the less dense continental crust itself could cause subduction shallowing or even stalling. Shallowing or stalling can lead to the generation of K-rich magmas which have been linked to the generation of magmatic Cu and Au deposits. During the collision, obduction of ophiolites from the overlying oceanic plate is a common feature and this may be accompanied by accretion of parts of, or even the entire volcanic arc. This will result in the preservation of mineral deposits such as ophiolitic Cr and VMS deposits that had already formed in the intra-ocean environment accreted to the continental margin. Arc reversal is a feature in some arc–continent collisions, and this effectively turns passive continental margins into sub continental subduction margins with an active continental arc. Continental arcs are important sites for porphyry Cu–Mo–Au and epithermal Au–Ag deposits and the arc reversal may result in melts being sourced from more fertile, previously metasomatised mantle resulting in larger and higher grade Cu and Au deposits. The actual collision itself is shown to result in the formation of orogenic gold deposits in some cases and finally post-collision extension can lead to the formation of VMS deposits in favourable settings.