METALLOGENIC EVOLUTION OF CONVERGENT MARGINS: SELECTED ORE DEPOSIT MODELS

Abstract

Metallogenic patterns in convergent margin arcs are related to thickness of the arc crust and its petrologic evolution, which are themselves linked by a positive correlation between average silica content of arc rocks and average thickness of arc crust. In general, tin, mercury, antimony and fluorine are most abundant in areas of thickest crust. Although the composition of ore deposits is strongly controlled by deep crustal and mantle features, the actual form and type of many ore deposits is controlled by upper crustal features, particularly the relative abundance of carbonate and silicate country rock. For instance, epithermal precious metal deposits are commonly associated with subaerial, silicic volcanism, which generally forms in the later stages of arc evolution. Deposits of this type will be absent if arc volcanism is terminated prematurely or removed if subduction of thickened or hot oceanic crust causes extreme uplift and consequent erosion of the upper parts of the arc. Similarly, chimney-manto limestone replacement lead–zinc–silver deposits are common only in terranes with extensive thick limestone units, which are commonly formed in shallow shelf environments. Several important types of convergent margin deposits remain poorly understood. Acid-sulfate deposits in primitive oceanic volcanic arcs are associated with bimodal volcanic suites that appear to have formed at an early stage in the evolution of the arc. Alkaline porphyry copper deposits are closely associated with calc-alkaline porphyry copper deposits, commonly in the same volcanic arc, although they appear to have formed farther back from the trench or during periods of oblique subduction or arc reversal. Massive iron oxide deposits and related base and precious metal mineralization are closely associated with silicic magmas that form during later stages of arc evolution. In addition to tectonic and petrologic factors, metallogenic patterns probably reflect global forcing mechanisms, the most important of which are global seawater anoxia and plate reorganizations. Global anoxia produces deep water environments that favor sedex mineralization and can promote subsurface ore deposition where anoxic bottom waters recharge underlying aquifers. Plate reorganizations can lead to ore deposition by providing extensional environments in the upper crust and possibly by facilitating partial melting of oceanic crust that has been modified by earlier subducting slabs.