Magmatism and ore deposit formation in SW Pacific island arcs.

摘要

Majority of the richest base and precious metal deposits on Earth are located in the Circum-Pacific region and are found either in a close proximity or within subduction-related volcano-magmatic complexes. It is not clear, however, why some of the volcanic centers contain ore deposits but others do not. This study utilizes a novel method for high-precision Pb isotopic measurements by multi-collector-ICP-MS and reveals a genetical connection between the ore-formation and magmatism in the Tabar-Lihir-Tanga-Feni (TLTF) island arc, SW Pacific. The developed model suggests that injection of a volatile-rich magma into an evolving magma body near the surface is the triggering event that ultimately results in the ore mineralization.; Isotope data for mantle xenoliths suggest that three possible end-members, including Pacific and Indian mantle, and Pacific sediments, can control the chemistry of the magmas in the region. Comparison between the isotopic compositions of lavas, and crustal and mantle xenoliths indicate that the incompatible elements in the TLTF magmas, including a major part of the ore metals, were ultimately derived from a subducted oceanic slab with Pacific mantle affinity. Regional and local isotopic trends in the volcanic rocks in the region indicate that subducted oceanic slabs overall control the composition of the island-arc magmas. Once the contribution from the subducting slab decreases, incorporation of a mantle component with Indian affinity can be identified in the isotopic composition of the lavas in the region.