Oceanic anoxic events, subduction style and molybdenum mineralization

Abstract

Nearly half of the world's Mo resources are hosted in Cenozoic porphyries along the east Pacific margin. In contrast, there are essentially no large Mo porphyry deposits along the west Pacific. We propose that this uneven distribution is mainly due to Oceanic Anoxic Events (OAEs) and the different subduction regimes in the eastern and western Pacific margins. Molybdenum is rare in the Earth, with an abundance of 50 ppb in the primitive mantle, and 0.8 ppm in the continental crust. Pre-enrichment is needed for such a moderately incompatible element to form porphyry Mo deposits. The oxidation–reduction cycle on the Earth's surface is the major process that concentrates Mo. Molybdenum was more intensively oxidized under high erosion rates during alternating high-atmospheric-CO2 and high-oxygen periods immediately before OAEs, resulting in high Mo concentrations in surface water and subsequently higher Mo concentrations in organic-rich sediments during the OAEs. Large amounts of Mo-enriched sediments formed on the Pacific Ocean floor during at least 9 major OAEs since the Late Jurassic. Given that Mo comes from chemical weathering, far more Mo-enriched sediment was available in the east Pacific because of the erosion of the continental arc. In contrast, due to backarc extension, erosions in the Western Pacific margin was less extensive, resulting in less Mo in OAE sediments on the Pacific Ocean floor. Importantly, the Eastern Pacific plate has been mostly subducted, with only Cenozoic crust left. During plate subduction, large amount of the Mo-rich sediments in the Eastern Pacific have been taken down to the mantle wedge. Subsequently these sediments were metamorphosed and then Mo was transferred to porphyry deposits through partial melting. In contrast, only a small amount of the sediment in the Western Pacific has been subducted, with most of the Jurassic and Cretaceous oceanic crust well preserved. Molybdenum deposits of the Eastern Pacific margin can be classified into three types: porphyry-Cu-Mo, high-F porphyry and low-F porphyry-Mo deposits. Large and superlarge porphyry-Cu-Mo deposits are usually associated with ridge subductions, and thus formed through partial melting of subducted oceanic crust and metamorphosed Mo-rich sedimentary material due to subduction erosion. The high-F porphyry-Mo deposits (e.g., Climax-type) were likely formed by partial melting of subducted and thus metamorphosed Mo-enriched sediments during slab rollback (usually associated with A-type granite). Low-F porphyry-Mo deposits were formed through direct partial melting of metamorphosed Mo-enriched sediments entrained into the mantle wedge through subduction erosion at slightly shallower depths (usually associated with arc granite). Porphyry-Cu (Au) deposits in the southwest Pacific margin are all associated with subduction of backarc basins younger than the last OAE (55.8 Ma). Therefore, porphyry Cu (Au) deposits in the southwest Pacific margin have no economic levels of Mo. The oxygen fugacity of northwest Pacific is lower than ΔFMQ +2, which inhibits the formation of porphyry deposits.

Keywords

• Porphyry Mo deposits
• Oceanic anoxic events
• Weathering
• Pacific
• Cretaceous
• Jurassic: black shale
• Subduction erosion