Did Paleo-Tethyan anoxia kill arc magma fertility for porphyry copper formation?

Jeremy P Richards,A.M Celâl Şengör

doi:10.1130/g38954.1

Abstract

The Tethyan orogen is host to numerous porphyry Cu ± Mo ± Au deposits, but the majority formed during subduction of the Neo-Tethyan ocean basin in the late Mesozoic–Cenozoic; very few deposits have been found associated with Paleo-Tethyan subduction. We propose that this sparsity is due to widespread anoxia in the Paleo-Tethyan ocean basin, leading to the generation of relatively reduced arc magmas that were infertile for porphyry Cu formation. A compilation of published geochemical data indicates that Neo-Tethyan arc rocks have higher average Cu contents and V/Sc and Sr/Y ratios compared to Paleo-Tethyan rocks, indicating higher magmatic oxidation states and greater fertility for ore formation during Neo-Tethyan subduction. Subduction of relatively reduced oceanic lithosphere, or reduction of normal moderately oxidized arc magmas by interaction with reduced lithosphere, can therefore destroy the ore-forming potential of arc magmatic suites.

Highlights

Phanerozoic arc magmas are generally hydrous and moderately oxidized, reflecting their derivation from partial melting of metasomatized suprasubduction zone asthenospheric mantle wedge material (Kelley and Cottrell, 2009; Zellmer et al, 2015)
Because porphyry Cu deposits require relatively oxidized, hydrous magmas to form, it has been proposed that their almost exclusive global restriction to Phanerozoic arc rocks reflects the delay of deep ocean oxidation, and oxidative seafloor alteration and subduction zone metasomatism, until after the Neoproterozoic oxygenation event (Evans and Tomkins, 2011; Richards and Mumin, 2013)
Extensive periods of deep ocean anoxia have occurred during the Phanerozoic; one of the best documented cases is the Paleo-Tethyan ocean basin

Summary

Introduction

Phanerozoic arc magmas are generally hydrous and moderately oxidized, reflecting their derivation from partial melting of metasomatized suprasubduction zone asthenospheric mantle wedge material (Kelley and Cottrell, 2009; Zellmer et al, 2015). It has been suggested that more reducing conditions may have characterized Precambrian subduction zones and arc magmas, explaining the rarity of Precambrian porphyry Cu deposits (Evans and Tomkins, 2011; Richards and Mumin, 2013).

Results

Conclusion