Lower crustal copper enrichment and mobilization indicated by Late Permian amphibole gabbros and granodiorites in the eastern Kunlun Orogen, northern Tibet

Abstract

Lower crustal fractional crystallization accompanied by sulfide saturation in thick continental arcs has been viewed as an essential step in forming porphyry Cu systems. These processes are supposed to lock Cu in the lower crust and, therefore, can be unfavorable to the generation of ore-forming magmas. In this study, we present in situ mineral major-trace elements, pyrrhotite S isotopes, and zircon UPb ages, as well as whole-rock major-trace elements and SrNd isotopes for amphibole gabbros and contemporary granodiorites in the eastern Kunlun Orogen to elucidate Cu variations during the transcrustal magmatic evolution of these rocks. The amphibole gabbros can be divided into two types based on field relationships with the granodiorites. These rocks were derived from an oxidized metasomatic mantle in a steep subduction setting at ∼250 Ma and experienced a transcrustal magmatic evolution. Orthopyroxene relicts, resorbed clinopyroxene, calcic plagioclase, high-Al2O3 amphibole, and magnetite in the amphibole gabbros of the first type are lower crustal cumulate minerals. Pyrrhotite with minor amounts of chalcopyrite, interstitial to cumulate amphibole or as inclusions in magnetite from the amphibole gabbros of the first type, is lower crustal Ni-rich monosulfide solid solution (mss) crystallized from immiscible sulfide melts with residual Cu-rich sulfide liquids. These lines of evidence together with field, mineralogical, and geochemical data indicate that the Cu-rich parental magmas experienced lower crustal MASH (melting, assimilation, storage, and homogenization) processes and became Cu-depleted. The evolved magmas after lower crustal evolution became moderately oxidized and Cu-normal according to amphibole, biotite, zircon, and whole-rock compositions. The moderately oxidized magmas scavenged and mobilized Cu from Cu-rich sulfide liquids to the upper crustal magma chambers where degassing dispersed the Cu and thus was unfavorable to porphyry Cu mineralization.