METASOMATIZED LITHOSPHERIC MANTLE AN UNLIKELY SOURCE FOR PORPHYRY CU DEPOSITS IN THE SOUTHERN BASIN AND RANGE

The source of Cu in porphyry copper deposits remains a point of debate, with researchers highlighting roles for the subducting slab, mantle wedge, subcontinental lithospheric mantle and/or continental crust. Based on the Cu-isotope compositions of collision-related, fertile and barren porphyry copper deposits in southern Tibet, Zheng et al. (2019; https:// doi .org /10 .1130 /G45362 .1) proposed that the Cu in fertile porphyries was derived from a re-fertilized subcontinental lithospheric mantle. Their interpretation was based on the observation that fertile porphyries and associated mafic enclaves have both high Cu concentrations and high δ⁶⁵Cu values relative to barren intrusions from the same region. In their model, oxidizing fluids derived from the subducting oceanic slab permeate and metasomatize the mantle wedge prior to continental collision. Primary sulfides in the mantle wedge peridotite are oxidized and dissolved by these fluids, which then migrate into the overlying subcontinental lithospheric mantle where they precipitate as ⁶⁵Cu-enriched, sulfide-bearing hydrous veins or metasomes. Partial melting of the refertilized lithosphere then produces hydrous magmas with high δ⁶⁵Cu as well as the high fO2 required for formation of large porphyry Cu deposits.

We evaluate the relevance of this model to porphyry Cu deposits in the western US based on lithospheric mantle xenoliths from the Geronimo Volcanic Field of SE Arizona, USA. We have analyzed both modally and cryptically metasomatized Type I (Mg- and Cr-rich) lherzolites and Type II Fe-Al-rich pyroxenites, the latter interpreted as the crystallization products of basaltic magmas in the lithospheric mantle, i.e., physical examples of mantle metasomes. We observe that both Type I and Type II lithologies range to isotopically heavier compositions than asthenospheric mantle (i.e., up to +1.12‰ and +1.44‰, respectively, vs. ~0‰ ± 0.2‰). In support of the Zheng et al. model is our observation from the GVF ultramafic xenoliths that the highest Cu contents (up to ~ 80 ppm) are observed in the hydrous Type II assemblages and the Cu-isotope compositions of these rocks are skewed toward higher δ⁶⁵Cu (i.e., up to +1.44‰). However, the sulfides in these rocks are Cu-poor (98% with <1wt% Cu), suggesting the lithospheric mantle is an unlikely source of Cu for large porphyry copper deposits in this region.