Lithospheric thickness controls the porphyry Cu mineralization: Evidence from neighboring arc volcanic rocks

• High-Al basalts in the Dananhu and Yamansu arcs originated from garnet peridotite and spinel peridotite sources. • The arc lavas were products of divergent double subduction of paleo-Tianshan oceanic slab during Carboniferous. • Magmas differentiation in the thickened arc lithosphere is important for porphyry Cu mineralization. It is widely considered that porphyry Cu deposits formed via oceanic slab subduction are closely associated with hydrous and oxidized arc magmas. Of note, two suites of neighboring (∼40 km apart) Carboniferous arc volcanic rocks in Northwest China show different extents of mineralization: volcanic rocks from the Dananhu arc (DNHA) host one of the most important porphyry Cu deposit belts in China, whereas those from the Yamansu arc (YMSA), adjacent to DNHA, are ore-barren. These arc volcanic rocks, thus, provide a precious opportunity to explore the main factor that controls the genetic links between coeval arc lavas and porphyry Cu mineralization. Here we report whole rock major and trace element compositions and Mg–Sr–Nd–Pb isotopic data, generating a comprehensive geochemical comparison for these two suites of volcanic rocks from basalt to dacite. The whole-rock geochemical analyses suggest that at a given SiO 2 content, the YMSA basalts show lower MgO, CaO, Fe 2 O 3 T , and TiO 2 contents than the DNHA basalts. The DNHA volcanic rocks have higher Sr/Y and (La/Yb) N ratios, which are positively correlated, indicating that these two suites of rocks were derived from different magma sources. The DNHA rocks are characterized by radiogenic Pb isotopic compositions with 206 Pb/ 204 Pb up to 1 9.4 57, clearly distinct from the YMSA volcanic rocks with less radiogenic Pb isotopic compositions ( 206 Pb/ 204 Pb = 18.1 46–1 8.4 87), suggesting variable assimilation of crust-derived components during magma evolution. The δ 26 Mg values of the DNHA rocks (−0.3 5 ‰ to +0.0 6 ‰) are largely similar to those of the YMSA rocks (−0.2 4 ‰ to +0.0 4 ‰), and both sets of isotopic ratio ranges have tendency toward heavy Mg isotopes, which could be attributed to serpentinite-derived high- δ 26 Mg fluids in their mantle sources. Both suites of arc lavas have constant Cu contents and Cu/Sc ratios, indicating inconspicuous pre-enrichment of Cu contents. Geochemical comparisons indicate that the DNHA rocks were derived from partial melting of peridotite at the depth around the spinel-garnet transitional stability field, whereas the YMSA rocks were derived from partial melting of spinel peridotite, and the DNHA magmas had a thicker overlying plate than that of the YMSA magmas. The thickened arc lithosphere facilitates water-rich magmas accumulation and garnet fractionation, driving the magmas to become more oxidized, thereby preventing sulfide segregation and releasing sulfide-bound Cu. Thus, magmas differentiation in the thickened arc lithosphere is a key factor influencing porphyry Cu ore potential.