Metasomatized mantle facilitates the genesis of magmatic nickel–copper sulfide deposits in orogenic belts: A copper isotope perspective

Abstract

The role of metasomatized mantle in the genesis of magmatic Ni–Cu sulfide deposits in orogenic belts remains vigorously debated. The greatest challenge in clarifying uncertainties is how to track the behavior of metals during metasomatic processes in the mantle. Copper isotopes can shed new light on this enigmatic topic as the transfer of Cu during metasomatic processes can significantly fractionate Cu isotopes. In this regard, we have characterized the Cu isotope and trace-element compositions of fertile and barren mafic–ultramafic intrusions, and coeval basalts in the Eastern Tianshan, Central Asian Orogenic Belt (CAOB). Fertile mafic–ultramafic intrusions are characterized by elevated and highly variable Ba/Nb (40–756, average of 172) and Ba/La (16–219, average of 48) ratios, and exhibit δ65Cu values (−0.97 ‰ to +0.57 ‰) that vary over 1.54 ‰. In contrast, barren intrusions and coeval Permian basalts are characterized by relatively low and invariable Ba/Nb (2–95, average of 29) and Ba/La (0.9–26, average of 8) ratios, and have δ65Cu values that are comparatively less variable (−0.06 ‰ to +0.49 ‰ for δ65Cu) and more similar to bulk silicate Earth (BSE, δ65Cu = 0.06 ± 0.20 ‰). The difference in Cu isotope compositions of fertile and barren intrusions, alongside the variability in Ba enrichment, suggests that a high degree of mantle metasomatism promotes the fertilization of primary magmas in metals and, thus, represents one of the first stages in the development of magmatic Ni–Cu sulfide deposits in convergent margins.