Tethyan mantle metasomatism creates subduction geochemical signatures in non-arc Cu–Au–Te mineralizing magmas, Apuseni Mountains (Romania)

Abstract

The history of processes that have affected the lithospheric mantle can be reflected in the chemical characteristics of magmas that are emplaced millions of years later during crustal extension. As a result, ore deposits normally associated with subduction can form in environments where they are not conventionally expected. Calc-alkaline magmatism in the Apuseni Mountains, Romania, occurred in response to Miocene extension and formed rich Au–Ag–Te epithermal and Cu–Au porphyry deposits. We present major, trace element and Sr–Nd–Pb isotopic data from a comprehensive suite of unaltered Apuseni samples. This dataset indicates that the Apuseni magmas were derived from partial melting of a lithospheric mantle source enriched in volatiles and incompatible elements. Mantle-like Sr and Nd isotopic signatures, combined with large ion lithophile element enrichments, Nb–Ta depletions, and Pb enrichments, are best explained by mantle refertilization during subduction. Subduction metasomatism of the Apuseni source region occurred at least 50Ma prior to extensional magma generation in the Miocene, most likely during NE-dipping subduction associated with closure of the Neotethys Ocean in the Mesozoic.We suggest that the different types of mineralization in the region are generally related to the degree of Miocene extension and consequent partial melting of the mantle source. Initial extension is correlated with the largest Au deposits in the Apuseni Mountains. The magmatic rocks associated with these deposits contain the most isotopically enriched Pb–Sr–Nd ratios, higher SiO2 and lower Mg-number, consistent with significant lower crustal influence during the initiation of extension and associated crustal anatexis. As extension progressed, increased mantle input resulted in magmas with more isotopically depleted Pb–Sr–Nd ratios, lower SiO2 and higher Mg-numbers. Uniquely rich Te-rich Au–Ag epithermal deposits are associated with these magmas, and are followed by porphyry Cu–Au deposits. Exceptional endowment in Te-rich epithermal deposits is common to other provinces associated with extensional magmatism preceded by much older mantle metasomatism, and we suggest that these deposits are ultimately linked to this metasomatism.