Multiple sources for ore-forming fluids in the Neves Corvo VHMS Deposit of the Iberian Pyrite Belt (Portugal): strontium, neodymium and lead isotope evidence

Abstract

Lead, Rb–Sr, and Sm–Nd isotopes have been used to constrain the sources and the timing of mineralisation at the Neves Corvo VHMS deposit of the Iberian Pyrite Belt (IPB). Sulfide- and cassiterite-rich ores, together with a mineralised felsic volcanic rock, yield a Rb–Sr errorchron age of 347±25 Ma with an initial 87Sr/86Sr=0.71031±65. The Rb–Sr results agree with palynological age constraints for ore formation at Neves Corvo, and are indistinguishable from several other mineralisation ages, indicating that major orebodies in the IPB formed coevally at ~350 Ma. In contrast, wide variations in initial 143Nd/144Nd indicate limited rare-earth-element redistribution during ore deposition. Initial eNd (350 Ma) values range from –0.2 to –9.7 for copper ores, –8.9 to –9.4 for copper-tin ores, and –6.9 (cassiterite) to –9.5 for tin ores, implying that neither the IPB volcanic host rocks [eNd (350 Ma) >–2.1; 87Sr/86Sr0<0.70664], nor contemporaneous seawater (87Sr/86Sr0 ~0.708), could have been the exclusive sources for the Neves Corvo ores. Distinct mixing arrays in the eNd (350 Ma)–Sm/Nd and 206Pb/204Pb–207Pb/204Pb diagrams demonstrate that sulfide and tin ore deposition involved ore-forming solutions from different sources. Whereas sulfide-ore compositional variations are consistent with significant incorporation of "typical" IBP volcanic-seawater derived hydrothermal components, the highly radiogenic lead and (exclusively) low-eNd values preserved in tin ores require a predominant derivation from external sources. This could be either a magmatic source (which must have been different from the typical IPB felsic magmas), or a metamorphic fluid deeply circulated through older basement rocks.