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

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 Sr-87/Sr-86 = 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 ore-bodies in the IPB formed coevally at similar to 350 Ma. In contrast, wide variations in initial Nd-143/Nd-144 indicate limited rare-earth-element redistribution during ore deposition. Initial epsilon (Nd) (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 [epsilon (Nd) (350 Ma) > -2.1; Sr-87/Sr-86(0) < 0.70664], nor contemporaneous seawater (Sr-87/Sr-86(0) 0.708), could have been the exclusive sources for the Neves Corvo ores. Distinct mixing arrays in the epsilon (Nd) (350 Ma)-Sm/Nd and Pb-206/Pb-204- Pb-207/Pb-204 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-epsilon (Nd) 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.