Vectors to ore in replacive VMS deposits of the northern Iberian Pyrite Belt: mineral zoning, whole rock geochemistry, and use of portable XRF

Abstract. Volcanogenic Massive Sulphide (VMS) deposits represent a major source of base, precious and other metals of economic and industrial importance. As in other mineral systems, progressive exhaustion of the shallowest and most easily accessible deposits is leading to increasingly complex exploration. In this context vectors to ore play a vital role. The Iberian Pyrite Belt (IPB) is an outstanding VMS district located in the SW Iberian Peninsula, which represents the main mining area in Spain and one of the main zones of base metal production in Europe. But the work on vectors to ore in the IPB is far from systematic or complete. In this work we have performed a detailed study of the main vectors to ore related to mineral zoning and whole rock geochemistry that are currently used in the exploration of VMS systems to a representative volcanic rock hosted replacive VMS deposit located in the northern IPB, the Aguas Tenidas deposit. Results have been compared to other deposits in the IPB and in other VMS districts. The investigated vectors include: mineralogical zoning, host sequence characterization and mineralized unit identification based on whole rock geochemistry, the study of the characteristics and behaviour of whole rock geochemical anomalies around the ore (e.g. alteration-related compositional changes, characteristics and extent of geochemical halos around the deposit), with definition of threshold values for the mineralization-related indicative elements, and application of portable XRF analysis to the detection of the previous vectors. In the footwall, a concentric cone-shaped hydrothermal alteration bearing the stockwork passes laterally, from core to edge, from quartz (only locally), to chlorite, sericite–chlorite, and sericite alteration zones. The hydrothermal alteration is also found in the hanging wall despite its thrusted character: a proximal sericite alteration zone is followed by a more distal albite one, which is described here for the first time in the IPB. Whole rock major elements show an increase in alteration indexes (e.g. AI, CCPI) towards the mineralization, with a general SiO2 enrichment, FeO enrichment in the central portion of the system, K2O and Na2O leaching towards the outside areas, and a less systematic MgO behaviour. Copper, Pb and Zn produce proximal anomalies around mineralized areas, with the more mobile Sb, Tl and Ba generating wider halos. Whereas Sb and Tl halos form around all mineralized areas, Ba anomalies are restricted to areas around the massive sulphide body. Our results show that proposed vectors, or adaptations designed to overcome p-XRF limitations, can be confidently used by analysing unprepared hand specimens, including the external rough curved surface of drill cores. The data presented in this work are not only applicable to VMS exploration in the IPB, but on a broader scale they will also contribute to improve our general understating of vectors to ore in replacive-type VMS deposits.