Hydrothermal phosphate vein-type ores from the southern Central Iberian Zone, Spain: Evidence for their relationship to granites and Neoproterozoic metasedimentary rocks

Abstract

Hydrothermal quartz–apatite veins, called “Iberian-type”, occur exclusively in the southern Central Iberian Zone. This study presents a multidisciplinary approach leading to the mineralogical, fluid inclusions, and geochemical characterization of these veins from two representative areas, Logrosán and Belvís–Navalmoral, in order to establish their relationship with nearby phosphorous-rich granites and apatite in Neoproterozoic metasediments. The mineral assemblage of these veins comprises quartz, apatite, minor sulfides, dolomite, Fe–Mg-carbonates and Fe–Mn-oxides. Three texturally different types of apatite have been recognized in the veins: apatite I showing prismatic habit associated with minor sulfides, apatite II occurring as white fibrous radial crystal aggregates called “dahllite”, and apatite III as grayish to greenish hexagonal crystals. Hydrothermal apatite from veins (H-apatite) is enriched in Sr and depleted in Mn–Y–REE–Th–U–Pb compared to magmatic apatite (M-apatite) from the granitic plutons. However, trace element compositions of apatite from metasedimentary phosphorous-rich levels or nodules (S-apatite) of the Schist–Greywacke Complex show similar characteristics to the H-apatite, although the H-apatite stands out for its relatively high Sr-contents. This relative Sr enrichment in H-apatite is interpreted as inherited from both phosphate in Neoproterozoic metasediments (S-apatite) and carbonate levels. REE, fluid inclusion and stable isotope data are consistent with a long episode of hydrothermal activity implying cooling and dilution processes while interacting with phosphate-rich shale and carbonate beds in the SGC. Fluid inclusion study undertaken on hydrothermal apatite and quartz reveals the presence of aqueous low salinity fluids (0.2–6.7wt.% NaCl equiv.) at moderate to low Th (125–350°C). All available data point at a recycling event of the southern CIZ metasediments (the SGC) during post-Variscan hydrothermal fluid circulation as the more plausible origin of the phosphate vein-type mineralizations.