PRIMARY AND SECONDARY PHOSPHATE MINERALS AS INDICATORS OF APATITE METASOMATISM AND REE MOBILITY WITHIN THE “TIN CORRIDOR” AT THE NEVES CORVO DEPOSIT (IBERIAN PYRITE BELT)

The Neves Corvo Cu-Zn-(Sn) deposit is rather notable by its size, Cu-Zn grades and tonnages, as well as the occurrence of Sn (in the form of cassiterite) mineralization. Both Sn and sulfide mineralization are hosted by felsic volcanic rocks and black shales of Upper Devonian to Early Carboniferous age. The Sn mineralization was structurally controlled and predated the main massive sulfide (Cu-Zn-Pb) mineralization along the “tin corridor” (Corvo orebody).

This research documents the presence of phosphate minerals (including apatite, florencite [LREEAl₃(PO₄)₂(OH)₆], and xenotime) associated with the Sn mineralization at Neves Corvo. We present a comprehensive set of in-situ measurements (microprobe and LA-ICP-MS). The occurrence of phosphate minerals is confined to the stockwork zone, with apatite being the predominant phosphate mineral. Apatite is found in association with quartz and/or cassiterite, forming aggregates of irregular, unzoned grains with dissolution rims, high porosity, and fractures, often hosting Na-sericite, chlorite, and/or florencite. Minute inclusions of cassiterite, pyrite, and chalcopyrite are also present. In contrast, florencite occurs either as inclusions in apatite or along late veinlets that crosscut the cassiterite. In the latter, florencite is associated with chlorite, sericite, chalcopyrite, and pyrite. Xenotime is rare, and its relationship with the other phosphates is unclear.

The analyzed apatites correspond to fluorapatites with excess Ca and a P deficiency, which indicates a shift from fluorapatite towards carbonate-fluorapatite endmember. In general, apatite has low trace element contents except for Y, Sr, S, and MREE (100-1000s ppm) and bell-shaped (MREE enriched) chondrite (C1) normalized REE pattern. The strong deformation, textural relationships observed, and REE patterns are suggestive of fluid-induced metasomatism. Depleted LREE patterns suggest that apatite was altered in the presence of Al-rich and NaCl-bearing hydrothermal fluids, resulting in the formation of Ce-rich florencite (± xenotime). This alteration is thought to be linked to the distal Na-metasomatic alteration halo associated with the stringer and massive sulfide stage (Lower Corvo feeder system) or the 320-300 Ma tectonic-metamorphic event that led to extensive Cu remobilization at Neves Corvo.