Preliminary work

Preliminary work conducted during the Melts-Fluids-Models (MFM) project funded by the first funding period of DOME suggests a ubiquitous occurrence of phosphate minerals (apatite, monazite, xenotime, florencite), hydrothermal zircon, and TiO2-polymorphs (Fig. 1), in volcanic rocks associated with VMS mineralization in the Iberian Pyrite Belt (IPB). At the Sesmarias prospect (Alvalade property detained by Avrupa Minerals), these phases are present in all three types of volcanic rocks identified (mafic, felsic, and mafic intrusive) as well as in volcaniclastic rocks. There is a clear predominance of apatite and TiO2-polymorphs in mafic (Fig. 1a) and volcaniclastic rocks, which may be related to a local higher availability of P and Ca for apatite formation. This is in line with experimental studies that showed that P solubility in volcanic rocks decreases with the increase of silica, e.g., [1]. Moreover, partial or total dissolution-reprecipitation of Ti-rich phases (e.g., magmatic titanite, ilmenite, rutile) to form TiO2-polymorphs and other Ti-rich phases (e.g., brookite, pseudobrookite) is a common feature. Even though apatite and TiO2-polymorphs are less common in felsic rocks, when present, they usually form clusters or occur along alteration planes with zircon (Fig. 1c, d). It is likely that due to high solubility in hydrothermal fluids, zircon and Ti-rich phases dissolve and reprecipitate locally, forming minute TiO2-polymorphs and zircon (or zirconia) during the hydrothermal alteration (Fig. 1c, d). Moreover, the coexistence of TiO2-polymorphs, apatite, and sulfide mineralization has been observed in one sample (Fig. 1b), where textural evidence suggests that apatite forms together with pyrite overgrowths and sericite.

At Neves Corvo, only samples with tin mineralization from the Corvo orebody have been studied [2], and therefore, it is unclear whether the volcanics also have considerable amounts of phosphate minerals and Ti-rich phases. In samples from the stockwork (Sn stringer mineralization), apatite coexists with cassiterite, quartz, and pyrite [2]. Late tectonic-metamorphic events are responsible for the fracturing of the grains (Fig. 1e), and the fractures are filled by a new generation of quartz and remobilized-reprecipitated chalcopyrite, stannite, sphalerite, and galena (not shown). One sample is also marked by the occurrence of xenotime associated with cassiterite, quartz, and minor pyrite and chlorite (Fig. 1f). The samples investigated are hosted by volcaniclastic rocks and black shales, and therefore, it is speculated that Y, REE, and P are mobilized from the host units into the stockwork during hydrothermal alteration. In addition, Florencite [REEAl3(PO4)2(OH)6] has been observed in the stockwork veinlets close to the contact with the host rock. Phosphates at Neves Corvo, in particular apatite and xenotime, have been identified for the first time during the MFM project; minor amounts of florencite were also reported by [3].