Phosphate Fractionation in Pegmatites of Pedra Da Moura Ii Claim – Ponte Da Barca

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PHOSPHATE FRACTIONATION IN PEGMATITES OF PEDRA DA MOURA II CLAIM – PONTE DA BARCA – PORTUGAL

Leal Gomes,C. Azevedo, A. Lopes Nunes, J. Dias, P.A.

Universidade do Minho – CIG-R – Escola de Ciências – Gualtar-4710-057 Braga, Portugal

Keywords – mingling corridor, inner granite pegamtite, triphylite, oligonite, fractionation

INTRODUCTION bodies located along the most leucogranitic stripe of an inner granite, mixing-mingling The Pedra da Moura pegmatite group corridor, which was established between - in the proximity of Ponte da Barca village, two distinctive facies of two-mica granites Northern Portugal (Fig.1) - is a rosary like – one coarse grained, essentially biotitic alignment of irregular shaped pegmatite and the other porphyroid also biotitic.

Pedra da Moura II claim 1

2 BRAGA 3 4

5 6 OPORTO

N AVEIRO 20 km

Fig. 1 – Location of the studied pegmatite group within the Pedra da Moura II claim. Symbols:

1- major tectonic lineaments; 2 – Post- tectonic granites in relation to D3 Variscan folding phase;

3 - D3 late tectonic granitoids related with ductile shear; 4 - D3 syntectonic granitoids related with

ductile shear; 5 – two-mica granites sin - tectonic in relation to D3; 6 – restitic-two mica granites and migmatites.

172 Estudos Geológicos v. 19 (2), 2009 C. Leal Gomes, et al.

Transition zones towards the hosting and graphic pegmatite with coarse, radial granites are enriched in white mica, garnet to plumous mica intergrowths (Fig 2).

Fig. 2 – Current asymmetrical transition between the two-mica granites, the leucogranite and the pegmatites. KF – microcline; Qz – quartz; AP – undifferentiated apatite mass.

Geological setting is complicated trajectory, 46º=>134º, parallel to the by the near presence of a Silurian magma ascent - planar arrangement of metasedimentary roof pendent and K-feldspar is 130º/90º. At the enclave several ﬂ ux-oriented swarms of xenoliths swarms, ascent lineation is mainly, suggesting the proximity of the cupola of 66º=>116º. The general orientation of the the magma chamber. Magmatic lineation mingling corridor, 132º, reveals a sense of (ﬂ ux lineation) is marked by megafeldspar injection to the NW (Fig. 3).

Estudos Geológicos v. 19 (2), 2009 173 PHOSPHATE FRACTIONATION IN PEGMATITES OF PEDRA DA MOURA II CLAIM PONTE DA BARCA – PORTUGAL

SSE

SE N NNW

20M

pegmatite inner units - quartz core magmatic flux lineation in garnet (Mn-almandine) + cordierite leucogranite pegmatite outer units - border and intermediate zones magmatic flux lineation in xenolith-rich leucogranite coarse grained two mica granite melanocratic restictic schlieren coarse grained, porphyroid two mica granite essentially biotitic megafeldspar flux lineation joint or fault surface

Figure 3 – Structure and anatomy of Pedra da Moura pegmatite group (inner granite), situated along a mixing-mingling magma corridor - 3d perspective bloc with the approximate location referred in ﬁ g. 1; 1999 is the reference for the image present in ﬁ g.4 and obtained in the year 1999.

Pegmatite bodies are heterogeneous, PRIMARY ACCESSORY MINERALS asymmetrically zoned and contact gradually with the different types of The mineral assemblages were granites, thus, the cinematic interpretation characterized and its suites and stable is compatible with a late-Variscan members discriminated by X-Ray emplacement and “in situ” gradual and diffractometry. Paragenetic relations were direct fractionation from the host stripe of established using geometric criteria and transitional leucogranite. the interpretation of crystal intergrowths. These pegmatites are quartz-rich. The major accessory minerals are At the zones with the coarser crystals, beryl, Nb-Ta-Ti oxides and mainly, primary inside de graphic boundary, quartz cores phosphates - such as F-apatite, isokite, frequently occupy more than 2/3 of the triplite-zwiezelite, triphylite-litiophylite, volume and the gemmy variety, pink eosphorite, or berlinite – as well as, several quartz, may be predominant, and typically sulphides, sulphosalts and carbonates, located at the periphery of the core. White mainly oligonite. Hypothetically and hyaline varieties are nuclear. Albite they correspond to early stages of the and orthoclase to microcline feldspar paragenetic evolution, predating the occur in equal quantities and phosphate centripetal completeness of primary assemblages can accomplish as much as fractionation. They are located mainly at 10% modal volume of coarse pegmatite. transition between inner intermediate zone

174 Estudos Geológicos v. 19 (2), 2009 C. Leal Gomes, et al.

and quartz core and so possibly represent an PARAGENETIC EVOLUTION equilibrium episode of depuration of excess phosphorous, materialized as a mineralogical The more stable primary phosphates threshold and assuming the form of a occur at the younger, sometimes miarolitic, structural front, coherent with the model of hydrothermal stages of quartz core formation “in situ” crystallization (fi g. 4). – berlinite (occasionally with pink gem Northwest pegmatites are more quality) and a late population of zoned fractionated (high Li in mica and lower K/ F-apatite are typical of this stage. Rb in microcline) and have more complex In the other cases, each one of the phosphate assemblages: triphylite- primary phosphates can generate a specifi c litiophylite > triplite-zwiezelite> F-apatite> and independent evolutionary sequence eosphorite> withlockite > reddingite - depending on peculiar combinations and TRIPHYLITE-LITIOPHYILITE SUITE. succession of deuteric to meteoric conditions, Sulphide minerals are rare. Typical multiple however, liquidus-subsolvus to subsolidus phosphate composite aggregates include and meteoric evolution are much more primarily and sequentially equilibrated: complex when affecting trifi lite-litiofi lite. triphylite-litiophylite => wolfeite => In liquidus state triphylite-litiophylite withlockite => F-apatite => eosphorite evolves to wolfeite and in liquidus to => reddingite =>oligonite => triplite – subsolvus conditions evolves to eosphorite, zwiezelite and F-apatite => wolfeite. sometimes collected as an interstitial phase The southeast bodies have less in triple junctions; in subsolvus to subsolidus diversifi ed assemblages and typically the same mineral tends towards a fairfi eldite sulphides are associated with phosphates + vivianite ± hureaulite (subsolidus, and quartz. F-apatite + pyrite ± sphalerite hydrothermal evolution and triphylite + arsenopyrite + quartz occur in great aureole formation) ± sicklerite assemblage; masses at the border of the quartz core. in subsolidus it evolves to heterosite and/or Besides apatite, triplite-zwiezelite and the purpurite. paragenetically dependent isockite are the In subsolvus conditions F-apatite can main phosphate phases determining the evolve to wolfeite and in subsolidus can

TRIPLITE-ZWIEZELITE SUITE: triplite – evolve to OH-apatite and later to CO3-OH- zwiezelite => isockite => F-apatite + pyrite apatite. => clorite (chamosite) + F-apatite ± pyrite, Frequently the change products are arsenopyrite, chalcopyrite, sphalerite. The maintained inside the original contour of the Pb – Bi – Ag sulphides and sulphossalts pristine crystal. This means that the cardinal coexist with sphalerite and quartz graphic minerals of the paragenesis (mainly silicates) intergrowths at the close vicinity of F-apatite persist in equilibrium. + pyrite intergrowths and include matildite, In what concerns hydrothermal cosalite, eskimoite, bismuthinite and alteration several phosphates can be affected galenobismuthite. From a spatial point of simultaneously for example when a given view, and from SSE to NNW, it seems to later fracture affects different mineral exist a trend of magma/ﬂ uid evolution and species. In this case evolutionary suites can crystallization in the sense of Li and Mn mix, resulting several textures, which can be enrichment at the phosphate assemblages. ascribed to individualized phenomena: This trend accompanies the sense of igneous A) Corrosion - explains some mass dislocation, which is deduced from cavernous cavities and box-work like geometric and cinematic analysis. textures circumscribed to earlier volumes

Estudos Geológicos v. 19 (2), 2009 175 PHOSPHATE FRACTIONATION IN PEGMATITES OF PEDRA DA MOURA II CLAIM PONTE DA BARCA – PORTUGAL of primary crystals; in these conditions Later hydrothermal to supergenic the departure volume of initial crystal alteration promotes a fi rst diverging is reduced and phosphate anion as well evolution, which produces corrosion cavities as Ca2+, Fe3+, Fe2+, Mn2+ are removed; with re-equilibrated walls and ion leaching. the resulting contaminated fl uids can re- The latest reactions are convergent and precipitate second generation phosphates promote ion fi xation and replenishment of inside the corrosion cavities. B) “In situ” the vacuoles. transformation – corresponds mainly Vacuolar like textures are enhanced to hydration processes with volume in supergenic state of evolution and enhancement, which can generate irradiating include a large set of minerals specially sets of ruptures at the surrounding minerals; when the hostage is triphylite-litiophylite: the fractures can be replenished with late sicklerite, heterosite-purpurite, frondelite- generation phosphates. Sometimes in almost rockbridgeite and barbosalite. Vacuole closed systems it is possible to identify replenishment includes deposition of phase transitions by direct hydration. This childrenite, ritmannite, mitridatite, is the case of direct transition from berlinite dufrenite, witmoreite, cacoxenite, strunzite, to variscite. C) Exsolution/exsudation jahnsite, strengite, hureulite II, cyrilovite, – refl ects some sort of unbalance and paravauxite, tynsleite, bermanite, fairfi eldite isochemical transformation; exsolution II, kryzhanovskyite, laueite, stewartite, generates a small volume reduction yet OH-ellestadite, hentschelite, kalunigite, suffi cient to attract fl uid infl ux capable of Fe-sicklerite, messelite, lithiophosphate, produce an aureole of newly precipitated, Mn-segelerite and lithiophorite. Supergenic reactional, assemblage – some vivianite can evolution seems to end up with supergenic be generated in this way. rockbridgeite-frondelite.

MINERAL LOCATION: GR – graphic texture AB – albite – pillow aspect MO – white mica R QZ – rose quartz QZ – other quartz varieties MI – microcline AB+OR – quartz+ortose+albite blocky intergrowth AP – graphic apatite intergrowth with sulphides PH – non apatite phosphates SU – sulphides concentration

MORFOLGY: hourglass form

HOST-ROCK: garnet, cordierite leucogranite

PARENTAL GRANITE: porphyroid, essentially biotitic, coarse grain

DOMINANT SUITE: triplite-zwiezelite

Figure 4 – Image of a mining front obtained during the year of 1999. The same number is used as a position reference in the bloc diagram of ﬁ g. 3.

176 Estudos Geológicos v. 19 (2), 2009