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The Phosphate Mineral Association of the Granitic Pegmatites of the Fregeneda Area (Salamanca, Spain)

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The Phosphate Mineral Association of the Granitic Pegmatites of the Fregeneda Area (Salamanca, Spain) The phosphate mineral association of the granitic pegmatites of the Fregeneda area (Salamanca, Spain) E. RODA Dept. de Mineralogfa y Petrologfa, Univ. del Pals Vasco/EHU, Apdo. 644, 48080 Bilbao, Spain F. FONTAN Univ. Paul Sabatier, URA 067, Toulouse, France A. PESQUERA AND F. VELASCO Dept. de Mineralogfa y Petrologfa, Univ. del Pals Vasco/EHU, Apdo. 644, 48080 Bilbao, Spain Abstract In the Fregeneda area different pegmatitic types can be distinguished on the basis of their mineralogy, internal structure and field relationships. The most common type corresponds with simple pegmatites with a homogeneous internal structure, but Li and Sn-bearing pegmatites are also relatively widespread, besides a minority group of Fe-Mn phosphate-bearing pegmatites that has recently been characterized. These pegmatites are located in an intermediate zone, between the barren pegmatites and the most evolved Li and Sn-bearing bodies, and they carry a complex association of phosphate minerals. The study of these phosphates has allowed the identification of the primary phases as wyllieite, graflonite, sarcopside, triplite-zwieselite and ferrisicklerite; the secondary phosphates are rosemaryite, heterosite-purpurite, alluaudite and v/iyrynenite. In this study, the main characteristics of these phosphate minerals are reported, including their chemical composition, analysed by electron microprobe, and their unit-cell parameters, calculated using X-ray powder diffraction techniques. A common transformation mechanism in this phosphate association is the oxidation of the transition metal cations at the same time as Na-leaching in wyllieite to generate rosemaryite, and Li-leaching in ferrisicklerite to generate heterosite. The occurrence of sarcopside lamellae in ferrisicklerite and heterosite is evidence of the replacement processes of the former by the latter. A Na-metasomatic replacement of the early phosphates as ferrisicklerite and graftonite, producing alluaudite, is also a well developed process. Phosphate minerals occur in pegmatites with an intermediate degree of fractionation, appearing between the barren and the more evolved pegmatites with Li and Sn, which is in agreement with the pegmatite field zonation established in the literature. KEVWORDS: Fe-Mn phophates, granitic pegmatites, Fregeneda, Spain. Introduction in the Hesperic Massif. This band is bordered by the Lumbrales granite to the south, and by the Saucelle IN the Fregeneda area different types of pegmatites granite to the NE. Both units are peraluminous, two- have been established according to their mineralogy, mica granites, and belong to a group of syntectonic morphology, internal structure, etc. (Fig. 1, Table 1). massifs, which have been deformed during the third This area is located in the NW part of the province of phase of Hercynian deformation (L6pez Plaza and Salamanca, Spain (Fig. 1), in the western extremity Carnicero, 1988; L6pez Plaza and Martfnez Catalzin, of a narrow metamorphic band with an E-W strike, 1988). The studied pegmatites intrude into rocks of Mineralogical Magazine, October 1996, Vol. 60, pp. 767-778 Copyright the Mineralogical Society 768 E. RODA ET AL. N PORTUGAL +Saucelle. .......... granite ++§+ + + DUero p,'l~e \ a 0 1 Km . / (TI) simple interior pegm. (T2) I"1 quartz+andalusite conformable d (T3) <> simple dykes and apophyses (T4) simple conformable pegm. (T5) A K-feldspar discordant dykc~ / (T6) simple discordant Ivegm / (T7) Li-beaxing discordant pegm. (T8) OSn-bearing discordant pegm. // § ~r247 +++§247 ++++§247247247247 + + ++ + + + + + ~Lumbral~ ~ranite ++++++§ +++++§247247247 §247247 SPAIN Biotite isograde [--I Schist-Metagraywacke-Complex -- Andalusite isograde -,~m:~Fracturecontainingquartzsegrcgations -- Sillimanite isograde FIG. 1. Distribution of the pegmatite groups recognized in the Fregeneda area (* numbers of pegmatite types as in Table 1). the Schist-Metagreywacke Complex, which in this (T6) crop out. The pegmatites of the T6 group that area consists of an alternation of quartzites, grey- appear near to the Lumbrales granite carry a complex wackes, schists and pelites, with abundant thin paragenesis of Fe-Mn phosphates, whereas in the T6 calcsilicate layers. Previous works on the description pegmatites which appear farthest from the Lumbrales of the pegmatite bodies, their mineralogy and their granite, amblygonite may be present. Finally, in the petrogenesis have been published (Roda, 1993; Roda area furthest from the granite, discordant Li-mica- et al., 1993, 1995, 1996). bearing pegmatites (T7) and pegmatites, consisting The different groups of pegmatites which crop out mainly of quartz, cassiterite and albite (T8) appear. in the area display a grade of differentiation which The main purpose of this study is to establish the increases away from the Lumbrales granite (Fig. 1; characteristics and relationships between the phos- Table 1). In the border zones of the granite phates that appear in the various different pegmatite intragranitic pegmatites (T1) occur, and these are bodies, as well as to specify the mineralogical and mainly composed of quartz and K-feldspar. Close to geochemical role of the phosphate minerals in the the Lumbrales granite, in the andalusite-cordierite evolution of the Fregeneda pegmatites. zone, dykes composed mainly of quartz and andalusite (T2) crop out. Another group of barren Data collection and analyses bodies, located near the Lumbrales granite, comprise dykes and apophyses showing aplitic and pegmatitic The studied phosphate minerals were mainly facies (T3), mainly composed of quartz, K-feldspar collected from the pegmatites of type 6 nearest to and muscovite. Also in close proximity to the granite, the Lumbrales granite (Fig. 1). In some cases, it was the conformable pegmatites, composed mainly of possible to separate the crystals by hand-picking quartz, K-feldspar, muscovite and albite (T4) appear. them under the binocular microscope and grinding by In an intermediate zone, between the barren and the hand in an agate mortar, but due to very fine-grained most evolved dykes, pegmatites composed mainly of impurities it was not possible to obtain pure amounts K-feldspar (T5) and discordant pegmatites that of all the phases. The various phosphate minerals consist of quartz, K-feldspar, albite and muscovite have been identified by optical properties in thin PHOSPHATES IN GRANITIC PEGMATITES 769 I .o= .,-, < z~ - i ~.~ ~-~== ~ ~ I o .=. '~ .~ .~ t-i o ." I I o~ o'~ e.- N ~ I I 0 "-d~ ~q o oo'F, em ~ V ~v ,-.M ~ -~.~ ~ ~ o =2 e-. o ~ ~ ~~ o ~ o= ~ o= == .o m ~g r~ o= ~2._= o < ~o ~ ~o ~.'~ ~ o.~ h~.~ o o t r~ o I ~.~ D~ 770 E. RODA ET AL. TABLE 2. List of the identified phosphate minerals in the different pegmatite types of the Fregeneda area Phosphate Ideal formula Pegmatite type minerals Wyllieite-rosemaryite Na2(Mn,Fe2+,Fe3+)2AI(PO4)3 Graftonite (Ca,Fe2+,Mn2+)3(PO4)2 I I Nil Sarcopside (FeZ+,Mn,Mg)3(PO4)2 I I i Ferrisicklerite Li,(Fe3+,Mn2+)3(PO4) I I M Sicklerite Li,(Mn3+,Fe2+)3(PO4) I [ IN Heterosite (Fe3+,Mn3+)(PO4) @ I I gN Purpurite (Mn3+,Fe3+)(PO4) [ [ I N Triplite-Wieselite (Mn2+,Fe2+,Mg,Ca) 2(POz)(F,OH) N Ni Altauadite (Na,Ca)(Fe2+,M nZ+)Fe~+(P403) NIN Apatite Cas(POa)3(OH,F) NNN @ Amblyg.-montebras LiAI(POa)(F,OH) Vtiyrynenite MnBe(PO4)(OH) section, X-ray powder diffraction techniques and transition metal cations at the same time as Na- electron microprobe analyses. Chemical analyses leaching, to generate rosemaryite (Roda, 1993; were performed at the Universit6 Paul Sabatier Fransolet, 1995). Both phosphate minerals constitute (Toulouse, France), with a Camebax SX 50 electron a little-known sequence, with problems similar to microprobe, using an operating voltage of 15 kV and those shown by the triphylite-ferrisicklerite se- a beam current of 20 nA for all the elements. quence (Quensel, 1937; Mason, 1941). Thus, Unit-cell parameters were obtained by means of rosemaryite shows a lower Na content, and this is X-ray powder~ diffraction using Cu-Ka radiation accompanied by a higher content of ferric iron to 0~= 1.5418 A, 40 kV, 20 mA) and graphite maintain the charge equilibrium in the following way monochromator. Runs were made by scanning (Fig. 2a): through 2 ~ to 70 ~ at 1~ 20 per minute for each Na (Mn2+, Fe2+)(Fe2+>Fe 3+) A1 (PO4)3 - sample, using silicon as an internal standard. Unit Na<l (Mn2+, FeZ+)(Fe 3+) AI(POa)3 cell refinements were performed according to the (wyllieite) (rosemaryite) procedure of Appleman and Evans (1973). This is the first time wyllieite and rosemaryite appear intergrown, a fact that allowed us to confirm Petrographic and chemical data the inter-relation between both phases, as has been The study of these phosphate minerals has allowed suggested by previous workers (Roda, 1993; Fontan the identification of the phases listed in Table 2. In et al., 1994; Fransolet, 1995). Wyllieite appears as the following, the more important phosphates will be xenomorphic grains, usually <1 cm, with a slight described, as they may provide valuable aid in pleochroism ranging from green to light-green. Some explaining the crystallization history of the phosphate of these grains show more pleochroic yellow to associations as well as of the pegmatites themselves. greenish-yellow areas of rosemaryite. Wyllieite-rosemaryite. This phase adopts the The finely intergrown nature of the wyllieite and following general structural formula (Moore and rosemaryite has not allowed us to obtain pure Ito, 1979): X(2)4 X(la)2 X(lb)2 M(1)4 M(2a)4 samples of such phases, in order to calculate their M(2b)4 (PO4)12, where X(2) = Ca, Na and K; unit-cell parameters. The compositional variation for X(lb) = Fe z+, Mn 2+, Ca and Na; M(1) = Fe z+ and wyllieites is small for some elements (Table 3; Mn2+; M(2a) = Fe3+, Mg, Li, Zn and FEZ+; and, Fig. 2a, b and c), with A1203 values that range from finally, M(2b) = AI, Fe 3 and Mg. Wyllieite can 6.12 to 7.84% and MnO from 18.35 to 18.92%; and undergo a topotactic alteration, with oxidation of the finally, Na20 varies between 4.75 and 6.40%.
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