Alfonso Pesquera

Phosphate mineral associations in the Cañada pegmatite (Salamanca, Spain): Paragenetic relationships, chemical compositions, and implications for pegmatite evolution

Abstract The Cañada pegmatite (Salamanca, Spain), a Li-P-(Sn-Nb ± Ta)-bearing granitic pegmatite, is intruded partly into a gabbro and partly into a leucogranite. Three phosphate associations have been distinguished, based on paragenesis, texture, and chemistry: (1) ferrisicklerite-magniotriplite-johnsomervilleite association (in the border zone), in which Mg-bearing phosphates and coexisting Fe-Mg silicates are abundant; (2) ferrisicklerite-graftonite association (in the transition zone) characterized by higher values of Fe/(Fe + Mg) than in the border zone; and (3) triphylite-sarcopside association (in an inner zone), which also includes minor montebrasite, ferrocolumbite, and cassiterite, typical of an evolved pegmatite facies. As a result of the decrease of Mg, the Fe/(Fe + Mg) ratios for phosphates, biotite, and tourmaline increase from the border to the inner association (e.g., for ferrisicklerite and graftonite, from 0.67 and 0.85 in the border to 0.94 and 0.98 in the inner association, respectively). This difference is particularly evident for biotite and tourmaline; for example, the Fe/(Fe + Mg) ratios for tourmaline range from 0.59 in the border to 0.86 in the inner zone. These variations seem to reflect contamination of marginal zones of the pegmatite by some type of reaction with the host gabbro. Thus, an evolutionary trend involving inward crystallization from the margins and contamination of fluids from wallrocks into pegmatite-forming melt may be a plausible genetic model. The occurrence of phosphates along with Fe-Mg silicates would indicate that the melt contained on the order of 1.3-2.4 wt% P2O5, based on experimental silicate-phosphate equilibria.

Origin and petrogenetic implications of tourmaline-rich rocks in the Sierra Nevada (Betic Cordillera, southeastern Spain)

Tourmaline-rich rocks (up to 60% tourmaline) associated with low–medium grade metamorphic assemblages occur in the Sierra Nevada area (Betic Cordillera, southeastern Spain). Tourmaline appears in a variety of forms: (1) stratiform tourmalinites; (2) quartz–tourmaline nodules; (3) porphyroclasts in felsic orthogneisses; and (4) disseminations in psammopelitic metasediments and gneisses. Tourmaline within these lithologic groups exhibits textural and chemical variations that reflect complex premetamorphic growth under open-system conditions, and subsequent changes due to Alpine regional metamorphism. Microprobe analyses of the tourmalines reveal a wide compositional variation between schorl and dravite end members with variable contents of X-site vacancies (av. 0.084–0.225 apfu), Ca (av. 0.095–0.269 apfu), and excess of Al (up to 6.588 apfu) compared with the theoretical value of 6 in ideal schorl and dravite. The amount of Ca may be significant in porphyroclasts from the gneisses. Fe/(Fe+Mg) ratios for tourmalines in tourmalinites, metasediments, and gneisses range from 0.34 to 0.95, 0.16 to 0.92, and 0.28 to 0.97, respectively. Na/(Na+Ca) ratios are also variable, mostly ranging from 0.5 to 0.9. Many of the tourmalines have complex chemical and colour zoning patterns, including significant fluctuations in Al, Fe, Mg, Na, Ca, Ti, and F. Based on petrographic and chemical data, three generations of tourmaline have been established. The first generation corresponds to magmatic–postmagmatic tourmaline that is represented by tourmaline porphyroclasts within the orthogneisses. The second generation of tourmaline formed during tourmalinization of psammopelitic rocks giving rise to tourmalinites. The third generation of tourmaline is represented by cellular textures, pale reaction rims and overgrowths developed during the Alpine regional metamorphic overprint. D 2002 Elsevier Science B.V. All rights reserved.

Chemistry and genetic implications of tourmaline and Li-F-Cs micas from the Valdeflores area (Caceres, Spain)

Abstract Pervasive metasomatism that involved the formation of tourmaline-rich rocks and influx of Li. F. and Cs into Ordovician psammo-pelitic metasediments occurred in the Valdeflores area (Cáceres. Spain). Numerous Li- and Sn-bearing. mineralized, greisen-type veins also can be observed here, in the vicinity of geochemically specialized granites. Tourmaline- rich rocks appear as: (1) massive, fine-grained, dark green to black rocks: and (2) fine- scale tourmaline-rich laminae, which alternate with quartz-rich layers parallel to the bedding. Electron microprobe analyses indicate that the tourmaline lies mostly within the space defined by the exchange vectors from dravite: FeMg-1 (schorl), ⃞AlNa-1Mg-1 (foitite), AlOMg-1(OH)-1 (olenite), and CaMgNa-1Al-1 (uvite). The Fe/(Fe+Mg) ratio ranges mainly from 0.87 to 0.54 and increases with Al in the Y-site. Analytical results and substitutional relations show an insignificant elbaite component. Mica in the tounnalinized rocks is very fine-grained (mostly <50 μm). White mica ranges from lithian muscovite-phengite to lepidolite/zinnwaldite, containing up to 8.40 wt% F. 6.0 wt% Li2O, and 10.73 wt% FeO. Dark mica shows a variable color and has compositions characterized by relatively high contents of Cs2O (1.14-2.78 wt%) and F (1.94-8.08 wt%), with a deficit in K2O (5.75- 9.04 wt%). Log (fH₂O/fHF) of fluids in equilibrium with biotite in the tourmaline-rich rocks was 4.02-4.17 at T ≈ 400 °C. Log (fH₂O/fHF) values of fluids in equilibrium with topaz (XF ≈ 0.8) in country rock adjacent to contacts with veins, and in equilibrium with am- blygonite-montebrasite (Xamb = 0.2) in the veins were about 4.30-4.60 and 6.4-6.7, respectively. Tliese variations denote the existence of gradients in relative aHF more than differences of temperature during metasomatism. The lack of tourmaline in the veins is interpreted to reflect the alkalinity and low Fe-Mg contents in the fluids, which precluded the formation of tourmaline. Consequently, most of the boron was expelled into metasediments where tourmaline was produced as a result.

Origin and internal evolution of the Li-F-Be-B-P-bearing Pinilla de Fermoselle pegmatite (Central Iberian Zone, Zamora, Spain)

Abstract The Li-F-Be-B-P bearing Pinilla de Fermoselle (PF) pegmatite occurs in the apical part of a leucogranite body. It shows a clear non-symmetrical vertical zoning from the contact with the leucogranite to a contact with the metamorphic country rocks. The pegmatitic facies evolve upward from (1) the undifferentiated Lower Border Zone (LBZ), with quartz, feldspars, muscovite, biotite, and black tourmaline, through (2) the Intermediate Zone (IZ), with quartz, muscovite, zinnwaldite, black tourmaline, and Fe-Mn phosphates, to (3) the highly evolved Upper Border Zone (UBZ), with quartz, albite, lepidolite, zinnwaldite, elbaite, and beryl. The composition of the pegmatite-forming minerals suggests that a residual melt become progressively enriched in F and Li until the crystallization of the apical UBZ, whereas P partitions in the melt only until the intermediate levels of differentiation attained in the IZ. Chemical variations in the mica and tourmaline as well as in the feldspar and Fe- Mn phosphate minerals are consistent with an internal evolution by crystal fractionation processes. A plausible model for the crystallization of the PF pegmatite involves a rapid, in situ, bottom-up crystallization from significantly undercooled liquids. The lack of metasomatic effects in the metamorphic host-rock and the estimated P content of the initial leucogranite melt suggest that the PF pegmatite mainly crystallized under closed system conditions.

The phosphate mineral association of the granitic pegmatites of the Fregeneda area (Salamanca, 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, graftonite, sarcopside, triplite-zwieselite and ferrisicklerite; the secondary phosphates are rosemaryite, heterosite-purpurite, alluaudite and väyrynenite. 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.

Chemical variations and significance of phosphates from the Fregeneda-Almendra pegmatite field, Central Iberian Zone (Spain and Portugal)

Abstract Field, textural, paragenetic, and chemical data for micas from pegmatites of the Fregeneda-Almendra pegmatitic field (Central-Iberian Zone) are used to characterize and evaluate their petrogenesis. These pegmatites show a zonal distribution from barren to evolved, with an increase in degree of evolution with increasing distance from the Mêda-Penedono-Lumbrales leucogranite. Five types of evolved pegmatites have been recognized: (1) petalite-rich, (2) spodumene-rich, (3) Li-mica + spodumenerich, (4) Li-mica-rich, and (5) cassiterite-rich pegmatites, plus six types of barren and intermediate pegmatites. Representative micas from the different pegmatite types and from the leucogranite were analyzed for major and trace elements. All micas belong to the muscovite-lepidolite series. Lithium is incorporated into Li-micas via the Li3Al-1□-2 and Si2LiAl-3 substitutions, where □ represents vacancies. The Al4Si-3□-1 and Al2□1R2+-3 substitutions, where R2+ = (Fe + Mg + Mn), account for the compositional variability of micas from the Li-mica-free pegmatites. The Li, Rb, Cs, Be, Ta, and Nb contents of micas increase in the order: leucogranites and barren pegmatites < intermediate pegmatites < spodumene-bearing and petalite-bearing dikes < Li-mica-bearing pegmatites. The Ba content decreases in the same order, and Sn and Zn are relatively abundant in the intermediate pegmatites. These variations are consistent with rare-element enrichment via fractionation processes combined with partitioning of rare elements from the pegmatite melt into the minerals and volatile phases. However, some pegmatite types occurring in this area, such as the cassiterite-rich dikes, do not seem to form part of the same evolutionary trend.

Mineralogy and geochemistry of micas from the Pinilla de Fermoselle pegmatite (Zamora, Spain)

The highly fractionated, Li-F-Be-B-P-bearing Pinilla de Fermoselle (PF) pegmatite crops out in the westernmost part of the Zamora province (Spain). This body appears as a cupola over the PF leucogranite, displaying a non-symmetrical internal zonation with a complete sequence from a barren pegmatitic facies near the granite, to a highly evolved zone in the uppermost part of the body. Representative samples of micas from the different pegmatite zones have been studied. Based on textural and chemical criteria, the micas may be grouped into two assemblages: Al-rich micas and Fe-rich micas. In general, Al-rich micas show a continuous evolution from muscovitic to lepidolitic compositions from the leucogranite to the most evolved zone. Fe-rich micas range from Fe-biotite in the leucogranite and in the least evolved pegmatite zones, to an intermediate composition between zinnwaldite and trilithionite in the most evolved pegmatitic facies. The incorporation of Li into micas appears to be controlled by the substitutions Si 2 LiAl -3 ,a nd Li3Al-1-2, AlLiR-2 ,S iLi 2R-3 ,a nd SiLiAl -1R-1, where R = (Fe 2+ + Mg + Mn). Paragenetic relationships and chemical variations in micas from different zones making up the PF pegmatite suggest that the pegmatitic system derived from a granitic melt and evolved upwards by fractionation processes. Evidence in support of this model comes from: (i) the gradual enrichment in Li, Rb, Cs and F, parallel to the decrease in Mg and Ti; (ii) the convergent evolutionary trends towards lepidolite showed by the Al- and Fe-micas; and (iii) the parallel decrease in the K/Rb ratio in micas.

Micas of the muscovite – lepidolite series from Karibib pegmatites, Namibia

Abstract Micas of the muscovite-lepidolite series are main constituents of the evolved pegmatites from the Okatjimukuju-Kaliombo portion of the Karibib belt, Namibia. The compositional variations shown by the micas from the intermediate zones are mainly controlled by the Li3Al−1□−2 and SiLi2Al−2□−1 substitution schemes, whereas for the micas from the core margins and the replacement bodies, only the first of these two exchange vectors seems to operate. The chemical composition of the micas not only depends on the degree of pegmatite evolution, but also on the position in the internal zonation of the pegmatite. Micas from the core margins and the replacement units are generally richer in F, Li, Rb, Cs and Zn than those from the intermediate zones. In general, the contents of these elements increase with decreasing K/Rb ratio. However, some data departing from this general trend are also observed, which could be related to subsolidus processes. Some pegmatite bodies show a complete internal evolution, developed from the margins to the core zone, which is reflected in the chemical composition of the micas. The regional distribution of pegmatites does not define a zonation, because an overlapping of pegmatites with different degrees of evolution occurs. This could be due to the high level of evolution attained by most of the rare-element pegmatites, and to their topography with respect to a dome structure of the basement.

Chromian tourmaline and associated Cr-bearing minerals from the Nevado-Filábride Complex (Betic Cordilleras, SE Spain)

Abstract Chromian tourmaline, in association with other Cr-bearing minerals (amphibole, mica, epidote, chlorite, titanite, rutile and chromian spinel), occurs in fine calc-schist levels within metacarbonate rocks from the Nevado-Filábride Complex, SE Spain. Electron microprobe analyses of tourmaline and coexisting minerals document both chemical differences dependent on the host-rock type and an irregular distribution of Cr at grain scale. Tourmaline is Na-rich dravite, with average Mg/(Mg+Fe) ratios of 0.83 and 0.63 a.p.f.u. and Cr contents of 0.32 and 0.18 a.p.f.u., in dolomitic and ankeritic marbles, respectively. Tourmaline contains small but significant concentrations of Zn (av. 0.01 a.p.f.u.) and in ankeritic marble it also contains Ni (av. 0.04 a.p.f.u.). Zn-rich chromian spinel appears as small relict inclusions in silicates, with average Cr, Fe, Al and Zn contents of 1.201, 1.241, 0.411 and 0.107 a.p.f.u., respectively. Amphibole, epidote, mica and chlorite show average Cr contents of 0.088, 0.138, 0.115 and 0.267 a.p.f.u., respectively, in dolomitic marbles, and 0.103, 0.078, 0.065 and 0.185 a.p.f.u., respectively, in ankeritic marbles. Cr-silicates formed through metamorphic reactions involving detrital Cr-rich spinel, in addition to clay minerals and carbonates. The B necessary to form tourmaline was probably derived from the leaching of underlying evaporitic rocks.

Tourmaline as a petrogenetic monitor of the origin and evolution of the Berry-Havey pegmatite (Maine, U.S.A.)

Abstract The Berry-Havey pegmatite (Oxford pegmatite field, Androscoggin County, Maine, U.S.A.), enriched in Li, F, B, Be, and P, is intruded in hornblende-rich amphibolite, with minor biotite or diopside. The pegmatite has a complex internal structure, with four texturally and compositionally different zones that show an increasing degree of evolution inward: wall zone, intermediate zone, core margin, and core zone. The main minerals are quartz, feldspars, Al-micas, tourmaline, with minor Fe-micas, garnet, beryl, amblygonite-montebrasite, Fe-Mn phosphates, and apatite. Tourmaline is present in all zones of the pegmatite, showing different textures: black anhedral crystals in the wall and intermediate zones; black prisms of up to 40 cm in length in the intermediate zone; black tapered prisms, surrounded by a pseudo-graphic intergrowth of quartz or albite with black ± green/bluish tourmaline, and constituting a continuous layer under the core zone; multicolored and “watermelon” zoned crystals in the core zone; and gemmy deep green and color-zoned “watermelon” tourmaline prisms, up to 15 cm length, inside the pockets. A complete chemical evolution from Mg-rich schorl in the wall zone to elbaite with an important deprotonation in the pockets inside the core zone is observed. The most plausible exchange vectors for this chemical evolution are FeMg-1, YAlWO[YR2+W(OH)]-1 and Al[X]X(R2+Na)-1 (where R2+ = Fe2++Mg2++Mn2++Zn2+), for the tourmalines from the wall and intermediate zones. In the core margin, tourmaline composition evolves from schorl toward Li-rich species through the substitution (YAlYLiYR2+-2). Later, during the crystallization of the core zone, this exchange vector combined with the substitution ([X] YAl0.5XNa-1YLi-0.5). Finally, the gemmy tourmalines from the pockets show a deprotonation related to the exchange vector YAlWO2YLi-1W(OH)-2 and may be classified as darrellhenryite. These substitutions may reflect an increase in oxygen fugacity and a decrease in Li and F related to the crystallization of lepidolite and amblygonite-montebrasite in the core zone adjacent to or within the pockets. The crystallization of these minerals would reduce the availability of Li and F for the very latest tourmaline crystals, growing inside the pockets, where the deprotonation becomes important. Chemical and textural variation in tourmaline is consistent with a fractional crystallization process for the internal evolution of the Berry-Havey pegmatite. Crystallization of the tourmaline layer under the core zone may be related to the exsolution of the fluid phase implied in the formation of pockets.