François Fontan

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 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.

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.

Occurrence and late re-equilibration of pollucite from the Koktokay no. 3 pegmatite, Altai, northwestern China

Abstract The Koktokay no. 3 pegmatite, Altai, NW China, is a strongly zoned spodumene-subtype pegmatite. Pollucite is the unique ore mineral exploited for Cs in this pegmatite. It occurs in internal textural zones of the pegmatite in different abundances. Primary pollucite is largely homogeneous, but it displays a broad range of composition between different textural zones with CRK [=100(Cs + Rb + K)/(Na + K + Rb + Cs + Mg + Ca)] = 74.86 and Si/Al = 2.20.2.51. Four principal types of pollucite were distinguished. (1) First is pollucite with blebby mosaic texture consisting of Na-enriched and Cs-enriched phases, which clearly resulted from local exsolution of primary pollucite in the sub-solidus state. (2) The second type is nearly end-member pollucite (CRK > 90) that occurs as an aureole of primary pollucite in contact with lepidolite or feldspar clusters. This is formed by dissolution/ re-precipitation of primary pollucite. (3) Symplectic pollucite is associated with quartz, observed in sub-parallel veinlets penetrating surrounding albite crystals; compositionally, this pollucite attains a CRK ratio of up to 96 and resulted from replacement of albite by Cs-rich fluids. (4) Oscillatoryzoned pollucite is the fourth type, typically restricted to the contact of pollucite with small miarolitic cavities. The zonation compositionally oscillates about the Cs/Na variations, and is simply related to locally changing fluid composition.

Elfstorpite synonymy with allactite; mineral and name discredited

Abstract Holotype material and other specimens of ‘elfstorpite’ from the Sjögruvan deposit, Örebro, Sweden have been characterized by powder X-ray diffraction and chemical analysis. The mineral is indistinguishable from allactite, which has priority, and consequently ‘elfstorpite’ should be discredited. The IMA Commission on New Minerals and Mineral Names has approved the proposition.