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Geochemistry of Cassiterite and Its Inclusions And 745 The Canadian Mineralo gist Vol. 34, pp.745-768 (1996) GEOCHEMISTRYOF CASSITERITEAND ITS INCLUSIONS AND EXSOLUTIONPRODUCTS FROMTIN AND TUNGSTENDEPOSITS IN PORTUGAL ANA M.R. NEIVA Departmentof EarthSciences, University of Coimbra,3000 Coimbra, Portugal AxSTRACT Euhedral cassiteritefrom Sn-bearinggranitic pegmatitesand stanniferousquartz veins in Portugal shows sequencesof altematingparallel darker and lighter zones.The darkerzones are strongly pleochroic,oscillatorily zoned,and have more Nb, Ta and Fe than the lighter zones,which are nearly pure SnO2.Darker zonesmainly show exsolvedcolumbite-tantalite and ixiolite, rarely ferrotapiolite, rutile and locally, ilmenite, which are less abundant in lighter zones. Columbite-tantalite inclusions were infrequently found; they generally have combined Ti, Sn and W contents lower than those of exsolved columbite-tantalite.Compositional features differentiate the columbite-tantaliteminerals and varietiesof ixiolite. Cassiterite from W-Sn-bearing quartz veins is generally zoned; darker zones are commonly homogeneousand slightly pleochroic,but chemically similar to lighter zones.This cassiteritehas mainly inclusionsof rutile and locally of ilmenite. Exsolvedrutile has similar or higher combinedM, Ta, Sn, W, Fe, Mn contentsthan included rutile. Euhedralto subhedralgrains of massive cassiteritefrom a volcanogenicdeposit are slightly pleochroic, and their color deepensfrom yellow to dark brown with increasingFe content;such cassiterite is devoidof inclusionsand products of exsolution. Keyvords: cassiterite,columbite-tantalite minerals, tapiolite, ixiolite, rutile, inclusions,exsolution, Portugal. : SolwtanB Des cristaux de cassit6riteidiomorphes provenant de massifsde pegmatitegranitique et de veinesde quartz stannifbresau Pornrgalmontrent une alternancede zonesde croissanceparalldles claires et plus sombres.Les zonessombres sont fortement plfochroiques,zon6es de fagon oscillatoire,et contiennentplus de Nb, Ta et Fe que les zonesclaires, qui ont une composition voisine du p61eSnO2. f€s zonessombres contiennent des produits d'exsolution, surtout columbite-tantalite et ixiolite, rarement ferrotapiolite et rutile, et, localementseulement, ilm6nite, qui sont moins abondantsdans les zonesclaires. l,es inclusionsde columbite-tantalitene sont pas courantesien g6n6ral,elles contiennentdes teneursen Ti + Sn + W plus faibles que dansles domainesde columbite-tantaliteexsolv6e. Des critbresgdochimiques diff6rencient les min6rauxdu groupecolumbite-tantalite desvari6tds d'ixiolite. Les cristauxde cassit6riteprovenant de veinesde quartzrnin6ralis6es en W et en Sn sonten g6n6ralzon6s. l,es zonesplus sombressont homogdnes et ldgbrementpl6ochroiques, mais leur compositionressemble i celle deszones claires. Ce type de cassit6ritecontient surtout desinclusions de rutile et, localement,d'ilm6nite. Le rutile exsolv6contient desniveaux comparablesouplus6lev6sdeNb+Ta+Sn+W+Fe+Mnquelerutileeninclusion.Lacassit6ritemassive,encristaux idiomorphesi sub-idiomorpheset provenantd'un gisementvolcanogdnique, est ldgdrementpl6ochroique; sa couleurpasse du jaune au brun fonc6 avec une augmentationde sa teneuren fer. Ce type de cassit6ritene contient aucuneinclusion ou lamelle d'exsolution. (Traduit par la R6daction) Mots-cl6s: cassit6rite,mindraux du groupede la columbite-untalite, tapiolite, ixiolite, rutile, inclusions,exsolution, Portugal. INrRoDUcrroN 1993).The veinsof graniticpegmatite cut gmnitesand surrounding country-rocks,which consist of pre- There are many occurences of cassiterite and Ordovician,Ordovician and Silurianmetapelites. The wolframite in northem and cenhal Pormgal,and most quartzveins cut thesecountry rocks at a distanceaway ofthem havebeen exploited. Cassiterite occurs in veins from the granites, and rarely cut the granites of granitic pegmatite, and wolframite, in veins of themselves(Cotelo Neiva 1944). Cassiterite also quartz. These veins are related to Hercynian S-type occursin tle massivevolcanogenic sulnde deposit at granites(Neiva 1983, 1984, 1987), except for the Neves{orvo, in southernPortugal, which is related W-Sn-bearing quartz veins flil > Sn) from Carris to olderHercynian felsic rocks. (Gerez),which are relatedto an l-type granite (Neiva Compositional studies of cassiterite are rather 746 TIIE CANADIAN MINERALOGIST numerous(e.9., Grice et al, 1972,Moore & Howie columbite-tantalite" rutile. ilmenite inclusions and 1979,Foord 1982,Cem! & Ercit 1989,(em! et al. products of exsolution (including ixioli@ also are 1985,Izoret et al. L985,Giuliani 1987.Gonzalez & presented.The element partitioning among Nb- and Revuelta 1989, Ogunbajo 1993, Gomes 1994). Ta-bearingphases also is discussed.Twenty-one Sn However,papers dealing with inclusionsand products and W depositswere selectedfor this study @g, l). of exsolutionin cassiteriteare scarce(Murciego er a/. 1987,Spilde & Shearer1992, Fuente & Izard 1994, PARAGENETTcRsr-arroNsHrps Suwinonprechaet al. L995),as are thoseon coexisting oF PoRTUGT]ESECAssnER[E columbite-tantalite and cassiterite (Cernf & Ercit 1985, eernj et at. 1986u b, Suwinonprechaet al. In Sn-bearingveins of gfanitic pegmatite of the 1995). Hercynianbelt in Portugal,euhedral cassiterite occurs This paper concerns the phase composition and mainly interstitially in associationwith albite - geochemistry of Portuguesecassiteriie on a broad muscovite - quartz, or replaces mainly quartz regional and parageneticbasis. The compositionsof and muscovite (Fig. 2a). The crystals, generally between2 and 10 mm across,rarely host microscopic (360 x 40 pm) inclusions of quartz, muscovite and columbite-tantalite.On the otherhand, they common-ly show exsolved Nb- and Ta-bearing oxide minerals (<350 x 150 pm), exsolvedrutile (3100 x 40 pm) is rarer, and only occasionallyare exsolvedferrotapiolite (<3 x 2 pm) and ilrnenite (<4 x 2 pm) found. Rutile inclusions (<20 x 20 pm) were found only in the cassiterite from Bessa, which does not show any Droductsof exsolution. +.4 +JI+ + a h I FIG. 1. Location of the twenty-oneSn and W depositschosen g for study. a: he-Ordovician metamorphic complexes, Paleozoic,some igneous and ultrabasicrocks and igneous A complexesfrom Alentejo; b: Marine Carboniferous metasedimentaryand metavolcanicrocks; c: Hercynian f granitic rocks; d: Mesozoic and Cenozoic stiilimentary rocks. Ore deposits: e: volcanogenic sulfrdes and Sn A deposits [2]: Neves Corvo (Sta B6rbara dos Padr6es, e Castro Verde)l; f: Sn-bearinggranitic pegmatitic veins [1: Cabragdo(Cabragdo, Ponte dr Lima); 2: Bessa (Morgade, Montalegre), 3: Fental (Sto. Aleixo do mnTdmega Ribeira de Pena), 4: Revel (Tr0s Mnas, Vila d Pouca de Aguiar), 5: Vieiros @ebordelo, Amarante)l; g: stanniferous quartz veins [6: Montezinho (Franqa [5 Braganga),7:Argozelo (Argozelo, Vimioso)l; h: W-Sn- bearing quartz veins [8: Carris (Cabril, Montalegre), c 9: Lombo de Boi (-amalonga, Macedo de Cavaleiros, 10: Linhares @rmelo, Mondim de Basto), 11: Fonte = Figueira (Aboadela, Amarante),12: Vale das Gatas b (S. Lourengo de Ribapinhdo e Souto Maior, Sabrosa;, 13: Muralha flilar de Magada Alij6), 14: Carvalhal tl (Cheires, Alij6), 15: Soutelinho (Favaios, Alij6), a 16: Filharoso (Presand6es,Alij6), 17: S. Domingos (Alij6, A1ij6), 18: Folgar (S. Mamedede Ribatua Alij6), 19: Bougo (S. Mamede de Ribatua, Alij6), 20: Panasqueira(Aldeia de S. Franciscoe S. Jorge da Beira, Covilhd e FundSo)1.Parish and boroughare given for the locality information. CASSITERITEAND ITS INCLUSIONS, PORTUGAL 747 Flc. 2. a Cassiterite (c) from the Vieiros pegmatite replacing quartz (q) and muscovite (m) (x55); b. euhedral grain of cassiterite(c) from the quartz veins from Argozelo, associatedwith a muscovite-richselvage (m) and qu*,rtz(q) (x27). In quartz veins, euhedralcassiterite occurs among SaMpLes.ANatYlcar METHoDS quartz crystals, mainly along the vein-schist contact, ANDTREAI'N{ENT OF DATA associatedwith a muscovite-rich selvage (Ftg. 2b). Cassiteritecrystals from the stanniferousquartz veins A total of 1738 compositions were obtained by from Argozelo are generally more than 1 cm across electronmicroprobe, 1100 on cassiteritein 63 selected and locally cut by fracture-filling veinlets of quartz, crystals,203 on their inclusions(82 of columbite- arsenopyriteand sphalerite.crystals of cassiteritefrom tantalite.108 of rutile, and 13 of ilmenite)and 435 of stanniferous quartz veins generally show exsolved their products of exsolution (293 of columbite- Nb- and Ta-bearing oxide minerals (<80 x 50 p-), tantalite,72 of.iJ{iohte, 15 of ferrotapiolite,43 of rutile, whereas tlose from W-Sn-bearing quafiz veins and 12 of ilmenite). commonlyhost inclusionsof rutile (<70 x 50 Fm) and The oxide minerals were analyzedfor major and rare inclusions of ilrnenite (<8 x 3 pm). However, minor elements on a Cameca Camebax electron exsolvedNb- andTa-bearing oxide mineralsand rutile, microprobe at Instituto Geol6gico e Mineiro, S. or rutile alone(<30 x 20 lrm), were locally found. Mamede de Infesta, Portugal. Analyses were In the volcanogenic deposit from Neves-Corvo, conductedat an acceleratingvoltage of 15 kV and a meter-thick massive lenses of euhedral to subhedral beam current of 20 nA. Standardsused include SnO2 cassiterite with interstitial quartz, carbonates and (Sn o), FqO, (FeKcr),MnTiO, (Mn-I(a and TiKcl), sulfideslocally overlie the massivecopper ore (Gaspar pure Ta (fo), Nb (Lo) and W (I{x) metals. Each 1991).This cassiteriteis intergrownwith or cut by elementwas countedfor 20 seconds.ZAF corrections veinletsof chalcopyrite,pyrite and carbonates.Neither were applied. inclusionsnor
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