Exsolution of Zirconian-Hafnian Wodginite From Manganoan-Tantalian Cassiterite, Annie Claim #3 Granitic Pegmatite, Southeastern Manitoba, Canada

Abstract

Primary (Mn,Fe,Ta,Nb)-bearing cassiterite with ≤0.21 wt.% ZrO 2 and ≤0.05 wt.% HfO 2 crystallized during the consolidation of the Archean Annie Claim #3 zoned, lepidolite-subtype granitic pegmatite, southeastern Manitoba, in amounts increasing from the outer to the inner pegmatite zones. Rare inclusions of tantalite and ferrotapiolite originated from transient local saturation in these phases. Sparse early cassiterite from the outer intermediate zone is rather rich in substituting elements and in part Fedominant, whereas the more abundant late cassiterite from the lepidolite core is strongly Mn-dominant and poor in the substituents. Exsolution in the primary cassiterite generated granular zirconian-hafnian ferrowodginite and wodginite enclosed in veinlets of substituent-depleted cassiterite. The Fe/Mn and Nb/Ta values of the wodginite phases mimic those of the primary cassiterite across the pegmatite zones, and Zr/Hf perceptibly decreases from the outer zones inward. Maximum contents of ZrO 2 and HfO 2 (5.98 and 1.59 wt.%, respectively; 0.31 Zr and 0.04 Hf apfu) are the highest so far encountered in wodginite-group minerals. These maxima characterize wodginite exsolved from early cassiterite; the levels drop off in wodginite exsolved in late cassiterite. Instead, hafnian zircon intergrown with abundant thorian coffinite becomes abundant in the lepidolite core, indicating a transition of Zr + Hf from octahedral sites in the oxide minerals to the much more common 8-fold coordination in the orthosilicate; this shift implies a transition from relatively alkaline to more acidic conditions. SOMMAIRE Une cassitérite primaire, enrichie en Mn, Fe, Ta et Nb, et avec ≤0.21% ZrO 2 et ≤0.05% HfO 2 (en poids), a cristallisé au cours de la solidification de la pegmatite granitique archéenne de Annie Claim #3, massif zoné, sous-type à lépidolite, dans le sud-est du Manitoba, en quantités augmentant de la bordure vers le coeur du massif. De rares inclusions de tantalite et de ferrotapiolite ont leur origine dans la saturation temporaire et locale du magma. La cassitérite, assez rare dans la partie externe de la zone intermédiaire, est relativement enrichie en éléments se substituant à l'étain, et elle est en partie à dominance de fer. Par contre, la cassitérite tardive, dans le coeur à lépidolite, est plus abondante, fait preuve d'une dominance de Mn, et est appauvrie dans les autres éléments substituants. L'exsolution a produit des inclusions granulaires de ferrowodginite et de wodginite zirconifère et hafnifère dans des veinules de cassitérite purifiée. Les valeurs Fe/Mn et Nb/Ta des phases du groupe de la wodginite simulent celles de la cassitérite primaire d'une zone à l'autre, et la valeur Zr/Hf diminue de façon subtile de la bordure vers le centre. Les concentrations maximum de ZrO 2 et de HfO 2 (5.98 et 1.59%, respectivement; 0.31 Zr et 0.04 Hf atomes par unité formulaire) sont les plus élevées qui soient connues dans les minéraux du groupe de la wodginite. Ces valeurs élevées caractérisent la wodginite exsolvée de la cass...

GEOCHEMICAL FRACTIONATION OF Nb-Ta OXIDES IN Li-BEARING PEGMATITES FROM THE BARROSO-ALVAO PEGMATITE FIELD, NORTHERN PORTUGAL

“…In the petalite pegmatites, the presence of CGM inclusions in cassiterite suggests that there may have been two distinct episodes of CGM crystallization. In this type of pegmatite, the CGM found in cassiterite are interpreted as inclusions, rather than a product of exsolution on the basis of our observations: 1) CGM have sharp contacts with cassiterite and do not occur as oriented exsolution-related "needles" or anhedral "grains" that exsolved from cassiterite, as described by Ramdohr (1980), and 2) there is a lack of chemical variation of cassiterite adjacent to the CGM inclusions [see Neiva (1996) and Masau et al (2000) for discrimination criteria]. These observations, together with the low Sn content for the inclusions (not detected or below detection limit), and relatively higher Sn content for the discrete grains (0.09-0.16 wt.% SnO 2 ), suggest that the CGM inclusions formed prior to cassiterite crystallization, and were later replaced by cassiterite.…”

“…Contacts between these grains and cassiterite are very sharp, as observed in Figure 3b. They occur disseminated in the cassiterite and are not associated with fractures or grain boundaries; thus they are not considered to be products of exsolution, as in other localities (e.g., Neiva 1996, 2008, Masau et al 2000. These observations suggest that these grains of CGM formed prior to cassiterite formation and were subsequently replaced by this mineral.…”

THE Ta-Nb-Sn-Ti OXIDE-MINERAL PARAGENESIS FROM LA VIQUITA, A SPODUMENE-BEARING RARE-ELEMENT GRANITIC PEGMATITE, SAN LUIS, ARGENTINA

“…Full occupancy of the C site is achieved by combining all of Nb and negligible W with the dominant quantities of Ta, and the remainder of Ta occupies the B site (up to 30% of its population). This site also hosts moderate to dominant Ti, major Fe 3+ calc , and significant Zr, which attains 1.63 wt.% in terms of oxides (0.350 apfu), almost in the range shown by the zirconian-hafnian wodginite exsolved from the Annie Claim #3 cassiterite in Manitoba (Masau et al 2000). Hafnium, not sought in the La Viquita samples, attains there up to 1.59 wt.…”

“…Sporadic, randomly as products of exsolution [cf. Neiva (1996) and Masau et al (2000) for discriminant criteria]. The compositionally pure, pale brown, euhedral crystals of cassiterite in quartz vugs of the core margin have probably formed from a postmagmatic vapor phase.…”

“…In individual rare-element granitic pegmatites described in the literature, the Ta-Nb-Sn-Ti oxide phases show a broad diversity in their mineralogy, chemical compositions and structural state, and in the evolutionary trends of their paragenesis (e.g., Černý & Ercit 1985, 1989, Černý et al 1986, Spilde & Shearer 1992, Černý & Nĕmec 1995, Lahti 2000, Masau et al 2000, Aurisicchio et al 2002, Zhang et al 2004). This diversity holds not only for pegmatite bodies from widely separated occurrences, but also within closely related local populations.…”