Exsolution of zirconian-hafnian wodginite from manganoan-tantalian cassiterite, Annie Claim #3 granitic pegmatite, southeastern Manitoba, Canada

Primary (Mn,Fe,Ta,Nb)-bearing cassiterite with less than or equal to 0.21 w t.% ZrO2 and less than or equal to 0.05 wt.% HfO2 crystallized during the c onsolidation of the Archean Annie Claim #3 zoned, lepidolite-subtype granit ic pegmatite, southeastern Manitoba, in amounts increasing from the outer t o the inner pegmatite zones. Rare inclusions of tantalite and ferrotapiolit e 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 Fe-dominant, whereas the more abundant late cassiteri te from the lepidolite core is strongly Mn-dominant and poor in the substit uents. Exsolution in the primary cassiterite generated granular zirconian-h afnian ferrowodginite and wodginite enclosed in veinlets of substituent-dep leted 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 per ceptibly decreases from the outer zones inward. Maximum contents of ZrO2 an d HfO2 (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 chara cterize wodginite exsolved from early cassiterite; the levels drop off in w odginite exsolved in late cassiterite. Instead, hafnian zircon intergrown w ith abundant thorian coffinite becomes abundant in the lepidolite core, ind icating a transition of Zr + Hf from octahedral sites in the oxide minerals to the much more common 8-fold coordination in the orthosilicate; this shi ft implies a transition from relatively alkaline to more acidic conditions.