OSCILLATORY ZONING IN STANNIFEROUS HEMATITE AND ASSOCIATED W- AND Bi-RICH MINERALS FROM CANADIAN CREEK, YUKON, CANADA

We document the first natural occurrence of oscillatory zoning in hematite, due to cryptic variations in tin content. Our studies bear on a placer grain ( ca. 0.5 × 0.7 mm) from a deposit located in Canadian Creek, Yukon, characterized by electron-microprobe analysis (EMP) and Raman spectroscopy. This grain was found in association with ferberite and hubnerite, which form an extensive and nearly continuous series (with up to 10–12 mol.% MgWO4 in ferberite), bismuthinite, daubreeite (?) [BiO(OH)], and tetradymite (Bi2Te1.9S1.1), which displays a minor deviation from stoichiometry owing to S-for-Te substitution. The compositional data, obtained via EMP traverses oriented across oscillatory zones (up to 10 μm thick, diffuse, and “bright” in back-scattered electron imaging) indicate that this zoning is a reflection of covariations in Fe, Sn, and, to a lesser degree, Al. The “bright” oscillatory zones are enriched in Sn, up to 2.26% SnO2, and in Al, up to 0.42% Al2O3, and are correspondingly poorer in Fe. The Sn-enrichment observed in hematite could be related to a solid-solution series from α-Fe2O3 toward a mixed-valence compound Sn2O3, yielding a mechanism (Sn4+ + Sn2+) ↔ 2 Fe3+. However, a regime of low values of oxygen fugacity would be required to stabilize Sn2+, which seems unlikely in this system. Instead, a series could extend from α-Fe2O3 toward SnO2 (cassiterite). Presumably, Sn4+ enters octahedral sites; a substitution Sn4+-for-Fe3+ is combined with a reduction of Fe3+ to Fe2+ in order to maintain charge balance. We prefer this mechanism, and suggest that the incorporation of minor Al3+ may occur to minimize lattice strain due to the incorporation of Sn4+ and Fe2+. This grain of Sn-bearing hematite may have been derived, along with the W- and Bi-rich minerals, from a mineralized zone associated with a fluid-rich environment, probably a hydrothermal deposit (possibly Au-bearing), which is related to Cretaceous felsic magmatism developed in the placer area.