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

Andrei Y. Barkov¹, Robert F. Martin^1,, Lang Shi¹, William LeBarge² and Yana Fedortchouk^2,¶

¹ Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7, Canada
² Ministry of Energy, Mines and Resources, Yukon Geological Survey, PO Box 2703, Whitehorse, Yukon Y1A 2C6, Canada

E-mail address: bobm{at}eps.mcgill.ca

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 x 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 hübnerite, which form an extensive and nearly continuous series (with up to 10–12 mol.% MgWO₄ in ferberite), bismuthinite, daubréeite (?) [BiO(OH)], and tetradymite (Bi₂Te_1.9S_1.1), 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 -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 -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.

Keywords: oscillatory zoning, hematite, tin, mechanisms of substitution, W–Bi mineralization, placer deposits, Canadian Creek, Yukon, Canada.