PATTERNS OF ZONATION IN MAGNESIOCHROMITE-CHROMITE FROM PLACERS OF BRITISH COLUMBIA, CANADA

Abstract

Patterns of compositional zoning are documented in nine grains of magnesiochromite–chromite (Mgc–Chr), 0.3–0.4 mm in size, subhedral or subrounded, recovered from six placer deposits in British Columbia. Their core zones correspond to Mgc or magnesian Chr, mantled by zones of Chr. A rim of Zn-rich chromite (9.1% ZnO) occurs in one grain. In most of these grains, Fe 2+ increases and Mg decreases toward the margin. In contrast, an “anomalous” rim, slightly richer in Mg than the core, is developed in one grain, in which an increase in Mg (≤2% MgO) is accompanied by a corresponding increase in Fe 3+ , Al, minor Ti, and by a decrease in Fe 2+ and Cr. In the other zoned grains, contrasting trends are documented for Fe 3+ and Cr, which decrease or increase during crystallization toward the margin. Aluminum covaries with Fe 3+ and Cr, as a reflection of different mechanisms of coupled substitution. In general, various factors may have exerted a control over the observed zoning, such as 1) loss of Mg during subsolidus re-equilibration between the Mgc–Chr and coexisting olivine, 2) the regime and variations in levels of oxygen fugacity, f (O 2 ), 3) a re-equilibration between the Mgc–Chr and a late melt enriched in Cr, and (4) accumulation of Al in a remaining melt or loss of Al during a re-equilibration at a lower temperature. The Zn was probably introduced into the rim by a reaction involving a late-stage fluid phase. The rim richer in Mg may reflect a diffusion-controlled mechanism of growth resulted from a late-stage reaction with an isolated portion of the more oxygenated melt rich in Fe 3+ , Al and Ti and relatively depleted in Cr. Two types of potential source-rocks, ophiolitic and Alaskan-type, are evaluated for these placer grains of zoned Mgc–Chr.