H漫画

B.Sc. (Honours) Thesis

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Chromite (FeCr2O4) is an oxide mineral and common accessory phase in ultramafic rocks. More unusually, chromite occurs in concentrated strata or lenses within layered mafic intrusions. The komatiite-hosted Blackbird Chromite deposit, located near McFaulds Lake in the James Bay Lowlands, Ontario, is one of five known chromite deposits within the Ring of Fire intrusive complex, and is the subject of this study. Chromite is the only chromium ore, and such deposits have great economic importance. Despite this, the conditions of formation of large chromite deposits are poorly constrained. The purpose of this investigation is to characterize the effect of magmatic water on the mineral phase relationships in komatiitic magmas. Anhydrous studies of komatiites have shown that olivine is the earliest-crystallizing phase in these magmas. It is known from studies of more felsic melt compositions that, while magmatic water inhibits crystallization of most silicate phases, it has a relatively lesser impact on oxide stability. Orthopyroxene, an important constituent in komatiites, readily accepts chromium into its structure, such that early-crystallizing pyroxene can prevent chromite from precipitating. Water is hypothesized to promote chromite crystallization by suppressing the growth of silicate phases that might otherwise compete for chromium in the melt, thereby extending the interval over which chromite may then accumulate, possibly as the only crystalline phase. Calculated phase equilibria suggest that, for magmatic water content greater than 3.5 wt%, the olivine liquidus may be sufficiently depressed to allow chromite to form as the sole liquidus phase. Experiments using synthetic komatiite with up to 4 wt% added H2O and varied Cr contents have been equilibrated at temperatures ranging from 1350-1600掳C. To accommodate water, experimental materials were sealed in graphite-lined platinum capsules and pressurized to 1 GPa in a pistoncylinder apparatus. Results from this study show that magmatic water increases the solubility of chromium in a komatiitic magma, which may have implications as a mechanism for chromium transfer.

Keywords: chromite, komatiite, experimental petrology, economic geology, ore petrology
Pages: 45
Supervisor: James Brenan