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Abstract

The massive Cu–Co–Zn sulfide ores at Outokumpu in Finnish Karelia are unusual among Cyprus type deposits by virtue of the high Cr content exhibited by both the ores and the host rocks. Throughout both the ore horizons and the metasedimentary host rock, bulk-rock Cr content averages 0.2 per cent Cr2O3. The high Cr contents result in an unusual degree of Cr substitution in a number of silicate and oxide phases. Cr2O3 analyses presented here include values up to 3.74 per cent in tremolite, 3.65 per cent in diopside, 26.90 per cent in uvarovitic garnet, 15.37 per cent in epidote, 24.63 per cent in muscovite, 8.56 per cent in biotite, 5.18 per cent in chlorite, 2.99 per cent in staurolite, and 72.65 per cent in chromite. Cr contents of these minerals can be extremely variable even on the thin-section scale. The high level of Cr in the metasediments, together with that of Ni, reflects their enhanced concentration in the original sediments due to presumed derivation from a Cr-rich komatiite-bearing substrate. High levels of both elements in the stock works indicate that they may have been partly re-distributed by the hydrothermal ore-forming fluids. The variability of Cr content within individual silicate grains suggests that they overgrew small-scale variations in sedimentary Cr content sufficiently rapidly to preclude diffusive smoothing of μCr gradients. This suggests that while Cr may be mobile in hydrothermal systems, it is immobile under metamorphic conditions.

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