The Cr-minerals of Outokumpu--Their Chemistry and Significance

The massive Cu–Co–Zn sulfide ores at Outokumpu inFinnish Karelia are unusual among Cyprus type deposits by virtueof the high Cr content exhibited by both the ores and the hostrocks. Throughout both the ore horizons and the metasedimentaryhost rock, bulk-rock Cr content averages 0.2 per cent Cr₂O₃.The high Cr contents result in an unusual degree of Cr substitutionin a number of silicate and oxide phases. Cr₂O₃ analyses presentedhere include values up to 3.74 per cent in tremolite, 3.65 percent in diopside, 26.90 per cent in uvarovitic garnet, 15.37per cent in epidote, 24.63 per cent in muscovite, 8.56 per centin biotite, 5.18 per cent in chlorite, 2.99 per cent in staurolite,and 72.65 per cent in chromite. Cr contents of these mineralscan be extremely variable even on the thin-section scale. Thehigh level of Cr in the metasediments, together with that ofNi, reflects their enhanced concentration in the original sedimentsdue to presumed derivation from a Cr-rich komatiite-bearingsubstrate. High levels of both elements in the stock works indicatethat they may have been partly re-distributed by the hydrothermalore-forming fluids. The variability of Cr content within individualsilicate grains suggests that they overgrew small-scale variationsin sedimentary Cr content sufficiently rapidly to preclude diffusivesmoothing of µ_Cr gradients. This suggests that while Crmay be mobile in hydrothermal systems, it is immobile undermetamorphic conditions.