Outokumpu
Outokumpu
Eastern Finnish ophiolite-derived ore district with minor gem occurrences
Outokumpu is a mining district in the North Karelia region of eastern Finland, historically one of the most productive copper and chromite ore bodies in northern Europe and the namesake of the Finnish mining and metallurgy company that grew out of the original ore find. The district enters the gemmological literature for the chromium-bearing silicate by-products of its ophiolitic host rock — chromian diopside, occasional uvarovite garnet, and rare chromite cabochon material — rather than for any commercially significant production of cut stones.
Geological setting
The Outokumpu ore body is a metamorphosed Palaeoproterozoic ophiolite assemblage tectonically emplaced into the Karelian metasedimentary sequence. The principal ore minerals are chalcopyrite, pyrrhotite, and pentlandite, with chromite as a chromium-rich accessory. Chromium liberated from the chromite during metamorphism colours the surrounding silicate matrix and produces the chromian diopside and uvarovite garnet that are the district's gemmological signature. The same metamorphic regime is responsible for the talc-tremolite-carbonate rocks that host the Outokumpu serpentinites.
Gemmological material
Chromian diopside from Outokumpu occurs as small green crystals and as massive vein material, occasionally cuttable in cabochon and rarely facetable. The chromium-related green hue is comparable to that of chromian diopside from Yakutia in Siberia, although Outokumpu material rarely reaches the saturation of the better Russian stones. Uvarovite from the district occurs as small drusy crystals on chromite or in cracks within the ore, comparable to the more familiar uvarovite drusy from the Saranovskoye mine in the Urals; faceted Outokumpu uvarovite is essentially unknown commercially because the crystals are too small to yield cut stones. Chromite itself appears in collector specimens; gem-quality material from Outokumpu is occasional rather than typical.
Production history
Mining at Outokumpu began in 1910, with the Keretti and Vuonos shafts working the principal ore lenses through the twentieth century. The original Outokumpu mine closed in 1989; the related Pyhäsalmi mine continued in production into the twenty-first century, and other ore bodies in the broader Karelian belt remain active. Chromite was a smaller component of the production but is the source of much of the chromium that subsequently enters the Finnish gem-mineral specimen market.
In the trade
Outokumpu is a name encountered principally in the specimen and reference collections rather than in the cutting trade. Faceted material from the district is rare and is held more often by mineralogical museums than by jewellers. The Geological Survey of Finland and the Finnish Museum of Natural History hold the most complete reference suites. The Mineralogical Record and the Lapis journal have published the principal occurrence descriptions.
Identification
Chromian diopside identification follows the standard routine for the species — refractive indices around 1.67 to 1.70, biaxial positive optic character, hardness 5.5 to 6.5, and specific gravity around 3.30. The chromium-related absorption in the red gives the green hue and is detectable spectroscopically. For uvarovite, the andradite-uvarovite series is identified by refractive index above 1.85 and specific gravity above 3.7, with chromium spectral features confirming the uvarovite end-member component.