The Tornio layered intrusion - a recently discovered intrusion with chromitite horizons in Northern Finland

Abstract

In the 1960's chromite boulders were found in the area between the Kemi chromium mine and the town of Tornio. In 1979 after many years of prospecting a layered intrusion with thin chromitite layers was found northeast of the town of Tornio. The stratified sill-like intrusion is situated between the Presvecokarelian basement complex and the Peräpohja schist formation. The age of the intrusion is very likely the same as that of the Kemi and Penikat intrusions, that is 2 440 Ma. The rocks both below and immediately above these intrusions are older than 2 440 Ma. The length of the intrusion is at least 6 km and it extends into Sweden. Several thin chromitite layers have been intersected by drilling, all in the ultramafic unit. The Cr2O3 content of the chromitite layers varies between 26 and 32%, the FeOtot content between 22 and 28% and the Cr/Fe ratio between 0.9 and 1.2. The chromites are Fe-rich and Mg-poor averaging 43 % Cr2O3. The chromite deposit of the Tornio intrusion, like those of the Kemi and Penikat intrusions, is of the stratiform type. The Tornio and Penikat intrusions resemble each other as do the compositions of their chromites.