Hydrocarbons in the Säviä volcanic schist zone, central Finland

Abstract

Light hydrocarbon gases have been analysed from 21 samples of hydrothermally unaltered and 11 samples of hydrothermally altered metavolcanic rocks from the Säviä volcanic schist zone in the southwestern part of the Proterozoic Vihanti-Pyhäsalmi-Pielavesi Cu-Zn ore province, central Finland. The altered rocks are enriched in the lightest hydrocarbons (propanes and lighter) and depleted in the heavier hydrocarbons (butanes and heavier) compared with the metavolcanic rocks. The difference is clearest for methane (297.3 ppb in the altered rocks vs. 150.3 ppb in the metavolcanic rocks), becoming less marked for the heavier hydrocarbons. A pre-metamorphic hydrothermal origin related to the alteration of the volcanic rocks is the most probable reason for the difference. It is suggested, in the light of the limited data available, that a negative methane anomaly (diameter about 8 km) and positive cyclopropane and pentane anomalies (diameters about 2 km) generated by ore forming processes exist around the Säviä Cu-Zn deposit.