Country-wide black shale mapping using airborne geophysics and petrophysical, geochemical and modeling studies

Black shales host polymetallic sulfide deposits and, moreover, they occur in the vicinity of other types of sulfide deposits and may serve as key horizons in exploration. Black shales are rarely outcropped in glaciated or deeply weathered terrains but can be traced with geophysical surveys. We have recently completed the mapping of Precambrian black shales in Finland and correlated airborne geophysics with the petrophysical properties and chemical characteristics of core samples. Systematic airborne geophysical surveys revealed stratigraphy-related, coupled magnetic and electrically conductive patterns. Electrical conductivity of black shales was related to their graphite and sulfide contents, producing continuous and bending electromagnetic anomaly patterns. The magnetic anomalies, if present, were caused by ferrimagnetic monoclinic pyrrhotite. For modeling, we classified the black shales by non-metric multidimensional method (NMDS). Then we applied this information to the Support Vector Machine (SVM) classification. The modeling distinguished black shale types with exploration potential. In addition to geologists and geophysicists, regional authorities currently use our country-wide black shale database in environmental permit procedures since sulfide-rich black shales may have environmental impacts during anthropogenic activities.