Characterization of sulfide deposits by airborne magnetic and gamma-ray responses in eastern Finland

Abstract

Integrated interpretation of aerogeophysical data permits geophysical characterization of sulfide deposits in eastern Finland. The structural control and the hydrothermal alteration of the rocks can be discerned from airborne magnetic and radiometric data. The Cu–Co–Zn deposits associated with serpentinite–dolomite–calc-silicate rock–siliceous rock–black shale settings of the Outokumpu type are locally characterized by reduced intensity of magnetization and electrical conductivity and reduced eTh and K radiation. Enhanced eU/eTh anomalies are commonly spatially related to major structural lineaments, such as the boundaries of structurally more coherent bedrock blocks or intersections of faults and shear zones. Electromagnetic surveys can be used to distinguish black shales and serpentinites in the region. The sulfide deposits in eastern Finland can be divided into two groups on the basis of airborne geophysical characteristics: (1) intense magnetization and high electrical conductivity and (2) moderate magnetization and electrical conductivity. In the sulfide deposits of group 1, ore-forming constituents mainly precipitated during primary sedimentation (for example, the black-shale-hosted Ni–Cu–Zn ore at Talvivaara). Group 2 is related to sulfide deposits where major ore-forming processes have affected the associated black shale formation after the sedimentation phase (Outokumpu-type Cu–Co–Zn deposits). In cases where the magnetization and electrical conductivity of black shales are especially weak, ore formation processes probably continued during late metamorphic or post-metamorphic deformation. The theoretical basis for the observed differences in magnetic response between different deposits has to do with the mineralogy of the black shales. Pyrite changes into ferrimagnetic monoclinic pyrrhotite during metamorphism above greenschist facies, especially along tectonic structures. If no further alteration occurs, pyrrhotite-bearing black shale formations appear as highly magnetic and electrically conductive zones on geophysical maps. Wherever intense tectonic deformation, emplacement of quartz–sulfide veins, hydrothermal alteration, or contact metamorphism has affected the black shale formation after sedimentation, both magnetic and electromagnetic responses may be weakened because of diminished amounts of the pyrrhotite-bearing and graphite-rich material. In addition, the presence of antiferromagnetic pyrrhotite of hexagonal crystal structure may decrease the magnetic response. The classification presented here applies specifically to eastern Finland, but may be widely applicable to terranes with abundant black shale formations that have undergone regional metamorphism of upper greenschist facies or higher. Association of sulfide ores with black shales is a common feature in the geological record. Our integrated geophysical method offers a quick way of examining large areas, and it should prove useful in the search for sulfide deposits in other parts of the world.