The ore potential of saprolites and variably ferruginised associated lithologies on Fosen Peninsula, Trondheimsfjord area, Central Norway

Abstract Results of a regional-scale lithogeochemical study of 57 saprolites and variably altered, spatially associated lithologies of a metavolcano-sedimentary succession in the Fosen-Trondheimsfjord area, Central Norway, are presented. Analyses of 53 elements after aqua regia digestion revealed Pb, Cu, Zn, Mo, As, Au, Ag, Bi, Cd, Mn, Sb, Se and V median concentrations in saprolite samples to be above those of spatially associated greenschist- to amphibolite-facies metabasite, gneiss and schist. Single analyses of saprolites are as high as 1055 ppb Ag, 87 ppm Pb and 8.8 ppm Se. These metal concentrations are subeconomic and below concentrations of geochemically fresh samples of known ore deposits in the Trondheimsfjord and adjacent areas. Based on the absence of clay minerals in the saprolites and other signs of weathering other than sulphide dissolution, supergene metal enrichment beneath the ferruginised zones of the Fosen-Trondheimsfjord area is unlikely to have occurred. Instead, saprolites and strongly ferruginised rocks of the study area are interpreted to be the product of superficial alteration and weathering of locally abundant primary pyrite in metabasite and pyrrhotite in metasiliciclastic horizons of the supracrustal lithologies. Distinct element associations in the various rock types are identified by cluster and robust principal component analyses, which may indicate that primary compositional features have been retained during the pervasive Caledonian metamorphism.