The Elsjø area is situated in the northem part of the Oslo Region and consists of a 2 km2 inlier of Palaeozoic metasediments. Cambro-Ordovician shales and limestones are enclosed in Permian intrusives and are contact metamorphosed and altered by metasomatic processes. The present investigation has defined three mineralisation types of interest: The heavy-metal contcnt of the Alum shale, sphaleritebearing skarn lenses within the Alum shale and molybdenite mineralisation in the underlying Permian granites.
The Alum shale of the Elsjø area has a maximum thickness of more than 150 metres as a result of overthrusting and folding during the Caledonian orogeny. The highest heavy-metal content is in Upper Cambrian strata. The greatest average content is in a 10.6 m thick layer with 160 ppm U, 200 ppm Mo,
930 ppm V, 11.6% C and 2.8% S. The Alum shale also contains lenses of hedenbergite-garnet skarn lenses with sphalerite mineralisation; these are alteration products of marble lenses in the shale. Similar sphalerite-bearing skarn mineralisation is also found in other stratigraphic levels and on shear zones. The Palaeozoic sedimentary rocks are underlain by three types of Permian granites. Molybdenite occurs as scattered disseminations in the phyllic alteration zones of the granites. Propylitic alteration is common in both the underlying intrusives and in the Palaeozoic sediments. The molybdenite mineralisation has similarities with porphyry molybdenum type occurrences, but can also be interpreted as a roof zone mineralisation in the granites.