LITHOGEOCHEMISTRY, METAL AND ALTERATION ZONING IN THE PROTEROZOIC TALLBERG PORPHYRY-TYPE DEPOSIT, NORTHERN SWEDEN

The Tallberg porphyry-type deposit is situated in the Early Proterozoic, syntectonic Jorn Granitoid Complex (JGC). The occurrence shows similarities with Phanerozoic porphyry type deposits: (a) low grade (0.27% Cu) and large tonnage, 44 x 10(6) metric tonnes, (b) stockwork as well as disseminated sulphides, (c) hydrothermal propylitic and phyllic alterations, (d) sulphides spatially associated to tectonic lineaments, and (e) genetically related to granitoid porphyry stocks. Lithogeochemical investigations indicate a metal zonation with (Mo + Cu + S) --> (Zn + Pb +/- Au) --> (As) from centre outwards. The host granitoid shows similarities with modern I-type granitoids, and the high primary content of Cu and Fe is possibly a discriminant factor for exploration for similar porphyry-type occurrences in Early Proterozoic volcanic are terrains. Late shear zones adjacent to postmineral dykes have high Au contents (2-3 g/t), which are interpreted as gold mobilized from the porphyry-type deposit.