ISOTOPE SYSTEMATICS OF THE KIRUNA MAGNETITE ORES, SWEDEN .2. EVIDENCE FOR A SECONDARY EVENT 400 MY AFTER ORE FORMATION

The Kiirunavaara magnetite-apatite ore occurs within, and is coeval with, an acid 1.89-Ga volcanic complex, the whole overlying the Kiruna greenstones. The ore experienced an intensive hydrothermal event at around 1.5 Ga, some 400 m.y. after ore formation. This event caused resetting of the Sm-Nd isotope system in the magnetite ore itself but did not widely affect host-rock Sm-Nd systems. The ore gives an Sm-Nd isochron age of 1.49 +/- 0.13 Ga (MSWD = 2.1). In contrast, the host-rock Sm-Nd systems are not reset but give an age consistent with U-Pb zircon ages. The initial Nd isotope composition (epsilon = -11.1) of the ore demonstrates that the rare earths in the ore had a long crustal residence and could not have been derived from the underlying Kiruna greenstones; it is similar to the intrusive igneous host rocks and a cogenetic origin is likely. Whole-rock Rb-Sr and U-Pb systems are reset in both the ore and host rocks. Resetting of the former system is observed on a regional scale throughout northern Sweden. The only nonisotopic trace of the 1.5-Ga event in the ore is the abundance of disseminated and vein pyrite (+/- calcite +/- chalcopyrite) which is locally concentrated in the primary magnetite-apatite ore, particularly along faults. The lead isotope compositions of these pyrites are similar to lead isotope compositions from ore and host rocks. They all plot on a secondary isochron with an age of 1.54 +/- 0.07 Ga (MSWD = 2.39). The porous textures of the magmatic magnetite primary ore as well as the faults may both have enhanced penetration of hydrothermal fluids, thereby localizing more intensive resetting. The evidence suggests that the late-stage pyrite-chalcopyrite mineralization in the Kiruna ores is not part of the primary magnetite paragenesis but was imposed upon the deposit at a later stage. The sulfide paragenesis appears to reflect a low-temperature hydrothermal event related to regional uplift and associated alteration of host rocks. Extension of Kiruna-type magnetite-apatite ores to embrace iron and copper sulfide facies may be misleading.