AGE AND GEOCHEMISTRY OF GRANITES ASSOCIATED WITH MO-MINERALIZATIONS IN WESTERN BERGSLAGEN, SWEDEN

The Bergslagen area, south-central Sweden, where large amounts of Fe, Cu, Zn, Pb, Au, Ag, Mo and W have been produced for several centuries, is one of the oldest mining districts in Europe. Two episodes of magm atism and ore formation can be distinguished: (1) an ''older'', Early Svecofennian phase that generated 1890-1850 Ma old calc-alkaline, cont inental-margin volcanic and plutonic units related to the Svecofennian orogeny; and (2) a ''younger'' phase comprising potassium-rich granit oids, which intruded over a period extending from 1825 to 1680 Ma duri ng the final stage of, or subsequent to, the regional metamorphism and deformation related to the Svecofennian orogeny. In recent years, con siderable confusion has dominated the discussion concerning the timing and setting of the Mo-W-forming granites in western Bergslagen. In th is study, one of the most important intrusions of undeformed granite i n western Bergslagen has been investigated in regard to geochemistry a nd U-Pb zircon age. The granite (Skalhojden) has a peraluminous, silic a- and potassium-rich chemistry as well as a uniformly flat REE-patter n with a marked negative Eu anomaly. The U-Pb data yielded an age of 1 758 +/- 8 Ma. These data show convincingly that the granite is compara ble to other 1800-1680 Ma Mo-W-bearing granitoids elsewhere in the Fen noscandian Shield but are in strong conflict with hypotheses on Early Svecofennian rift-related crust- and ore-forming processes recently pr oposed for western Bergslagen. A revision of the models of crustal gro wth and ore formation in that area is suggested.