Age of porphyry-type deposits in the Skellefte District, northern Sweden

Abstract

The Skellefte District in northern Sweden consists of metamorphosed Lower Proterozoic submarine volcanic, sedimentary, and intrusive rocks. Several massive sulphide deposits occur in the volcanics, and several small porphyry-type deposits exist in the oldest granitoids, which are coeval with the volcanics. The volcanic rocks have been dated previously at 1.88 Ga while the oldest granitoids have an age of 1.89 Ga. The aim of this study was to establish the timing of the porphyry systems. For this purpose, an intrusive quartz-feldspar porphyry, associated with the Tallberg porphyry-type deposit, has been dated. The U-Pb zircon age is 1886⁺¹⁵ _-95 Ma which is within the error limits of both the host tonalites (the oldest Jörn granitoids) and the lower Skellefte volcanic rocks, hosting the massive sulphides. While the massive sulphide deposits were formed on the 1.88–1.89 Ga old, early Proterozoic seafloor, the porphyry-type deposits formed farther north inside the marginal arc, both types of deposit occuring in a tectonic regime similar to modern destructive plate margins.

Key Words:

• Porphyry copper
• Proterozoic
• age determination
• U-Pb zircon
• 1886 Ma. Skellefte District
• Sweden