Source of metals and age constraints for epigenetic gold deposits in the Skellefte and Pohjanmaa districts, central part of the Fennoscandian Shield

Abstract

Abstract. The lead isotopic compositions of galena in Early Proterozoic gold deposits have been determined for three districts in northern Sweden and central Finland. The deposits are hosted by a variety of ~ 1870-1890 Ma Svecofennian host rocks including the volcanosedimentary succession within the Skellefte District island arc in Sweden as well as I-type tonalites at J6rn (Sweden) and Pohjanmaa (Finland). The deposits are epigenetic in relation to these Svecofennian rocks and are part of a goldbearing metallogenetic belt, which can be followed for 600 km parallel to the southwestern margin of the Archaean Domain. In spite of these epigenetic relationships, the lead isotopic data indicate that the deposits are not dramatically younger than the ~1870-1890 Ma Svecofennian host rocks (probably not exceeding 10-20 million years). Two principal lead sources were activated when the gold deposits were formed.-The most significant source is represented by the I-type tonalites, which constitute a relatively primitive (/~ = 9.3) and widely distributed source in the entire metaUogenic belt. In addition, the volcanic components in the westernmost part of the Skellefte District constitute an extremely primitive (~ < 9.0) source, which only locally was an important contributor to the epigenetic deposits in this metallogenetic belt. The significantly different lead isotopic composition estimated for these sources indicates that the volcanic rocks in the western part of the SkeUefte District were not comagmatic with the I-type tonalites recognized at J6rn and central Finland.partly related to volcanogenic massive sulphide deposits of the Skellefte type, partly related to synkinematic intrusions of the J6rn type and partly related to major shear zone structures. This complex metallogenetic province can be followed over a length of about 600 km and includes the Skellefte mining district in northern Sweden, and the 'main sulphide belt' of central Finland (Fig. 1). Volcanogenic massive sulphide deposits (VMS) and Niores were until recently the only ore types which were mined in this province, but during the last decade, increasing interest has also been focused on gold lode deposits and two gold mines have come into production since 1988.The origin of the massive sulphide deposits and the Ni-ores has been discussed in a number of papers (see e.g.

Fluid inclusions, stable isotopes and gold deposition at Bj�rkdal, northern Sweden

“…The age of this intrusive-hosted deposit is constrained to c. 1886 Ma (U-Pb dating of the related porphyritic intrusive; Weihed and Sch6berg 1991). This adds further support to the importance of early plutonic processes and linked gold mineralization (compare Sundblad et al 1993) and the hypothesis that surface-derived waters were important for the concentration of gold.…”

Geology and Age Constraints on the Origin of the Intrusion-Related, Sheeted Vein-Type Åkerberg Gold Deposit, Skellefte District, Sweden

“…Groves et al [65] suggested that the presence of W-Sn deposits in the magmatic province and a continental tectonic setting well inboard of convergent plate boundaries as diagnostic for intrusion-related type of deposits. There are no W-Sn deposits in the Åkerberg area, although such mineralizations occur in the so called -Au line‖ west of the Skellefte district where some potential intrusion-related deposits occur [14,24]. However, Åkerberg is tentatively situated in a back-arc region [34] which appears to be consistent with a typical tectonic setting for the intrusion-related group of deposits.…”

Synvolcanic mixing of ore lead and the development of lead isotopic provinces in the Skellefte district, Sweden

“…2. Additional data are from Vaasjoki and Vivallo (1990) and Sundblad et al (1993). The bulk of the lead is fixed in galena and sulphosalts and unless otherwise stated, the following discussion refers to such data.…”

“…This highlights the need to characterize the individual source components that have contributed to the a The analyses have been performed at Helsinki (G-numbers), at Geospec, Canada (Sk-numbers) and at LIG, Stockholm (LIG-numbers). Helsinki data have been published by Vaasjoki and Vivallo (1990) and Vindelgransele data by Sundblad et al (1993) b Unless otherwise stated the mineral analyzed is galena; sp, sphalerite; py, pyrite; su, sulphosalt c Model ages and #-values (present-day 23aU/2~ have been calculated according to Stacey and Kramers (1975) analyzed lead mixtures and to derive a model for the lead isotopic evolution that is consistent with the local geology.…”