A GENETIC REINTERPRETATION OF THE FALUN AND AMMEBERG ORE TYPES, BERGSLAGEN, SWEDEN

The stratiform sulphide and oxide ores of Bergslagen, south-central Sw eden, constitute the largest concentration of base metal and iron ores in northern Europe. They are hosted by Early Proterozoic metamorphose d volcanic and sedimentary successions, which (together with later gra nitoids) belong to the Svecofennian Domain. An earlier genetic model s uggested that two principal types of sulphide ores existed in Bergslag en (Falun and Ammeberg), which had been formed through two contrasting granitoid-related processes, whereas the iron oxide ores were conside red exhalative-volcanogenic. The prevailing view for the Bergslagen or es is that all stratiform sulphide (and oxide) ores were formed by exh alative-volcanogenic processes from one homogenous metal source. In th is paper are presented new high-precision determinations of the ore le ad isotopic composition of twentytwo stratiform sulphide and oxide ore s in Bergslagen (among them Falun, Zinkgruvan-Ammeberg and Dannemora), in order to provide an improved base for their genesis. The results s how that significant variations in metal sources existed in Bergslagen for the volcanogenic ores. Most ores (including the Falun Cu-Zn-Pb de posit) were formed from a major isotopic reservoir that occurs in the interior parts of Bergslagen. This source is defined as the Falun rese rvoir and is dominated by calcalkaline felsic volcanic rocks. A variab le input from a more evolved source component (recycled pre-Svecofenni an crustal components) is locally important in sediment-dominated area s, particularly the Stockholm archipelago. A third source, representin g relatively primitive metabasalts, influenced the lead isotopic patte rn of ores in westernmost Bergslagen. The composition of the Zinkgruva n (Ammeberg) ore lead is distinctly different from that of the Falun r eservoir, but forms, together with other sulphide deposits along the s outhern margin of Bergslagen, a pronounced linear trend in standard is otope diagrams. The linear trend is interpreted as a mixing line and s hows that the lead in these ores were derived partly from evolved and partly from primitive sources. The evolved end member has an isotopic composition, which is comparable with the sediment-dominated sources i n the interior parts of Bergslagen and in the Stockholm archipelago. T he primitive end member is represented by tholeiitic volcanics, which are more abundant in the southern margin of Bergslagen than elsewhere in Bergslagen. A significant variation with respect to metal sources a nd depositional environments can thus be recognized for the Bergslagen ores and a renaissance for the genetic concepts Falun and Ammeberg ty pes is suggested.