Recent research has shown that Cypriot copper, which was naturally of great purity, was being traded to Scandinavia about as far as 1700 BCE, / 3700 BP.
The interesting thing is Scandinavian rock carvings which not only show ox-hide ingots but Mediterranean style ships, indicating that at least some ancient Swedes travelled to the Med from about that time.
Representations of oxhide ingots in Scandinavian rock art: the sketchbook of a Bronze Age traveller?
Johan Ling and Zoa Stos-Gale
Antiquity / Volume 89 / Issue 343 / February 2015, pp 191 - 209 DOI: 10.15184/aqy.2014.1, Published online: 30 January 2015:
http://journals.cambridge.org/abstract_S0003598X14000015