There is no universal currency in pre-modern societies, and what uniformity there was broke down with the fall of the western Empire.

The literal gold standard of coinage in the Mediterranean basin and beyond was the Byzantine solidus, which was originally of very high quality (~95% purity). For all of late Antiquity, wealth of any larger scale was calculated by the solidus. The solidus was primarily minted in Byzantium, but there are some examples of western imitations into the sixth century.

Most of the western economy was based on silver coinage. Silver coins of various types and purity were minted by whatever local potentate could afford to do so.

In either case, the ability to mint coins was a sign of authority, and the ability to mint gold coins was a sign of imperial authority. For example, Charlemagne re-started the minting of gold coins to legitimize his imperial claims.

Bronze and copper-alloy coins died out in the west in the late 6th century and did not reappear in force until the second millennium.

So, to answer your question, it's most likely that you would have found mostly have been silver with some copper-alloy. Very wealthy persons might have used gold in some circumstances.

See:

• Brown, Peter. Through the Eye of a Needle: Wealth, the Fall of Rome, and the Making of Christianity in the West, 350-550 AD. Princeton, N.J: Princeton University Press, 2012.

• Naismith, Rory. “Gold Coinage and Its Use in the Post-Roman West.” Speculum 89, no. 02 (April 2014): 273–306. doi:10.1017/S0038713413004533.