The silver denarius was the principal coin of the Roman Republic and Empire. The denarius remained an important Roman coin until the Roman economy began to crumble. As the Roman economy crumbled, the denarius which had proliferated the empire began to be adapted to local needs. The medieval period for almost all regional economies is marked by re-use of Roman coinage. It's legacy can be seen in that the word was preserved in most Romance languages; denier in French, the dinero in Spanish, denari in Italian, and denar in Hungarian.
The later 5th and 6th centuries are very murky in almost every way, and coinage is no exception. The once vigorous late Roman monetary system lay in tatters, with almost no new minting and very little importation of new coins. Nevertheless, it is apparent that coinage never faded away completely, and that re-use of the existing supply of coinage continued throughout the period.
It is worth noting that few gold coins were struck in the West since the fall of the Roman Empire. Gold coins were in circulation, of course, but the West simply made use of existing coins as well as new coins from Byzantium or Islam. There actually was a ‘gold’ phase of currency in England, which began with an increase in the rate of importation of continental gold, principally in the form of tremisses. By the middle of the 7th century the quantity of gold in these coins had declined rapidly, and by the 670s they were more or less completely silver. Existing silver seems to have been sufficient, even with debasement occuring.
The first European gold coin struck in sufficient quantities to play a significant commercial role since the seventh century was the Florin, provided by Florence in 1252. Gold coins had also been occasionally struck as commemorative pieces for big events, but this was the first that was intended as currency.
A wonderful insight into the history of the english coinage during the period you are interested in:
History of English penny (600-1066)
History of the Gold Florin