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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 Islamic Caliphates. There actually was a βgoldβ 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 debas*m*nt occurring.
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:
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One of the fundamental reasons why the Roman Empire fell is the shortage of precious metals. The lack of coin is not the effect of the Empire's crumbling, it is it's cause. The shortage was both of silver and gold. Under Augustus the silver denarius was pure. Nero added 5-10% alloy. Trajan raised the alloy content to 15%. Marcus Aurelius to 25%. Under Severus, around 200 A.D., half of the denarius was base metal. Sixty years later, under Gallienus, the antonianus (the new coin) had only 5% silver. Gold coins suffered a similar fate.
There are two fundamental reasons for this shortage. First is the exhaustion of the Mediterranean mines. Second is the unilateral trade with China and India, which along the centuries became sinkholes for Roman coin.
"We already find Tiberius complaining in his day that the Romans were giving away their money to foreign peoples for jewels, and under Vespasian the imports from the east amounted to no less than 100 million sesterces (22 million marks) annually. In the two centuries from Augustus to Septimius Severus, something like 4 billion marks of valuable metals could have drifted from the Roman Empire to India and East Asia."
["The Barbarian Invasions" - Hans Delbruck]
We could also derive a third one from the second, that is - the coin given to the barbarian mercenaries (as pay and later as tribute) also did not return to the Roman economy.
The continuous debas*m*nt of the currency went hand in hand with the State's incapacity to pay it's troops. The result was allowing the troops stationed on the borders to practice farming. Some emperors even allowed them to live with their wives. From the psychological military point of view, this was a catastrophe. A farmer-soldier is cannon fodder for the semi-savage, naturally war-like barbarian peoples. The result was that the emperors started hiring barbarian mercenaries, given that the Roman troops became ineffective. This was the beginning of the end for the Roman Empire, and it began in the 3rd century.
The reason why so much non-debased coin is found in Scandinavia/Britain is that the barbarians demanded imperiously pure silver/gold. The emperors had to pay them that, draining to the utmost the already scanty reserves which flowed in the civil economy. That's why you won't find pure coin on the territory of the Empire itself.
Now, to answer your question itself: When precious metals started flowing anew in Europe, it was because mines were being rediscovered, especially in Central Europe: Bohemia, Austria and Southern Germany. These mines were mainly of silver. This is the basic answer.