The farmer in the village of Ueken first stumbled upon the "green-tinged" coins in a mole hill in July, the canton said on Thursday.
He contacted Aargau's archaeological department, which sent out staff to look at the “exceptionally well preserved” coins.
Over three days in September volunteer researchers found more coins under a search directed by cantonal archaeologist Georg Matter, the canton said in a news release.
A total of 4,166 coins made of bronze and silver were collected in a find that “exceeds all expectations by far — something you experience as an archaeologist rarely more than once in a career,” Matter said in a statement.
It is one of the largest discoveries of ancient coins made in Switzerland — all found within an area of a few square metres.
Archaeologists have already started to examine the coins that date from between 270 and 294 AD.
Coin expert Hugo Doppler has examined 200 of the coins, examples of the so-called Antoniniani used during the third century of the Roman Empire, the canton said.
They bear the images of different emperors, including Aurelian (dating from 270-275), Tacitus (275-276), Probus (276-282), Carinus (283-285), Diocletian (284-305) and Maximianus (286-305).
The coins have a silver content of five percent and are well preserved because they were immediately withdrawn from circulation, Doppler said in a statement.
“The owner must have deliberately chosen to hoard these coins for the silver in them guaranteed a certain value in a time of economic uncertainty.”
Researchers believe the treasure was accumulated over several years and buried in the ground.
While the original value of the coins is difficult to estimate, the experts believe they represented a considerable fortune at the time, “in the order of average earnings for one to two years”.
Research is continuing on the find, made south of the municipality of Frick where a few months earlier parts of a Roman settlement were excavated.
Other Roman remains have been found in the area, also known for its dinosaur discoveries, where the Romans had an iron ore mine.
From the first century BC until 401AD the area covered by present-day Switzerland was part of the Roman empire and vestiges of Roman structures can be found across the country.
Photo: Canton of Aargau