The Russian tycoon who has bought a £120m stake in Arsenal, the Premier League leaders, has been accused in court papers of “fraud” and “unjust enrichment” in a dispute over one of the world’s most lucrative diamond mines.

Alisher Usmanov has been named in documents filed by lawyers acting for a firm controlled by the Oppenheimer family, the billionaire dynasty behind the De Beers diamond corporation.

The court case, being brought in America, will add to the controversy surrounding the Russian tycoon, whose personal wealth is estimated at £5 billion. His 23% stake in Arsenal, acquired last month, makes him the club’s second biggest shareholder.

Last week he flew a party of journalists by private jet to the Moscow headquarters of his iron, steel, media and sports empire, and put them up at the five-star Kempinski hotel. He complained that he had been wrongly portrayed in Britain, saying: “I’m not a whipping boy, I expect to be treated with the same respect that I show other people.”

His recent acquisition of the entire art collection of the late cellist, Mstislav Rostropovich, was seen by some as a statement of intent. He spent more than £20m on the 450 artworks, and announced that he would present them to the Russian state to keep the collection intact.

The possibility that he might launch a hostile takeover of the club has rekindled interest in Usmanov’s business past. Peter Hill-Wood, the Arsenal chairman, has said he opposed Usmanov’s involvement because it was not sufficiently clear how the businessman amassed his fortune in Russia and Uzbekistan.

The latest controversy concerns a court action in Denver, Colorado where hearings are due to start next month. At stake is the ownership of the so-called Grib Pipe, a fabulously rich diamond mine in the Arkhangelsk region of northern Russia.

The mining firm, Archangel Diamond Corporation (ADC) in which De Beers owns a controlling stake, claims it is entitled to an interest in the Grib Pipe, which was discovered in 1996. The Grib Pipe is now said to be one of the largest diamond mines in the world, with a prospective value of £4.5 billion.

But in the Colorado court papers, ADC has alleged that Usmanov and other Russian interests “engaged in fraud in order to deceive” it over an agreement it says it had to take a 40% interest in the mine.

Rollo Head, a spokesman at the public relations firm Finsbury, which represents Usmanov, said the case was not against his client and he was not a defendant. “Mr Usmanov refutes and rejects any allegations of fraud or other wrongdoing in relation to the case against [a Russian oil firm],” he said.

An attorney close to the case said that the role of Usmanov was still at issue, because he was at the time deputy chairman of AGD, a Russian firm which is alleged in the court papers to have benefited from the alleged fraud.

According to the Colorado court papers, the de Beers firm says it spent more than $30m in a joint venture with AGD.

Usmanov is alleged in the documents to have personally represented AGD in talks with ADC. His former company is alleged in the court papers to have benefited from “unjust enrichment” because of the scheme.

De Beers, which coined the slogan “a diamond is forever”, controls more than 40% of the world’s diamond supply. Forbes magazine says Nicky Oppenheimer, its chairman, is the 158th richest person in the world with a fortune estimated at £2.5 billion. He spends his time between Johannesburg and England where the family have a farm which specialises in organic foods.

The case stems from a decision in the late 1990s by the government of Boris Yeltsin to strip the assets of Western diamond firms and hand them over to a clique close to the Kremlin.

Foreign firms were encouraged to develop exploration and mining concessions with a view to improving the local economy. They say few in Russia expected that anyone would strike diamonds in the remote area.

Many companies, including Rio Tinto and BHP, withdrew after finding little of value. But in 1996 ADC unexpectedly struck a rich vein.

It was shortly after this, the court papers allege, that Usmanov and others who were involved with a big Russian oil firm became party to a scheme to drive ADC out of Russia and take over the diamond project for themselves. The Russian firm in the joint venture was privatised and after this, ADC was denied access to develop the mine.

The De Beers firm claims it has lost the $30m investment “as well as over $400m in profits” which it expected to win as a result of its 40% stake in a joint venture. It claims it lost a further $800m in profits from other potential mines in the same area.

When the purchase of his initial 14.6% stake in Arsenal was announced last month, Usmanov instructed a firm of London lawyers to warn the media to be careful when reporting the fact that he had spent eight years in prison in Moscow in the late 1980s “for various offences”. These are said to include fraud, corruption and theft of state property.

The tycoon insists he was a political prisoner and his lawyers say he was “fully pardoned” after president Mikhail Gorbachev took office. He said last week that the charges had been cooked up by his enemies in the KGB.

However, parts of his account have been challenged by Craig Murray, the controversial former British ambassador to Uzbekistan, the country in which Usmanov was born.

Murray has made a series of allegations about Usmanov’s business conduct, including one which relates to his alleged financial ties to Gulnara Karimova, the glamorous daughter of Islam Karimov, the strongman president of Uzbekistan.

Usmanov’s lawyers wrote to the internet service provider that hosts Murray’s website demanding that it take down a posting detailing further allegations relating to Usmanov’s business and personal life.

At the press conference in Moscow last week Usmanov warned he would resist anyone who tried to push him out of Arsenal. “People talk about me as an Uzbek businessman involved in narcotics and in a shady regime. It is beyond my dignity to respond to all these allegations . . .

“I don’t want even to qualify what Mr Murray says about me.” He added: “My parents did not bring up a gangster and a racketeer.”

Usmanov is expected to meet the Arsenal board next month to discuss his plans for investing in the club. He has plans to float his Gazmetal mining company in London next spring.

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