WILMINGTON, Del. (AP) _ In a ruling that could have wide-reaching implications for corporate takeover wars, a Delaware judge has upheld a controversial move by Newmont Mining Corp. to thwart a hostile buyout by a T. Boone Pickens-led investor group.

Vice Chancellor Jack Jacobs on Thursday upheld a ''street sweep'' in which Newmont's biggest stockholder, Britain's Consolidated Gold Fields PLC, scooped up 15.8 million Newmont shares in two days to prevent the Pickens group from acquiring a controlling stake in the company.

The ruling also threw a major obstacle in the way of the multibillion- dollar takeover bid by the investor group, Ivanhoe Partners.

Ivanhoe had sued to void the stock purchases on the grounds that Newmont breached its fiduciary duties to shareholders by allowing Consolidated to acquire the shares.

Ivanhoe announced today that it intends to appeal Jacobs' ruling to the Delaware Supreme Court. In order to accommodate the appeal, Ivanhoe Thursday extended its cash tender offer for 28 million common shares of Newmont until Oct. 29. The offer was to have expired at midnight this morning.

Meanwhile, Wall Street reacted to the latest developments by sending Newmont's stock down $8.62 1/2 a share by midday, to $50 per share.

Consolidated's purchases raised its stake in Newmont from 26.2 percent to 49.7 percent.

Ivanhoe is offering $72 a share to acquire 28 million of Newmont's 66.8 million common shares outstanding, which combined with its present holdings would give it a 51 percent stake in Newmont. If the tender offer succeeded, Ivanhoe then would move eventually to acquire the remaining Newmont shares for $72 apiece, giving the takeover a value of more than $4.3 billion.

The case had been watched closely by stock speculators and others, because of the possible implications of the use of the ''street sweep'' as a defensive measure.

The Securities and Exchange Commission also has said it might outlaw street sweeps as unfair to shareholders, on the grounds they did not allow enough time for shareholders to evaluate the offering price.

Like many other major corporations, New York-based Newmont is incorporated in Delaware. As a result, Delaware court rulings can have far-reaching implications on corporate law.

Jacobs, in a 60-page ruling, denied a request by Pickens for a preliminary injunction and also vacated previous orders that barred Consolidated from exercising its stock voting rights.

''To require Consolidated Gold Fields to divest its 23 percent block of Newmont stock would not only undo an act that was legally proper, and that would have occurred irrespective of any standstill agreement, but it would also effectively grant plaintiffs all the relief they hope to gain after a final hearing,'' Jacobs said.

No street sweep previously had been challenged under Delaware corporate fiduciary principles, the judge noted, adding that recent federal court challenges of the anti-takeover defense had met with little success.

''To enjoin Gold Fields' now completed stock purchases is unnecessary. The street sweep was a legally proper transaction that did not, by itself, involve actionable wrongdoing,'' the Chancery Court judge wrote.

Jacobs did express concern about the ambiguity of a Newmont-Consolidated agreement over whether the British company could designate up to 40 percent of Newmont's board before Newmont's certificate of incorporation was amended to authorize cumulative stock voting.

That concern could be satisfied if the parties reached an agreement to preserve the board's composition pending litigation or were able to resolved the ambiguity on an interim basis, the judge stated.

''Unless and until the court is advised that the parties are unable to agree upon such interim arrangements, it would not be appropriate to impose preliminary injunctive restraints,'' Jacobs said.

On Sept. 28, Jacobs had ruled that Consolidated could consumate its purchase of Newmont shares, but had to hold the shares separately until a decision was made on Ivanhoe's legal challenge. He vacated that order on Thursday.

Newmount is a major energy and mining company, which through its Newmont Gold Co. subsidiary holds the largest gold reserves in North America. The company has stiffly resisted the takeover bid by Ivanhoe, and was joined from the outset by Consolidated Gold Fields.