WILMINGTON, Del. (AP) _ A state judge on Thursday upheld the legality of a British company's purchase of a major chunk of Newmont Mining Corp., in a key ruling against a hostile takeover attempt by T. Boone Pickens.

Vice Chancellor Jack Jacobs, in a 60-page ruling, denied a request by a Pickens-led investor group to void stock purchases by Newmont's ally, Consolidated Gold Fields PLC, that gave the company a 49 percent stake in Newmont.

The investor group, Ivanhoe Partners, had sought an injunction to invalidate the purchases on the grounds Newmont had breached its fiduciary duty to shareholders by allowing Consolidated to scoop up 15.8 million Newmont shares in a two-day ''street sweep'' last month.

The $1.5 billion in open-market purchases raised Consolidated's stake from 26.2 percent to 49.7 percent of Newmont, a New York-based company incoporated in Delaware.

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 51 percent of Newmont.

''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,'' Jacobs wrote.

''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.

Ivanhoe officials and attorneys could not immediately be reached for comment.

Newmont stock dipped 12 1/2 cents to $58.37 1/2 in New York Stock Exchange trading. The ruling was released after the market had closed.

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

Ivanhoe Partners contended the agreement under which Newmont allowed Consolidated to buy up to 49.9 percent of its stock amounted to an illegal scheme that would hurt Newmont shareholders by preventing Ivanhoe's $72-a- share takeover offer from proceeding.

Jacobs ruled that except in one area, the challenged transactions were not shown to violate any Delaware law or fiduciary principle.

The judge said that sole area related to ambiguity in the Newmont- Consolidate d agreement over whether Consolidated may designate up to 40 percent of the Newmont board prior the time Newmont's certificate of incorporation was formally amended to authorize cumulative voting.

That concern could be satisfied if the parties reached an agreement to preserve the status quo of the board composition pending litigation or were able to resolve the ambiguity on an interim basis, the judge ruled.

''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 he ordered that the shares be placed in escrow or be held separate until Ivanhoe's legal challenge could be decided. That order was vacated.

The order had also required that the Pickens' group take no further action other than extending its tender offer for 28 million shares of Newmont stock.

Newmount is one of the nation's wealthiest mining companies, owning major North American gold reserves through its Newmont Gold Co. subsidiary.

In addition to Pickens, Ivanhoe includes Harbert Corp., a Birmingham, Ala. real estate and construction company; Dallas-based NRM Energy Co.; and Galactic Resources LTd., a Vancouver, British Columbia mining company.