Consolidated Gold Fields PLC and Newmont Mining Corp. said they filed a federal suit Tuesday to block a $3.5 billion hostile buyout bid by a South African conglomerate, on grounds the offer violates federal securities and antitrust laws. The U.S. District Court suit was the latest action the companies have taken to thwart the overture by South African-controlled Minorco SA, which has offered $3.5 billion for the 70.6 percent of Consolidated Gold Fields it does not already own. London-based Gold Fields owns a 49.3 percent stake in Newmont. Gold Fields and New York-based Newmont last week asked President Reagan to block the takeover on national security grounds, contending a buyout would endanger U.S. access to strategic metals such as gold, platinum and zirconium. Gold Fields has asked the British government to review the bid because of the South African connection, and the South African government also is investigating the proposed acquisition. Named as defendants in the suit filed Tuesday were Anglo American Corp. of South Africa Ltd., De Beers Consolidated Mines Ltd., and Minorco. Luxembourg-based Minorco, which owns 29.4 percent of Gold Fields, is controlled by Anglo American and De Beers Consolidated Mines, the gold and diamond mining interests of the wealthy Oppenheimer family of South Africa. Anglo American and De Beers have said they would relinquish control of Minorco if the bid succeeds. Minorco has said it plans to sell Gold Fields' South African interests and Gold Fields' stake in Newmont if it acquires the company. The Gold Fields-Newmont suit contended the defendants had conspired to monopolize the gold market and that a successful acquisition likely would substantially lessen competition in that market. Gold Fields controls 8 percent of the world's gold production. It also produces rutile and zircon, which are raw materials for metals used in military aircraft and submarines. The suit claimed a takeover of Gold Fields by Minorco would give the South African syndicate control over a third of the world's gold market. The suit stated the Oppenheimer syndicate has major investments in companies that control more than 20 percent of the West's gold production, has interests controlling more than 50 percent of the free world's platinum production and has what it claimed the Justice Department and Federal Bureau of Investigation consider ``an absolute monopoly'' over the diamond business.