JOHANNESBURG, South Africa (AP) _ The black mineworkers' union and management traded accusations of harassment today and both sides said no new negotiations were planned to end the nation's largest legal strike ever.

The National Union of Mineworkers also threatened to expand the strike to the Rand Refinery's gold processing plant, which processes all of South Africa's gold for international markets. The plant has about 190 black workers and a similar number of white workers.

The mineworkers' union said a strike ballot was set for today at the refinery, but the company said negotiations were still underway and a walkout at this stage would be illegal.

The mineworkers' union said Monday that 340,000 workers were on strike and 44 of the 46 targeted gold and coal mines had been shut down since the stayway began late Sunday to press demands for a 30 percent pay increase.

The Chamber of Mines, which represents major mining firms, disputed the union figures, saying about 230,000 of the nation's 600,000 black miners were striking. It said 13 of 55 coal mines under its jurisdiction and 16 of 44 gold mines had been ''significantly affected'' by the strike.

The chamber unilaterally implemented 15 percent to 23.4 percent wage increases on July 1.

There have been several clashes since the strike began, with more than 30 miners and three mine security officers reported injured. Mine owners accused strikers of intimidating and assaulting miners who wanted to work, while the union said more than a dozen union officials had been arrested and many others harassed.

The union said today that 14 of its workers were injured in clashes with mine security officials on two mines near Secunda in the eastern Transvaal province.

In addition, nine workers were hurt at the Kinross gold mine and five injured at the Bracken mine, said union spokesman Marcel Golding. Both mines are run by Gencor.

The union also said its offices in Carletonville were surrounded by police and that union officials were refused access for more than an hour.

Anglo American Corp., which produced 39 percent of South Africa's gold output of 638.2 tons in 1986, said today all its mines were affected.

The company complained of intimidation by union officials, who it claimed were interfering with workers' access to hostels where they live.

''This action is preventing workers from exercising their free choice to work,'' the company said, without citing where the alleged incidents occurred.

Johann Liebenberg, a Chamber of Mines official, said it was too early to assess the strike's impact on production.

About 80 percent of South Africa's exports are derived from mining. The country is the world's leading producer of gold, which provides more than half its foreign currency earnings. For the most part, gold production has escaped the sanctions imposed on many other South African exports.

Unions have moved to the forefront of the anti-apartheid movement in the 14 months since President P.W. Botha declared a state of emergency to quell black unrest.

Scores of labor leaders were among the estimated 25,000 people detained during the emergency. But black unions weathered the crackdown better than many other organizations opposed to the white-led government's system of race separation, or apartheid.

The mineworkers' union confines most of its current contract demands to traditional workplace issues. However, it recently endorsed the political platform of the outlawed African National Congress guerrilla movement, including calls for black majority rule and nationalization of industries.

There have been bigger walkouts by black workers before, but those were brief, illegal protests linked to political issues rather than to contract negotiations.

This strike is legal because the union declared a dispute over specific grievances, reached a formal deadlock during bargaining, balloted its members and received a majority vote to strike.

The chamber says black miners are paid about $245 a month, one-fifth the average white miner's wage. The union says that the monthly salary is less than $200. The union also wants danger pay for black workers.