Sen. Richard G. Lugar (R-Ind.), chairman of the Senate Foreign Relations Committee, plans to urge President Reagan to send a high-ranking administration envoy to South Africa this summer to explore whether the United States might host negotiations -- alone or with other western governments -- between black and white South African leaders, aides said yesterday.

Lugar and Sen. Nancy L. Kassebaum (R-Kan.), chairman of the Senate subcommittee on African affairs, will suggest to Reagan in the next few days that an administration envoy such as Vice President Bush or Secretary of State George P. Shultz go to South Africa "to see whether there is any chance of a negotiating process under the auspices of the United States acting alone or with the Germans and British," Mark Helmke, a Lugar aide, said.

The White House said last week it had "noted" the idea of sending an envoy to Pretoria but had no current plans to do so. Republican sources said the idea was still alive and that Reagan was under pressure from various Republican leaders to take some initiative to defuse the mounting pressure in Congress for the imposition of new U.S. sanctions.

Lugar is anxious to have the administration explore "all the avenues" of possible policy alternatives to imposing new economic sanctions before the Senate Foreign Relations Committee opens hearings on this issue July 22, Helmke said.

The House has already passed on a voice vote a bill that would require all U.S. businesses in South Africa to leave immediately. The Senate is not expected to go along with the bill but is now under considerable pressure to support some kind of sanctions legislation.