Brazilian authorities are investigating reports that six tons of arms seized from a ship here last weekend were bound for Ghanaian rebels seeking the overthrow of military rule there.

Federal police said the Norbistor ocean-going tug bearing the arms left Argentina March 1. The crew of 10 Argentines and eight Americans told Brazilian authorities the weapons were of Argentine make and bound for Ghana, on the west coast of Africa.

In Accra, Ghana's Foreign Ministry asked Brazil for cooperation in investigating charges of a plot involving U.S. veterans of the Vietnam War to overthrow the Accra government, the Ghana News Agency said.

[Accra Radio said Tuesday night that Ghanaian dissidents linked to what it termed foreign intelligence agencies planned to use the arms to overthrow the four-year-old regime of Flight Lt. Jerry Rawlings. It identified the dissidents as led by Godfrey Osei, 40, who was arrested in April 1983, in one of several reported attempts to overthrow Rawlings. Osei escaped two months later.]

Press reports here said Ghanaian diplomats had alerted the Brazilian Foreign Ministry that a shipment of "clandestine arms" was passing through Brazilian waters. The Ghanaian Embassy in Brasilia officially denied knowledge of the arms shipment.

Maritime authorities said the crates currently in the hands of the Brazilian Army contained weapons meant for an assault by sea, including automatic revolvers, machine guns, grenades and inflatable launches with outboard motors.

Crew members have said the ship was bound for Ghana but anchored here for needed repairs. Police seized the vessel after a raid on a Rio night club, where crew members were found without Brazilian visas.