Hecla Mining Co. has exercised its option to purchase the Apex germanium-gallium mine and processing plant near St. George for $5.5 million through an asset purchase agreement with St. George Mining Corp., which is owned by Musto Explorations Ltd. of Vancouver, British Columbia.

Hecla, based in Coeur D'Alene, Idaho, said the purchase is subject to approval by Musto shareholders. Hecla said it decided to buy the property after seven months of metallurgical testing determined that mining and processing efficiencies could be improved.Although not currently in operation, Hecla officials say the mine will be returned to production late this year and should be up to full production by June 1990, employing about 90.

"The purchase of the Apex mine is in line with Hecla's goal of entering the specialty metals business," said Arthur Brown Hecla's chief executive officer.

He said the number of high technology uses for germanium and gallium is growing, and Hecla plans to fill a niche in supplying those materials.

Germanium and gallium are specialty metals, used in high-tech applications such as fiber-optics, light-emitting diodes and infrared lenses.

Hecla Mining Co. is a domestic producer of newly mined silver and gold, and a supplier of ball clay and volcanic scoria. The company has operations and projects under development in 12 states and four Canadian provinces.