By Devon Maylie

JOHANNESBURG--South Africa's gold mines could be hit by another strike, just as the mine industry body said Monday operations are back to normal following last week's strike by one of the country's biggest unions.

Members of the National Union of Mineworkers accepted a revised gold wage increase of between 7.4% and 8% on Friday, ending a strike that started last Tuesday.

But up-and-coming rival to the NUM, the Association of Mineworkers and Construction Union, said Saturday that it rejected the same offer and would issue a strike notice if companies didn't come up with a better deal.

"We have voted for a strike," AMCU President Joseph Mathunjwa told a group of members in Carletonville, a gold mining town outside Johannesburg. "You have mandated us to continue to engage with the Chamber of Mines with your demands...failing which we need to serve the Chamber with a 48-hour strike notice."

The outcome of these negotiations is key to the country's economy, which is expected to grow just 2% this year, down from 2.5% growth last year. Mining remains a key pillar of growth and employment in South Africa, but new union dynamics threaten to exacerbate the strike unrest.

The wildcat strikes last year cost the mining industry 15.3 billion rand ($1.5 billion) in lost production, while the mining industry as a whole accounts for 6% of gross domestic product and is a major contributor to government export revenue.

The strikes last year worsened dramatically after 34 protesters at one of the country's bigger platinum producers were killed when police fired into the crowd of striking miners. Since then tensions at the country's mines have risen and AMCU's power has increased.

"Don't force your 8% agreed with NUM down the throats of AMCU," Mr. Mathunjwa told the country's state broadcaster on Saturday.

The Chamber of Mines, which is coordinating the gold salary talks on behalf of South Africa's biggest producers, said AMCU has until Monday night to respond to the 8% offer. The NUM represents about 65% of all gold miners but AMCU's presence is growing at gold mines. AMCU is now the majority union at most of the country's platinum mines and wage talks in the platinum sector are just beginning.

Write to Devon Maylie at devon.maylie@wsj.com.com

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