* Earnings up 21%
* Helped by lower costs, higher gold prices
* Kibali output up 12%, Geita up 6%, Mponeng down 4% (Adds detail, quotes)
JOHANNESBURG, Aug 8 (Reuters) - AngloGold Ashanti Ltd reported a 21% rise in interim earnings on Thursday as a solid performance from its Kibali joint venture, lower costs and a high gold price helped counteract lower output in South Africa.
Its best performing mines included the Kibali mine in Democratic Republic of Congo, Geita mine in Tanzania and Tropicana mine in Australia, AngloGold Ashanti said.
"Kibali has been a strong performer, Geita has been very positive, Tropicana as well... I'm pleased with the emphasis on costs, and that's going to continue," Chief Executive Kelvin Dushnisky told reporters on a call.
Kibali is co-owned by AngloGold, Barrick Gold and DRC's state-owned Societe Miniere de Kilo-Moto.
"We're very pleased with the rising gold price environment, but... we're not changing our focus one degree. We're going to continue to keep a tight rein on costs and capital (and) we're going to keep a close eye on the margins,"
The South African gold miner's headline earnings per share (HEPS) for the six months ended June 30 rose to 29 cents from 24 cents a year earlier, in line with the company's guidance.
HEPS, which strips out certain one-off items, is the main profit measure in South Africa. The company posted a 4% improvement in cash costs.
Dushnisky, who was appointed September 2018 and previously served as the president and executive director at Barrick Gold , has spearheaded the strategy to streamline the portfolio.
The miner employs around 6,000 people in South Africa where mining companies have faced volatile labour relations, rising costs, regulatory disruptions and technical issues.
AngloGold said in May it would review divestment options for its Mponeng mine, the world's deepest gold mine, and other South African assets to focus on higher returns elsewhere.
Those assets include Mponeng, a surface rock dump processing business and a mine waste retreatment operation. In contrast to output gains of 12% at Kibali and 6% at Geita, production at Mponeng fell 4%.
"We're very pleased with the high level of interest... we're going to let the process play out," Dushnisky said.
"If we don't receive a bid that we think is adequate, we're also happy to own these assets."
Overall the firm's gold output at retained operations fell 1.5% to 1,554,000 ounces. (Reporting by Alistair Smout; additional reporting by Tanisha Heiberg; editing by Uttaresh.V and Jason Neely)