* Second worker shot dead in about a week
* No sign of let-up in union turf war
* Labour uncertainty causes worries for investors
By Sherilee Lakmidas
JOHANNESBURG, Aug 12 (Reuters) - A worker was shot dead at Lonmin's Marikana mine in South Africa's restive platinum belt on Monday, just days before the first anniversary of the killing of 34 striking miners by police.
The off-duty female worker was shot in the head near a shaft where rival unions have offices, police said. They would not speculate on whether the killing was related to a deadly turf battle between the National Union of Mineworkers (NUM) and the Association of Mineworkers and Construction Union (AMCU).
"She was returning home after buying a chicken and somebody came from nowhere and shot her in the head before fleeing the scene," said police spokesman Thulani Ngubane.
Local media said the worker was affiliated with the NUM, something union officials were not immediately available to confirm. The shooting happened about a week after an AMCU-affiliated miner was killed near Marikana.
Lonmin spokeswoman Sue Vey said the reasons for the latest shooting remained unclear.
Gangland-style killings have become a feature on the platinum belt, which lies about 100 km (60 miles) northwest of Johannesburg, as the protracted turf war between the two unions shows no sign of easing.
More than 50 people have been killed in the labour unrest since last year, including the 34 workers shot dead by police on Aug. 16, 2012, the deadliest security incident since the end of apartheid in 1994.
The violence has sapped investor confidence in Africa's biggest economy and President Jacob Zuma's government, which has been criticised for being slow to respond, and out of touch with the social problems in the massive mining sector.
A government-appointed commission investigating what has been dubbed the "Marikana massacre" has made little headway and was recently suspended due to bickering over lawyers' fees.
Union leaders say the violence is now out of hand, even as police have characterised shootings as isolated incidents.
"Our shop stewards are still under attack," said AMCU President Joseph Mathunjwa, adding that union leaders lived in fear for their lives.
AMCU has steadily poached members of the NUM since early 2012, recently eclipsing it as the majority union at the world's top three platinum producers.
The two unions blame each other for the violence.
"Both unions are totally unabashed in their use of violence against each other. Rivals are often killed to advance self-serving interests," Philip Frankel, a former professor at the University of Witwatersrand in Johannesburg said in the Sunday Times newspaper this weekend.
"Both unions see an unstable situation as a source of political capital."
NUM's troubles have also had political ramifications because the union is closely connected with the ruling African National Congress and has been a training ground for party leaders.
Lonmin, the world's third largest platinum producer, was forced to halt production for weeks last year due to the union fight. The National Treasury said labour strife had cut into mining production enough to slow economic growth.
The labour uncertainty also sparked a sell-off in the rand currency and downgrades of South Africa's debt.