LONDON (Reuters) - 

Never a destination for the faint-hearted investor, the Democratic Republic of Congo is again worrying those betting on its mines and vast mineral wealth, as rebels battle government troops in the country's east.

Goma - a lakeside trade town at the centre of an eastern region that once exported significant quantities of tin, gold and tantalum, a metal used in electronics - has been taken by the M23 fighters, who are now progressing south.

The eastern Kivu regions had seen exports tumble even before the recent months of fighting, as tighter controls on companies' supply chains hit the mostly artisanal mining operations. Without tags to certify origin, buyers like electronics powerhouses Apple or Intel steer clear of the region's metals.

Industry sources and analysts say the fighting - in an area that has seen recurring ethnic strife - is unlikely to spread far beyond the region, certainly not as far south as the major copper mines of Katanga, a region more than 1,000 kilometres (625 miles) south of Goma that has been the heart of Congo's mining industry.

"This is not national. You can't correlate it with (the civil war of) 2003; it is a different conflict," said Mark Bristow, CEO of Randgold, which works in Mali and Ivory Coast and is developing the Kibali project in Congo's northeast.

"Without wanting to downplay the seriousness of this - and it is serious - it is fairly well defined to the region. We don't see it spreading, and it is very encouraging the population at large is clear about (wanting) national unity."

But campaigners and industry analysts say the unrest will set back efforts to boost legitimate mining in the east, and to clean up Congo's tarnished reputation.

Unchecked progress towards Bukavu - 200 km by road from Goma, south along Lake Kivu - will bring the rebels closer to Canadian miner Banro's Twangiza operation, Congo's first new gold mining project for more than half a century, and hailed at its debut last year as a symbol of revival in a war-scarred region.

Banro's shares have lost more than a quarter of their value since reports of fighting began to emerge this month, though the company has said operations are normal