* Aspen offers to buy Arixtra and Fraxiparine, plus factory
* Sales of drugs 420 mln pounds in 2012 but in decline
LONDON, June 18 (Reuters) - GlaxoSmithKline is discussing the sale of its thrombosis drug brands Arixtra and Fraxiparine, along with a related French factory, to Aspen Pharmacare in a deal that could be worth some $1 billion.
Britain's biggest drugmaker said on Tuesday that Aspen had offered to buy the medicines, sales of which are in decline, and it was now in exclusive talks with the South African company about a deal, which is subject to consultation with employees.
Arixtra and Fraxiparine had worldwide sales of approximately 420 million pounds ($660 million) in 2012, down from 510 million in 2011, and revenues are expected to slide further in 2013.
Neither company put a value on the proposed deal but assuming Aspen pays twice this year's anticipated sales, it could be worth around 700 million pounds - after stripping out sales in China, India and Pakistan, which GSK will retain.
About 1,000 GSK employees would transfer to Aspen, if the deal goes through, the majority of whom work at the Notre Dame de Bondeville production site in France.
GSK said the proposed disposal fitted with its strategy of focusing on products with the most growth potential and delivering its new-drug pipeline.
For Aspen, which is already Africa's biggest maker of generics drugs, the acquisition will fuel its expansion in overseas markets.
Investors in Aspen - 19 percent owned by GSK - warmed to news of the likely deal and its shares rose 4 percent by 1230 GMT, while GSK was little changed.
Declining products such as Arixtra and Fraxiparine are seen as a drag on GSK's growth at a time when new drugs are set to reach the market, and the company said in April it was bundling many of its older medicines into a new established products unit.
That decision prompted speculation that more old drugs might be sold off to generate windfall gains, which could be returned to shareholders.
GSK is also divesting its energy and fruit drinks Lucozade and Ribena in an auction that is expected to get under way later this summer.
Aspen and GSK have a long history of doing deals together. Last year, the British group sold 25 older brands marketed in Australia to Aspen for 172 million pounds in another clear-out of non-core drugs.