* Agreement offers lifeline to London-listed firm
* Exploitation of Guinea mine previously too costly
* Sable flags announcement; shares soar 300 percent (Adds deal signing, updated stock price, quotes)
MONROVIA, Jan 23 (Reuters) - Sable Mining signed a$1.3 billion deal to export iron ore via Liberia on Friday,offering the London-listed firm the transport link it has so farlacked to exploit the site in Guinea where the ore is mined.
The agreement was signed by the head of the Liberia NationalInvestment Commission George Wisner and Sable non-executivedirector Aboubacar Sampil in the nation's capital Monrovia.
The 25-year deal, which mandates the firm to expand raillinks between the two countries, should allow it to begintransshipping ore from the Mount Nimba mine later this year,Wisner said.
Shares in AIM-listed Sable soared more than 300 percent onFriday before closing up 156 percent.
Wisner said the agreement still needs parliamentaryapproval.
The Guinean government had been reluctant in the past toallow transshipment, insisting mining firms operating in Guineaship their ore through the port of Conakry, which would havemade Mount Nimba too costly for Sable to exploit.
But in October 2013 it granted permission for Sable toexport via Liberia.
Wisner said Sable would invest $300 million in the firstfive years of the project and $1 billion in the remaining 20,and would ship its ore via the Liberian port of Buchanan.
Sable is expected to rehabilitate and expand an existingrail link from Yekepa in northern Liberia to Buchanan inaccordance with third-party access rights between the Liberiangovernment and ArcelorMittal, the world's largeststeelmaker, which operates the line.
Wisner said the Liberian government was taking the lead infacilitating the negotiations over third-party access.
"Under our constitution, the rail is owned by Liberia. Theground is owned by the republic of Liberia. I do not want topre-empt the negotiation, but I do not foresee any hurdle in thethird-party negotiation," he told Reuters.
ArcelorMittal has previously said that companies in Guineawould have to wait years to make use of the existing rail linkthrough Liberia because there was no spare capacity.
But the LNIC said Sable was also mandated to construct a newline from Guinea via Tokadeh in northern Liberia to link withthe existing line at Yekepa. (Reporting by James Harding Giahyue and Alphonso Toweh; Writingby Bate Felix and Daniel Flynn; Editing by Joe Bavier, JohnStonestreet and David Evans)
