Shares in Sula Iron & Gold lost more than a third of their value on Tuesday after the Sierra Leone-focused miner said a study had cast uncertainty over part of a potential iron ore deposit at its Ferensola project.Sula said studies by SRK Consulting had shown an easily accessible surface resource of about 55m tonnes at Ferensola, with an extra potential deposit of up to 250m tonnes accessible by deep drilling.But it added: "SRK notes that the exploration target is conceptual in nature, that there has been insufficient exploration to estimate a mineral resource in this area and that it is uncertain if further exploration will result in the estimation of a mineral resource in this area."The stock dropped 1p or 40.4% to 1.47p at 13:33 in London.