* China’s GEM signed a 1-year deal to buy ERG’s cobalt -sources LONDON/JOHANNESBURG, Jan 11 (Reuters) - Swiss-based commodity trader Telf AG will sell cobalt produced by Eurasian Resources Group’s operation in Democratic Republic of Congo, equivalent to 10% of global supply in 2021, three sources with direct knowledge of the matter told Reuters. Under the deal, expected to run from 2021 to 2023, Telf will market 18,000 tonnes of cobalt for ERG this year - worth some $657 million at London Metal Exchange prices. Demand for cobalt, a key ingredient for the rechargeable lithium-ion batteri...