BEIJING, July 10 (Reuters) - Copper prices in London and Shanghai extended gains for a second day on Tuesday, with investors lured to buy low after a trade war-fuelled sell-off last week. The United States on Friday slapped tariffs on $34 billion of goods from top metals consumer China, which responded in kind. Fears that the trade spat could dampen demand for industrial metals had weighed on copper prices. "It's already maybe a bit oversold," said Helen Lau, an analyst, Argonaut Securities in Hong Kong. The prospect of strikes by labour unions in South America has "dragged on a bit too long," which is also supporting prices, she added. Labour negotiations at BHP Billiton Plc's Escondida copper mine in Chile, the world's largest, are entering the final three weeks before a 30-month contract expires at the end of July. FUNDAMENTALS * LME COPPER: Three-month copper on the London Metal Exchange was up 0.4 percent at $6,418 a tonne, as of 0414 GMT, extending a 1.7-percent gain from the previous session. The metal lost more than 5 percent last week. * SHFE COPPER: The most-traded September copper contract on the Shanghai Futures Exchange gained 0.6 percent to reach 49,850 yuan ($7,549.48) a tonne by the mid-session interval. * ZINC: There was no such respite for Shanghai zinc , however. The metal fell as much as 2.3 percent to 21,725 yuan a tonne, having dropped 5.3 percent last week on trade war fears and expectations of higher supply. * ZINC FUNDAMENTALS: "A lot of zinc concentrate is coming on line from mines across the world," this year, Lau said, adding that demand for zinc in steel galvanising in China was seeing "a bit of a slowdown." * RUSAL: United Company Rusal, the world's second-biggest aluminium producer, in May increased aluminium exports to 197,000 tonnes, up almost threefold from April, Interfax news agency reported. {nL5N1T72B2] * BLOCKCHAIN: Online metal concentrates exchange Open Mineral is seeking to build a consortium of mining companies and financial institutions to create a blockchain system for minerals trading and logistics.