By Pratima Desai LONDON (Reuters) -Aluminium prices dropped on Monday as inventories in London Metal Exchange (LME) warehouses jumped, fuelling fears of unwanted Russian-origin metal in the LME system, traders said. Benchmark aluminium on the LME was down 2.3% at $2,252 a tonne at 1037 GMT. Western countries have imposed sanctions on Russian banks and wealthy individuals connected to President Vladimir Putin since Russia’s invasion of Ukraine in February, but so far there are no restrictions on buying Russian metal. However, metal industry sources say there is concern that Rusal will be unable…