LONDON: Leading mining companies are struggling to balance investor expectations for hefty returns with paying the necessary premiums to buy pure play copper companies as global demand for the metal sends valuations soaring. Big diversified miners including Rio Tinto , BHP Group and Glencore, pressured by a slowdown in global economic growth and falling commodity prices, are watching rival copper producers gradually grow beyond their reach, with shares benefiting from the metal’s robust outlook. While shares of Rio, BHP and Glencore have slumped between 10 per cent and 15pc this year, the valuations of pure play copper producers including Freeport-McMoRan, Ivanhoe Mines and Teck Resources have risen, even as benchmark copper prices retreated after hitting a record high above $11,000 per tonne in May this year. “Engaging in large copper deals makes the boards (of directors) nervous when fluctuations in other commodities, like iron ore and coal, are likely to persist,” a banker, who has worked on several mining transactions, told . “And since copper companies have performed better, diversified miners find it challenging to pay massive premiums when their share prices have dropped more in comparison,” the banker added.