Utilize the input text as a context reference, choose the right relationship between South32 and nickel from the options. Options: product/material produced, manufacturer, distributed by, industry, position held, original broadcaster, owned by, founded by, distribution format, headquarters location, stock exchange, currency, parent organization, chief executive officer, director/manager, owner of, operator, member of, employer, chairperson, platform, subsidiary, legal form, publisher, developer, brand, business division, location of formation, creator.

But Citi also said there was potential South32 would run the ruler over long-life base metal assets that would fill the company's growth "void" and offset the relatively short life of its Cannington silver, lead and zine mine in Queensland and declining grades at the Cerro Matoso nickel mine in Colombia.

Answer: product_or_material_produced