South32 picked up a 15% stake in the Toronto-listed zinc player for $110 million, or $2.45 per share, in May. Overnight the company exercised its top-up right to subscribe for another 1.43 million shares at $3.14 per share to maintain its 15% stake.

South32 is keen to further increase its interest in Arizona, which is worth C$935 million at current prices.

In September it entered into an over-the-counter equity forward contract to acquire up to another 15 million shares over 12 months to reach 19.9%. The 19.9% is the maximum allowed under the agreement.

Last month South32 CEO Graham Kerr noted there was no clear path to ownership of Arizona. The company does have a seat on the Arizona board, recently appointing former Coalspur CEO Gillian Winckler as its nominee.

"We like the deposit, we like the participation that we have," Kerr told reporters in late November.

"We have a good relationship with them and we'll see how the results and things pan out over time."

Arizona owns the Hermosa project, south of Tucson, containing the Taylor zinc-lead-silver sulphide deposit, which has measured and indicated resources of 72.4 million short tons at 10.5% zinc equivalent, and 38.6Mt of inferred resources at 11.6% zinc equivalent, using a 4% zinc cut-off.

A preliminary economic assessment released in April returned capital costs of US$457 million for a 19-year operation producing 287Mlb of zinc, 286Mlb of lead and 5.5 million ounces of silver per annum at all-in sustaining costs of 61c per pound of zinc.

The PEA returned a post-tax net present value of $1.3 billion, an internal rate of return of 42% and a 1.7-year payback.

Arizona is aiming to deliver a definitive feasibility study in the first half of next year, targeting first production in 2020.

Former Barrick Gold co-president Jim Gowans is CEO.