The Hermosa base metals project comprises the development of two polymetallic ‘deposits, namely Taylor and Clark, in Arizona, US. South32, a mining and metals company based in Australia, is the owner and developer of the project.

A pre-feasibility study (PFS) of the Taylor zinc-lead-silver sulphide deposit was released in January 2022. It calls for a large-scale underground mining operation with a nameplate production capacity of 4.3 million tonnes per annum (Mtpa).

The Taylor deposit is expected to produce 111,000 tonnes (t) of zinc, 138,000t of lead, and 7.3 million ounces (Moz) of silver a year over a mine life of 22 years, with an estimated capital investment of $1.7bn.

The final investment decision (FID) on the Taylor underground mine is expected after the completion of a feasibility study (FS) in 2023. Construction works are expected to start in 2024, with ore production targeted to begin in 2027.

South32 also completed a scoping study of the Clark zinc-manganese-silver oxide deposit in January 2022. It indicates the potential to produce battery-grade manganese, along with zinc and silver. A PFS for the Clark deposit is expected to be completed in 2022. Clark represents the second stage development option at Hermosa.

Location, geology and mineralisation

The Hermosa base metals project is located in the Patagonia Mountains, 80km south-east of Tucson, in Santa Cruz county, Arizona, US.

Taylor is a zinc-lead-silver massive sulphide deposit hosted in Permian carbonates of the Pennsylvanian Naco Group. It is characterised as a carbonate replacement deposit (CRD), with mineralisation extending more than 1.2km from the near-surface area. The polymetallic ore body has a strike length of 2.5km and an average width of 1.9km.

Clark constitutes the oxidised portion of the CRD style Taylor massive sulphide deposit and is found hosted in Mesozoic Hardshell volcanic rocks and the underlying Concha carbonate stratigraphy. The zinc, manganese and silver mineralisation at the deposit spans more than 1.3km from the near-surface.

Reserves at Hermosa

Mineral resources at the Taylor deposit stand at 138 million tonnes (Mt) grading 3.82% zinc, 4.25% lead and 81g/t silver.

The Clark deposit was reported to hold 55Mt of mineral resources grading 2.31% zinc, 9.08% manganese and 78g/t silver as of May 2020.

Mining and ore processing

Longhole stoping underground mining method will be employed to extract ore from the Taylor deposit.

The extracted ore will be crushed underground and hoisted to the surface for screening, grinding and processing. Lead, zinc, and silver concentrate will be produced by using the conventional flotation method.

The tailings from the flotation plant will be sent to one of the two dry stack tailings storage facilities at the project site.

Contractors involved

SRK Consulting was engaged by South32 to provide mineral resource estimates for the Clark and Taylor deposits.

AMC Mining Consultant was previously engaged by Arizona Mining to provide mineral resource estimates and preliminary economic assessment (PEA) of the Taylor deposit.

Hermosa base metals project background

Asarco, a mining, smelting and refining company based in the US, conducted exploration drilling at Hermosa between 1950 and 1991.

Arizona Mining (formerly Wildcat Silver), a mining company based in Canada, purchased the property from Asarco in March 2006 and conducted diamond core and reverse circulation (RC) drilling at the deposit between 2007 and 2018.

South32 acquired the project from Arizona Mining in August 2018.