McArthur River (HYC) Deposit Model

The McArthur River (HYC) zinc-lead deposit is one of the largest in the world. Current mineral resources (Xstrata Annual Report 2006) are 157 Mt @ 11.3% Zn, 4.9% Pb and 49 g/t Ag.

It is an example of a sediment hosted (SEDEX) zinc-lead deposit, which are known from around the world. Sedex deposits are widely distributed in Northern Australia in the Mount Isa – McArthur River region, eg. Mount Isa, Hilton, George Fisher, Lady Loretta, Dugald River, Century, and McArthur River.

Deposit features include:

• Fine-grained galena and sphalerite, with pyrite and pyrrhotite
• Good geophysical targets (eg. EM, IP, gravity, conductivity).
• Generally there is either a iron-manganese or a silicate alteration halo.
• Syn-sedimentary and replacement ore textures.
• Comprise 50% of the world’s zinc and lead reserves, and 25% of world zinc and lead production.

Myrtle Similarities to McArthur River (HYC) Deposit Model

McArthur River Basin rocks occur widely within the Reward EL 10316. The Barney Creek Formation and associated units is widespread.

A schematic east-west cross-section through the McArthur River (HYC) deposit is shown below. It shows the HYC deposit occurring at or near the base of the HYC Pyritic Shale Member and comparison with the Myrtle prospect shows a close similarity. The proximity of faults (eg. Western, Emu Fault) is regarded as being important in the genesis of the HYC and associated (Ridge, Cooley) deposits. Similar faults have been interpreted at Myrtle.

Other prospects in the Reward EL, including Berjaya, Buffalo Lagoon, and Barney Creek Sub-basin, show similarities to the HYC deposit model also, although exploration is at an early stage.

Schematic E-W Cross-Section through the McArthur River (HYC) Deposit (after Williams 1978)

Myrtle Differences to McArthur River Deposit

While there are a number of similiarities to McArthur River, there are some important, and perhaps fundamental differences, which greatly enhance the value and development potential of Myrtle. These include:

• Coarse sulphide grainsize at Myrtle (>100 microns) compared to McArthur River (<10 microns), meaning that metallurgical recoveries and processing costs will be much more favourable at Myrtle.
• Higher Zn:Pb ratio at Myrtle (4:1) compared to McArthur River 2:1, meaning that it is more likely that separate zinc and lead concentrates will be able to be produced and metallurgical recovery maximised.
• The host rock at Myrtle is different to McArthur River, being a tuffite rather than a shale. This simply means that while the geological processes of deposit formation may have been similiar (i.e. SEDEX style), the provenance of the host rock sediment at Myrtle was of a more volcanic origin than McArthur River. The tuffite may have been more permeable, allowing the sulphide grains to grow to a coarser size at Myrtle than at McArthur River.