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.

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 in Figure 1 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.

Figure 1: Schematic E-W Cross-Section through the McArthur River (HYC) Deposit (after Williams 1978)

A model for the deposition of McArthur River has been developed by Ireland et. al. (2004) and is shown in Figures 2 and 3 below. The source of metals (Zn and Pb ions) is from a fault system tapping these at depth in the Earth’s crust. The metal ions are circulated into a shallow (~600-800m) marine basin with restricted circulation, and precipitate to form sulphide minerals (ZnS and PbS) at a depth below the Redox front (where free oxygen no longer exists).

Figure 2: Cross Section of McArthur River deposit model showing the source of metal ions and the interpreted depth of sulphide precipitation (after Ireland et. al., 2004).

Of interest, in terms of exploration, is that the “fringe” zones to the highest grade mineralisation at the bottom of the basin is the development of “nodular carbonates” at shallower depths where some oxygen still exists in the form of SO₄^2- ions. This zone generally contains zinc mineralisation in the 5-7% grade range, and is very similar to what has been found at Myrtle so far. When viewed in plan (Figure 3) this nodular carbonate zone at McArthur River can be seen to surround the higher grade zone.

Figure 3: Plan of McArthur River deposit model showing the central high grade laminated siltstone surrounded by the medium grade nodular carbonate zone (after Ireland et. al., 2004).

The interpretation is that the drilling so far at Myrtle has largely been within the nodular carbonate zone, and drilling further to the west of current drilling is required to properly test the model for higher grade mineralisation (Figure 4). This central part of the basin is clearly seen on aerial photographs (Figure 5) and is also indicated by the metre-percent contours of drill intercepts as shown in Figure 5, open to the north-west of current drilling (upper left hand corner of photograph).

Figure 4: Cross Section at Myrtle showing the interpreted location of the potential higher grade zone in the central portion of the Myrtle sub-basin.

Figure 5: Aerial Photograph of Myrtle showing drill hole locations, metre percent contours of drill results and the potential centre of the basin in the upper left hand corner. The newly discovered Eastern Zone is shown central right.