The Portia (gold) Deposit occurs at the base of the Eyre Formation (of Tertiary age) and immediately above a weathered basement profile of pyritic albitite, graphitic pelite, quartz albitite, and another pelite unit, all of which dip at about 40 degrees.

The gold occurs as granular aggregates sitting in a deflation surface that has formed a slight depression in the basement topography. Weathering occurs for many tens of metres below the Tertiary - basement contact in the pyritic albitite but is less deep in the graphitic pelite, quartz albitite and other pelite unit. The base of the oxidised zone also occurs significantly shallower in the pyritic albitite.

The Eyre Formation is overlain by clayey sediments of the Namba Formation, which extends from surface to about 70 metres. The following cross section shows the base of Namba Formation, base of Eyre Formation, the base of the Tertiary gold horizon, dipping basement units, the base of the oxidised zone, and the base of the weathered zone.

The Portia and North Portia prospects are overlain by about 50-75 m of Quaternary and Tertiary sediments. The Proterozoic host and footwall rocks are intensely albitized and oxidized (hematite- and magnetitebearing). Prior to albitization, the host rocks were finely laminated carbonaceous shales and evaporite- and carbonate-rich beds of saline silt and shale (Teale, 1999). The hanging wall is carbonaceous phyllite intercalated with albite- and muscovite rich beds.

The regolith (Figure 3) includes, from the top down:
Soils, developed in degraded dunes and present-day floodplain deposits, consisting of 1-2 m of light brown, fine sands, with soft, powdery carbonate and groundwater gypsum in the subsoil.

Quaternary fluvial sediments, 3-8 m of moderately sorted, unconsolidated fine- to medium-grained olive brown sand and clay.

The Tertiary Namba Formation (50 m thick), comprised of various grey smectite-bearing clays with red and brown mottles. The sandrich upper Namba Formation is thought to be a fluvial and flood plain deposit, whereas the clay-rich middle and lower units were deposited in low-energy shallow lacustrine environments (Tan, 2001).

The Tertiary Portia Unit, light grey clays with abundant kaolinite, variable amounts of mica (5-20%) and minor quartz (<5%), but with up to 20% quartz sand at the base of thicker parts of the unit. The Portia Unit is probably transported, filling a palaeovalley across the prospects, and sourced from proximal saprolite.

Saprolite, over 90 m thick, developed on the Proterozoic bedrock, comprised of quartz and kaolinite, with variable amounts of goethite, hematite and mica.

Proterozoic phyllites, containing K-feldspar, albite, biotite, chlorite, scapolite, actinolite, pyrite, Fe-oxides, rutile, quartz, and vein and matrix calcite and dolomite.

Primary mineralization only occurs at North Portia and consists of chalcopyrite and molybdenite, with minor bornite and Au, in pyritic albitite host rocks. Some coarse-grained Au occurs in sulphide-poor veins in the hangingwall carbonaceous phyllite. There are two populations of Au grains, with mean compositions of 9.5% and 24% Ag for the first and second population respectively (Teale, 1999). Native Cu, covellite and chalcocite also occur in the saprolite from 85 to more than 100 m depth, i.e., below the local water table at about 70-80 m.

At Portia, irregular grains of Au (with 4-8% Ag) occur in the basal sand-rich part of the Portia Unit. Some contain arsenopyrite inclusions and appear to represent placer deposition (Tan, 2001). Some parts of the grains have Ag depleted rims which, together with numerous etch pits, grooves and voids on their surfaces, indicate dissolution. Minor amounts of small (1 µm) possibly diagenetic Au particles also occur in the Portia Unit.

In the saprolite of the Proterozoic bedrock at Portia, irregular Au grains (large grains have dimensions of 800x1400 µm) contain <10% Ag (i.e., similar to some of those in the unweathered bedrock at North Portia) and a few have galena and altaite (PbTe) inclusions. These are probably in situ primary grains, although some have secondary Ag-depleted rims (Tan, 2001).

Saprolite of Proterozoic bedrock
High Cu, Au, Mo, As, Ni and Co concentrations occur in the mineralized pyritic saprolite (Summary Table), hosted by primary and secondary minerals including chalcopyrite, chalcocite, covellite, native copper, electrum, molybdenite, arsenopyrite, pyrite (Ni, Co) and sphalerite. Where these minerals are oxidized, element concentrations decrease sharply, except in the ferruginous saprolite, which is distributed across a 250 m wide zone up-dip of the mineralized sequence. In contrast, Zn appears to be concentrated in the ferruginous saprolite as the Zn abundance in the pyritic saprolite is less. Overall, the footwall and hanging-wall units are poor in these elements.