The Lake Carnegie Potash Deposit is a brine, containing the target potassium and sulphate ions that could form a potassium sulphate salt. It has potential for potash mineralisation with a wide range of exploration targets from shallow brine within the lake sediments to deeper brine horizons in the palaeochannel basal sand aquifer. Advisian name five different prospective exploration targets for obtaining brine supplies associated with the Lake Carnegie project, which include:

• Evaporite sand layers in the lake sediments,
• A minor aquifer associated with alluvial and colluvial deposits,
• A sandy horizon at top of palaeochannel stratigraphy,
• Basal palaeochannel sand; and
• Fractured-rock aquifers at the southern and northern edges of the lake.

Lake Carnegie is part of an extensive palaeodrainage system that was active in the area until the middle Miocene. The palaeodrainages are infilled with a basal palaeochannel sand that is overlain by dense plasticine clay, and a variable thickness of alluvium and colluvium. Numerous tributaries incising the Earaheedy Group and draining towards Lakes Carnegie and Wells were once connected to this system and are now represented by calcrete-rich valley floors, and isolated areas of playas and associated material.

There are also a wide range of weathering profiles and weathering products derived from the basement lithologies, as well as associated with the deposition of the lake systems. Valley floors and marginal sloping areas contain large areas of depositional regime regolith including colluvial, sheetwash, floodplain and alluvial material. In places, calcrete has developed in the major valleyfloors and drainage systems and is well developed in floodplain deposits. Distal sheetwash and colluvium grades into dune and playa terrain associated with Lake Carnegie.

Dolerite
Dolerite and gabbro sills and dykes are widespread, where they outcrop as concordant or slightly discordant intrusions. These intrusive rocks are fresh to weakly altered, and largely composed of glagioclase (locally sericitized), clinopyroxene, and titanomagnetite. Accessory amounts of apatite and pyrite are found in parts. Several dolerites have well-developed granophyric layers and veins at the top. AGSO airborne magnetic data indicate the likelihood of a large subsurface dolerite sill near Top Fourteen Mile Well along the southern shoreline of Lake Carnegie

Paterson Formation
The Paterson Formation is an Early Permian, flat-lying glacial and fluvioglacial succession, which can be divided into three lithofacies - tillite (non-bedded, poorly sorted boulder conglomerate to pebbly, clayey siltstone), cross-bedded conglomeritic sandstone of fluvioglacial origin, and lacustrine siltstone. Deep weathering is common, and in many places a resistant silcrete or laterite cap has developed resulting in mesas with steep-sided breakaways. Locally developed crossbedding indicates a north to north-northeast transport direction, with many rock types sourced from the Archaean Yilgarn Craton.

Cenozoic sediments
Early Tertiary sedimentary deposits of Middle to Late Eocene age have infilled the Cretaceous or Early Tertiary valleys. The sediments typically comprise a basal fluvial sand overlain by lacustrine clay. These are overlain by an interfingering sequence of alluvium and minor colluvium of later age, possibly Late Tertiary that is locally replaced or displaced by calcrete.

Alluvial deposits form a thin veneer over most basement rocks in the area. The alluvium form outwash fans on the flanks of the trunk valleys. Thicker deposits of colluvium also occur in tributaries, especially within the greenstone belts where the valley sides are steep.

Early Tertiary sediments
The sediments comprise a basal sand overlain by an interbedded sequence of dense, plastic clay with minor interfingering sand lenses. The palaeochannel sand at the base of the sequence occurs as a sinuous stringer sand unit, bounded by relatively steep topography, on the underlying bedrock surface.

The palaeochannel sand consists of predominantly very fine to coarse-grained quartz sand with minor silt, gravel and carbonaceous horizons, which were deposited in a combination of fan-type and braided channel-type alluvial structures. They may be up to 40 m thick, from 100 to 1,000 m in width and in sections of the palaeodrainage become locally thicker, broader and coarser. A thin bed of rounded grey quartz cobbles often occurs at the base of the unit.

The overlying clay, possibly of lacustrine origin, rests on the basal sand with a gradational contact comprising several metres of dark-grey clayey sand. The unit grades downward from sandy clay to a uniform, light-grey plastic clay interfingering sand horizons up to 10 m thick occur throughout the clay and are believed to have been contributed from lateral tributaries. In the Northern Goldfields, the clay is present only in palaeochannels and is not known to outcrop.

Calcrete/dolocrete
Partial replacement of valley-fill material has produced extensive areas of cavernous sandy calcrete to dolocrete. Calcrete/dolocrete is a carbonate rock formed by the in-situ replacement or displacement of the alluvial and colluvial deposits by magnesium and calcium carbonate precipitated from percolating carbonate-saturated groundwater.

Alluvium and colluvium
Alluvial deposits form the upper portion of the Cainozoic sequence within the palaeodrainages and include interfingering minor colluvium. Alluvium occurs as channel fill associated with palaeodrainages and the lower parts of the tributary valleys. The variation in thickness is largely dependent on position in the drainage system with the thickest sequences often coinciding with the axes of the Tertiary palaeochannels. The depositional environment is like that found in presentday outwash alluvial fans and minor creeks.

Lake sediments
There is up to 12 m of shallow Quaternary sediments associated with lake deposition and the reworking owing to deflationary influences. This zone is characterised by playa features that are heavily influenced by extreme salinity, as well as repetitive wetting and drying cycles of the salt lake. Most of the material was deposited by sheet flood and slope wash, but gypsum crystals are precipitating in the lake sediments from the highly saline ground water. There are likely to be variable thicknesses of gypsiferous sand that are typically saturated and may yield large quantities of brine.

There are two principal methods applicable to extract the brine from the surrounding sediments:

• pumping from wells in the basal sands (lower aquifer) plus leakage from potential brine bearing segments within the clays;

• pumping from trenches inside the alluvial sediments (upper aquifer).

It is likely that both methods will be used because of the properties of the different aquifers. The design of the bore field will be based on the brine demand and aquifer conditions.

The general mineral processing concept is comprised of the following areas:

• Brine winning
• Brine concentration and crystallization of solid raw materials for the purification plant
• Purification plant
• Utilities

According to the composition of the deposit brine, the SS process design considers the recovery of SOP as the principle product. Potential by-products might incude Epsomite, Magnesium Hydroxide, Bischofite and Magnesium Oxide.

The process begins with brine entering the evaporation ponds whereby water is removed by solar evaporation. This causes different salts, starting with gypsum and halite, to crystallise subsequently in the first set of ponds. Unless determined economical to process, these salts are left within the ponds, and will be harvested once the salt layer gets to high. The remaining brine crystallises in the next set of ponds producing a Kainite Type Mixed Salt (KTMS) consisting of sulphatic and potassium contain ........