The Lake Mackay Uranium Project covers 3286km² of highly prospective ground in central-eastern Western Australia, 600km west of Alice Springs. Remote location, lack of geoscientific baseline datasets and historic access issues, have limited previous exploration in the region to reconnaissance surface sampling and scattered aircore drilling, none of which was directed towards uranium. Access and publically-available geological/geophysical data has improved considerably in recent years and led Toro Energy into the area.

The terrain is generally a flat, undulating sand plain with some very low hills comprising Proterozoic in the south and southwest and Palaeozoic sediments in the west.  There is extensive aeolian sand dune cover between the Proterozoic hills and the edge of Lake Mackay to the north.

The 14 tenements forming the Lake Mackay Project cover a total area of approximately 3286km².  The Project is located within Native Title Determined Lands overseen by the Ngaanyatjarra Land Council (NLC). An agreement is in place with the Traditional Landowners, the Kiwirrkurra people, which allows Toro to explore for uranium and other metals on reserve lands. 

GEOLOGICAL SETTING

Palaeoproterozoic metamorphic rocks of the Arunta Complex are comprised mostly of greenschist facies schist, after a protolith of greywacke, siltstone and quartzite, modified locally by thermal metamorphism associated with granitic intrusions. The unconformably overlying Pollock Hills Formation consists of acid volcanics. Highly altered, flow-banded and brecciated lava is overlain by sandstone or less commonly by agglomerate or lapilli tuff. The Pollock Hills Formation is intruded by the high level, comagmatic Mount Webb Granite (1640Ma).

The Neoproterozoic Heavitree Quartzite is the basal formation of the Amadeus Basin sequence, and crops out to the south and west. The main outcrop area is in the Pollock Hills area, where the formation is about 500m thick. Bitter Springs Formation conformably overlies the Heavitree Quartzite.  Examination of the regional magnetics also suggests the presence of Devonian-Carboniferous sediments of the Amadeus Basin within the central part of the Project area.  Local outcrops are mapped as conglomerate, lithic sandstone and mudstone of unassigned Palaeozoic age.

Most of the project area and beyond is covered by Cainozoic sediments. Calcrete, commonly associated with chalcedonic silica, is developed on carbonates of the Bitter Springs Formation and along former drainage channels and in old lake beds.

EXPLORATION TARGETS

Most notable in the region is an aerially-extensive high-amplitude uranium channel radiometric anomaly located over the southern part of Lake Mackay. The radiometric ‘plume’ is likely due to radionucleiide daughters that have been discharged into the Lake via modern groundwater flow. The ultimate source of the radionucleiides is speculated by Toro to be a concealed uranium mineral system to the south of the Lake. Three potential geological scenarios are envisaged.

Figure 1: Uranium channel Radiometric and DTM drape image (DOIR stitch) for Lake Mackay Project, showing the large anomaly over the southern Lake Mackay. Toro detailed mag-rad survey outline in yellow.
 

Figure 2: Regional magnetic image (DOIR stitch) and Exploration targets for the Lake Mackay Project, showing various target zones. IOCGU targets are the magnetic highs in the southern and northern parts of the project area.

Mesoproterozoic granitic and metamorphic basement rocks of similar age and character to those that host the Iron Oxide-Copper-Gold+/-Uranium (IOCG-U) deposits of SA’s Gawler Craton, including Olympic Dam, underlie the Lake Mackay Project area.

Baseline geochemical studies by Geoscience Australia and cursory exploration by Aurora Gold and BHP in the late 1990s suggest the presence of large-scale alteration systems and low grade gold and copper mineralisation around Mount Webb, immediately south and west of Toro’s tenements. Ashburton Minerals has drilled wide intersections of Cu-Au mineralisation at Pokali South, following up strong surficial mineralisation, (Quarterly Report to ASX, 30^th October 2008).  Historically, samples have not been routinely analysed for uranium. Based on this data, Toro’s conceptual target at Lake Mackay is a large basement IOCG-U system, buried beneath Cenozoic dune sand or Amadeus Basin sediments.

The Neoproterozoic to Palaeozoic Amadeus Basin, including Carboniferous foreland sediments, has been folded into a broad syncline occupying an east-west ‘graben’ immediately south of Lake Mackay, transecting the middle of the project area (Figure 2). Geophysical data suggests this graben has a steep structurally complicated northern margin and a shallow-dipping southern margin. Toro will be targeting the margins and fold closures for structurally-enhanced redox (‘roll-front’) type uranium deposits similar to Bigryli and Angela in the Ngalia and Amadeus Basins respectively.

The majority of the project area is covered by a veneer of Cenozoic sand dunes. Sporadic saline lakes and calcrete pavements are exposed in the interdune areas and indicate the presence of a larger buried ‘fossil’ calcrete system. Anecdotal accounts of a yellow uranium secondary mineral (carnotite) developed at surface along the southern edge of Lake Mackay point to the possibility of a buried calcrete uranium system in the vicinity. Toro believes the geological setting is analogous with Toro-owned Lake Way and Centipede deposits (Wiluna project) in Western Australia, where uranium mineralisation has developed at the deltaic interface of palaeodrainages and saline lakes (see comparison figure below). The dimensions of the radiometric anomaly at Lake Mackay are much larger than the surface expression at Lake Way-Centipede and provide encouragement for potential new mineral deposits to be discovered.

 
Figure 4:  Uranium channel Radiometric images for: (1) Lake Way-Centipede and other nearby uranium prospects and; (2) radiometric anomaly at Lake Mackay Project (same scale and algorithm of DOIR stitch)

PREVIOUS EXPLORATION

There has been almost no exploration over the area encompassing the Lake Mackay Project, but there has been limited exploration on adjacent ground. To the west, BHP was granted three exploration licenses in 1996, targeting IOCG mineralisation of the ‘Olympic Dam type’ related to the Mount Webb granite. A large regional gravity anomaly (GA data) and the identification of appropriate granite type and alteration were the principal reasons for exploring the area. An aeromagnetic survey was flown and geophysical interpretation completed. Work completed from 1996 to 1997 comprised negotiating Aboriginal access agreements, and the interpretation of airborne magnetic data. During 1998 to 1999, work completed on the project comprised collection of ground magnetic, gravity and EM sounding data. A large gravity high anomaly was identified to be tested further.  Due to the stalling of access negotiations and difficulty getting a drill rig to the target zone, two exploration licenses were surrendered and only the ground containing the gravity anomaly was retained. A further ground magnetic traverse across the gravity anomaly indicated a very weak anomaly. Drill testing was recommended.  However, no further work was completed and the tenement was surrendered.

Aurora Gold held a large package of ground immediately south of Toro’s project from 1995 to 1999. That company completed rock chip sampling and acquired airborne magnetics-radiometrics, ground magnetics and remote sensed data. Aurora also drilled fences of shallow aircore holes and identified anomalous gold and copper at Pokali and Mantati prospects. The best rock chip interval was 4m @ 9.1 % Cu, 3.1 g/t Ag and 0.38 g/t Au.

WMC held substantial ground from 1997 to 1998 targeting gold on the north-western shoreline of Lake Mackay. Access could not be negotiated and therefore no exploration was carried out.

BHP was granted three tenements immediately west of Toro’s project in 2001 in search of diamonds in a series of small, high-frequency magnetics anomalies. Ground work was never completed due to access issues arising from floods in 2001 and difficulty in coming to an agreement with Traditional Owners.

In 2008, Ashburton Minerals reported anomalous Cu and Au in surface samples collected at the Pokali Prospect immediately adjacent to the south of Toro’s project. It subsequently drilled 5 Reverse Circulation holes, all reporting thick anomalous Cu-Au mineralisation, with the best results including:

 
PKC001           205m @ 0.1% Cu from surface

PCK002           16m @ 0.51% Cu from 30m

PCK003           44m @ 0.3% Cu from 146m

PCK004           10m @ 0.6% Cu from 156m

PCK005           14m @ 0.51% Cu from 124m

 
EXPLORATION COMPLETED BY TORO

2008

Airborne magnetic-radiometric survey

An airborne 100m spaced, magnetic and radiometric survey was completed over approximately 50% of the tenements, covering 10,270 line kilometres.
This survey has enabled Toro to:

• Refine the structural architecture of Amadeus Basin to assist in the generation of Angela-style targets;

• Resolve magnetic features that may relate to IOCG-U mineralising systems, including fault intersections;

• Identify and resolve surficial radiometric and magnetic targets for detailed ground traversing.

• Locate a new generation of palaeochannels that may host tabular uranium mineralisation.

2009

Surface Sampling
1,892 Surface samples were collected. Sampling failed to detect obvious bedrock uranium anomalies.

Airborne Electromagnetic (TEMPEST) Survey
During November/December 2009, Fugro Airborne Surveys Pty Ltd undertook an airborne TEMPEST electromagnetic and magnetic survey over the Lake Mackay Project area. The survey resolved large magnetic bodies akin to IOCGs of the Gawler Craton, and a delicate drainage network emanating from south. Strongly conductive clays in the top 50m dominate the TEMPEST data.


Figure 7: Airborne electromagnetic (TEMPEST) image from the 2009 survey.

Drilling
52 aircore (5193m), one diamond and two RC holes along two east/west traverses were drilled in October, 2009 through the sand cover to extend or locate potential host facies in the Tertiary and Devonian-Carboniferous sequences and any other targets recognised. Anomalous U was intersected in a number of aircore drillholes with some encouraging results in two drillholes spaced over 3km.

 
LP0029            6m @ 216ppm U₃O₈ from 103-109m

LP0031            2m @ 646ppm U₃O₈ from 106-108m or

                        4m @ 428ppm U₃O₈ from 104-108m


Figure 8: Location of 2009 Lake Mackay aircore drillhole traverses

2010

Airborne Magnetic-Radiometric Survey

During May and June 2010 GPX Surveys Pty Ltd completed a100m spaced multi-channel radiometric, magnetic and digital terrain data survey adjacent to the 2008 survey area. The new survey is approximately 1,374 km² in area, and together with the 2009 survey will deliver Toro detailed radiometric and magnetic coverage of approximately 70% of the project area. The Theseus Prospect lies within this survey area. Interpretation of the new geophysical data will allow:

1)  Identification of subtle high frequency magnetic patterns that will assist in resolving buried palaeochannels that are prospective for uranium, including the recognised zone at Theseus;
2)  Refinement of the structural architecture of the Amadeus Basin to generate Angela or Bigrlyi style targets and;
3)  Resolution of deep magnetic features that may relate to IOCGU mineral systems.

Drilling