The 250km2 Wallbrook tenement package is considered highly prospective for the discovery of significant gold mineralisation.View ASX Announcements
The Wallbrook tenement package covers 250km2 and is an amalgamation of tenements acquired from Newmont and Saracen Minerals in 2018.
In November 2018, Nexus completed a Tenement Sale and Purchase Agreement to acquire from Newmont the surrounding 190km2 tenement package adjacent to Nexus’ existing 60km2 Wallbrook Gold Project. Nexus acquired from Newmont the surrounding three tenements covering 190km2 for a total consideration of $13,100 and a 2% net smelter royalty on all mineral product extracted and recovered from the tenements. (ASX release 29 November 2018 – Nexus Completes Wallbrook Tenement Acquisition from Newmont).
Nexus had previously acquired the central Wallbrook exploration tenements from Saracen, in January 2018 for consideration of 1,490,000 Nexus shares. (ASX release 23 January 2018 - Nexus Wallbrook Gold Project Update).
Regional and Local Geology
The region is dominated by metamorphosed volcaniclastic sediments, andesite, basalt and other sedimentary and variously metamorphosed rock units. These rock units have been intruded by numerous small, variably mineralised, porphyritic intrusions. The majority of these intrusions identified to date are under cover or very weathered. Their composition varies from syenite to quartz monzonite to tonalite. These intrusions are discordant with respect to the lithological layering and major regional structures, suggesting emplacement late in the tectonic evolution of the greenstone belt, indicating mineralisation associated with the intruded porphyries may be relatively “local” to the position of the intruded porphyry.
Gold mineralisation has been identified and mined historically in a belt stretching from the Porphyry mining district to the Wallbrook area 8km to the southeast. Over 350,000 ounces of gold have been produced from multiple operations from 1921 to 2012. This same region today contains a current JORC 2012 mineral resource inventory of 31Mt @ 1.5g/t for 1.5M ounces gold (all Saracen Resources, see their 2017 Annual Report). None of these historically mined ounces nor the current mineral resource inventory are contained on Nexus tenements.
The main sites of gold mineralisation in the district are on the margins of porphyritic intrusions. Mineralisation is known to occur within the margins of these porphyries themselves, and also developed within the intruded sheared and altered greenstone sequence lithologies and as mineralised quartz vein stockworks.
Nexus Exploration Activities
Results from Nexus drilling show that the prospective rocks are being mapped quite accurately with the use of gravity and IP geophysics. These prospective altered rocks occur over an extensive corridor striking 10’s of kilometres with widths of up to 1km within the Nexus tenement package and are widely gold mineralised within this zone. This now provides ‘proof of concept’ and focus for ongoing exploration planning.
Targeted exploration of the project area utilizing surface geology and geochemistry, in conjunction with gravity / IP survey results and historical aeromagnetic data, to guide future exploration drill programs commenced during the December quarter 2018 (ASX release 24 January 2019: Quarterly Activity Report For The Period Ending 31 December 2018).
A historical data review has now been completed with various data sets being assembled. These data sets were brought into a central database system for more efficient management and interpretation. The main sites of gold mineralisation in the district are on the margins of porphyritic intrusions. Mineralisation is known to occur within the margins of these porphyries themselves, and also developed within the intruded sheared and altered greenstone sequence lithologies and as mineralised quartz vein stockworks. Exploration activities were developed to target these styles of mineralisation.
Geological mapping of the tenement package began on completion of the transaction and is ongoing. A gravity survey was undertaken over the area highlighted in Figure 3. Gravity is an effective tool used to map the location of non-outcropping porphyritic intrusives and dyke stocks above larger intrusive bodies. The gravity survey comprised 2,011 stations with 400m line spacing and 100m station spacing along the lines. The survey successfully mapped out the broader areas of interest with gravity lows outlining intrusive bodies.
Three Induced Polarization / resistivity lines were completed over prospective areas outlined in the gravity survey. IP resistivity highs being associated with possible intrusive bodies or siliceous alteration and chargeability highs with disseminated sulphides often associated with mineralisation.
Utilizing surface geology and geochemistry, in conjunction with gravity / IP survey results and historical aeromagnetic data, an initial 18 hole RC drill program was designed and undertaken in June 2018 for a total of 3,992m.
Gold enrichment encountered in this first pass drill program provided a broad low-grade mineralised halo, as identified by 4m composite sampling of all drill holes (>0.1g/t Au). Drilling by Nexus returned mineralised intervals including 64m @ 0.32g/t Au, 48m @ 0.36g/t Au, 32m @ 0.43g/t Au and 40m @ 0.24g/t Au.
Within these broad low-grade zones narrower higher-grade zones were identified and these zones were re-sampled using 1m intervals. The higher-grade mineralised zones, often exhibiting brick-red coloured hematitic alteration, have returned results including:
- 3m @ 19.36g/t Au (incl 2m @ 27.18g/t Au)
- 3m @ 6.04g/t Au (incl 1m @ 14.76g/t Au)
- 3m @ 4.88g/t Au (incl 1m @ 9.70g/t Au)
- 2m @ 3.65g/t Au (incl 1m @ 6.03g/t Au)
- 2m @ 2.93g/t Au (incl 1m @ 4.75g/t Au)
Gold mineralisation in the Wallbrook area is known to be closely associated with quartz +/- pyrite and brick-red coloured hematitic alteration of high level porphyry intrusives and their volcanic/ sedimentary host rocks.
Drilling intersected the altered intrusive and volcanic rocks down to 300m depth across a width of approximately 800m and contain broad zones of prospective brick-red hematitic hydrothermal alteration and associated elevated mineralisation.
Previous Exploration Activities
The Wallbrook project contains no prior commercial mining operations. A number of shafts exist at the Eastward Reefs prospect, which was mined by prospectors in the early 1900’s.
An extensive search of the Western Australian Department of Mines and Petroleum (DMP) database has shown Wallbrook to have undergone a number of periods of exploration activities by different exploration companies between 1977 to 2011. These activities including geological mapping and rock chip sampling, geophysical surveys, as well as RAB, RC and diamond drilling campaigns. The majority of the Wallbrook geology is under cover with limited areas of outcrop identified at the Eastward Reefs, Jedi and Paint prospects.
There are 552 rock chip results available with 79 samples returning >1g/t Au and 34 samples returning >3g/t Au.
There have been 12 series of drill programs between 1977 and 2011. RAB drill holes represent 1772 holes for 39,000m, with the majority of these holes less than 10m in depth effectively representing surface geochemistry. RC drill holes represent 275 holes for 21,000m (average depth 76m), and a sole diamond drill hole 159m deep.