Sorby Hills Joint Venture Project
Boab owns 75% of the Sorby Hills Joint Venture Project. Sorby Hills is located 50km from the regional centre of Kununurra. There are existing sealed roads to transport concentrate from site to the facilities at Wyndham Port, a distance of 150km, and established infrastructure allows for fast track production. Our joint venture partner, with a 25% contribution is Henan Yuguang Gold Lead Co. Ltd. Henan Yuguang is China’s largest lead smelting enterprise and the largest silver producer in China.
Regional Geology (Geological Setting)
The onshore Bonaparte Basin is a remote and under-explored mineral province prospective for carbonate-hosted Zn-Pb (Ag) mineralisation.
The Late Palaeozoic rocks (Devonian to Carboniferous, 380 to 320 Ma) of the Bonaparte Basin were deposited on the margin of a rift basin bounded by Precambrian basement. Map 1 below shows the onshore triangular shape of the basin and its opening and deepening to the north. In the south and east, fault reactivation extending from the Halls Creek Orogen controlled its intra-basin geometry.
The Precambrian Pincombe Inlier was an emergent palaeo-high during Late Devonian to Early Carboniferous sedimentation and separates the Burt Range Sub-basin in the east from the Carlton Sub-basin in the west. The early sediment horizons of the Burt Range Sub-basin are referred to as the Cockatoo, Ningbang and Burt Range Formations, which are made up of basal clastic rocks (sandstones) and dolomitized limestone and clastic units, representing a continuation of continental to marine deposition. Within the project area the succession measures a total thickness of about 300 m .
The Bonaparte Basin sediments are the time equivalents to the Lennard Shelf sediments which host the Mississippi Valley-type (MVT) of the Lennard Shelf between Fitzroy Crossing and Halls Creek in Western Australia.
Map 1. Regional Geological Map
The Devonian-Carboniferous sea appears to have been inundating an incised coastline around a pronounced palaeo-high (Pincombe basement, Map 2 below). The oldest sediments include basal clastic rocks with a gravel beds onlapping on the basement inlier to the east. A thin succession of intertidal, and shallow marine to lagoonal facies is initially deposited within the project area while eastwards, in the Burt Range Sub-basin the basin floor subsided rapidly accumulating in excess of 2 km of marine sediments as indicated by seismic data. The articulated paleao-coast line is well illustrated W of the Norton deposit where an embayment of Devonian strata extends several kilometres SW over the basement rocks.
Prior to the onset of Carboniferous sedimentation, the marine platfroma the palaeo-topography effect appears removed and a rapid . Correlation between down-hole gamma profiles shows a gradual thickening of Carboniferous strata towards the east within the project area.
The mineralisation at Sorby Hills is located in about 10- 20 m of transitional facies between the Knox Siltstone in the hanging wall and the Sorby Dolomite in the footwall and is made up of reworked clasts of Sorby Dolomite. The extensive dolomitization is inferred to be genetically related to the mineralisation and has completely recrystallized the primary calcite-rich rocks, creating porosity estimated to be up to 15% in part of the Sorby Dolomite (Ferguson 1999). A secondary dolomitization event is indicated by coarse dolomite in fracture linings and vughs.
Map 2. Project Area Geological Map
Since the commencement of exploration and resource delineation work by Boab in 2018, the Company has completed 251 drill holes for just under 20,000 m over four drilling campaigns of which the majority are diamond drill holes. In the process the Company has reached several critical milestones towards development. The aggressive work schedule has served multiple technical purposes and resulted in the release of three mineral resource statements (MRE).
Boab’s work programs have advanced the Sorby Hills Pb-Ag (Zn) project significantly towards the completion of an Optimised Pre-Feasibility Study (OPFS) by facilitating increased levels of confidence in the areas of:
- Hydro-geology and
The drilling work completed by the Company has demonstrated the lateral continuity of mineralisation, has increased the footprint of economic mineralisation, has enabled a 3D geological interpretation and opened the opportunity for further discovery.
- ACD046 intersected 20.0m @ 7.3% Pb, 56g/t Ag and 0.4% Zn
- AF012 intersected 15.0m @ 5.8% Pb, 81g/t Ag and 0.1% Zn
- AF005 intersected 11.7m @ 10.8% Pb, 105g/t Ag and 0.4% Zn
- AI010 intersected 9.4m @ 6.6% Pb, 53g/t Ag and 0.9% Zn
- AI011 intersected 12.3m @ 5.5% Pb, 0.23% Zn and 42g/t Ag
- SH_PD_A-18 intersected 18.0m @ 5.08% Pb, 36 g/t Ag and 0.2% Zn
- SH_PD_A-31 intersected 10.0m @ 7.15% Pb, 383 g/t Ag and 0.43% Zn
- SH_PD_A-15 intersected 6.0m @ 9.53% Pb, 55 g/t Ag and 0.32% Zn from 47m
- SHDD032 intersected 15.0m @ 3.8% Pb and 22 g/t Ag from 24m
- SHDD035 intersected 9.0m @ 4.8% Pb and 33 g/t Ag from 36m
- SHDD043 intersected 11.0m @ 7.4% Pb and 29 g/t Ag from 35m
- SHDD039 intersected 21.0m @ 3.4% Pb and 15 g/t Ag from 31m
Figure 1. 3D Oblique view of the Project Area and Resource Outlines
- Geological & Resource Model: a new geological and resource model have been constructed using new drilling data together with historic information
- Rock Density: Additional rock density data together with the revised geological model have led to a revised density algorithm and an appropriate density model
- Resource Classification: the improved geological interpretation and the density model have led to a re-classification of the resources and for the first time “Measured” category resources have been declared by the QP
- Resource Expansion: Since Boab has taken over the Sorby Hills Project the Company has been able to expand by 66% on its original resource estimate and the metal content (Pb) by 33%
- Metallurgy: Metallurgical recoveries after geo-metallurgical classification and additional testwork are higher with 84% silver and 94% for lead.
The Company announced the results of an updated Mineral Resource Estimate (‘MRE’) in April 2021. The Global Resource is now 44.9Mt at 4.3% Pb equivalent (3.2% Pb, 37g/t Ag) and 0.5% Zn.
In August 2020, Boab released the results of an updated Pre-Feasibility Study (‘PFS’). Working with an update of the most recent MRE, the PFS envisaged a 1.5Mtpa throughput rate over an initial 10 year mine life. The proposed mine plan targets high grade lead-silver mineralisation within the Indicated Resources category. The PFS combines ore drawn from three open pits with a processing plant employing conventional milling and flotation with a beneficiation plant between the coarse crush (dry end) and the grind and flotation circuit (wet end).
The PFS indicated steady state annual production of concentrate containing approximately 50ktpa of lead and 1.5Mozpa of silver. Pre-production capital expenditure is estimated to be A$183M and lead C1 direct cash costs of production of US$0.40/lb (inclusive of silver by-product credits). Using a lead price of US$0.95/lb and silver price of US$21.10/oz with an exchange rate of A$1= US$0.70 results in an estimated NPV of A$303M (at an 8% discount rate) and an IRR of 46%.
Key outcomes of the PFS are presented in Tables 2 and 3 below.
During the period, Boab released results from the Phase I metallurgical testwork program conducted by ALS Metallurgical Services (under management by Simulus) to assess beneficiation options and support previous flotation performance.
Results confirmed the potential for the Project to produce high quality lead concentrate containing appreciable silver credits. Flotation recoveries were higher than the assumptions used in the recent PFS. Cleaner flotation testing confirmed that a final concentrate grade of 65% Pb can be produced. Meanwhile, rougher flotation testing with sulphidisation indicated up to 96% Pb and 95% Ag recovery on Fresh composite and up to 91% Pb and 95% Ag recovery on Oxide composite. Favourable results were received from further heavy liquid separation testwork to upgrade the ores with lead losses lower than the 10% assumed in the recent PFS.
A lead upgrade resulted for both fresh ore (1.85x with lead losses of 3.3% at Specific Gravity (‘SG’) 2.75 and 3.36x with lead losses of 6.6% at SG 2.93) and oxide ore (1.22x with lead losses of 4.1% at SG 2.75 and 2.15x with lead losses of 9.1% at SG 2.93).
Preliminary ore sorting testwork returned 1.3x and 1.4x upgrades for lead and silver respectively using XRT with lead losses of 2-3%.
Ongoing testwork will support both the Optimised PFS and the Definitive Feasibility Study planned to commence in 2020 using samples from the Phase II drilling program.