The results from THB’s Induced Polarisation (IP) survey at Kookabookra have confirmed an outstanding set of significant chargeability and resistivity anomalies that are strongly indicative of sulphides, and are set in highly favourable geological settings. None of these targets have been tested, and historical drilling at the project has repeatedly intersected what can now be interpreted as smoke near a fire of what is likely more concentrated mineralisation. $300 million market cap LRV recently revealed near identical looking IP results 60km south of Kookabookra which contained assays as high as 7m at 22.28g/t AuEq.
The go-to analogues for the project are the high-grade 2Moz Hillgrove mine (orogenic), and the gigantic 5Moz Kidstone mine (IRGS) – and the IP survey has further bolstered the potential for both types of mineralisation to be present at Kookabookra. You can read our initial take on THB here.
Despite none of the main chargeability anomalies at Mannix being tested with historical drilling – all drill holes at the project have intersected gold, with several terminating in mineralisation. Notable intercepts include 12m at 0.53g/t Au from 10m, finishing with 1m at 1.62g/t Au and 15m at 0.37g/t Au from 7m, ending with 1m at 3.42g/t Au. This is a powerful indicator of a large and pervasive hydrothermal system, and given Kookabookra has been able to hit these assays outside of the main anomalies – a major discovery could very well be lying beneath it right next door.
Mt Secret’s results are also very exciting, with 3 strong chargeability anomalies that are indicative of sulphides – two of which that are large and adjacent to the Glen Bluff Fault (which has been interpreted as a NE-trending fault), as well as a geological contact and anomalous gold in soil. A high chargeability target to the north is also right near what can now be interpreted as 7 near-miss drill holes that went to a maximum depth of 79m, and hit 7m at 0.43g/t Au from 10m and 6m at 0.67g/t Au from 69m.
The gold is interpreted to be mostly hosted within the Black Knob Monzogranite, which is associated with sericite alteration, fracturing, quartz veining and disseminated pyrite. This is especially interesting due to much of Kidston’s 5Moz worth of mineralisation being able to be seen on IP, which has been attributed to pyrite alteration.
It’s important to note that THB has only tested a tiny fraction of Kookabookra’s 618km2 land package of highly prospective ground in NSW’s famed New England Orogen with IP – many of the project’s high grade historic mines are still yet to be tested.
The array of chargeability anomalies at Mannix can be seen below, in conjunction with drill results:
Source: THB
Mt Secret’s three anomalies can be seen below, in conjunction with the Glen Buff fault and historical drilling:
Source: THB
The Hillgrove Analogue – A Validation of the Technique in the New England Orogen
LRV has repeatedly validated IP’s strong correlation between known mineralisation and high chargeability and resistivity responses. Just 60km south of Kookabookra, at LRV’s Clarks Gully prospect, the company recently hit extremely high grade assays of 7m at 22.28g/t AuEq, 6m at 15.5g/t AuEq and 7m at 9.34g/t AuEq.
As can be seen below, the main body of mineralisation at Clark’s Gully cuts right through the section of strongest chargeability:
Source: LRV
It also aligns with the highest level of resistivity, as seen below:
Source: LRV
It’s important to note that the potential for seriously high grade ore at Kookabookra has already been proven. Just one of the project’s prospects, Bear Hill, has historical production of 3,100 ounces at 24.4g/t Au, and THB have found high grade rock chips up to 153.5g/t Au at Mt Secret – which is about to be drilled.
Other prospects include Butchers Reef which historically produced at 32.5g/t Au, as well as Welcome Stranger Reef which has 42.6g/t Au in vein quartz and is where THB have recently recorded additional samples of 8.56g/t Au and 2.09g/t Au.
The Kidston Analogue – A Multi-Million-Ounce IRGS Blueprint
At Kidston, historical IP work showed a low-chargeability, silica-rich core surrounded by a higher-chargeability ring – which ultimately hosted the bulk of the ounces in sulphide minerals such as pyrite and arsenopyrite. The mine held many high grade gold shoots in excess of 3g/t and major bulk tonnage areas of 1.5g/t.
At Kookabookra, the geology is continuing to line up with that IRGS model. Historical logs at Mannix describe mineralisation with sericite alteration, quartz veining and disseminated pyrite, which is the exact kind of sulphide-bearing, quartz–sericite assemblage you expect around an IRGS centre.
IRGS deposits are genetically linked to intermediate felsic, reduced intrusions and commonly develop inboard of active margins. The New England Orogen hosts Permo-Carboniferous granitoids intruding late Paleozoic metasediments, which is a setting well known to generate IRGS-style systems – a local example being the 397koz Timbarra deposit.
Crucially, Kookabookra’s Black Knob Monzogranite has been identified as a reduced I-type granite, which is the right setting to indicate the flow of mineralised fluids. Reduced magmas concentrate gold with elements like Bi, Te, W, As and Sb, and those fluids move into fractures and receptive wall rocks – precipitating sulphides and gold. The presence of this reduced intrusion at Kookabookra is a key observation that supports the Kidston-style analogue.
The Inner Workings of Induced Polarisation
Gold and antimony mineralisation is overwhelmingly associated with sulphide minerals in the NEO, which has been strongly proven for orogenic-style deposits, such as the Hillgrove mine. For land in the NEO, this makes IP chargeability not just an indirect exploration tool – but a direct and low cost detection method for the mineral assemblage that hosts the target commodities. IP has also proven extremely effective for IRGS deposits like the 5Moz Kidston mine, which while further North of the NEO – has many clear characteristics that make it a plausible comparison.
The Kookabookra survey successfully defined several anomalies with chargeability values greater than 10msec, a threshold considered highly indicative of significant sulphide concentrations.
To fully appreciate the significance of the Kookabookra results, it is essential to understand the geophysical technique employed. IP is a very well understood geophysical survey method that dates back to the 1950’s, and is a cornerstone of modern exploration for sulphide-associated mineral systems, which measures two key electrical properties of the subsurface – chargeability and resistivity.
Chargeability reflects how strongly and how long the ground holds charge after the current is switched off. Disseminated sulphides act like microscopic capacitors, releasing charge slowly, which means that higher chargeability generally means more sulphides, with certain caveats such as graphite and some clays also being able to elevate chargeability.
Resistivity is the inverse of conductivity and captures how readily current flows. High resistivity is typical of quartz-rich/silicified zones and many fresh granites, while lower resistivity often marks clay-altered, graphitic or sulphide-bearing zones, but also can reflect saline groundwater. Interpreting chargeability together with resistivity, along with structure and geochemistry, significantly sharpens target definition.
The litany of rock chips and soil samples at Kookabookra, along with consistent mineralisation from drilling around key IP anomalies, combined with the highly favourable structural geology – make THB’s raft of high priority targets very worthwhile drilling. If any of them strike economic mineralisation, then there is potential to deliver serious value to investors given THB’s $5 million market cap.
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