The challenge

Charters Towers has produced over 7 million ounces of gold since 1872, most of it before 1917. It remains highly prospective. It is also close to untargetable.

The gold sits in pods and lenses of massive sulphide — galena, pyrite, sphalerite — inside quartz veins typically under a metre thick and under 50 m in their longest dimension, at depths past 400 m. Grade varies at the metre scale: the same reef yields 0.5 m at 0.1 g/t Au in one place and 0.3 m at 30 g/t Au a few metres along. And the primary target area lies directly beneath a country town, which rules out most surface geophysics on access grounds before physics even enters the argument.

The operator had tested the field methodically: borehole radar, surface magnetics, radiometrics and gravity, borehole induction and gamma, DHIP, surface and borehole TEM, DCIP, regional MT and government-funded deep seismic. None defined a drillable target. Down-hole IP returned a 52% anti-correlation between its predicted mineralised zones and the mineralisation actually present in core — marginally worse than drilling at random.

What we did

Atomic Dielectric Resonance transmits a coherent, low-power pulsed electromagnetic beam from the surface — 1 to 70 MHz for deep mineral work — and analyses the returned energy, frequency and phase spectra to build a virtual borehole: a vertical log of dielectric contrast with depth, acquired without breaking ground.

The team was given one drill core to calibrate against. No information on the location of sulphides was supplied. Several scans, A56 among them, were shot over ground where the operator’s own geologists had no knowledge of mineralisation and where drilling was scheduled to begin only after the survey was delivered.

Over two weeks the crew averaged eight scans a day and completed more than eighty — a vertical sampling extent equivalent to roughly 80,000 m of drilling, or about 2,300 rig-days with a single diamond rig. Nine processed scans across the Warrior, Imperial and Central areas are reported here.

Results

Scan A56 · Imperial · 467.91 mA prediction made before the hole existed

A56 returned a single anomaly at 467.91 m. Diamond hole CT8205 was collared 75 m away and drilled to test it. It cut a roughly 40 cm intercept at 463 m down hole, assaying 37.9 g/t Au and 7,200 ppm Pb. Measured against the projected E07 structure surface rather than the down-hole depth, the difference between the ADR pick and the expected intersection is under 2 m.

Scan A63 · Central · 664.77 m / 695.15 mTwo lodes to within a metre — but a retrospective match

The Central area sits under the town itself, where land access forecloses almost every alternative. Its two lowest energy values fell at 664.77 m and 695.15 m. The DD93_QF5 holes pre-date the survey, so this is a match against known intercepts rather than a prediction — the most accurate result in the set, and the weakest test in it. The DD93_QF5 drill series intersected mineralisation at 664 m, assaying 183 g/t Au and 12,700 ppm Pb, and at 696 m, assaying 15.8 g/t Au and 8,150 ppm Pb. The picks are out by 0.8 m and 0.9 m.

Scan A30 · Imperial · 510 m / 608.9 mA structure that was not on the map

A30 returned anomalies at 510 m and 608.9 m against modelled expectations of 501 m and 600 m. Hole CT8204 was drilled afterwards as part of the operator’s resource expansion. It intersected the E07 hanging wall at 514.27 m — and a second, previously unknown E07 footwall structure at 589.5 m. The geometry of E07 at that location had been poorly constrained until the hole was completed.

The comparison that matters most

After CT8205 was drilled it was cased with PVC and logged with down-hole magnetic susceptibility and conductivity, scanned twice at 5 cm intervals with a Mount Sopris tool.

Where ADR fits your programme

Not as a replacement for drilling — as the step before it. A virtual borehole is acquired in a day, from the surface, at a fraction of the cost of the hole it informs.