Twin hole drilling (twin) is a technique used for validation by testing the historic drill hole with another adjacent hole.
This paper is a case study of the importance of resource due diligence based on two separate twin hole verification programs in PNG, the first a twin as part of a resource to reserve upgrade at the Morobe Heavy mineral Sands project, and the second a sanity check on a resource drill out for the offshore Fly River iron sands project.
A twin’s aim is to demonstrate that geology, sampling and assaying are repeatable. The successful verification underpins confidence in the drilling and sampling integrity, but this is not always the outcome.
The planned twin program at the onshore Morobe chromite and Iron sand project was part of a feasibility study with the initial drill out being mostly from churn drilling on a 200 x 100 m. The churn drilling was validated by 3 sonic core twin holes, 2 m of the original drill hole.
The planned twins at the Fly River offshore iron sand project was part of the initial resource infill drill out of a 200 x 200 m submarine vibracore and intertidal Bangka drilling which was tested with by vibracore twins within 1 m of the resource drill hole.
The two different programs increased the uncertainty in the resource estimates and highlighted critical project risks. The Morobe twins compared to the original churn drilling demonstrated vastly different geology, completely different mineral distribution and downgraded the original assay results.
The Fly River twins tested the resource drill holes in large offshore sandbars and showed a large discrepancy from the initial resource holes, in some cases having higher grade and in others no grade at all. A follow up program of further twin (triplet) holes a month later showed wild variation to the initial and even the twin. A trial twin program in the same spot after each tidal cycle demonstrated that the resource grade was changing with each tide cycle and flagged the risk in reliance iron sand resource estimate.
Twin hole drilling is useful to provide data to inform key decisions and in these two cases flagged critical risks in grade overestimation and grade continuity.
