Orebody Detrital Model, The Pilbara, Western Australia

The Orebody 25 open-pit mine is a small iron-ore pit that forms part of a series of deposits known as the Orebodies situated 10 km of the town of Newman. The site is at the junction of three major fault systems, the Whaleback, Fortescue River and Homestead faults. Archean bedrock sequences have been tightly folded into an overturned synclinal fold. Thick clays, minor conglomerates and calcrete, all inferred to be Cenozoic age detrital units, unconformably overly the bedrock and comprise the southern hanging wall of the final wall cutback.

PSM was engaged to interpret the nature and origin of geotechnical rock and soil mass units in the detritus above the bedrock and to model their distribution.

Located near the Ophthalmia Dam, the mine is continually recharged with groundwater. There is strong calcrete overprint of detritals with significant seepage along calcrete margins.

Engineering and technical opportunities included:

• Limited cored drilling before the PSM study
• Poor historic geotechnical understanding of detritals at the site
• Recommendation of previous designs for conservative shallow slope angles, whereas excavation at higher angles was being achieved.

Following a rigorous assessment and modelling, PSM’s tailored approach included:

• Training of site staff in geochemical interpretation of detrital units
• Use of sonic coring techniques and Hylogger geochemical spectral imaging analysis for maximum cost-effective recovery and analysis of detritus
• Geochemical comparisons of the Orebody 25 clays with other clays of known detrital origin and weathered shales to establish geochemical affinities and interpret origin
• Recognition of the clays as a preserved weathered bedrock shale landslide based on form and geochemical relationships
• Assessment of calcrete geochemistry to infer relative rock mass strength properties.

Working closely with BHP Billiton Iron Ore, PSM developed a sound knowledge of Cenozoic detritals which form portions of final walls with increasing frequency across their deposits.  PSM contributed their expertise in Pilbara detrital and bedrock geochemistry to develop a model that characterised the unit origins and distribution. Subsequent excavation of the cutback proved the model accurate.

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