A 2.1 Mt gold resource at Bétaré-Oya: concentrate bias or real ore?
A 2.5 km² gold prospect near Bétaré-Oya, eastern Cameroon, has been sampled with 96 auger holes on a 200 m grid to an average depth of 3 m. Visible gold was panned from 18 holes. Fire-assay results on 1 kg bulk samples range from 0.03 to 0.60 g/t Au (mean 0.18 g/t), but assays of pan concentrates from the same holes range from 5 to 42 g/t Au. The heavy-mineral suite is quartz, magnetite, ilmenite, zircon, goethite, abundant kaolinite and monazite; some monazite grains give 600–1,200 cps on a scintillometer. Gold grains are 50–300 µm, and some remain attached to quartz. The company has issued a press release claiming 2.1 Mt at 1.6 g/t Au (about 3.4 t Au) and is seeking an investor for a 100 t/day gravity plant.
1. Explain why the bulk fire-assay and pan-concentrate results differ so strongly, and state what must be done before the 2.1 Mt at 1.6 g/t Au figure can be treated as a mineral resource.
Read the worked answer
The two datasets measure different things. Panning concentrates dense minerals and free gold, so a 5–42 g/t assay is a concentrate grade, not an ore grade. The 1 kg bulk fire assay is much closer to an ore grade but is still vulnerable to a nugget effect: 50–300 µm gold grains are coarse relative to a 1 kg sample, so repeat assays can scatter widely and under-report gold. Before the 2.1 Mt at 1.6 g/t claim can be treated as a resource:
- collect a bulk sample of several tonnes, or use metallic-screen/cyanide-leach assays on 10–30 kg samples, with duplicates, blanks and certified standards;
- establish a geological model of the auriferous horizon (colluvial, saprolite or quartz-vein hosted) and measure bulk density;
- confirm that the mean grade is not calculated from concentrate assays;
- classify the estimate only after independent QA/QC and a qualified-person review.
On the present evidence the 1.6 g/t figure is unsupported. The bulk assays suggest a much lower grade, and the prospect may be a small, nuggety occurrence rather than a 2.1 Mt resource.
2. Recommend a processing route for the proposed 100 t/day operation and identify two environmental or regulatory issues that must be addressed before permitting.
Read the worked answer
Gold is 50–300 µm and partly locked in quartz, so a gravity concentrator (jig or Knelson) plus shaking table could recover coarse free gold, but tailings would still need cyanidation or sulphide flotation. The abundant kaolinite will cause blinding and high water demand, so washing and desliming are needed before gravity. At only 100 t/day, a gravity–cyanide hybrid may be difficult to finance; testwork should compare gravity plus intensive cyanidation with whole-ore cyanide leach.
Two issues must be addressed:
- Monazite is radioactive (600–1,200 cps). Concentrates and tailings are NORM, so handling, storage, worker protection and transport require a radioactive-materials permit and monitoring.
- Cyanide or mercury use, tailings containment and water discharge require an environmental impact assessment and community consultation. The 18 g/t pan results should not be used to size the plant or to attract investment.