Required grams/tonne of AU at depth for commercialisation.9 Sep 2025 08:01
I’m no geologist or gold expert, but using ChatGPT to try and get some understanding of commercialisation:
Got it — let’s re-frame the whole first answer using US$3,600/oz gold instead of US$1,800/oz.
Short answer: At $3,600/oz, an underground gold mine at ~700 m depth might be viable at ~0.8–2.5 g/t Au, depending on operating costs. That’s much lower than the ~2.5–5 g/t range usually needed at $1,800/oz.
Why there’s no single fixed g/t: Cut-off grade depends on many variables: mining method, cost per tonne (which rises with depth), metallurgy/recovery, processing costs, royalties/taxes, and desired profit margin. Depth alone doesn’t set the grade.
Examples: underground mines often report cut-offs ~1–2 g/t at today’s high prices; very selective narrow-vein mines still need much higher grades, but the economics are more forgiving when gold doubles.
Back-of-envelope illustration (at $3,600/oz)
Gold price = $3,600/oz ÷ 31.1035 g/oz = $115.74 per gram
Recovery = 90% → effective value ≈ $104.17 per gram
Break-even grade ≈ OPEX per tonne ÷ (value per gram)
OPEX (US$/t) Break-even grade (g/t)
80 0.77 g/t
120 1.15 g/t
150 1.44 g/t
250 2.40 g/t
So, if your underground costs at 700 m are ~US$150/t, you’d only need ~1.4 g/t to break even at $3,600/oz.
Implications
Higher gold prices lower the economic hurdle dramatically — turning sub-2 g/t underground ore into potentially profitable material. Still, mining costs at depth matter: if your OPEX creeps toward US$250/t or more, you still need ~2.5 g/t+ to be comfortable. Companies usually use a conservative cut-off grade below spot prices to avoid overestimating reserves.
Can any informed posters shed further light on this?