[defrost]

> If it wasn't a dense deposit, then did the costs substantially increase with cyanidation? (Not total cost, but cost per unit production.)

I made a broad long term statement that's true over multiple decades and centuries - if you take a keyhole view there will be times when the long term trend is bucked.

I don't specifiaclly know the exact answer to your question (although it can be worked out by a research student with a month or two to spare) but I would hazard that profits from gold mining were dwindling with a high cost of getting some value from fines .. and then cyanidation made things profitable again.

It's a market with supply | demand and a finite amount of gold in the crust - nuggets are no longer laying aboutto be picked up, and now many tonnes of sand and grit need to be centifuged | screened | shaken to get a concentrate .. and as the profit from that dwindles and price/kilo rise due to limited supply - it become possible get more gold from the concentrate with a little additional cost (in time + chemical) and profits rise again.

Whatever specifically happened in a short time window in a specific location though; the long term trend remains, more effort for less return of product.

[eesmith]

I understand how you can think of it as a keyhole view.

Instead, I think it's that I want "dense deposit" to mean something fixed, so we can look at a Roman gold extraction operation and say "yes, that is a dense deposit" or at a South African mine and say "no, that is not a dense deposit" independent of the technology in use.

Here's a thought experiment for that research student - which would cost more using current wages:

- extract 1 ton of gold from a deposit as rich as (say) the Dolaucothi Gold Mines when it used by the Romans, and using only Roman techniques.

- extract 1 ton of gold from a deposit equivalent to a South African mine in 1900, using cyanidation techniques of that era.

(I don't know if 1 ton is too low or too high to be reasonable.)

> nuggets are no longer laying about to be picked up

This isn't entirely true. People do still fund nuggets by happenstance. A news search finds things like https://www.yahoo.com/entertainment/family-finds-24k-gold-nu... from a few years ago.

But yes, they aren't the types which kick of a new gold rush.

In any case, Roman gold extraction wasn't just from picking up nuggets either, so I'm not sure that's quite the right comparison.