I got downvoted here, so I thought I'd expand. If we replace "South American gold mine" with "the release of the iPhone" or "the creation of Google" perhaps my point is clearer - i.e. the creation of value should have economic impacts. Even in a fiat currency situation, the discovery of a massive South American gold mine would have an economic impact as someone now has access to more valuable resources than they did previously.
Now, if the currency was based on a commodity that has no value, then maybe your argument makes some sense, but precious metals were chosen specifically because they are valued irrespective of their role in currency. Besides, if an enormous amount of gold was discovered, sovereigns and governments still had control of the currency through debasement.