I think people would be surprised. The western US between the Sierra Nevada and Rocky Mountains inclusive is one of the largest and richest mineral fields in the world. It is also so sparsely populated and difficult to traverse in many areas that much of it has never really been walked over in modern times. If you have some basic geology and mineral knowledge, it is surprisingly easy to find incredible mineral deposits simply by wandering around the outback off the beaten path.
I used to do a lot of overlanding in remote parts of Nevada, just to see what was out there (not prospecting). Gold, silver, and copper are ubiquitous in that State and easy to find. We discovered crude oil while exploring an abandoned gold prospect (there is now a company developing that oil field). We found opals and turquoise in abundant quantities.
Last year while hiking around the Cascades, I found the most concentrated copper deposit I have found anywhere in the US. In the same area, I found molybdenite of a scale and quality such that I can’t find pictures of anything comparable on the Internet (I did not know what it was when I found it). Both of these finds, while difficult to get to, were within line of sight of a popular hiking trail.
It is important to understand that economic viability of mineral deposits is a function of scale and value per ton. Concentrated deposits that are not large enough cannot offset the capital overhead of developing the deposit. Big mining operators tend to prospect in areas where the geology is conducive to scale, but small operators occasionally find excellent deposits in unexpected areas that big companies won’t waste their time on.
I am not a prospector and I have never disclosed the locations of where some great mineral samples I possess were collected. It is really just a fun and geeky adjunct to my other outdoorsy activities. The US mountain west is one of the world’s best places to go prospecting for minerals. If you are out there and keep your eyes open, you will find some eventually.