The old miners weren't careful with or knowledgeable about their ore. They wasted many valuable, unidentified minerals and left platinum nuggets in the streams in the mining of other, more common minerals.
Since they didn't know about PGMs back then, I think there is a lot of opportunities to find it in old and current mines and in the unexplored places it is found. I am not as pessimistic as you because I have witnessed the vast waste and poison the old mines unleashed on the land. 150 years ago or 1888, PGM were first identified and no production of it existed anywhere before then or until at least 1939.
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