[hanamizu]

Yeah, because ancient coin collections are often found the backpacks of itinerant miners.

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The Egyptian/Ptolemy coins are big, heavy and impressive, and surprisingly, not all that hard to come by. Lots of people were in Egypt digging up things 100 years ago and Ptolemic coins aren’t that rare. One could well be a lucky charm a miner might carry with him. Much more likely than the other scenarios.

[SunkenCiv]

Sure, because prospectors can afford more gold than they're ever likely to find in a year. IOW, no, it is not only not likely, it verges on impossible. It requires that Australia wasn't invisible to seafarers for over 40,000 years, which isn't any great leap.