In regards to the use of a VLF, as I haven't hunted for platinum before, I am relating information from others who have. It would make sense if you think about it. PI machines miss the smaller weaker signals, as well as not being very good at hitting gold mixed with other material, where as a VLF will. The fact that these guys (one owns a Lbo) said that it gave off a weak signal, would be a good indicator, that a PI machine will struggle with it. I would suppose that if a nugget is big enough, then you may hear it, but can give no clue as to if that is possible or not.
Sorry that I can't give more accurate information than that, but it is the best that I can do for you.
Something that you may need to factor into hunting in South America. Check out on how civilised the place is before you go. I read about a chap who went to Papua New Guinea, having the the circumstances as yourself. He had heard that there was plenty of gold to be had there, got very excited and went there, planning to spend 6 to 12 months nugget hunting. After he had arrived over there, he realised the danger that he had placed himself in. There is no law and order over there. He realised that if the locals had got wind that he had a metal detector and that it could find gold, then they would have killed him for it without hesitation. He was back in Australia within 3 weeks. When you consider how gold fever affects us even when we have good law and order, then you can imagine how it will effect someone who has nothing and nothing to lose and fear of prosecution. I'm not suggesting that you shouldn't go, just look into it so that you know if you will be safe. My inlaws have travelled to most counties in South America a few times, and found them to be, a very friendly bunch, but that doesn't factor in greed, when it can raise it's ugly head. Think of the Gold fields back in the 1850's and how civilised they were.
Something worth thinking about.
Mick Evans.