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Post by aleksi on Jun 12, 2018 7:03:41 GMT 12
Hi everybody, I joined a group of people in Mexico, involved in different placer mining activities (industrial). They have few dredges they are not using and are asking me to see if we could put them to use on one beach of the Pacific showing nice fine colors in the pan. The thing is, their dredges are kind of customized according to what the sellsmen seemed to find more profitable to sell... for example, the 6" has two 23hp engine, which I find ridiculously big, but they said that in the context of beach placer it will allow them to get deeper. Does it make sense ? I am just thinking that all the fine gold might just get washed out of the recovery system. They also have a 8" dredge, and two spare 10hp engine that I am thinking to put on the 6" and use the two 23hp for the 8" instead. Your opinion ? Speaking of recovery system, what are the recommandations in the case of beach placer, fine gold and loads of heavy black sand ? If you guys know someone knowledgeable about beach placer mining, I would be very glad to be able to speak with him. I apologize for my english, as I am just a french guy  . Thanks everybody ! |
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Post by aleksi on Jun 14, 2018 4:28:02 GMT 12
Hi everybody !
Anybody from the forum working in central or south America ?
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Post by RKC on Jun 14, 2018 12:35:39 GMT 12
G'day aleksi, Any off-shore dredging, utilizing conventional suction/eductor dredges, will require a very different approach compared to dredging an active in-land river. Because of the success off-shore dredging, with small suction dredges, has achieved over recent years at None in Alaska, its only inevitable that miners would want to try the same successful approach in other parts of the world. However ... the gold deposits off- shore at Nome are, I believe, fairly unique. The Nome gold deposit is actually a convention auriferous placer deposit that just happens to be submerged under the sea. Most of the other world wide off-shore gold deposits are made up of extremely fine gold (New Zealand's West Coast for example) that has been washed down rivers and dumped on the beaches, and off-shore. Because its a placer deposit in Nome, the gold dredged off shore can be quiet chunky, with enough weight, that the recovery systems of a convention river dredge can easily retain the gold in a conventional sluice box. So, the approach you decide to take will very much depend on if the gold in your area of interest is in a submerged Nome type placer deposit (unlikely), or if its all fine gold (fine and mustard gold) typical of a beach deposit. Many years ago I consulted on an off-shore dredging operation in Indonesia. The dredge used was a conventional home built, but quiet large, suction dredge (pictured below) that would be typically used for production dredging of active inland rivers. They knew the gold was there just off-shore from the coast, and they knew the deposit was very rich (which it was), but they found as soon as they started dredging that the gold (picture below) was just going straight through the sluice box and back into the sea. Black sand was not a major problem, and what ever they did to improve the sluice box, the gold was still being lost. The sole problem was that the gold was too fine to be caught by any sluice box. The only suggestion I could come up with was to use the sluice box solely to classify, and then to pump (probably with a gravel or slurry pump) to the beach where a more involved recovery system could be set up ( img.photobucket.com/albums/v521/NZGOLD/Alluvial%20gold%20mines/P1010039.jpg ). But that would have changed the whole economics of the operation, which was being run by indigenous artisanal miners. So, that problem was not able to be solved. If you also have to contend with great amounts of heavy black sands, this will complicate things significantly. My personally preferred solution to solving any problems with black sands is to use hydraulic riffles (you will find previous posts on hydraulic rffles by doing a search of the forum). But even if you used hydraulic riffles you would most probably still lose gold (if the gold is fine). So using hydraulic riffles is something only to consider after other considerations are made. What stage are you at presently? Has any prospecting being done to find out the deposit is all fine gold, or not? img.photobucket.com/albums/v521/NZGOLD/Alluvial%20gold%20mines/NZGOLDffggrrr138.jpg  Regards, Rob (RKC) |
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Post by aleksi on Jun 15, 2018 6:32:30 GMT 12
Hi Rob, thanks a lot for that interesting answer. So far, I just went paning the beach : lots of black sand, very very fine gold. Pretty much what you describe.
I have an acquaintance here claiming that years ago on a similar beach placer fuether south from where I am, he could work with a 5" Dahlke dredge using no matting whatsoever, and gold concentration would double every meter down. We are going for a test day tomorow with a regular 6" dedge. I'll let you know about the results.
As for beach placer, I am interested about it anyway for futur insvestigations. You speak about Nome as a quite unique kind of deposit. Do you think, or ever heard of, the possibility of some similar deposit down in Patagonia and tierra del fuego ?
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Post by RKC on Jun 15, 2018 10:23:34 GMT 12
Hi Rob, thanks a lot for that interesting answer. So far, I just went paning the beach : lots of black sand, very very fine gold. Pretty much what you describe. I have an acquaintance here claiming that years ago on a similar beach placer fuether south from where I am, he could work with a 5" Dahlke dredge using no matting whatsoever, and gold concentration would double every meter down. We are going for a test day tomorow with a regular 6" dedge. I'll let you know about the results. As for beach placer, I am interested about it anyway for futur insvestigations. You speak about Nome as a quite unique kind of deposit. Do you think, or ever heard of, the possibility of some similar deposit down in Patagonia and tierra del fuego ? G'day aleksi, It looks like you are at the very start of what will involve a lot more work to come! I have always firmly believed that the prospecting and evaluation phase of any mining project is the most important part of a mining project. And panning is only a beginning of the prospecting phase. A dredge can operate with no matting (and I'm presuming also with the removal of the riffles) if the power is reduced significantly to control the flow to just a mild current running down the box. But ... then there is not enough power to lift the wash from any depth. The most used hand-feed beach boxes in New Zealand don't have riffles and the wash is just run over a mat (see pictures below). When you are testing with the 6-inch dredge, and because its fine gold that is expected to be recovered, its essential you regularly pan the tailing's from the dredge. There is always 'some' fine gold in the top riffles (which has just not had enough time to move down through the riffles), so just seeing some fine gold in the top of the box does not tell you anything conclusive. The question to be answered is, how much fine gold is going through the dredge. Many amateurs (and some dredge manufactures  ) will look in their dredge and when they see some fine gold behind the first few riffles come to the wrong conclusion that the dredge can catch nearly all the fine gold. There is also a reasonable theory that the velocity of water traveling down a dredge sluice box increases as the water travels further down the sluice box. So, once the gold gets past the first few riffles the increased velocity of the water will carry the gold more quickly out of the box the further down it gets in the box. I can't help you with any information on the areas you mention. I have never been to South America. However I do vaguely remember reading, a long time ago in the California Mining Journal, an article about bucket dredging in tierra del fuego. Maybe someone from there who is reading this might like to make a post. If you find a deposit in your area of interest like None then it would be better that a typical beach fine-gold deposit (much simpler to work). Extensive prospecting will tell you that. If I was you I would be first trying to find any geological reports on your area. I'm looking forward to reading the results of your prospecting with the dredge.    Regards, Rob (RKC) |
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Post by aleksi on Jun 15, 2018 11:23:59 GMT 12
Hi Rob, thanks again for this very informative post. To be honest, if it were only me I would not focus on beach placer here in Mexico but on the rivers. I am now just trying things that comes on my way... I do have some promissing geochimical datas and mining history in the area where I am, but it is also a hot spot for narcos as soon as you enter the sierra. And I did meet them to many times already and do not specialy like it. I am missing a bit of intel about Mexico even if I thought I had hard datas. I hope someone around can read this and maybe we could figure out something . Anyway, I'll let you know about the ocean dredging once we give it a try ! Thanks again. |
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) will look in their dredge and when they see some fine gold behind the first few riffles come to the wrong conclusion that the dredge can catch nearly all the fine gold. There is also a reasonable theory that the velocity of water traveling down a dredge sluice box increases as the water travels further down the sluice box. So, once the gold gets past the first few riffles the increased velocity of the water will carry the gold more quickly out of the box the further down it gets in the box.