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Sampling Methods For New Placer Mine

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Any one ever use a standard earth auger for punching a grid of samples? I have a hyd auger with a 6" and 10" bit. It goes on my mini excavator with rubber tracks. Some miners dig test holes and some do ditches. A ditch would be best down in the creek bottom but I want to test up on the hills with minimal damage. Is a auger just gonna jumble my sample with inaccurate tests? I want to gps and log in 5 gal buckets and take to creek on wheeler to run through a dredge



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Carter / Ronald,


Lot's of variables here so I can only speak about what I am doing.


I have a Track-Mounted Hydraulic Auger. Drilling 9 inch Holes down to as much as 20ft. 20ft., right or wrong, is my Cut-Off in terms of how much Frozen Overburden I would be prepared to shift to get at Pay. If I'm not in Pay by 20ft. and the ground is not changing much then I'm out and moving. For example, I have just moved to a new area where a 1907 USGS Bulletin states that Bedrock was reached in a Shaft @ 97ft. The top 40ft. is Gravel but if I haven't hit Pay by 20ft. then I'll move. All the ground is Frozen. If I'm in Pay @ 20ft. I'll keep Drilling.


No need for Casing. Lot's of extra work. Frozen Ground holds up really well apart from the expected Thawing around the collar. Thawed ground is also quite stable. This could vary greatly depending of course on what and where you are Drilling. A Gravel Bar in the River would be problematic for example and apart from the Hole caving, I would expect there would be water ingress at some depth. This tends to wash Sample off the Auger flights.  


As far as the accuracy of an Auger, I haven't had any problems and in fact, Sample results can be remarkably accurate and consistent. One way I realised this quite quickly is when I wanted to do some Sample checking so I Drilled a series of Holes on close spacing then Logged the results and found them to be all very similar. I.e. Similar amount of Gold at the same depth sitting on a Clay layer. Barren Gravels still barren. 2ft. Of Clay above Bedrock. Zero colour on Bedrock etc. etc. The talk about Dilution or having additional Sample or even Gold drop off the Flights and fall down the Hole to a deeper layer is not necessarily nonsense but is not a major problem and with your own experience you will soon get to notice anything unusual or out of place in your Samples.


If you are Drilling Gravel you will naturally come across all the various materials such as sand, silt, fine gravel, pebbles, cobbles, greenstone, limestone, quartz, etc. etc. and usually with various amounts of Clay intermixed. We Drill through everything including cobbles up to 8" diameter or larger. I don't know if I have been through much larger than that or not as the larger rock gets ground up in to chunks so maybe I've just been lucky but I do know that the right type of Auger, i.e. Bullet-Tooth which is an aggressive bit configuration, will wear away and break up large Gravel to a certain limit. You will have to sit on it for several minutes grinding away and slowly increase the down-ward pressure until the rock starts to break up but it is do-able and is better than having to abandon a Hole then move and start a new one. These things will get easier with practice. Every Hole presents different challenges. The worst of which for me is Frozen Clay. Sometimes 10 to 12 Ft. of it.


Each Machine will also have it's own limitations and ground conditions will vary widely but it ( Auger Drilling ) can be a very useful way to get a good look at what is below the surface and in my own experience appears to be quite representative although Bulk Sampling of larger volumes will still be required if a Block of Pay can be outlined with the Drilling.


Good luck with it and I for one would be very interested to hear about your experiences.                  

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thanx Steve and Geo,


I am very familiar with the finger bits but on a very large drill, 100K lb.+ machine at a coal mine, ive drilled and been a welder rebuilding the bits. Can drill a 12 1/4" hole 140' in 12-15 minutes, and not have to change bit for a few days, thats drilling sandstone, but one 2ft layer of gravel with ruin the teeth on the bit which are cone shaped carbide. Typical bit has 9 teeth.

 My auger has the spiral center bit carbide, and the cutters are bolted on flat pieces, I may have to modify and use carbide tips, I just hope what works in gravel will do ok up out of the creek in the tundra because I bet it will be frozen clay just under the surface

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Are you processing / panning the samples from the drill directly in the field or do you log and collect them in bags for offsite processing?
Its good to hear that the drill / sampling results are repeatable from auger holes in close proximity.

Often, then only statement on gold grades comes from counting the colors in a pan. With the time and effort spent in the field to execute the drlling, a more detailed analysis of the placer gold sample may be justified though, i.e. using an optical image analysis process, which basically replaces the observers eye vision with a systematical process, that counts gold particles, calculates their weight (and thus the gold grade) and produces data on the particle size distribution and shape.

The drill sample would still have to be panned to a common pan concentrate volume (thumbsize or so) and should not contain any shiny sulfides (like pyrite), but that's normally not the case in placer deposits. The optical image analysis might be a good fit for anyone involved in professional placer exploration, i.e. drilling programs: www.higrade.tech



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