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  • Writer's pictureLori Dee

Advanced Sampling Techniques


If you are serious about getting all the gold from your chosen prospecting site, a good sampling technique is essential. Every commercial mining operation relies upon the data obtained from their sampling program, because they do not want to waste a lot of time, money, and energy digging in dirt that has no gold. This technique is not only used by miners but also by metal detectorists and even archaeologists.



Let's look at some numbers. The standard placer gold claim is 20 acres per individual claimant. This differs from lode claims. Lode claims are measured from the discovered gold vein, and may not exceed 1,500 feet by 600 feet from the center point of the vein or lode. Placer claims cover an area. Since we are dealing with placer mining, we will focus on that.


20 acres equal 871,200 square feet. If you dig a hole with a shovel, the hole will be about one square foot in size. That means that if you wanted to search every bit of a 20-acre placer claim, you would need to dig 871,200 holes in the ground. How long would that take?


As you might know by now, there is no need to sample every square foot of a claim. Like commercial operations, you don't want to spend a lot of time processing dirt that has no gold. So you can look at topographic maps, geology maps, and aerial photos, and walk the ground to use what you know will be the most likely places to find gold.


For example, this is an aerial image of a creek that I have worked quite thoroughly. Some things you may notice are the bend in the creek, and the rapids at the bend. Reviewing what we learned, we know that as the water flows around the bend, the water on the inside of the bend flows slower than on the outside. As the flow slows down, it loses power and starts dropping the heaviest materials. Other rocks and heavy gravels start to build up, and rapids can form. Rapids make a great natural riffle system that traps gold.

When scouting this location on the ground, I noticed that three ridges are running across the creek caused by raised bedrock. These bedrock ridges contributed to slowing down the water flow and catching other rocks as they were pushed downstream. We know that this creates drop zones where heavy gravel (and gold) will drop in and around those rocks.

Now that we have a pretty good idea of where we want to dig, are those the only locations? Consider this. We know that placer deposits form when a lode deposit erodes and the gold works its way downhill into the streambed. We also know that gold is very heavy, so will tend to be pushed along in a straight line. We know that gold will drop on the inside bend of a turn, but does that mean it will NEVER be on the outside edge? It most certainly could be.


What if the gold lode was uphill from the outside bend of the creek? The arrows show the straight line that gold would be pushed along the creek bottom. So if the gold entered the creek on that upper shore, it could end up on the outside of the bend. And, thanks to my sampling technique, I discovered there was gold in that exact spot. Otherwise, I never would have looked there.

There are two important parts to a good sampling system:

1. Be consistent and thorough, and 2. Document the results. (When documenting, be consistent and thorough.)

You will want to divide your area into grids, and then sample each grid location. It doesn't matter how big the area is, just add more sample points to your grid. Metal detectorists do this to ensure they don't miss an area. Archaeologists do this to ensure they don't miss an area and to document where everything was found. That is what you want to do too.

With rivers, it may not be possible to sample certain locations due to high, fast water or other safety concerns. Maybe an area along a shore is not accessible, like a cliff. I mark those areas with a zero. Then, sometime later if the water level is low, or conditions make it possible, I can get back there and sample those points. Those locations could be quite rich because no one else could get out there to prospect them.


When you are sampling each spot, be sure to record the results separately from any other spot. You also want to note the depth of each find. Typically, I will dig a hole just a couple of inches deep, pan it, then dig it 4 inches deep, pan it, then 6 inches deep, etc. before moving on to the next hole.


As you complete your notebook, you may begin to see a pattern. For example, maybe the pay streak (the line the gold was pushed along), is along line C at a depth of 8 inches. Once I identify where the biggest, chunkiest gold is located, I begin loading the sluice box. In one case, that hole ended up about three feet deep and 15 feet in diameter. It paid very well.

Later, when you have that area cleaned out, you can still use your notes to "point" to the next place to look, upstream or downstream. And you will have an idea of what depth the good gold is at. I don't know how old the layer is, but the best gold I have found at that location was at knee-deep. Unfortunately, when the water is running high, that means I am almost up to my armpits in the water. In the late Summer, or during drought, when the water is low, I can get back in that spot and dig some more.


Keep good records. Document your finds. Take lots of photos on the ground, and use Google Earth for aerial views. Pay attention to how this information relates to your previous research (geology maps, mining journals, etc.). You may find a gold deposit that was active 50 years ago and today everyone has forgotten about it.


And maybe you'll have as much fun as I have.


Good luck!



 

All content Copyright 2021- 2024 - Lori Dee


 




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