Saturday, July 12, 2008

Placer Gold Sampling, Part I



Do It Right the First Time

Sample, sample, sample.The importance of thorough sampling techniques in successful placer gold mining operations (whether large or small) cannot be overstated. Even newcomers to recreational mining seem to understand this concept intuitively and will take pan fulls of gravel from various locations in a streambed in their search for color. This approach, however, is usually performed in a "helter-skelter" fashion where luck is the main qualifying factor and stream hydraulics, deposition physics, and proper sampling techniques take a back seat.

With that in mind, here are some basic sampling principles:

1) Make sure you have the appropriate knowledge and experience of the physics of placer gold deposition and stream hydraulics before sampling. This entails a clear understanding of how gold moves and is distributed in a streambed and the effects of water flow on gold deposition in terms of force, speed, pressure, current, eddy, and so on. Without a good working knowledge of these principles you are setting yourself up for potential failure and lots of frustration.

2) Since most gold placers are composed of many layers and materials (assorted sands, gravels, rocks, boulders, clays, bedrocks, and "false" bedrocks) your samples should be relatively large in number and quantity. For example, 10 samples will give a better overall indication of potential gold values in a specific location than just 2 from that same location. The same is true of quantitative samples. Five pounds of sampling material from one location will provide more accurate test results than one pound from that same location.

3) Some placer gravels will contain gold that is consistently distributed throughout streambed gravels. If you are lucky enough to locate one of these evenly distributed placers, you can lower your sample numbers and quantities. But when sampling placers where the gold is unevenly or erratically distributed (like those in dry areas or with low annual water flow) you should increase your sample numbers and quantities to ensure a reasonably accurate estimate of the gold values present.

4) During sampling, pay close attention and take note of what types of stream materials are included your samples. Ask yourself these types of questions because they are flashing red lights that should not be ignored:

Are you finding greater amounts of gold in sample pans containing pieces of lead shot or fishing weights?

Does an increased amount of gold in your sample pans seem to be associated with increased amounts of rusty (oxidized) metal such as old square-headed iron nails or conglomerations of rusty iron that have been fused together?

Are your best sample pans those that contain the greatest amounts of very coarse black sands?

5) Remember that, in most instances, your best gold values will appear in samples taken from just above or directly on bedrock (or false bedrock layers in certain locations).

Basic Elements of an Effective Sampling Program:

1) Research the mining history of the area you want to work:

What did the old timers find in terms of gold values and where were the best gold values located?

Was the gold coarse and nuggety, or was it primarily recovered in grains and small flakes?

What materials were associated with the best paystreaks and pockets?

Were the best-producing areas upstream, downstream, on bedrock, in nearby "benches" or elsewhere?

2) Closely study the terrain around you for unique or unusual physical characteristics (colors, changes in vegetation, rock slides, large boulders or obstructions, etc.). The topography and geology around you can provide important clues to where the best gold values may be and where you should start your sampling.

4) Choose the sampling method that works best for you in terms of location and that requires the least amount of hassle and expense. For must of us that means a pick and shovel or small hand-held digging/crevicing tools. In other instances it may mean using core augers, drills, or even heavy equipment.

(c)  Jim Rocha (J.R.)  2008

3 comments:

  1. Hello,
    I am new at prospecting for placer gold. Please clarify for me the process of sampling. You say that the most gold is going to be on bedrock or false bedrock. Does that mean the samples should come from the bedrock area? Or do you sample as you dig down to the bedrock?

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  3. I apologize but I don't have the room to repsond in full to your questions here. However, I have written a number of posts that deal with the topic(s) at hand..you may want to read those first. Secondly, gold is very heavy and dense so it always headds downward over time until it can move no farther...thus, the value of bedrock or false bedrock in retaining good gold values. J.R.

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