Saturday, July 12, 2008

Placer Gold Sampling, Part II



The Sampling Process

In an earlier post I discussed the overall importance of proper sampling and the general principles involved in establishing a successful approach. Now we'll examine the sampling process itself, primarily from the small-scale, hand tool approach which most recreational placer miners would employ. Sampling by suction dredge is, in some respects, a different ball game so I will relegate it to another post at a later time.

Please note that the following sampling approaches are useful in both wet and dry placer locations. Additionally, each of these approaches is contingent on the premise that the sampler has chosen a likely area for placer gold deposition based on stream structure and hydraulics, and the physical laws governing depositional physics.

Sampling Approach 1, Lateral or Perpendicular

This approach involves taking a series of samples in a lateral sequence perpendicular to the flow of the stream (or wash). There is no solid dictum regarding a starting point (i.e., extreme left, right, center, etc.) along this lateral line, but I tend to start my sampling sequence in low-pressure areas of the stream where material is being deposited, directly behind large obstructions such as boulders, or where bedrock is visible or sitting under shallow overburden. This then becomes my starting point and I will subsequently take samples at consistent intervals and depths from this central location.

Sampling Approach 2, Parallel

In this approach a sequence of samples is taken in a likely area parallel to the direction of the stream or wash flow. This method is particularly useful in evaluating the gold values contained in areas such as gravel bars (inside bends) or low-pressure areas studded with smaller obstructions. It can also be effective in determining the overall consistency of pockets or paystreaks laid down in the direction of the stream flow (something which does occur, although not as frequently as deposits or paystreaks deposited across the stream flow).

Sampling Approach 3, Grid

This approach employs a grid pattern for sampling and can be very effective in determining the gold values contained within a localized area of the stream or wash. The first step is to lay out the boundary of a large box or rectangle. Samples are then taken from each of the four corners of the box and, if needed, from midway points along each side of the box. The interior of the box can then be gridded out into smaller boxes for more detailed (and accurate) sampling.

Sampling Approach 4, Diagonal

Here a sequence of samples is taken diagonally across the stream or wash. Though similar in function to Sampling Approach 1 (lateral), this approach combines sampling elements of both the lateral and parallel (Sampling Approach 2) approaches and can reap the same benefits as these if applied conscientiously.

Sampling Approach 5, Getting Struck by Lightning (not for serious miners)

This is what I term the "getting struck by lightning" approach. Invariably you will see it being employed by novices to the world of recreational mining or by those who are just too lazy to do it the right way. In this approach there is no rhyme, reason or logic to the way samples are taken. The sampler simply runs to and fro in a haphazard manner, grabbing a pan full of dirt here and another there, paying no attention at all to stream hydraulics or deposition physics. Or alternatively, the individual plops down in one location from where he or she cannot be budged and then proceeds to spend the entire day searching for the "big one" in that very same spot. Later, this same person will complain bitterly about the lack of gold in that particular stream or wash and tell everyone else what a lousy location it is. (Of course, some people actually get struck by lightning, but how many do you know personally?)

Make sure you tag or identify all your samples or employ some means of distinguishing which ones came from what locations and at what depths. One way this can be done is by placing the samples in small or medium plastic bags (such as trash bags) and using a piece of tape and a Sharpie or Magic Marker to number or identify them. Pan or process each sample carefully and note how much gold each contains and whether the better gold values are associated with the following:

large amounts of coarse black sands,

pieces of rusty (oxidized) iron or nails,

conglomerations of oxidized metal, or

lead shot or fishing weights.

Additionally, pay close attention to where the best samples are coming from and what sample depth appears to be the most productive. Once you have performed all aspects of one of these systematic sampling approaches, you should have a clear idea where to start your actual mining procedures and what piece of mining equipment is best suited for maximum gold recovery.

(c)  Jim Rocha (J.R.)  2008

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