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Geology/understanding sediment data


My question concerns using USGS sediment samples from the 1970's NURE (National Uranium Resource Evaluation) studies to predict the mineralization of an area today.

Their findings are shown in ppm.  Several streams in my area show samples between 3ppm and 8ppm for Gold.  It is my understanding that gold is not visible to the naked eye until it is 30ppm or larger.. Is it probable that in the areas sampled above, larger gold would be located if I dug deeper in the stream bed? I haven't been able to find anything that tells how deep their samples were taken. Is there a standard depth when taking stream sediment samples?

Thank you for any wisdom you can impart!

I am not personally familiar with the procedures from the NURE sampling program; however, I was able to find at: the following description.

Each rock was analyzed for 31 elements using semiquantitative emission
spectrography (Grimes and Marranzino, 1968). The analytical values were
reported as the midpoints of intervals. These midpoints make up a series of
numbers called steps: 1, 1.5, 2.0, 3.0, 5.0, 7.0, 10.0, 15.0, 20.0 etc. The
precision of the values generally is within 1 step of the reported value
approximately 83 percent of the time, or within 2 steps of the reported value
approximately 96 percent of the time (Motooka and Grimes, 1976). The upper
and lower limits of determination for each element are shown in table 1. In
addition to emission spectrography, many of the rocks also were analyzed for
particular elements using chemical methods listed In table 2.

Although this particular Report was from Alaska the procedures were probably all the same.  The essence from these ANALYTIC PROCEDURES would lead me to believe that the samples were rocks/gravel taken from the stream.  This procedure would work a lot better for uranium than it would for gold (as far as prediction), since uranium has six valances and gold has none. The difference being that uranium changes chemistry every time that the eH (oxidation) or pH changes whereas gold almost never changes chemistry.

30 ppm is in the range of 1 oz./ton.  I haven't done much placer mining but I ran a gold mine in Colorado for a number of years, in hard rock, and I could see gold beginning in the range of 20 ppm or slightly less with the necked eye.  I also used 10X, 30X, and 40X microscope lenses.  I could, within a few months predict, with the microscope, within 10 percent of the analysis in the sample.

Because of the weight of gold (about 20 - 22 Specific Gravity) it does not travel real well.  You will probably be able to find gold under big boulders and on the downstream sides of same if they have not already been looked at. And yes you may find more gold deeper in the sediments.

Hope this helps  


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