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Geology/understanding sediment data

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Question
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!

Answer
I am not personally familiar with the procedures from the NURE sampling program; however, I was able to find at: http://www.dggs.alaska.gov/webpubs/usgs/of/text/of88-0240.pdf 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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Joe Norris

Expertise

I am an economic geologist. An economic geologist does mineral evaluations and appraisals of mineral or mining properties. I can tell you if your deposit has value - remember that a mineral deposit, no matter how good, only has value when mined. Any value assigned to a mineral deposit, in the ground, is only the speculative value that deposit.

Experience

I have been a economic geologist for most of my 35 year career. Although I have done work in perhaps 45 states and numerious countries much of my work has been in Appalachian coal, intermountain west gold and silver, and Arizona uranium.

Organizations
Past President of the Virginia Section of the American Institute of Professional Geologists and a certified geologist in twelve states.

Education/Credentials
BS Degree from Eastern Kentucky University. Work on MS Degree @ Eastern Kentucky University, Colorad School of Mines & Marshall University Numerious short courses on the value of mineral deposits and how to value same. Also several short courses dealing with the different types of geologic processes; sedimentary, igneous, metamorphic along with the mineral associated with each.

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