Multi element assay as prospecting tool

Welcome To The Canadian Gold Prospecting Forum. Please consider registering to gain full access! - Its free and only takes a few seconds to complete - Click Here.

Gold Prospecting Forums, Gold Prospecting Forum

[Guest]

A A A A

[aumbre]

Can anyone offer any input on the use of multi-element assays to help in directing efforts toward more efficient prospecting? I have lots of veins with variable mineralization but the gold is spotty or enriched only within certain areas and I’m trying to prioritize the areas/veins with the higher potential.
These 5 samples were taken from the dump pile of our old mine by my partners in 1993. I don’t have much info on what they are supposed to represent, but at least some were probably hand selected for sulfide mineralization.

http://i890.photobucket.com/albums/ac102/aumbre/new/assay.jpg

[aumbre]

Hey, I knew this would be a hot topic! Sorry that the assay certificate is hard to read in this format but you can enlarge it on your computer (zoom) and see it ok.
Also please note that the lower ½ of the page is just a continuation of the sampled elements for the same 5 samples.

I’m looking for a concentration range of indicator minerals that accompany potential ore.
.
Samples #1, #2, and #3 show a good grade of ore that averages 0.37 oz / ton gold with over 3.69 oz / ton silver.

Samples #4 and #5, not so good, average a mere 0.051 oz / ton gold and but 0.892 oz / ton silver.

Silver; elevated silver levels appear to be a good indicator of gold ore. If you have a vein in a gold district that is carrying silver but not much gold, don’t give, up there may be more gold nearby and you can have a lot of fun digging up unprofitable silver bearing vein material.
Bismuth; Higher levels of Bi accompany the gold ore.
Cobalt; might be a better indicator for nickel, chromium, iron, copper, and platinum.
Chromium; These 5 samples show an inverse relationship of Cr to Au.
Copper; Clear correlation of elevated gold with Cu over 1%. It’s a good one to watch for as many copper minerals can be easily recognized in the field. Keep a lookout for green stains in the oxidized material.
Iron; the good gold ore comes with a higher percentage of Fe. Iron can come in several different forms and is one of the most common and persistent element of gold bearing veins. As a general rule a quartz vein should have at least some visible metallic mineralization to be of interest.
Manganese; I don’t know if Mn is going to be something to watch.
Phosphorus; I’ll kind of keep my eye on P.
Tungsten; All the high gold samples have much higher W levels.
Lead and Zinc; all the samples are carrying significant levels of Pb and Zn which doesn’t surprise me as these samples were probably selected because of obvious shiny mineralization. There are a lot of veins out there that have nice galena but no gold.
I’ve heard arsenic, antimony, and mercury are also useful indicator minerals.

[GollyMrScience]

Ambre I am a big believer in multi element assays to get good info but the first thing I would observe is that you have single assays on each sample.
Without at least three assays on each sample the natural variables in an ore - including nugget effect - can bump your result and throw the interpretation sideways.
As for what I see right off the bat I would note that the calcium in directy co-related to the gold in the first three samples but that link gets tenuous when the other samples are added to the mix. The other elements are not so definite but I have just glanced at this so will take some time later today to really check it out. In the meantime you may have a calcite link in the vein that would deserve sme attention. Could also be calcium scale from groundwater invasion.
The calcite (if that is what is giving the calcium bump) might have been deposited along with the gold or after so that would need a look see. If it is hydrothermal calcite there may be a feldspar (adularia) link as well.

[aumbre]

I would have never thought to have 3 analyses done on the same sample. Is the whole problem the nugget effect due to the small sample size, or are inaccuracies involved with the assay that can be averaged out? The physical samples themselves are rather meaningless at this point as there are no records or descriptions other than they were all taken from the same waste dump pile from the same mine.
The assay report seems useful as a starting point since it shows a distinction between good grade and a low grade samples.
I have the report from 18 more samples that I took in June this year. All were low on gold and 1 had a few ounces of silver.
Most of my new samples came from different veins and perhaps different types of veins so any comparisons will probably be like apples and oranges.

[GollyMrScience]

There are always variations on assays but one assay could be skewed by nugget effect or it could be just lower than it should be. With one assay you would never see the nugget effect. With two you might spot it if one was really out of line. Three gives a better picture.
Taking samples from a mine dump I usually am after examples of what the mine took out and what it means. I take fist size samples and make sure that I keep an example of what the raw ore looks like I can spot ot when I see it in the mine. It also gives me something  have thin sections taken from if I want to get microexamination.
I get the rest of the sample crushed and split for three assays at least. I try to avoid the mini assays so common with assay labs. Often they go for half assay tons but the little extra of a full assay ton sample is worth it
I also like to go for a larger assay series. 32 elements for instance. Better chance to spot trends and besides you could find that while looking for a gold mine a vanadium mine might break out. 

[PlacerPal]

The most significant assay results I see in the assay report besides the gold and silver,
are the greater than 1% lead and zinc. Silver usually goes with lead and so does arsenic.
I would be exploring the lead and zinc results because with the high test results there
may be a silver-lead-zinc mine with the silver a bonus to offset the overhead.

Gold in the same area is not to be ignored either. Why? A hard rock mining company
I used to work for in the Assay Dept. discovered a zinc ore body of 16% - 18% zinc.
Further diamond drilling below the zinc ore body which is well defined discovered a
huge copper and gold deposit immediately below the zinc ore body. There is a billion
dollars worth of gold in the  huge copper-gold ore body. Now the company has a nice
problem because they are set up for zinc mining and refining and having recently shut
down their copper smelter due to the Fed reducing the allowing SO2 emissions another
mg/ton!! Anyway, many more years of operation for the 83 year old mine that no geologist
would buy a home in the town because the mine may run out of ore any day!  

 

[aumbre]

Placerpal, I think that you are right about the lead and zinc. The productive veins in the area were classified as “base metal veins with varying gold” or something like that. Most of the early mining was done on the shallow oxidized parts of the vein and recovery dropped of quickly in the unoxidized, deeper parts of the vein. I’m pretty sure that most of the precious metals would be recovered in selective flotation concentrates. I really have no idea of any idea of any depositional models, it is where it is and there are a lot of places where it maybe is.



Partially oxidized sulfide ore from the mine dump.

http://i890.photobucket.com/albums/ac102/aumbre/new/DSCN0711.jpg

[aumbre]

I’ve been looking over the assay report for the 18 samples from this summer and unless there is any interest in seeing the actual report I’ll just summarize my observations.
 5 of the samples contained precious metal values of between $40.00 and $210 per ton (Au @ $1,000 and Ag @ $15 per ounce).
Bismuth: Direct relationship of the 3 highest Bi with the 3 highest precious metals.
Copper: The 5 highest in Cu (586 ppm. to 8,370 ppm.) correspond with the 5 highest precious metals.
Lead and zinc: Good relationship, although the one highest sample of Au, Ag shows only medium levels of Pb, Zn. High cadmium levels correspond with Zn and Elevated levels of Cu, Pb, and Zn pretty much hang together.
Calcium: There is some indication of low Ca to high Au, Ag but there are some blips and relationship may be somewhat vein specific.
Arsenic and Antimony together were anomalous, perhaps with Iron in a couple of   samples from generally good neighborhoods but with low reported precious metals.
Mercury was not tested (extra cost).

A blast from the past.


Things have changed since 1962.

http://gpex.ca/image-sharing/images/assayglg.jpg