Bill,
I once met a man, Vaugn Hunter. He spent all of his life learning how to assay precious metals while with Giant Yellow Knife in Canada, Inco in Sudbury, Canada and he spent his last 38 working years doing precious metals analytical research for the South African Chamber of Mines. His comment to me was that he knew more when he was 20 years old than he does now. That was 20 years ago. When he was 20, he had explanations for everything, now everything he had learned was faulted. He said he knew hundreds of different ways to assay PGMs and gold ores, some methods take more than a week to perform but yield the most values and therefore are the best methods if being close to total values are required. At that time the Chamber of Mines in South Africa looked at a good PGM assay as costing about $450 to perform That would probably be $5000 today. A large precious metals lab at a major refinery in Switzerland charged about $1950 in US currency for a total metals assay. That was a couple of years ago. I never could duplicate their results no matter what I did. I actually did not believe them. I thought they knew bullion, they couldn't assay ore. Well, I am a little wiser today and am not so quick at the trigger.
By the way, in this day of not trusting the assayer, AA analysis are out except for ore control during production where you have produced results to prove your assay. In that application an AA is quite valuable, but an extractive assay that produces the metals in hand is still the most valuable tool if trust has to be a part of the report.
Just for others that do not know the difference, a fire assay is an extractive assay that produces the precious metals into a dore' bead containg gold and silver. The dore' metal bead is refined to gold and weighed, the metal that is dissolved away is the silver. PGM assays can be done also as extractive assays. An old rule of metallurgy is there is no such thing as a 100% extraction process. Extractions can range from 0 to 99%. You never know for sure. An instrumental assay like neutron activation, induced coupled plasma (ICP) are instrumentsl assays. I believe that all instrumental assays are subject to the fatal flaw of having potential interferences resident in the natural sample. Interferences can either add values or take values away from the report. You never know for sure until the product is produced, refined and weighed. The trouble with an AA is that it is an extractive assay that is followed up with an instrumental spectrographic analysis. The current devices that do the instrumental work are computerized so as to minimize the interferences. I have found the AA work that the old timers did where they extracted the interferences before doing the instrumental analysis were much more accurate than the current high tech devices. In those years we dissolved the values, or what we could, into aqua regia acid. The acid went through chemical cleaning of interferences and filtering to remove as many interferences as was reasonable. The results seemed to be better. But to do that, the expense is put back into a cheap assay. Bummer. In any case the AA starts as a chemical extraction and is subject to extractive loses. The error is compounded by interferences in the leachant. If you know your ore and adjust the instrument or procedure or calculations with known standards of material from that deposit when preparing standards, AA analysis are very good. But what commercial lab goes to that end in checking their results. Most assay business is to the lowest bidder.
I used to say that the assay is only as good as the quality of the person doing it. An extractive assay, by wet chemical or fire that results in refined values, is only a minimum if done correctly. But more important, the prospector should know as much as he can about the value to be placed upon the analytical method and the flaws resident in each procedure. Then, the weight he places on the results will be buffered by his experience and knowledge. And he should not get his analytical knowledge from the commercial labs either. They are in business with a sell job and the people they hire are not usually as knowledgeable as the guy that meets you at the door. An assayer that meets you at the door in a white shirt and tie, is not an assayer. He is a sales person or management. Very few analytical labs have assayers that have analytical research experience doing the work. Some have no experience in chemistry at all before getting the job. If they haven't spent the thousands of hours struggling with results and inconsistencies as part of the learning process of the trade, they are probably incompetent except for the easiest of analysis. And when they tell you this is a good assay method and they (the assayers)don't find any problems with it, grab your sample and run away. If they have not had problems, they are new in the business, they are bogus or they have never seriously looked for problems. If they want to see problems, there are a number of DD companies that could introduce them to the tough rocks. That is if they will get in and get dirty and find out for themselves. mike |
