Desert Dirt Gold Guru Contest
A rich man once said "never invest in anything where you have to entirely rely on others for advice". Gold is one of the toughest metals to predict and mining company performance is even tougher. He loved to play mining stocks because he knew the business and felt his experience gave him an edge on the competition.
I suggest that every miner and mine speculator should start with the common assay. There are only three gold assays that one should be aware of. Each can be further subdivided, but lets start with the basics. They are:
1. The Extraction Assay-The best and most reliable in my opinion is the Fire Assay. It follows the extractive rules of pyro-metallurgy. The first rule is there is no such thing as 100% extraction in any extractive assay. In this assay, a weighed amount of material is melted with various flux ingredients and usually litharge (lead oxide). Carbon in the assay causes the lead oxide during fusion to reduce to the metallic melted lead form, collect the gold as an alloy and bring the gold with it to the bottom of the crucible. Certain metals other than gold and other precious metals can also be delivered to the bottom of the crucible. These should be removed if the next step (called cupelling) is to be successful. My favorite is to pour the melt, cool the charge, break the glass and clean the lead button. The lead button is then added to hot melted litharge and cooked until the new charge is quiescent. The charge is poured into a cone and cooled again. The button is again cleaned and wrapped in lead foil with silver and sometimes gold inquarts. The inquarts help to determine what is called the old Roman Rule of Inquartia. That is it takes like metals in a certain ratio to insure that the gold will coagulate during parting and not be lost into acid during the next step. The lead button and inquarted foil is placed in a furnace again and the lead is oxidized back into litharge. Precious metals are not supposed to oxidize under high temperatures and therefore remain as refined precious metals dore' as the litharge is adsorbed into the cupel. The bead of silver and gold is cleaned, hammered flat, weighed and added to hot nitric acid. The acid dissolves the silver. The remaining weight of metal is pure gold. Take the total bead weight and subtract the weight of the inquart and gold and you know how much silver was present. Subtract the weight of the inquart and silver and you will know the exact amount of gold. The process is called by difference. --------------------- Liquid extraction methods where the gold and silver are fired and weighed are also called extractive assays and although they don't always represent the total amount of metal present, they do represent minimums.
2. The Instrument Assay- These assays are the curse of mankind. The sample is submitted to radiation or heat and the returning radiation or the spectrum of light is analyzed for gold by instrumentation. Neutron Activation is an example. When we radiate the rocks with neutrons, the gold sends back its radiation fingerprint. Sounds good, but the problem is there are neutron absorbing minerals and elements that can either cause false positive readings or mask real ones. Pulse height alpha spectrometry is a little more reliable for gold, but only a few labs outside the government nuclear facilities have them.
3. Extractive with instrumentation-A popular method of assay for gold is atomic adsorption spectrometry (AA) and Induced Coupled Plasma (ICP). These methods use a liquid extraction to extract the gold and the liquid is analyzed by spectro-analysis. Here we have the risk of a poor extraction followed by interferences that effect the reliability of the report. ICP can also be run as an instrument analysis without being preceded by liquid extraction (emissions spectrometry) but it is usually with a liquid extraction.
The information above will help one understand the differences in gold analysis. PGMs are in my opinion more believable when done by Fire Assay extraction methods. Here it takes a specialist to perform his art. Mike Thomas is, I believe, one of the best. He has been mentioned in Desert Dirts before.
The "standard fire assay" is an abortion of the fire assay process. It does report gold for some ores, but all too often it fails to report gold with ore suites too numerous to mention. If it works, it is cheap to perform and it does report weighed gold if used the way it should be done. An assayer can increase the reliability of this assay by doing six or seven slag correction assays to insure there are no gold losses left as residue in the slags during fusion. This takes "cheap" out of the process.
Hey Desert Dirt Gold Gurus, these are only the opinions of a man with too many years of experience in business and in gold mine and lab management also.
Later we will introduce the "precious metals process quiz" as the next learning curve. |
