***Interesting Stuff on Fire Assays...
J.B.P. Sawyer, Consulting Geologist
From: Annels, A.E. (ed.) 1992, Case Histories and Methods in Mineral Resource Evaluation, Geological Society Special Publication, No. 63, pp 37-45
< Abstract: This paper is mainly concerned with the assaying of precious and noble metals, gold, silver and platinum group metals.The determination of the amount of saleable metals ... in an ore is the most important step in evaluation of a mineral deposit. Most assays, even if well done, will not tell you how the material occurs in the rock or how much of it can be recovered, but if the amount that is present in the first place is unknown, it is impossible to make any comment as the efficiency of a recovery system...
Fire assaying, with its origins in antiquity, is widely regarded as the best, if no the only, way to determine precious metals in naturally occurring ores. There are a lot of misconceptions about the universal applicability of fire assay methods and the more recent combination of traditional fire assay collection techniques with laboratory instrumentation has given rise to a serious, if not ludicrous, situation in the case of some materials.
There are some naturally occurring materials which cannot be successfully determined by fire assay. Some of those same materials can only be determined by instrumental techniques if their chemistry is understood. Even the most sophisticated instruments in the world will not yield the correct answers if the chemistry of the ores being tested is not taken into consideration in designing the sample preparation and instrumental procedures to be used. Such considerations can only be made if the chemistry is understood, and such proper understanding requires the availability of a mind open to the fact that there may indeed be something new (to man) under the sun. It is important to appreciate the implications of the fact that naturally occurring materials may be in an unusual form; hence the need for a clear as well as an open mind...">
Just a few highlights:
<"...generally speaking there is no problem in determining the amount of zinc or lead in a Mississippi Valley type ore... or of copper in porphyry copper ore, or of Uranium at Key Lake, Blind River and Rossing... There are, however, some precious metal deposits which do present considerable problems in their accurate determination, and it is on these that this paper is focused.
The term "some precious metal deposits" refers to gold, or gold plus PGM occurrences, which do not respond in the normal way to standard assay procedures. In fact, in our present state of knowledge, they cannot be satisfactorily determined by standard assay laboratory procedures; specifically fire assay will not produce the correct result. Now in many jurisdiction, and indeed in many corporate offices, 'them's fighting words', and if one is rashenough to utter them in audible tones one runs the serious risk of being branded a knave or a fool, or both. However there is plenty of evidence to demonstrate that they are true and will find acceptance by those willing and able to approach the matter with an open mind.
...Fire assaying is not the only problem area... but it is the one which attracts the most attention, and causes the biggest controversy...
...Two important things happened in the years immediately prior to the 1970's and the 20 years or so since then. First, we lost a whole generation, or perhaps two generations, of fire assayers: with the price of gold fixed at $35 an ounce, it did not matter very much whether the amount of gold in a sample was .01 or .1 ounce per tone and thus fire assaying became something of a lost skill. Many of the old time assayers... could tell if an ore was acidic or basic and would adapt the flux accordingly, and if they did not get it right the first time they would repeat the assay. ...They know fire assaying was not a universally applicable technique as indicated by some of the old texts on assaying... The second important factor was the replacement of fire assayers with instrument technicians who are trained to know which buttons to push and which digital read-outs to record but who, for the most part, have no chemical background.
...The modern assay laboratory is designed to handle several hundred... or thousand samples a day and their price structure is based on volume. One has only to review the current price lists to see the problem... (Canadian $) when the poverty-line salary in
Canada is $26,000 pa, what kind of assay are you going to get for $15? Certainly not one that requires more than cursory inspection... Anyone who thinks that a $15 precious metal assay on a complex ore is going to be accurate on any but an accidental basis is not living in the real world. Even worse (some labs) are offering instrumental precious metal assays, acid digestion/ICP or similar procedure of $3.75 and they would have you believe they have a foolproof computer program that will correct for all the matrix effects and interferences!>
The fellow goes on to give a lot of background on fluxes and all types of assays... and then most interestingly goes on to describe spiking experiments which showed that certain types of complex ore could mask the presence of gold in a known sample.
I've been told the article was controversial -- there exits strong opposing points of view -- Sounds just like IPM.
More to come on meeting Dr. Shaw at IPM on my trip later...
Lew Green |
