Judging by the queries from forum members there is a need for basic gold analytical information regarding ores.
It should be noted that there are always specialised exceptions to the general rules but these exceptions are unlikely to be encountered by the average prospector.
Most queries start along the lines of how much gold is in the ore sample.
An experienced person can estimate the grade of visible free gold in an ore sample. They know what gold in ore looks like and from having previous samples assayed they can estimate the grade.
If gold is not obviously present as coarse free particles then it is necessary to call in a professional assayer.
There are three types of assay for gold which are applied according to what the prospector wants to know.
Keep in mind that most labs charge a fairly hefty batch fee to cover the cost of doing paperwork so it pays to look at pricing to see if it is near the same cost overall to do more than one assay type on an ore sample.
The first assay type is the standard fire assay. This will tell you how much gold is present in your ore and is the basic assay format. Unfortunately it does not tell you anything more than the gold grade. An experienced assayer should be able to look at an ore sample and identify the presence of sulfides and anything else obvious which may affect the fire assay and use the appropriate assay flux accordingly.
The modern trend is to use only two flux types covering ores with and without sulfides present, this has been brought about by the shortage of assayers able to formulate fluxes and the high cost of doing these formulations compared with just taking a scoop of premixed flux from a commercially purchased container of such.
That said, the standard flux formulations will give you a go/no go result for possible further work on a sample.
Always keep in mind that human nature is to take the best looking samples and have them assayed so your first samples assayed should be treated as indicative only, the results are not necessarily applicable to the whole deposit.
So now we have done the basic fire assay and registered gold in viable quantities. The next thing we want to know is where in the ore sample is the gold; is it in the quartz matrix for example or is it associated with sulfides or is it all as free gold.
Most labs will offer you a range of acid type assays along with reasons for having them done on your samples.
For the average prospector just wanting some basic information these assays are bewildering and expensive.
What is required is a basic aqua regia digest assay. This assay type will show gold which is either available as free or exposed gold particles or is in a matrix such as sulfides which can be dissolved by the aqua regia.
The aqua regia values are usually slightly less than fire assay values due to fire assay being able to access the gold locked in ore particles which the aqua regia cannot access.
If we have a large difference in values between fire assay and aqua regia digest then the usual reason is that the bulk of the gold is encapsulated in a matrix which aqua regia cannot dissolve such as quartz.
In the above case we now have the information that there is a strong possibility that fine milling of the ore may expose enough gold values to allow viable treatment of the ore.
What the above tests do not tell you is if the gold is free or is in a matrix which aqua regia can dissolve, both the fire assay and the aqua regia assay will show you the above gold values.
Often gold in sulfides is in the form where it is as small particles down to atoms locked inside the sulphide crystal matrix. You are not going to recover much of this gold without either smelting the sulfide in the presence of a collector metal or by chemically destroying the sulfide matrix.
The third form of assay has two uses; it tells you what gold recovery can be expected with cyanide and it tells you what level of the gold is locked in sulfides or similar by comparing the results with those of an aqua regia assay.
This assay form is a cyanidation assay, it is usually offered in scale from a few grams to several kilograms.
The smallest scale is often a few grams of ore stirred in a beaker but it is usually run as a bottle roll of 50 grams plus of ore run for 24 hours.
Because the bottle roll is an alkaline leach it has little dissolution effect on most but not all sulfides.
If you have highish gold values from fire and aqua regia assays but low gold values from cyanide then it is a fair assumption that the gold is locked in sulfides and that recovering the sulfides alone will recover most of the gold. This will give you a concentrate which can be sold or treated in a separate stage.
None of the above is totally definitive but is the basis for test work to set out an operating plan.
Deano
It will at least let you know if doing further work on a deposit is warranted and will do it at the lowest cost.
It should be noted that there are always specialised exceptions to the general rules but these exceptions are unlikely to be encountered by the average prospector.
Most queries start along the lines of how much gold is in the ore sample.
An experienced person can estimate the grade of visible free gold in an ore sample. They know what gold in ore looks like and from having previous samples assayed they can estimate the grade.
If gold is not obviously present as coarse free particles then it is necessary to call in a professional assayer.
There are three types of assay for gold which are applied according to what the prospector wants to know.
Keep in mind that most labs charge a fairly hefty batch fee to cover the cost of doing paperwork so it pays to look at pricing to see if it is near the same cost overall to do more than one assay type on an ore sample.
The first assay type is the standard fire assay. This will tell you how much gold is present in your ore and is the basic assay format. Unfortunately it does not tell you anything more than the gold grade. An experienced assayer should be able to look at an ore sample and identify the presence of sulfides and anything else obvious which may affect the fire assay and use the appropriate assay flux accordingly.
The modern trend is to use only two flux types covering ores with and without sulfides present, this has been brought about by the shortage of assayers able to formulate fluxes and the high cost of doing these formulations compared with just taking a scoop of premixed flux from a commercially purchased container of such.
That said, the standard flux formulations will give you a go/no go result for possible further work on a sample.
Always keep in mind that human nature is to take the best looking samples and have them assayed so your first samples assayed should be treated as indicative only, the results are not necessarily applicable to the whole deposit.
So now we have done the basic fire assay and registered gold in viable quantities. The next thing we want to know is where in the ore sample is the gold; is it in the quartz matrix for example or is it associated with sulfides or is it all as free gold.
Most labs will offer you a range of acid type assays along with reasons for having them done on your samples.
For the average prospector just wanting some basic information these assays are bewildering and expensive.
What is required is a basic aqua regia digest assay. This assay type will show gold which is either available as free or exposed gold particles or is in a matrix such as sulfides which can be dissolved by the aqua regia.
The aqua regia values are usually slightly less than fire assay values due to fire assay being able to access the gold locked in ore particles which the aqua regia cannot access.
If we have a large difference in values between fire assay and aqua regia digest then the usual reason is that the bulk of the gold is encapsulated in a matrix which aqua regia cannot dissolve such as quartz.
In the above case we now have the information that there is a strong possibility that fine milling of the ore may expose enough gold values to allow viable treatment of the ore.
What the above tests do not tell you is if the gold is free or is in a matrix which aqua regia can dissolve, both the fire assay and the aqua regia assay will show you the above gold values.
Often gold in sulfides is in the form where it is as small particles down to atoms locked inside the sulphide crystal matrix. You are not going to recover much of this gold without either smelting the sulfide in the presence of a collector metal or by chemically destroying the sulfide matrix.
The third form of assay has two uses; it tells you what gold recovery can be expected with cyanide and it tells you what level of the gold is locked in sulfides or similar by comparing the results with those of an aqua regia assay.
This assay form is a cyanidation assay, it is usually offered in scale from a few grams to several kilograms.
The smallest scale is often a few grams of ore stirred in a beaker but it is usually run as a bottle roll of 50 grams plus of ore run for 24 hours.
Because the bottle roll is an alkaline leach it has little dissolution effect on most but not all sulfides.
If you have highish gold values from fire and aqua regia assays but low gold values from cyanide then it is a fair assumption that the gold is locked in sulfides and that recovering the sulfides alone will recover most of the gold. This will give you a concentrate which can be sold or treated in a separate stage.
None of the above is totally definitive but is the basis for test work to set out an operating plan.
Deano
It will at least let you know if doing further work on a deposit is warranted and will do it at the lowest cost.
