Fire assay (Vic)

Does any one know where to get a fire assay done in Victoria. And cost and how much sample they need..

Can you describe exactly what you mean by this and on what do you want assayed ?

I got a few rock samples that i want tested to see if there any gold value in them.

Oh ok.
You could dolly them up and pan it to see if there is any gold in your sample.

Yeah this is true but i would like to know the gpt

ben2363 said:

Yeah this is true but i would like to know the gpt

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If you have enough of them and have dollied up to see if there IS any gold first , then CPG Group or Melbourne Gold Company do assays.

You wont get an accurate figure from a small sample , you may need to be talking 10 - 20 kg of ore .

I dont know if they do fire assay , you will find spectroscopy is the tech these days

Crack a chat with them.

Ps , some people who spend a bit too much time out bush are a bit suspicious that they will be ripped off , it doesnt happen in Oz , you _can_ trust them.

There are many places advertise fire assay (it is a well known term that involves mixing sample, flux and lead oxide together and putting in a furnace). The product is metallic lead that concentrates all the gold within it, and this lead "prill" is then analysed by techniques such as atomic absorption spectroscopy (AAS). The beauty of the method is that a tiny bead of lead has concentrated all the gold from a large sample into the tiny bead or prill, making it easier to assay accurately (it is easier to accurately analyse a high concentration of gold than a low concentration of gold). You then relate the amount of gold back to the original sample size (mass), to get the gold concentration of the original sample. Sensitive and accurate - and fairly expensive.

There are cheaper methods of analysis which we use in exploration (e.g. I am often happy to know I have 3 g/t gold and don't care that it might really be 2.5 or 3.5 g/t gold. But if I want to accurately determine a grade (for mining - or reporting to the public), I use fire assay.

The results can be significantly different - on one job where the gold was fine and locked in pyrite, a simple aqua regia attack followed by AAS analysis underestimated the gold by a third. This is common when the gold occurs in this manner.

Assay labs vary in quality and there are some shockers in Victoria. All charge like wounded bulls (but having run a lab I understand why). I have used ALS a lot and found them reliable, but usually through ALS in Orange and Brisbane - they do have an environmental lab in Springvale (Melbourne), but they possibly do not run gold (but might). If not, you will probably find that they do sample preparation (which greatly decreases the mass to transport), then send them to one of their interstate labs to analyse. There are other excellent labs in WA and SA (perhaps also Victoria).

I had one done many years ago at the Uni in Bendigo,you need good over sample of about 100 grams grounded very fine,not that they need that much.

A mate works at ALS Orange in the lab, occasionally sees some nice prills. 5kgs of well split sample max, (5g used for analysis).

thedigger said:

I had one done many years ago at the Uni in Bendigo,you need good over sample of about 100 grams grounded very fine,not that they need that much.

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Two different issues. The size of your sample, which you need to get down to a small size (say a few 100 g) by careful splitting. Versus the size of the sample used for analysis that is taken from it (25g or 50g). Unfortunately the "nugget effect" makes consistent assay results difficult. I have tried sample sizes as large as 50 kg for crushing or quartering, then used things like a bottle roll in cyanide solution for a sample split from it (the larger the sample actually analysed the better, and you can analyse a large sample that way).

goldierocks said:

There are many places advertise fire assay (it is a well known term that involves mixing sample, flux and lead oxide together and putting in a furnace). The product is metallic lead that concentrates all the gold within it, and this lead "prill" is then analysed by techniques such as atomic absorption spectroscopy (AAS). The beauty of the method is that a tiny bead of lead has concentrated all the gold from a large sample into the tiny bead or prill, making it easier to assay accurately (it is easier to accurately analyse a high concentration of gold than a low concentration of gold). You then relate the amount of gold back to the original sample size (mass), to get the gold concentration of the original sample. Sensitive and accurate - and fairly expensive.

There are cheaper methods of analysis which we use in exploration (e.g. I am often happy to know I have 3 g/t gold and don't care that it might really be 2.5 or 3.5 g/t gold. But if I want to accurately determine a grade (for mining - or reporting to the public), I use fire assay.

The results can be significantly different - on one job where the gold was fine and locked in pyrite, a simple aqua regia attack followed by AAS analysis underestimated the gold by a third. This is common when the gold occurs in this manner.

Assay labs vary in quality and there are some shockers in Victoria. All charge like wounded bulls (but having run a lab I understand why). I have used ALS a lot and found them reliable, but usually through ALS in Orange and Brisbane - they do have an environmental lab in Springvale (Melbourne), but they possibly do not run gold (but might). If not, you will probably find that they do sample preparation (which greatly decreases the mass to transport), then send them to one of their interstate labs to analyse. There are other excellent labs in WA and SA (perhaps also Victoria).

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Are they all NATA certified , if not what do the others use for QA ?

I have found On Site Laboratory Services at 2 Abel Street Bendigo to be both efficient and friendly. The last lot I sent in
averaged around 2 kg a specimen. But they will take smaller ones if you get in touch. Other assay offices I approached
had a minimum of around 50 specimens between 2 and 3 kg. Cost around $30 per specimen.OSLS charged around $17. I dropped that lot in but I have sent in smaller lots by mail.Give Rob a ring and discuss what you have.(o3 54 416 418).Thy operate 7 days.

Dunno about Vic

Here in WA you can get an aqua regia digest done for any sample expected to be up to 1ppm (a gram a ton)

If you expect more a fire assay is done

Supply a sample up to 3kg dont worry about crushing it as it needs to be pulverised and its only about $2 a sample anyway

These cost about $15 each

A geotech sample looking for gold and 40 odd elements is around $38 per sample, best for looking for the path finder elements as well

Cheers Jim

Hotrock said:

Dunno about Vic

Here in WA you can get an aqua regia digest done for any sample expected to be up to 1ppm (a gram a ton)

If you expect more a fire assay is done

Supply a sample up to 3kg dont worry about crushing it as it needs to be pulverised and its only about $2 a sample anyway

These cost about $15 each

A geotech sample looking for gold and 40 odd elements is around $38 per sample, best for looking for the path finder elements as well

Cheers Jim

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It is not so much the gold concentration that determines the assay method, but the size of the sample to be analysed. One speck of gold in a sample makes a difference if you are only analysing say 25 g of sample - if the speck occurs in the 25 g sample that you analyse, then you will get a much higher result than if the speck does not occur in that 25 g (i.e. you may have crushed 1 kg of sample, but you often take 25 g of sample from this to to analyse). This is more an issue if the gold is a coarse speck (in some deposits the same amount of gold will instead be present in specks of microgold, perhaps each less than one micron in size, so there is more chance of a number of specks in the 25 g taken)). So it is good to use a technique that analyses a larger sample than 25 g (e.g. fire assay).

A second issue is whether the gold is locked as fine gold in sulphide minerals in the sample - some assay techniques under-report the gold content in such cases. I have compared fire assay with another common digestion in such a case (e.g. aqua regia followed by AAS or ICP), and found that gold was underestimated by a third in the latter method.

So fire assay tends to be used for any sample for which the gold content needs to be accurately known (e.g. when estimating ore reserves). Other techniques use methods such as a bottle roll in a cyanide solution (which permits use of very large samples).

In many cases you just want to know if significant gold is present, not its exact amount - acid digestion followed by ICP can be useful in such cases as you can also get assays for 15 or more other elements for comparable cost. However the gold concentration will often not be accurate (for those more technically-minded, it may nevertheless be precise, so is useful in exploration rather than evaluation). As Jim says, methods such as ICP are often used for samples containing less than 1 ppm (1 g/tonne), as the technique is sensitive at very low levels (it does not mean it is accurate, but you may only want to know that gold is present at above-normal levels). Commonly exact grade does not matter in such cases (few mines operate at average grades below 1 g/t - the lowest grade gold-only mine I know is Fort Knox in Alaska that was operating at 0.39 g/t average grade).

Crush 20kg and if you get less then a gram, there would need to be a fair amount of easily accessible ore for it to be a profitable one or two man show. If it's not free milling gold it can be very difficult to extract the gold and probably not worth it unless of course there's many tonnes of it or it's very high grade ore.

DykeHead said:

Crush 20kg and if you get less then a gram, there would need to be a fair amount of easily accessible ore for it to be a profitable one or two man show. If it's not free milling gold it can be very difficult to extract the gold and probably not worth it unless of course there's many tonnes of it or it's very high grade ore.

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half a gram of gold in 20 Kg of rock is 25 g/tonne ore which is higher grade than most modern mines - one gram would be fantastic grade. One gram in 20 kg would usually be quite profitable in even a small mine.

That's interesting. I guess it depends a lot on the size of the shoot. A recent source I traced is going much better then that but is only 2m long with varied thickness max being maybe 250mm. I wouldn't believe that would be payable. I mean it could be although it would be quiet a gamble to throw money at it in my opinion.

DykeHead said:

That's interesting. I guess it depends a lot on the size of the shoot. A recent source I traced is going much better then that but is only 2m long with varied thickness max being maybe 250mm. I wouldn't believe that would be payable. I mean it could be although it would be quiet a gamble to throw money at it in my opinion.

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Yes, definitely. To work 2 g/tonne ore profitably it needs to be open pit and millions of tonnes. Once you got to 50 g/tonne you can probably high-grade it further visually by selective mining at surface, so for a lone prospector hundreds of tonnes might be interesting (you will see reference to old mines around Melbourne that went "9 lb of gold per bucket" i.e. 4 kg - they never produced much gold but if those buckets were yours it certainly paid the groceries). A lot of discussions I see on this site do not make this distinction - they do not appreciate that if a prospector made a rich strike, often the government geologist would go out and have a look for a day or two and write a report that they were producing 5 oz/tonne. But of course by next week it might be 0.2 0z/tonne, and they might only have hauled a few tonnes the previous week. Many "mines" reported as having 3 oz/t or more grade never produced 200 oz in their (short) lifetime. Some of the Bendigo mines produced 0.7 to 1 oz/tonne for years - the Long Tunnel at Walhalla produced more than one million ounces at an average grade way above one ounce per tonne. They were the sort of mines that mattered and that kept towns like that (and Stawell, Chewton Ballarat etc.) alive for decades - because in those days they often only produced a few tens of thousands of ounces per year, but it was all labour-intensive mining). Such mines went to more than 1,000 m depth.

As for labor-intensive mining in those days - it is worth keeping in mind that at one stage Ballarat (for example) was averaging 30 deaths per month on its mines. The average miner was very lucky to get to 40 before lung disease stopped them working (and average life spans were less than 50). They called the early dry rock-drills "widow-makers". They worked in semi-darkness to the glow of a candle in a "spider", or a carbide - lamp flame. Even today most companies are not fans of narrow-vein mining where only a single operator with an air-leg drill can fit at the face. Many more accidents. An individual needed to average an ounce per week, and educating each of your children at primary school could cost 8% of your earnings per child, when people commonly had 5-10 children. You went down the shaft at first light and came up at dusk. No wonder many changed to farming. Reality is often not as much fun as the history books suggest.....

Yeah,I think narrow-vein mining = hobby mine if you don't mind burning some cash.