Paul Morris: Using fine fraction geochemistry to explore under wind-blown sand

Paul Morris talks about tackling the problem of detecting low metal concentrations in wind-blown sand, culminating in dating and geochemical work that had significant implications for gold exploration in areas under sand cover.

Transcript

00:01 Paul

Some of these samples had an excess of 2000 parts per billion gold in them. They had incredibly high concentrations of gold in them.

00:08 Julie

Welcome to The Rocks Beneath Our Feet. In this series, five geologists talk about their years devoted to working for the Geological Survey of Western Australia. From understanding early life, to the tectonic processes that shaped our planet, and making the maps that unearth our understanding of Western Australia’s geology, they reveal their shared passion for discovering the stories in the rocks beneath our feet.

I’m Julie Hollis.

In this episode Paul Morris talks about tackling the problem of detecting low metal concentrations in wind-blown sand, culminating in dating and geochemical work that had significant implications for gold exploration in areas under sand cover.

00:47 Paul

The exploration incentive scheme started at the geological survey, which is a State Government funded initiative to generate interest in mineral exploration. And the money came through for a lot of programs at the survey. We decided that here’s an opportunity to do another one of these remote area programs. And the Kiwikurra program we did in a pretty remote area showed that we could get some pretty good results out of the chemistry. Looking at the aqua regia results gave us a lot better trace element numbers than if we’d analyzed the sample where you dilute the sample with all this barren quartz sand in there.

We thought where's an area that we can do this and perhaps promote a little bit more the prospectivity the state? So what we looked at was the margin of the Yilgarn Craton. The reason for this is that the Yilgarn is world renowned for a number of deposits, especially nickel and gold deposits. But trying to get a tenement on the Yilgarn is just about impossible. The ground is taken up. And if you want to look anywhere you have to move right out on the margins. And even then, it's getting more and more explored.

However, on the eastern margin, in particular, if you look at some of the geophysics, you can see that the Yilgarn is covered up by some reasonably shallow, younger Paleozoic, Proterozoic sedimentary rocks and regolith. You can see the traces of these more magnetic rocks these greenstones going underneath this cover. So the question was put up, is it possible to detect some of these rocks and therefore potentially buried mineralization using regolith?

Now, at the time that we were looking at this, CSIRO were also looking at this issue of doing regional surveys in regolith-dominated areas. But they were concentrating on things like hydrogeochemistry using water to find out if rocks were mineralized, biogeochemistry using regional vegetation surveys. The geological survey was funding some of these some of these programs. The problem in the areas that we looked at on the eastern margin, it's called the East Wongatha area. It's about 180, 200 kilometres east of Kalgoorlie, right on the edge of the Yilgarn Craton.

02:50 Julie

Yep.

02:51 Paul

The problem in this area is that it's about 90% regolith, largely sand plain. And I think I would probably get more for water out there than I could get for an ounce of gold. There is no water out there at all. So hydrogeochemistry is not, water is not a viable sample medium. Vegetation surveys, the vegetation is is pretty scrappy and it's not uniformly distributed. So if you say looking for bluebush or something like that, you might find some near some lakes, a particular species of bluebush. And you think I'll sample that but then you find for the next 50 square kilometres there isn't a bluebush at all that you can sample. So the only thing you can sample is the regolith.

03:28 Julie

Right.

03:29 Paul

So this was a bit of an issue. Should we take this coarser fraction, we used a less than two millimetre fraction on the Kiwikurra survey, using aqua regia or should we look at something else? I was sort of toiling with this a few years before. Shortly after joining the regolith program, I joined the Association of Applied Geochemist, which is an international association devoted to exploration geochemistry. And every two years they have an international conference. And I used to routinely apply to go to this and I was funded to go to every one of them from 1999 through to 2017.

04:02 Julie

Wow.

04:02 Paul

So I went to all of them. And they were all over the place. They were in eastern Canada, there was one in British Columbia, there's one is in Santiago, there was one in Rovaniemi in Finland, there was one of Dublin, Spain, Northern Spain, I went all over the place. I gave talks at them, and we had posters and things like that. And I've been a member and an office bearer in the association for the last 20 odd years. It's a small association, but it's got some pretty high-powered geochemists in it, some exploration geochemists. At the at the Spain meeting, which is in northern Spain in Oviedo, we’d gone out for an Astorian night, this is Astorius country we've been out and drink large amounts of cider and eat local food. And I was sitting outside the hallway and I was talking to this guy, Ron Clusman, who is an organic geochemist from Canada. And Ron had given a talk at the meeting about how organic gas bubbles, they're so big and so strong, they can carry metal particles with them through soil. And he'd been doing the sort of work for a while. I was talking to him about this and I said I was quite fascinated by it. And he said, “Oh”, he said, “it's not only organic molecules that can do this”. He said, you know, big gas bubbles. He said, “it’s clay” he said, “you need to look at clay”. He said, “clay has got all this stuff stuck on it in it”. He said, “that's what you should be looking at”.

So I thought about this and I realized that we should perhaps be concentrating on a much finer fraction in these quartz-dominated areas. So here's this east Wongatha program, east of Kalgoorlie. We went through the usual situation with this. We did a meeting with the traditional owners in Kalgoorlie. They said yes, you can do this. We went out to the area. And just before we started the program with the helicopter, we gave them the helicopter for two days.

05:43 Julie

Okay.

05:44 Paul

We said take it. You go where you want to go and have a look and tell us which areas we can't go to. They came back after a day and a half and they sat down and had a look at the map. And they said, “Yeah that that's okay”. And then off they went home.

So we were based at an exploration camp, on the western part of the area we were looking at, an area of about five 100,000 sheets. I think we picked up about 700 samples, all told, on a density of about one every twelve and a half square kilometres. One helicopter. It waits while we collect a sample, but this time we decided we’d try and collect the sample uniform depth. So we took a posthole bore with us a power auger.

06:20 Julie

Yep.

06:21 Paul

And we had a basket on the side of the helicopter we could put it in. So this thing goes down about 90 centimeters. It’s got a big auger bit on the bottom of it with a petrol driven motor on. So when we got to the site, field assistant gets the auger, we pick a site, he or she drills a couple of holes down as far as they can go, sometimes not all the way because there might be few rocks below the surface. They fill a bag up with a sample. In the meantime, the geologist is busy making notes, location of the sample. After a few minutes back into the helicopter, no doors on there, flying around off to the next site. And so we would do this.

So sometimes, the if there was a geologist, two geologists on the on the aircraft, one was the field assistant, one was the geologist, we'd swap halfway through the day. So after lunch, I might be digging all the holes. And I've got a guy who's 20 years younger than me who's making all the notes and here's me trying to keep up with it. You know, I'm hulking this bloody auger around, and trying to fill these bags, and it's about 35 degrees.

07:19 Julie

Oh God.

07:20 Paul

There's dust all over the place, and the noise is unbelievable. The aim, as I said was to try and see if we could detect signs of these greenstones through this cover. There was a small amount of drilling had been done. And the suggestion was these greenstones were up to 90 metres below the surface.

So

07:39 Julie

Right.

07:40 Paul

we were keen to see if this if this could be done. When we got back to Perth, the samples were put into the laboratory and we screened out the less than 50 micron fraction.

07:49 Julie

Yep.

07:49 Paul

So that's point o five of a millimeter. It's very fine and it's the silt and clay fraction. The reason we did this is that we didn't want to sort of adulterate the sample by putting it in water or any solutions to get the fine fraction out. We dry screened it. We dry sieved it. And if you go finer than 50 microns with a nylon sieve, you actually distort the sieve mesh. You can go down to 30 microns but you distort the sieve mesh. Just the motion of sieving the sample will distort the mesh and you don't get a 30 micron fraction, you get a mixture of 30 to 40 microns or something like that.

08:24 Julie

Yep.

08:24 Paul

Less than 30 or 40 microns. So we change the cloth for every sample. It's an expensive process, but it's a reasonably quick process. So we got the less than 50 micron fraction out and we treated it with aqua regia, this these two acids which ignore the silica fraction so they're not dissolving the silicates. And we got the results back. We did some fire assay work as well on the samples for gold, platinum and palladium, but gold was also analyzed by aqua regia. And we got values, I think the highest value on the sheet was something like 23 parts per billion, which is pretty low. But when we did the statistics on it, we argued that statistically anything over about 14 parts per billion was statistically anomalous.

09:06 Julie

Okay.

09:07 Paul

And when we plotted that up, we found that these anomalous samples, the high proportion of these anomalous samples, were sitting directly over the top of these greenstones.

09:13 Julie

Nice.

09:14 Paul

So we were quite excited by all this. And we thought, here's a potential here for doing some exploration. I think the key was, we weren't using any special techniques, you know, screening of samples has been done for years. Aqua regia digestion has been around for years. ICP analysis was routine at this stage. It was a cheap process, we could get 50, 60 elements for less than $50, you know, so it was a very cheap way to get lots of samples done.

09:43 Julie

Yep.

09:44 Paul

The sampling was pretty straightforward augering the sample. So there was nothing special about what we did. So, there was a fair bit of excitement about this. I gave a talk at the annual lectures about it and the Open Day. And argued an approach that could be well worth looking at in terms of looking at these heavily regolith-dominated areas, especially the ones where the regolith is transported.

After we published the results, I was talking to Ryan Noble, who was a geochemist at CSIRO about this. And after some discussion, Ryan agreed to analyze about 10 or 15 of the higher gold samples we’d got from the east Wongatha area. But he did something a little bit different. What he did was separated out the clay fraction. This is the less than two micron fraction. And he did this by putting the sample in a column of water. And you agitate the sample in the water and you leave it and the sample settles. And using Stokes Law, which is the law which calculates the density or the grain size of a sample that will sink over time, after a period of time, you can pipette it off a certain amount of material from the column of water, and that is the one that contains a particular grain size fraction.

10:59 Julie

Right.

11:00 Paul

So he was after the minus two micron fraction, the clay fraction. He pipetted these out, he dried them on a filter paper. And he had them analyzed. And we found out the some of these samples had an excess of 2000 parts per billion gold. They were hugely gold rich, you know, some of the ones that had high gold in the aqua regia digest that we'd done also had incredibly high concentrations of gold. So this was, I think, some evidence that, that this was a really worthwhile thing to do. It  was an independent justification of what we did.

About four or five years later, which was in 2015, we went back to the east Wongatha area. This process of gold migration had been quite a controversial thing. What is the process? Is it just by soil gas. Is it by osmosis? Is a fluid involve? 

So we decided we should do something about this. So late in 2014 we went back to the east Wongatha area, but this time we did it with a ground base vehicle I took this a good friend of mine from ANU with me Brad Pillans. Now Brad is a is a professor at ANU and I've known Brad for a long time. When I was finishing my PhD in New Zealand at Wellington, Brad had just arrived over for his first job in academia and was going to take up a teaching position. So we crossed briefly in Wellington, but of course me being a hard rock geologist I wouldn't have anything to do with him because he worked on soil.

12:25 Julie

Right.

12:26 Paul

You see, we just didn't have anything to do with these people. You know that these were people who played in the dirt For God's sake. But now here was Brad and I, out in the field. We had Nadir de Souza Kovacs with us who was working at the geological surveys or regolith geologist and Carmen Krapf, who now works at the South Australian survey.

So the idea was to go out there to a couple of areas and basically dig a big hole in the ground and sample down the hole to see if we could find out if there was a change in gold concentration going down the hole, that is as the regolith got older, but also to take some samples for dating.

13:03 Julie

Dating.

13:03 Paul

Now, dating samples is particularly difficult in these areas, because all you've got to work with is quartz sand. But there is a technique which is optically stimulated luminescence dating, which is, without going into too much detail, tells you the age at which the sand was last exposed to sunlight.

13:20 Julie

Yep.

13:21 Paul

So when you collect it, you have to collect it in the dark. In this east Wongatha area went to two places, one was near the camp where we stayed. And the reason we sampled there is because we had a really quite a well exposed section already. It was in the rubbish dump. They’d dug this big hole to put all their rubbish in using a front end loader and part of the face was exposed. But unfortunately, there was no anomalous gold there. But we took a lot of samples there and we and got the face exposed. I took some samples for geochemistry. Brad was in there collecting samples for OSL dating, which involves hammering a tube into the face, and then putting a cap on the end of the tube to exclude the sunlight. And then pulling the tube out and capping the other end. And then you actually look at the sample which is right in the middle of the tube, which of course has not been exposed to sunlight at all since it was buried.

14:07 Julie

Right.

14:08 Paul

So we did that. And then we went back to one site where we'd got elevated gold, some high values in this area, which was actually very close to the unconformity between the sediments and the Yilgarn Craton, this cover sequence in the Yilgarn. And we dug this hole in the middle of nowhere. It was about 1.8 meters deep. And believe me it takes a long long time to dig a hole with a spade which is 1.8 meters deep. Because you can't just dig a small hole. You have to dig a big pit so you can actually climb down inside it. And then you've got three people watching to make sure the pit doesn't collapse on you. But we got down to about 1.8 meters, did the same thing, samples for geochemistry. Brad collected some samples for dating. We found a lab in Adelaide that would do the OSL dating for us, I had these samples which we sent off to the lab. And the same thing. We screened out the less than 50 micron fraction and we analyze the with aqua regia below, but I also analyzed another part of the sample with deionized water, because I thought if this gold is actually held on clay particles, I should be able to just wash the sample and I can wash the gold off.

15:15 Julie

Right.

15:16 Paul

So and that would show that was really loosely bound gold on there. So I sent them off to the lab, and we got the geochemistry back first. And it was really quite surprising. There was an increase in gold concentration at the sandpit site that we called it, where we'd had elevated gold, an increase in gold concentration from about 10 parts per billion at about 50 centimetres down to 33, 34 parts per billion at about 1.8 metres, a gradual increase in the aqua regia gold. But also a gradual increase in the amount of gold with deionized water digestion - much lower concentrations, but there was an increase in concentration with depth. So the suggestion here was that as you go deeper, you're getting closer and closer to the mineralization. The signal becomes weaker as you go to the surface as the regolith is deposited. The signal is progressively weaker as you go close to the surface.

16:09 Julie

Yep.

16:10 Paul

An interesting thing was that, as I mentioned before, when we augered, we went down to 90 centimetres. And when we'd all get at this one particular site and got we got 14 parts per billion at the bottom of the auger hole. But if we'd have gone, probably about 30 centimetres higher, we would have only got about eight or nine parts per billion. And we wouldn't have noted that site has been anomalous at all. So the depth of sampling is really quite critical in these areas.

So we were buoyed by the geochemical data. And after several months, we got the dating back, and some really wonderful results, you know. Ages of seven or 8000 years towards the top of the regolith sequence, gradually increasing to 172,000 years at the bottom of the sandpit hole.

16:59 Julie

Wow.

17:00 Paul

A gradual increase in regolith age to the bottom. So some of the first stratigraphically controlled ages on Quaternary sands in Western Australia. And of course you can then tie the rate of change of gold concentration with a change in age. So I thought that this added more strength to the argument that there was some kind of migration process and the fact that the gold was, could be washed out with water suggested it was very loosely bound on the finer fraction of the regolith. 

17:29 Julie

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