Tim Griffin: Modern clues to interpreting ancient rocks

Former Director of GSWA, Tim Griffin, talks about studying and mapping very young rocks in Queensland and Papua New Guinea, which gave him clues to interpreting some of Western Australia’s oldest rocks, in the Yilgarn, during his early days as a mapping geologist at GSWA.

Transcript

00:01 Tim

I started in 1980 on the Widgiemooltha quarter million sheet. I finished two hundred thousand sheets, which were the first of the systematic hundred thousand mapping in the Geological Survey of Western Australia.

00:13 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, former Director of GSWA, Tim Griffin, talks about studying and mapping very young rocks in Queensland and Papua New Guinea, which gave him clues to interpreting some of Western Australia’s oldest rocks, in the Yilgarn, during his early days as a mapping geologist at GSWA.

01:15 Tim

Well, my name’s Tim Griffin. My geological career started by going to ANU for my undergraduate and honours work. And in honours, I ended up working on granites and working on the I and S granite system of Chapel and White.

From there, because of the lack of work in industry, I went to James Cook University to do a PhD, and worked on some really young volcanic rocks. And these rocks included some quite dramatic cinder cones with nice little dimples in the top. 

Yeah, so I worked on this young volcanic province. It was actually a large sort of dome of mafic volcanics. And was characterized by these long lava tubes with collapses, creating openings into some significant tubes up to a kilometre long, in some cases.

02:05 Julie

Wow.

02:06 Tim

There was quite a large lava crater and the reason for the long flow was continuous eruption and insulation of the flow as it went down in valley systems. But in addition to that, there was another volcano which had a quite a young lava crater and the lava from that still had its ropy texture, although the glassy skin was missing.

And I tried to date it using potassium-argon, it's come back now at about 7000 years. And there was stories when we were there that the Aboriginal people had remembered volcanic eruptions in this area. They were actually younger eruptions further south. So it's not, wasn't unique in that sense.

But that was an interesting diversion, going from granites to young volcanics. And then I went to Papua New Guinea and worked with the geological survey in Papua New Guinea. And that was working not only in young volcanics, but everything was very young. We had granite intrusions into Miocene limestone and the associated skarns and porphyry copper was of great interest

03:17 Julie

Wow.

03:17 Tim

up there. So that was a field mapping program or several programs I worked on, introduced me to very severe terrain, very severe weather conditions, and the use of helicopters, which was absolutely essential to our work,

03:34 Julie

Yeah.

03:34 Tim

and the logistics about getting fuel and supplies in. So having worked there for about four years, I moved to Western Australia. Initially, I applied for a job as a petrologist. But by the time I got to WA they needed a field mapper in Kalgoorlie. And actually, I was keener to do field mapping rather than looking down a microscope.

03:53 Julie

Yeah.

03:54 Tim

And so from this very young terrane to an Archean terrane in the eastern goldfields with highly deformed rocks, and also deeply weathered, very deeply weathered. And so that was a real challenge. So that's how I came to the geological survey.

04:08 Julie

Yeah.

04:09 Tim

So I came into doing some field mapping on the Widgiemooltha quarter million sheet. And there was a program that was starting because the quarter million mapping had been completed across Western Australia. They decided that those sheets that had been mapped some time ago, or where they realized there was some serious problems, in terms of understanding the rocks, they should be remapped, but also those that were particularly interesting areas for the exploration industry. And Widgiemooltha was chosen. So I was put to map that at one to 250,000 scale. 

I started in 1980 on that project. I couldn't have asked for a better project. And primarily because the Kambalda nickel region is on that map. And what I did very early on, was to go and talk to the Kambalda geologists, and one of the geologists in particular was very generous and said, we have a hanging file of geological maps, which are the best of maps we've been able to collect for this region and beyond. And you're more than welcome to come down and make use of those. So that was great to see that I didn't have to go and source the best company mapping that had been done elsewhere, although there were companies that were still holding on to the mapping very secretively. And I did manage to get access to some of that over time.

But the interesting thing to me was coming from Papua New Guinea, where they'd finished their quarter million mapping, and we went back into the Ok Tedi area, and we decided we should map that at one to 100,000 scale. And the reason being is that Kennicott had pulled out of that area and the PNG government were trying to get the porphyry copper up and running.

So I came to WA and came to Kalgoorlie and said, Well, if I'm going to map the quarter million sheet, I really probably need to map it at 100,000 scale across it and then I can compile that and reduce it to quarter million scale. It was particularly evident when you looked at the detail that the Western Mining Company geologists had

06:11 Julie

Right.

06:12 Tim

and the amount of mapping that you can generate a very good 100,000 from their work and a little bit of my own work. And so I went about creating my first piece of the map and I went down to the Perth at the annual lectures and presented my draft 100,000 sheets. And I was told, “You're not there to map 100,000 sheets, you're there to do a quarter million sheet”. I said, “Well this is just the stage towards it”. So I diligently went back to Kalgoorlie and I finished two 100,000 sheets and reduced those to quarter million and mapped the other two thirds at quarter million scale and produced a quarter million sheet, which was published prior to the 100,000 sheets, which were the first of the systematic 100,000 mapping in the Geological  Survey of Western Australia.

06:54 Julie

Oh, wow.

06:55 Tim

I think it was recognized it was a much better scale, and the industry came out and they were the ones that said, No, this is the scale we want, we want more detail. And particularly where a lot of exploration had taken place. And there was a lot of detail available to put onto the sheets.

07:10 Julie

Yeah,

07:11 Tim

And I suppose in the goldfields, it was a real challenge, because the outcrop is so poor. But people had learned to interpret these deeply weathered rocks very accurately and, with the easy access to things like geochemistry, that helped determine rock unit types and things like that. It made a big difference.

07:29 Julie

Yeah.

07:30 Tim

The first phase of detailed airborne magnetics was flown over the eastern goldfields by Aerodata, which was a private company. And they were selling that to the industry and the industry were valuing it very highly because it allowed you to see through the regolith. And they could see where there were major structures, where there were minor structures, where there were things like ultramafic rocks, particularly. We looked at this and said, This is fantastic!

07:57 Julie

Yeah.

07:58 Tim

It's really useful. So we pushed to get the survey to buy it. All we got in our purchase were hardcopy contour maps.

08:08 Julie

Right.

08:09 Tim

Over time, of course, we got the digital data, and we were able to manipulate that, but it was a big breakthrough in the goldfields mapping.

08:16 Julie

Yeah, I bet.

08:17 Tim

And I guess the other thing that came out of that work, which I think from a geological point of view was critically important, was the identification of the concept of terrane mapping. So it was Coney, Peter Coney, someone James, and there was a Canadian fellow as well. And I been introduced to this by a colleague I went to university with who was working with BHP and they’d employed Peter Coney as a consultant and he was telling me about the whole concept. So I looked into it and with the other people working in the goldfields, we said, This is the way to do it, because the goldfields geology, up until that time, was considered as a cyclic process of greenstone development.

09:01 Julie

Right.

09:02 Tim

And this was because wherever they went, they saw a similar sequence of ultramafic, mafic, felsic or sedimentary rocks, and they kept seeing repetitions of this. So they invoked sort of a cyclic model because these things seem to be sitting on top of each other.

09:19 Julie

I see.

09:20 Tim

Up until that point, we largely looked at rock sequences and said, What other rock sequences look the same? This was before we had good geochronology. In particular, in Archean terranes, we had no fossil control. And so it was a matter of correlating similar sequences, and similar deformation histories,

09:38 Julie

Yep.

09:39 Tim

and say well they probably are the same. Whereas with the terrane model, you said, if you come to a major boundary, a fault, particularly, then you don't automatically go across. You've actually got to study the rocks on the other side of that terrane  boundary and demonstrate that they're very, very similar in many aspects to the one and then you can make a tentative correlation. So in the goldfields, you would correlate along the main structural trends, but crossing the major faults, you couldn't do it. And of course, now we have terranes and super terranes in the goldfields, and we use the terrane concept quite widely.

10:13 Julie

Yep.

10:14 Tim

You know, it was just a terrific breakthrough. To me, it was a bit like plate tectonics. Sort of, all of a sudden, you can put this together and talk about it in a sensible way without forcing correlations.

10:25 Julie

Yep.

10:26 Tim

Of course, the introduction of geochronology was a big breakthrough that helped, you know, further define those terranes and allow correlations, which we perhaps wouldn't have recognized otherwise.

The other concept, I suppose, that came out of the goldfields was the structural repetition of rocks. We could see the repetitions of cycles. And so was it cyclic? Or was it structural. And within a few weeks of me coming to Western Australia, I went out to Kalgoorlie with a couple of the godfathers of the geology of the goldfields. The conversation got to a point where they were saying, ‘Some of our recent PhD students working out here are sort of invoking thin-skinned tectonics. They reckon there’s structural repetition on shallow faults through here. Of course, this is just the flavour of the month. Why would we really take much notice of it? It's just a fad.’

11:20 Julie

Right.

11:20 Tim

Well, of course, you know, I'd come from a terrane where I'd seen this happening in real life.

11:26 Julie

Right.

11:26 Tim

We had a granite in Papua New Guinea that that had intruded into the Miocene limestone, and it was mineralized. So the company had drilled through the granite and gone into Miocene limestone and the age of the Miocene limestone below the granite was the same as the limestone, the granite had intruded. So the whole thing was a several hundred metres thrust of limestone that carried the intrusion with it.

11:48 Julie

Yep.

11:49 Tim

So, you know, I sort of just listened very politely and didn't say anything. But over time, I started to recognize real structural repetitions, even in company drill holes. You'd see them log the core, and they'd log exactly the same sequence again.

12:03 Julie

Right.

12:04 Tim

So that was another way, you know, we recognized there was structural repetition and so it wasn't a separate cycle. One of the other interesting things from the goldfields mapping, people were sort of saying with the nickel mineralization in the goldfields seemed to be in thick ultramafic flows. And these flows, when they mapped them out, even though it was a deformed terrane, seemed to be in a trough in the landscape, the low point in the landscape. And so, the theories evolved around the geochemistry of these. And said that initially, they were sulfide-rich, carrying the nickel. But because they're so hot, they were eroding the substrate, and that was changing the chemistry that forced precipitation of the nickel sulfides.

And some of the people were saying, ‘Well, we should go to Hawaii and see these lava flows that are, you know, eroding their base’. And I said, ‘You don't have to go that far. You can go to North Queensland in the McBride Province in walk into a lava tube’. And you can actually see where the lava that's come out of the volcano has been shielded for so long and flowed for so long, it's eroded the underlying flows.

13:10 Julie

Yep.

13:11 Tim

And you can actually identify the flows in the wall of the lava tube. And so, which is sort of interesting to see that you can see that in real life almost. And then it took a while to understand that might be a potential model for mineralization in the goldfields. So you know, it's just fascinating. The way geology translates from a super modern terrane to a super old terrane. 

13:33 Julie

You’ve been listening to The Rocks Beneath Our Feet. You can discover more about GSWA by visiting dmp.wa.gov.au/gswa or find GSWA on LinkedIn and Facebook. If you like what you’ve heard, give them a follow.