Background to an essential IOCG primer

"Whatever you like!" said Cait.

Dr Cait Stuart, of the Northern Territory Geological Survey, had just asked me to give a talk to the Geological Society of Australia, Northern Territory Division. I was really excited to be asked. It's an honour to be asked to give a scientific talk anytime, but to get the chance to give a talk in the NT - a place I've loved since the great family trips my partner and I had done with our two children back in the 2010s - was a double honour.

That set me thinking.

There's great geology in the NT. They've got everything from Archean gneisses and Paleoproterozoic gold, through to Mesoproterozoic base metals... Not to mention an incredible record of continental tectonics in central

Australia from the Paleoproterozoic to the Paleozoic. Plus there's lots of outcrop and beautiful scenery. They even have iron oxide related copper and gold deposits in and around the Tennent Creek region.

The Tennant Creek deposits are an interesting economic connection between the geology that I know reasonably well in southern Australia and NT geology. So I decided I would give a talk about the IOCG deposits of the Gawler Craton as a way to give my NT geo colleagues something to compare their copper-gold systems to.

Heavitree Gap in Alice Springs, Mparntwe, an incredibly beautiful part of central Australia and one of the reasons why I love the Northern Territory.

The Darwin dimension

It was so hot. The drive from Jabiru back to Darwin was comfortable but as soon as we stepped out of the car the humidity hit us southerners again, like walking into a sauna without walls. We had just been in a fantastic tourist ride on the east Alligator River, hosted by the wonderful guides from Guluyambi Tours. We were so happy to have the chance to revel in the splendour of Kakadu. Although at this time of year the crocs were dispersed around the park in the extensive wetlands across the Top End so we didn't see as many as we had on past trips. And the spectacular Ubir Rock Art site was another fantastic treat. What an incredible record of habitation, of culture, of connection. It's a real privilege to go to that site. We were there first thing in the morning and had the entire place to ourselves this time. How quiet those shelters are. How incredible the rock art.

But now I was sitting in a small apartment in Darwin, under a lazy ceiling fan, sweating away to prepare a set of slides for my talk. Now I was reminiscing about things other than Kakadu dreaming.

Mantle to deposit scale indicators of goodies in the Gawler Craton

So it came to be that I was thinking about the mantle beneath the Gawler Craton. All the geochemical data from mafic rocks suggests it is enriched in large ion lithophile elements, basically elements that shouldn't really be there and suggesting they were enriched through metasomatism as a result of ancient subduction processes. The geophysical data suggests the same as it shows us that some zones are more conducive than others.

The massive transient remobilisation event that occurred at around 1590 million years ago resulted in a huge outpouring of mafic and felsic lava and a huge thermal spike across the region. Those drove high temperature metamorphism and a remarkable set of deformation systems. In this context, tapping of the copper and gold rich mantle was almost inevitable as was the focussing of fluids and magmas into weak zones in the eastern Gawler Craton at the edge of the more resistive cratonic core.

These fluids formed a whole range of alteration minerals from pyroxene to chlorite, from garnet to sericite and in the context of these silicate assemblages a whole raft of sulphides were formed that formed the major ore deposits themselves - from Olympic Dam and Prominent Hill to Carrapateena, Oak Dam and Hillside. Such was the intensity of the crustal reworking at this time.

In the end, this is what I came up with. A distillation of what I've been thinking about on the Gawler Craton's iron oxide - copper - gold deposits:

https://youtu.be/99x0yj3tIX8

Screenshot of part of the talk available on YouTube.

Please have a listen in to the talk and let me know what you think. I'm sure I've not covered all the details and have made some mistakes but hopefully the talk was useful for those present and now that it's up on YouTube, anyone can take a moment to review what I was thinking about on that afternoon in Darwin earlier this year (2023).

Thank you to the numerous colleagues who both invited me to sunny Darwin and have helped get this online, including Cait Stuart, Jo Whelan and others. Many colleagues at the Geological Survey of South Australia contributed over the years to the ideas in the talk, including most especially Claire Wade, Stephan Thiel, Adrian Fabris. I always also give credit to Dr Louise Corriveau of the Geological Survey of Canada, whose ideas and enthusiasm have heavily influenced my thinking on mineral systems and alteration facies. I would also like to thank Dr Kathy Ehrig of BHP who, along with her research team, has provided us with an incredibly rich description of the Olympic Dam orebody and regional geological context.

Here's to the next chapter of discovery in the Gawler craton because there's more to be found out there. And I would love to get involved with companies working elsewhere, such as the Northern Territory, looking for similar and related deposits.

Keep in touch and see you in the NT again soon!

Anthony enjoying a cup of tea in Elliott on the Stuart Highway on the long drive to Darwin from Alice Springs.