A new technique for geochronologists and sedimentologists will be put to the test in late 2019, providing unique information on the source of sedimentary rocks throughout Western Australia
Accurately dating and tracking down the origin, or provenance, of rocks is critically important knowledge for explorers when targeting deposits. During the past two decades, geoscientists have used uranium-bearing minerals called zircon in sediment (detrital zircon) for the isotopic dating of rocks, because the uranium isotopes change to lead at a well understood rate.
Uranium-lead zircon dating has come to be known as the ‘gold standard’ for geochronology, but it is far from perfect, as the Geological Survey of WA (GSWA) Chief Geoscientist Dr Simon Johnson found when trying to establish the provenance of sediments in the Edmund Basin. That quest led to a geological detective story, which highlighted using lead isotopes in potassium (K) feldspar – one of the most common minerals in the earth’s crust – as a more accurate alternative in certain situations.
Dr Johnson said zircon suggested that the Edmund Basin sediments had come from the Gascoyne Province to the south, despite evidence from rocks showing the direction of river currents, which indicated they came from the north. Being extremely hardy, zircons can survive a number of cycles of uplift, erosion and deposition, sometimes ending up in a location nowhere near their source.
‘We knew that using detrital zircons can be an issue because they get recycled, and then recycled again and again, and sometimes they’re telling you the wrong answer when you’re looking for their provenance,’ Dr Johnson said.
‘We wanted to work out a way to use other minerals or methods to support or reject the story being told by the zircon.’
In a happy coincidence, a former colleague at GSWA, Professor Chris Kirkland, who is now working with the Centre for Exploration Targeting (CET) at its Curtin University node, was looking into the lead isotope composition of K-feldspar as an alternative to compare with zircon. He wanted a project to test the new method and approached Dr Johnson, who jumped at the chance to use the laser-ablation mass spectrometer at the John de Laeter Centre to solve the Edmund Basin mystery.
The K-feldspar compositions proved that the zircon and K-feldspar were definitely coming from different sources, but it did not completely solve the story because it did not prove where the sediments were actually coming from.
The question nagged at Dr Johnson and Professor Kirkland.
‘It’s not coming from the south. We proved that – it’s clearly coming from the north, so we had to think about what sources of K-feldspar there would be in the north,’ Dr Johnson said.
‘We hunted around in our collections of samples from the core library to find a potential source and in the end, the only samples left to analyse were granitic rocks from within the sedimentary basins which are part of the Pilbara Craton.
‘We did a few of those, and then we got that perfect match, proving that the sediments were coming from the north.’
But the detective work wasn’t quite over – where had the zircons really come from?
‘The zircon story is ultimately correct, the zircons are really coming from the south, but it’s a two-stage process,’ Dr Johnson said.
‘They came from the south from an older period of time, where the granitic rocks were uplifted and the zircons eroded into a sedimentary basin in the north, and then during the deposition of the Edmund Basin, the northern basins were uplifted and the zircons were recycled.’
In a complementary study, Dr Imogen Fielding, a PhD student at Curtin University at the time (now GSWA’s Phosphate Geochronologist), was examining the timing of gold mineralisation in an area of the Pilbara hosting the Paulsens, Belvedere and Mount Olympus gold mines – the same area that was uplifted to provide the sediment to the Edmund Basin.
‘The combined studies show that precisely defining the source region of a sedimentary basin may aid in identifying regions of highly prospective crust.’
‘Dr Fielding’s study has shown that uplift, erosion, sediment transport and gold mineralisation were synchronous,’ Dr Johnson said. ‘The combined studies show that precisely defining the source region of a sedimentary basin may aid in identifying regions of highly prospective crust.’
These results sparked publication of a paper – ‘The complexity of sediment recycling as revealed by common Pb isotopes in K-feldspar’ – with Dr Johnson and Associate Professor Kirkland as lead authors, in the prestigious Geoscience Frontiers magazine.
Three-year project to create statewide lead isotope map
As a result of the Edmund Basin breakthrough proving the worth of measuring the lead isotopes of K-feldspar, the GSWA is partnering with the Australian Research Council (ARC), Professor Kirkland at Curtin University, and medium-tier gold miner Northern Star Resources to carry out this technique in the Eastern Goldfields and the Northern Carnarvon and Canning Basins.
The three-year, $682 000 project is being funded by the ARC ($352 000), the State Government’s Exploration Incentive Scheme ($180 000) and Northern Star Resources Ltd ($150 000), and will start this financial year. The project will also involve the creation of a lead isotope map of WA. The map project will run in conjunction with a Geoscience Australia project which is collecting lead isotope data in mineral deposits across Australia. ‘Both our datasets will be amalgamated to produce a state and nation-wide lead isotope map,’ Dr Johnson said.
This article originally appeared in the Winter 2019 issue of Prospect, published by Department of Mines, Industry Regulations and Safety, Government of Western Australia.