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Telling the time of South Australia

Geochronology layer added to SARIG.

Figure 1 Zircons from Atacama heavy mineral sands prospect in the Eucla Basin, sample 1924236.
Figure 1 Zircons from Atacama heavy mineral sands prospect in the Eucla Basin, sample 1924236. (a) Transmitted light image. (b) Cathodoluminescence image: circles indicate a laser ablation pit and its analysis number.

South Australia has a new tool for capturing and delivering the excellent inventory of geochronology data built up by the Geological Survey of South Australia and collaborative partners. Geochronology is the science of determining the age of rocks based on the natural radioactive decay of unstable elements such as uranium. The SARIG geochronology layer captures the ages of different rock samples determined by geochronological techniques such as U–Pb and K–Ar methods, and draws its data from a new geochronology module within SA Geodata (our chief repository for most forms of geoscientific data). Geochronology provides the point of truth for interpretations of the age of geology across South Australia, and is especially important where there is no outcrop that would assist direct correlations between units.

South Australia’s crystalline basement history spans an incredible length of time. From the oldest granites dated to around 3150 million years ago in the Gawler Craton, to granites that intrude deformed sedimentary packages on the eastern margin of Gondwana around 480 million years ago, South Australia has a range of rock types and geological events with a diverse range of mineral prospectivity. Geochronology is one of the key tools that geoscientists use to reconstruct the geological evolution of a terrane.

One of the chief methods for determining the age of rocks is analysis of the U–Pb method in accessory minerals such as zircon or monazite. Zircon in particular is a very useful mineral for determining the age of crystallisation of igneous and metamorphic rocks.

Zircon can also be a useful tool to understand the source of detrital minerals in sedimentary rocks. This has been particularly useful in assisting with models for the formation of the world-class mineral sands district of the Eucla Basin (Fig. 1).

With the new layer, we now have the capacity to visualise the spatial distribution of these different events across the state and a way to correlate different geological events and processes across regions with little or no surface exposure. In this way, the geochronology layer in SARIG provides a complementary dataset to the geophysical and geochemical data now available for mineral explorers to evaluate mineral potential in their region of interest.

The geochronology layer provides a summary of the age determined by geochronologists along with details of the mineral analysed, the analytical methodology applied to derive the age and the publication reference that contains further information about the rock sample and date (Fig. 2). In this way the SARIG layer provides a snapshot of the main pieces of information a non-specialist user of geochronological information needs, and with the option to track down directly any Geological Survey derived reports in which the geochronology is published if they seek more detail.

Figure 2

Figure 2 SARIG screenshot showing the new geochronology layer.

New geochronology data is being added progressively to eventually provide a complete coverage of published data from South Australia.

– Anthony Reid, Ursula Michael, Liz Jagodzinski and Peter Buxton

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