Research Update: Isotopic Mapping

Craton-scale module: Isotopic mapping of the Superior Craton

A research update by Metal Earth researcher David Mole. 

David Mole is planning, designing and implementing a research program that will allow Metal Earth to collect large volumes of isotopic data on hundreds of samples, quickly and efficiently across the Superior Craton. This will be the largest high-quality geochemical and isotopic dataset for any Archean craton. This offers unique opportunities to understand the tectonics that drove geological change and activity in the early Earth, the formation and evolution of the crust, and timing and localisation of major mineral provinces, which form in response to those processes. 

Background

Previous isotopic mapping in Western Australia demonstrated that cratons (such as the Yilgarn and Pilbara), have a cryptic history and corresponding architecture that can be shown best by mapping their isotopic data spatially. The craton architecture corresponded to the location of different types of mineral deposit. In the Yilgarn, orogenic gold and komatiite-hosted Ni-Cu-PGE systems occurred in juvenile, young, mantle-derived crust, adjacent (at the margins of) older crustal blocks within the craton. In contrast, banded iron formation (BIF) deposits were concentrated within the older crustal regions. These correlations suggest that isotopic mapping could make a powerful area selection tool, for the strategic ranking of exploration targets.


What is isotopic mapping?

Using a spatially-extensive isotopic dataset (in this case Lu-Hf) to map variations in crustal age, source and evolution, and image the crustal architecture of a region.

Why is crustal architecture important?

Archean cratons are heterogeneous in many geological attributes, especially mineralisation. The constraining architecture helps us understand time-space variations in the evolution of the craton. 

Why use radiogenic isotopes?

They are time-resolved – hence they show the architecture as it was in the Archean

What about other isotopic systems?

There are plans further into this project to collect O-isotopes on the zircons – this will constrain temperatures and sources of external components


Project Objectives

One of the primary project objectives is to provide the exploration community with an isotopic map that can be used in a similar way to pre-competitive data provided by the geological surveys.

This will be a new and valuable tool that will help drive expansion of greenfields exploration into new or under-explored areas.

Further to this, the dataset the team is building will allow Metal Earth to investigate spatial and temporal development of archean crust in unprecedented detail. As well as offering the potential for important academic discoveries related to the establishment of the current habitable Earth, this study will also drive a greater understanding of the fundamental first-order processes behind the development of major ore provinces.

 

Who will benefit from these findings the most?  
"The fundamental dataset behind this project offers exciting developments for both economic geology and blue-sky science." "We hope the dataset and the maps we build will have real effects and implications for industry and explorers, and a tool they can use to further refine selection decisions. Further to this, we also hope the data will aid in the fundamental understanding of how major mineral provinces form, and the science behind the correlations of certain ore deposits and crustal history. Finally, the data collected in this project will allow fundamental insights into the understanding of tectonics and crustal evolution in the early Earth, and by proxy the development of the atmosphere-biopshere system. Ultimately the understanding of these processes will help us constrain the development of the Earth into a habitable planet full of life."

 

The vast majority of all isotopic and geochemical data will be collected on zircons from felsic volcanics and granitoids from all over the Superior Craton. These rocks represent the crust of the craton, and hence data from their zircons can be used as a medium for the nature of the crust from which they formed. This project will be collecting data in three isotopic systems (U-Pb, Lu-Hf, and 18O/16O), as well as trace elements in zircons. The new laser ablation split stream (LASS) system at Laurentian University will be used to collect U-Pb ages and Lu-Hf isotopic data synchronously, from the sample laser pit.

This will allow the team to data the rocks, as well as understand the age and nature of their source (i.e. mantle-derived, or crustally-reworked). Zircon trace element data allow the investigation of processes and features important for mineralisation potential, such as hydration and oxidation state of the magma. Oxygen isotopes will be collected separately on an ion microprobe to allow the team to constrain whether a low- or high-temperature hydrothermal component has been incorporated into the magma, and whether the crust was extracted from the mantle, or reworked within the crust itself. The combination of these processes have important implications for the history and origin of the crust in space and time, and mapping these allow us to map the prospectivity of the crust.

Figure 1.  Lu-Hf (εHf) map of the Yilgarn Craton at 2,720–2,600 Ma. (A) Hf isotope map with the location of sample sites and komatiite localities. (B) Interpretive map of the area, showing the individual crustal blocks identified from the Hf isotope map and corresponding probability density plots. The blue curve represents the median εHf for discrete temporal groups (ng), whereas the red curve represents all of the individual grain analyses (na). Dark gray polygons shown in the background of all maps represent supracrustal belts (Mole et al., 2014).

 

Utilizing the Superior Data Compilation  
"The release of the Superior Data Compilation has been very important for this project in particular." "As our work covers the entire craton, it is vital to have a good, craton-wide geological understanding of the area, and this must be embodied into a spatial form. Without this detailed craton-scale geology we would not be able to make efficient comparisons between the basic geology and the isotopic data. We feel that the integration of the Superior data compilation and our isotopic mapping will be a valuable tool to industry and academic in Canada, and worldwide, and result in a step-change in our understanding of the Earth’s largest Archean craton."
 

Project Duration

The project is estimated to take 4 years with an initial first version of the Hf-isotope map completed. Further time will be required to improve and finalize the map as well as integrate other datasets such as the O-isotopes, zircon trace elements and whole-rock geochemistry.  The length of the project does not mean tangible outcomes will not be felt sooner. The project is compartmentalised into a number of zones or quadrants, i.e. the Abitibi is one such quadrant. New isotopic data, and subsequent maps, from these areas will be presented as they are finalized allowing the data to be utilized by explorers as quickly as possible.

Preliminary Results

At this stage the team is designing a protocol to collect and prepare samples, when, where and how to run those samples and in what order. This stage of the project is almost complete, with the first analyses from Abitibi rocks expected to take place in July.

Figure 2.  Sm-Nd (εNd) map of the Yilgarn Craton from Mole et al. (2013).

Figure 3. Distribution of BIF-hosted iron and orogenic gold deposits in the Yilgarn Craton, shown on top of the εNd isotope map (Mole et al., 2013). Note gold deposits prefer the juvenile regions, whereas iron deposits prefer more reworked crustal blocks.

Update by Metal Earth researcher David Mole

 






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