Fathom Geophysics Newsletter 23

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Field Geochem News: Not all cruddy carbonate is calcrete

MINERAL explorers who use calcrete geochemistry as a vector to ore now have a way to avoid wasting their time and money sampling weathered carbonate rocks of primary marine origins.

It turns out that straightforward calcium-to-strontium ratios should allow explorers to quickly distinguish between Quaternary calcrete and weathered Cenozoic marine carbonate rocks. [1]

The findings were made by South Australia-based researchers, who looked at rocks in South Australia's Yorke Peninsula region.

Calcrete, also known as pedogenic carbonate-bearing rock, is a target for some mineral-exploration sampling programs. It forms when calcium is chemically leached or mechanically removed from pre-existing rocks and re-cemented as a carbonate.

In their paper, the researchers said discriminating between the two carbonate rocks types was important because pedogenic carbonate-bearing rocks had the potential to carry a geochemical signature transferred from any underlying mineralization, while the signature of marine carbonate-bearing rocks usually only reflected the chemistry of the water in which they formed.

They listed the Gawler Craton's Challenger gold deposit as one notable discovery that followed on the heels of a regional carbonate-sampling campaign.

Difficulties arise in being able to tell between calcrete and weathered marine carbonate when rock texture has been destroyed, such as when rock chips that are gravel-sized or finer are brought to the surface using rock-breaking-intensive drilling methods.

In addition, the weathering of marine carbonates — which includes recrystallization and other overprinting processes — can turn these rocks into calcrete look-alikes.

Among their results for Myponie Point rocks, the researchers found the Ca/Mg ratio range of limestone to be 82-109, while the range for pedogenic carbonate rocks was 6-19. The Ca/Sr ratio ranges there were 2,233-3,095 for limestone and 184-235 for pedogenic carbonate rocks.

They found similar significant contrasts in the ratio ranges occurring in rocks from from Point Turton.

The researchers said they favored the use of rocks' Ca/Sr ratio, because of (1) its larger overall range across the rock types they studied, (2) the larger gap in values for the two distinct carbonate-rock populations, and (3) their knowledge of other research data in the scientific literature showing that the use of Ca/Sr ratios was applicable to regions outside of the Yorke Peninsula.

"Calcium and often strontium data are routinely collected in laboratory whole-rock geochemical analysis, and generating this data is significantly less time-consuming than [say] analysis of strontium isotopes," they said.

They added that, based on the concentrations of calcium and strontium seen in the rocks they studied, portable X-ray fluorescence instruments had sufficiently low detection limits that immediate analysis of Ca/Sr ratios was achievable as a part of exploration campaign fieldwork.


[1] K. Wolff, C. Tiddy, D. Giles, S.M. Hill and G. Gordon (October 2017) "Distinguishing pedogenic carbonates from weathered marine carbonates on the Yorke Peninsula, South Australia: Implications for mineral exploration", Journal of Geochemical Exploration, 181, 81-98.

About Fathom Geophysics

In early 2008, Amanda Buckingham and Daniel Core teamed up to start Fathom Geophysics. With their complementary skills and experience, Buckingham and Core bring with them fresh ideas, a solid background in geophysics theory and programming, and a thorough understanding of the limitations of data and the practicalities of mineral exploration.

Fathom Geophysics provides geophysical and geoscience data processing and targeting services to the minerals and petroleum exploration industries, from the regional scale through to the near-mine deposit scale. Among the data types we work on are: potential field data (gravity and magnetics), electrical data (induced polarization and electromagnetics), topographic data, seismic data, geochemical data, precipitation and lake-level time-lapse environmental data, and remotely-sensed (satellite) data such as Landsat and ASTER.

We offer automated data processing, automated exploration targeting, and the ability to tailor-make data processing applications. Our automated processing is augmented by expert geoscience knowledge drawn from in-house staff and from details relayed to us by the project client. We also offer standard geophysical data filtering, manual geological interpretations, and a range of other exploration campaign-related services, such as arranging surveys and looking after survey-data quality control.