Fathom Geophysics Newsletter 24

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Exploration-related news: Exploration labwork pays off

APPARENT mineral-chemistry trends that seem to point toward mineralization may evaporate upon closer scientific scrutiny. But that closer scrutiny may also uncover a legitimate exploration vector to mineralization, as a study at the Marathon copper-palladium deposit in Canada has revealed.

The study looked at whether vein-hosted mineral chemistry could be used as an exploration vector to mineralization in the search for low-sulfide mafic-conduit-related copper-platinum-group-element deposits. [1]

The researchers analyzed diamond drill core from the Marathon copper-palladium deposit, which is located in the Canadian province of Ontario, directly southeast of Bamoos Lake. All but one of the drill holes examined contained mineralized intervals.

The Marathon deposit is hosted by the Two Duck Lake gabbro within the Mesoproterozoic-aged midcontinent-rift-related Coldwell Alkaline Complex.

The mafic-ultramafic rock package that includes the Two Duck Lake gabbro was thought to have formed when mineral crystals and interstitial sulfide melt accumulated near narrow, high magma-throughput conduits, also known as feeders. Researchers have posited that feeders at Marathon may potentially occur where depressions in the contact between the Complex's rocks and Archean country rock coincide with linear structures that trend east-west and northwest-southeast. [2]

The Marathon deposit has in recent years been the site of exploration for high-grade feeders that would bolster the deposit's project economics. [3]

Vein chemistry is deceptive: Researchers

Researchers looked at the major- and trace-element abundances of vein minerals using Raman spectroscopy, X-ray diffraction, scanning electron microscopy in energy-dispersive and wavelength-dispersive modes, electron microprobe, and laser ablation inductively-coupled plasma mass spectroscopy.

The researchers said veins were ubiquitous in the gabbros surrounding the Marathon deposit, and generally contained fine-grained acicular chlorite, usually along with minor amounts of fine-grained aggregated and disseminated calcite. Most veins were 20-400 micrometers thick.

Because the minerals were so fine-grained, results for major-element analyses represented the chemistry of small aggregates of crystals, the researchers said. The major-element chemistry of vein-hosted minerals showed that they headed away from ideal chlorite compositions, leaning toward compositions expected for saponite, serpentine, and amphibole.

In examining the trace-element chemistry of vein-hosted minerals, the researchers only considered veins that traversed plagioclase grains. This was because they found other host-grain types (namely magnetite and pyroxene) influenced vein compositions.

When they looked at results for the whole population of trace-element analyses obtained from across multiple drill holes, the researchers found that cobalt, nickel, and zinc abundances of veins tended to be higher the further they were situated above mineralization zones. So on the face of it, this seemed to be a potential trace-element trend that might help explorers zero in on mineralization.

However, within individual drill holes, the researchers found no correlation between vein-hosted mineral chemistry and distance from mineralization. They also found a negative correlation between cobalt, nickel, and zinc abundances and whole-rock copper abundance. These additional findings, among others outlined in their paper, likely indicated that the fluids creating the veins didn't derive their cobalt, nickel and zinc contents from copper sulfide mineralization, they said.

So what was the explanation for the apparent broad relationship between vein-hosted trace elements and vein location relative to their distance above mineralization zones?

The relationship didn't having anything to do with the mineralization zones, but rather was a function of the height in the mafic-ultramafic pluton at which the vein minerals precipitated, the researchers said.

This was corroborated by the close similarity between plots of compositional variation of vein-hosted minerals as a function of distance above mineralization, and plots of compositional variation as a function of distance from the pluton's footwall contact with the Archean country rock, they said.

They said the height-related variation in the metal content of vein fluids was likely to have been caused by the progressive leaching of elements from the pluton's pyroxene, olivine, and magnetite grains as the fluids migrated upwards.

"These results act as an example of how mischaracterization of the small- and large-scale controls on mineral chemistry can lead to erroneous interpretations and should be viewed as a cautionary example for the application of [vein-hosted] minerals as exploration tools in other systems."

Vein abundance is a more reliable indicator at Marathon

Because the source of most of the cobalt, nickel, and zinc in the fluid that precipitated the vein minerals had nothing to do with sulfide mineralization, the abundance of these elements therefore couldn't be used as an exploration vectoring tool, the researchers said.

The good news was that they were able to confirm an alternative phenomenon at Marathon that could be applied as a legitimate exploration vector to mineralization.

"The [abundance] of veins, measured as the number of veins per three-meter interval of drill core, is lower in areas where mineralization is present. The relationship between vein [abundance] and mineralization can therefore be used to predict if a drill hole hosts mineralization, and roughly where that mineralization is located," they said.

They noted that while the presence of visible sulfides was an obvious indication of mineralization, in zones such as Marathon's W Horizon, where platinum-group-element grades are high and sulfide content is very low, mineralization was only identifiable via whole-rock assays, which were not immediately available during the core logging process.

Vein abundance, which can be measured routinely during core logging, could be used as a preliminary mineralization indicator and would help in the selection of appropriate drill core sections for assaying, they said.


[1] M.J. Brzozowski, I.M. Samson, J.E. Gagnon, R.L. Linnen, D.J. Good, D.E. Ames and R. Flemming (2018) "Controls on the chemistry of minerals in late-stage veins and implications for exploration vectoring tools for mineral deposits: An example from the Marathon Cu-Pd deposit, Ontario, Canada", Journal of Geochemical Exploration, 190, 109-129.

[2] D.E. Ames, I.M. Kjarsgaard, A.M. McDonald and D.J. Good (2017) "Insights into the extreme PGE enrichment of the W Horizon, Marathon Cu-Pd deposit, Coldwell Alkaline Complex, Canada: Platinum-group mineralogy, compositions and genetic implications", Ore Geology Reviews, 90, 723-747.

[3] Sibanye Stillwater company website, Marathon project information page, www.sibanyestillwater.com/our-business/americas/projects/marathon, accessed 25 September 2018.

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.