Fathom Geophysics Newsletter 24

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Research news: Understanding of VMS deposits grows

VOLCANIC-related massive sulfide (VMS) deposits in Canada are offering up important clues that may be helpful to explorers of this ore type.

Recent work on two VMS-producing hydrothermal systems has led in one case to the recognition of nearby potentially favorable terrane, and in the other case to the formulation of a mineral-composition tool that may one day prove deployable elsewhere as an exploration vector to undiscovered deposits.

Kidd Creek-generating system much bigger than once thought

Correlates of rocks hosting the copper-zinc-silver Kidd Creek volcanic-related massive sulfide (VMS) deposit are situated directly to the northwest of the mine, Canada-based researchers say. [1]

The researchers said the full extent of the hydrothermal system responsible for Kidd Creek was still unknown, but their model of the system had more than doubled the research community's previous estimate.

Kidd Creek is the world's largest Archean-aged VMS deposit and is located in the Abitibi Greenstone Belt near Timmins, Ontario.

The correlative package of rocks is known as the North Rhyolite.

They said that the North Rhyolite's stratigraphy was identical to the Kidd Creek mine stratigraphy.

In order of decreasing age, that stratigraphy included komatiites accompanied by interflow rhyolites, massive aphyric rhyolite, brecciated aphyric rhyolite, aphyric rhyolitic volcaniclastic rocks, massive quartz porphyritic rhyolite, quartz porphyritic rhyolite volcaniclastic rocks, and basalts.

The researchers' findings were made via detailed underground mapping and logging of drill core.

VMS deposit formation was largely controlled by synvolcanic subsidence-related faults, the researchers said in their paper.

They said their study showed that a graben fault wall — which helped funnel hydrothermal fluids to Kidd Creek's North and Central orebodies — did not entirely block off wider development of the mine stratigraphy, the rhyolite field and the hydrothermal alteration zone.

The whole system clearly continued some two kilometers to the north [of Kidd Creek] and some two to three kilometers to the northeast, to reach the area of the North Rhyolite, the researchers said.

They said that while alteration beyond the graben wall structure was much less pronounced than alteration in the Kidd Mine stratigraphy, and while no significant mineralization had been found yet within the North Rhyolite, the increased known extent of the Kidd Creek system expanded the zone of potential VMS mineralization.

White mica compositions point to VMS: Researchers

In other VMS news, researchers from government and academia in Canada's eastern province of New Brunswick have found that the geochemistry of hydrothermal alteration-associated white mica would make a useful vector-to-ore tool for explorers looking for VMS mineralization. [2]

They looked at white mica compositions determined by laser ablation inductively coupled plasma mass spectrometry. Samples came from drillcore obtained at seven VMS sites in the Bathurst Mining Camp: Brunswick No. 12, Heath Steele B, Halfmile Lake Deep, Key Anacon East, Louvicourt, Armstong A, and Restigouche.

They found that hydrothermally-formed white mica grains typically showed anomalous compositions within 20 to 50 meters of a massive sulfide horizon, in both hanging wall and footwall zones. Compositions involved elevated thallium, antimony, and mercury abundances, along with varying degrees of enrichment in arsenic, tin, and indium.

And in locales that were up to several hundred meters away from mineralization, the sum of thallium, antimony, tin, and mercury abundances was the most appropriate measure to use.

Fathom Geophysics notes that geochemical spatial information of this kind may lend itself to the formulation of a 3D deposit footprint model for VMS deposits in a similar manner to what was done for porphyry copper deposits (see our explanatory write-up).

The study's authors said that even though later episodes of deformation and metamorphism affecting the Camp area had impacted white mica compositions locally, significant compositional trends could still be seen.

White mica compositions may also work in the search for VMS deposits outside of the Bathurst Mining Camp, but further development of the technique was needed first, they said.

They noted in their paper that the small grainsize of the Camp's hydrothermal white mica and the presence of other minerals, such as chlorite and pyrite, in and around the mica grains meant that the elemental abundances they determined may have been influenced by these other minerals. The ablation spot-size they used was 66 micrometers.

References

[1] Y.M. DeWolfe, H.L Gibson, and D. Richardson (October 2018) "3D reconstruction of volcanic and ore-forming environments of a giant VMS system: A case study from the Kidd Creek Mine, Canada", Ore Geology Reviews, 101, 532-555.

[2] A.S. Dehnavi, D.R. Lentz, C.R.M. McFarlane, and J.A. Walker (May 2018) "Quantification of fluid-mobile elements in white mica by LA-ICP-MS: From chemical composition to a potential micro-chemical vectoring tool in VMS exploration", Journal of Geochemical Exploration, 188, 290-307.

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.