Ann: Drone magnetic survey at Mongoose completed, page-5

Understanding Magnetic Susceptibility in Chalcopyrite Orebodies

Disclaimer: While I've done my best to understand and explain the provided text, a deeper geological understanding would be beneficial for a more comprehensive explanation.

Key Points from the attached video by Jim Austin:

• Magnetic Susceptibility and Density: These physical properties are used to study the evolution of a mineral system.
• Cloncurry Example: The Cloncurry region in Northwest Queensland, known for iron oxide copper gold deposits, is used as a case study.
• Different Mineral Systems: The region has various mineral systems with varying magnetic properties, such as magnetite-rich and pyrite-rich deposits.
• Reactive Transport Model: This model suggests that alteration processes create fluids that precipitate minerals and evolve over time.
• Alteration Stages: The video describes different stages of alteration, each with its own mineral assemblages and magnetic properties.
• Magnetite and Chalcopyrite: Magnetite is a magnetic mineral, while chalcopyrite is a copper-rich mineral. The evolution of the system affects the balance between these minerals.

How Magnetic Susceptibility Relates to Chalcopyrite Orebodies:

1. Early Stages: In the early stages of alteration, magnetite-rich deposits form.
2. Later Stages: As the system evolves, oxidation processes can destroy magnetite and form hematite, a non-magnetic mineral.
3. Chalcopyrite Formation: Chalcopyrite often forms in later stages of alteration, sometimes associated with hematite.
4. Magnetic Signature: The magnetic signature of the orebody changes as the mineral assemblage evolves.

In essence:

• High magnetic susceptibility in early stages indicates the presence of magnetite.
• Decreasing magnetic susceptibility in later stages suggests the formation of chalcopyrite and the destruction of magnetite. Copper mineral formation!!!

Non-Geologist Perspective:

Imagine a rock as a mixture of different ingredients. At first, it might have a lot of a magnetic ingredient (magnetite). Over time, the rock changes, and the magnetic ingredient breaks down, while a new ingredient (chalcopyrite) forms. This change affects how magnetic the rock is.

Conclusion:

Magnetic susceptibility is a valuable tool for understanding the evolution of mineral systems like chalcopyrite orebodies. By studying changes in magnetic properties, geologists can infer the sequence of alteration events and the formation of valuable minerals.

The plot below shows the magnetic susceptibility (SI) versus density for various ore bodies.

• Very magnetite (Fe3O4) rich deposits like Ernest Henry, Swan, E1 and Osborne having higher magnetic susceptibility (high SI) values (linear grey line).
• Pyrite (FeS) -Chalcopyrite (CuFeS2) and Hematite (Fe2O3) rich deposits like Great Australia (Pyrite-Chalcopyrite) & E1 (Hematite) exhibit zones of low magnetic susceptibility (low SI) values over a range of densities.

The image below illustrates the evolution of the mineral system - essentially reducing conditions (to the left) have a high magnetic signature due to the presence of Magnetite + Pyrrhotite. As you move to a more oxidised system (to the right) then a highly oxidised mineral assemblage system will exist containing Hematite + Pyrite which has a low magnetic signature and an improved environment for forming copper. The gravity profile remains relatively unchanged between the reduced & oxidised states.


Hopefully RNX are applying the learning's from the CSIRO research around the Cloncurry district to accurately identify mineral halos in rocks surrounding the Mongoose Deeps ore zone.



Reference:
https://www.youtube.com/watch?v=03_HL-C2-rc

https://www.academia.edu/84074966/Cloncurry_Metal_Ernest_Henry_Petrophysics_Final