The magnetic properties of rocks, which are reflected in maps of the magnetic field variation, are determined by the physical and chemical histories of those rocks. This segment looks over these influences.

There is also a segment referring to methods of structural geophysics, and some references to case histories.

This page is part of an edited version of a class presentation, not a full set of subject notes.

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Mineral Susceptibilities

• Most rockforming minerals are diamagnetic
  • they produce weak induced fields
• Some minerals (clays) are paramagnetic
  • but induced fields are still weak
• A few minerals are ferromagnetic
• Magnetite is the most important

More-significant minerals

• Magnetite (most susceptible)
  • seldom associated with remanence
• Pyrrhotite (much less susceptible)
  • may have significant remanence
  • can be metamorphosed to magnetite
• Hematite (weakly susceptible)
  • may have significant remanence
• Ilmenite/Titanohematite (weakly susceptible)
• Maghemite (weathering product)

• A very general overview of relationships follows
  
• Since magnetic map is "a map of the magnetite distribution… "
• You should understand what affects that–
  
  • the geological significance of magnetic minerals

Geological Influences on Magnetic Mineral Distribution

• Primary Processes
  • occur during
    crystallisation of magma, or
    deposition of sediment
• Secondary Processes

  • All post-depositional processes

  • Metamorphism
  • Metasomatism
  • Deformation
    

Magnetite in Sediments

• is very unusual! (it is unstable under weathering conditions)
• Exceptions:
  • banded-iron formations (chem precipitate)
  • volcanogenic (chem precipitate)
  • beach sands, river/stream placers (detrital)
    • More common iron oxide forms are haematite, goethite, limonite, maghemite
    • maghemite has most significant susceptibility among these

• most often stratiform (deposition-related)
• may be related to shearing (secondary processes)
• may be related to granitization (secondary process)
  
  • (new work in Broken Hill region shows magnetic patterns there not bedding…
  • the anomaly at Archery Range (SA in the diagram, which is about 20 km across) is attributed to magnetite in steeply-dipping S3 shears, rather than to shallow bedding).

• Total Iron content
  • obvious key factor
• Geochemical Environment
  • partitioning of iron into different phases
• Igneous Crystallisation Environment
  • unexpected mafic/felsic outcome

• Phases expected as fO₂ increases are:
  • Fe Silicates > Titanomagnetites > Titanohaematites > Haematite
    • (see FMQ diagram - or recap on 625-222!)
• and with fractionation:
  • less Fe > higher Fe/Ti ratio > higher fO₂
• so:
  Fe in mafics likely to be in Olivine;
  more Fe in felsic rocks likely to be in Magnetite

A synoptic view of (fO₂,T) for many igneous rocks, and the FMQ buffer

Secondary Influences–Metamorphism

• Metamorphism includes magnetite-producing and -destroying reactions
• Magnetite relatively common
• Formation depends on oxidation state (usually inherited from sediments)
  • magnetic response often delineates premetamorphic sedimentary environments
    • several other factors also probable

• Fugacities determine stable iron phases
  • High fO₂                            haematite formation
  • Moderate fO₂                    magnetite formation
  • Low fO₂ (and high fS₂)    pyrrhotite/pyrite formation
    
    
• State may be inherited by metasediments

Iron and Oxidation Ratio Diagram

Oxygen/Sulphur Phase Diagram

Secondary Influences – Metasomatism

• Metasomatism is likely origin of most magnetite/pyrrhotite associated with ore deposits
  • - but is not considered further here

• Deformation can affect magnetic response through gross disruption:
  • Physical reorientation of remanently magnetised rocks is factor
  • Fault-related alteration may also be significant on small scale, and observable as lineaments
• and at grain scale
  • susceptibility and remanence depend on size of magnetic mineral grains

• Grain-growth in high-grade ductile settings yields different behaviours, because of magnetic domain behaviour in crystal grains
  • small (<0.03 mm)     superparamagnetic, high k
  • intermediate              low k but high remanence
  • larger (>0.06 mm)    moderate k, low remanence

• Geometry is main conclusion from regional magnetics interpretation
• Primary Fe variations, fO₂ variations also discernible where appropriate
  
  
• Other effects may allow constraint of (for example)
  metamorphic isograds, or
  alteration haloes,
  
  with subsequent detailed work

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Interpretation

• Interpretation can now involve
  • Assessment of the structure from magnetics images
  • Assessment of the structure from gravity images
  • Detailed ground mapping in key areas, returning closely-controlled geological data
    
    
• Synthesis of these inputs is vital

• Noddy is a package developed by Mark Jessell of Monash University, to
  • Model deformations applied to an original geological model, ranging from folding to intrusion
  • Predict geological-map and structural data resulting from the deformations, and
    
    
• Predict the geophysical response of the same model

Follow through by visiting the Noddy web site.

As a sample, here are

• a geology map,
• a gravity map,
• a magnetic map
• and a block model

constructed by deforming a simple, defined, layer-cake model with a sequence of folds, tilts, faults, intrusions, and erosional events which I defined.
Not shown here are other geological observations, such as bedding dips and strikes, which Noddy can compute.

• Look for these tools for your work!

Integrated Exploration

• The next two screens show gravity, magnetics, and geology images for an area near Parkes in which there are several known mines.
• Where would you look for the next prospect?

The samples are from a Computer-assisted Learning package developed by Learning Curve PL of Canberra, with mining-industry sponsorship.

• The package, on CD-ROM, is intended for self-teaching by technical staff. It can be loaned to interested students. See LT for more information.

• Discovery with magnetics:
• Elura Zn-Pb-Ag deposit (the image has already been shown.)
  • For details, see Exploration Geophysics v11, #4

Woodlawn

• The case shown here is a "Non"discovery

  • but see more information in the "Woodlawn" volume (ed. Whiteley)
  
  Woodlawn was a base-metal orebody near Canberra

• Cu-Pb-Zn Sulphide Deposit
• Discovery by joint use of geology, geochemistry, geophysics (electrical methods)
• Used as test site for geophysical methods for search for further volcanogenic, stratiform targets.

Woodlawn Magnetics

Gossan marked is surface expression Actual Orebody dips vertical-to-W Ore is not magnetic Origin of magnetic anomaly? Is it Irrelevant? Is it Cogenetic?

Detailed, numerical inversion

Interpretation of this result

• Location, depth, trend, susceptibilities indicate anomalies associated with dolerite intrusions
• Dolerites postdate mineralization
  
• Magnetic anomalies thus unrelated to mineralization in this case
  
  
• Analysis does help with structural studies of environment

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