CSIRO EARTH SCIENCE AND RESOURCE ENGINEERING · Understanding IOCG magnetic targets from the inside out . ... • Juno is typical of an intermediate type ... CSIRO Earth Science and

Case Studies from northern Australia

CSIRO EARTH SCIENCE AND RESOURCE ENGINEERING

James R. Austin and Clive A. Foss

Email: [email protected]

Introduction

• Targeting magnetic anomalies for IOCG mineralisation is common. • There are generic models to explain the geophysics of IOCGs • In reality the term “IOCG” is applied to a range of architectures

and mineralogies. • Hence IOCGs have a variety of magnetic anomaly styles. • Inner workings of IOCG magnetic anomalies are often overlooked: • remanent magnetisation • self-demagnetisation, • relationships between mineralisation and magnetisation, • zonation of magnetic minerals,

Understanding IOCG magnetic targets from the inside out

Aims of this talk

1. Characteristics of magnetic minerals

2. Examples of geophysically different end-members of the “IOCG” family

3. Review some “IOCGs” from the Tennant Region and Arunta Block.

4. Hopefully provide some insights into geophysical signatures.


Induced vs Remanent Magnetisation

• Rocks contain two magnetic components, induced and remanent magnetisation, • Induced magnetisation = magnetic susceptibility x

inducing field (Earth’s Field).

• Remanent magnetisation is retained in a rock, i.e. it is not induced by the earth’s field

• Remanence in Mt-rich IOCGs: • 1/5th of induced magnetisation • 60-80% is viscous remanence in MD magnetite • meaning it is unstable. • Usually aligned with the Earth’s field, • thus mimicking induced magnetisation.


Remanent magnetisation in Hematite and Pyrrhotite • Hematite and pyrrhotite in IOCG systems

are more likely to retain remanence. • At Monakoff remanence in hematite iron

formation is 13x stronger than induced (Q=13)

• At Brumby, remanence in pyrrhotite-rich zones is 90x times stronger than induced (Q=90).

• Neither contribute significantly to a magnetic anomaly if magnetite present.


Self demagnetisation in IOCGs • Self-demagnetisation occurs in

all magnetic bodies. • But it is negligible for

susceptibilities < 0.1 SI. • The demagnetising field within a

body opposes the inducing field, • effectively decreases magnetisation • reduces the measured field

• Caused several deposits to be mis-modelled, e.g., Osborne & Candelaria ->

Figure 5: Magnetic model of the Candelaria IOCG, showing a shallow-dipping sheet architecture – from Austin et al., (2014b)


IOCG EXPLORATION Geophysical criteria commonly applied

to exploration for IOCGs: 1. Gravity anomalies -> indicating Fe-oxide

± Fe-sulphide (e.g., Olympic Dam) 2. Magnetic anomalies -> indicating

substantial magnetite (e.g. Osborne) 3. Proximity to crustal-scale geophysical

lineaments# (e.g., Olympic Dam, Ernest Henry, Candelaria).

crustal-scale faults that act as pathways mineralising fluids

Let’s look at some end-member types

#( )

O’Driscoll, 1986 Understanding IOCG magnetic targets from the inside out

Hematite Dominated Breccias

• Can have huge gravity anomalies (20 mGal)

• Often have subtle magnetic anomalies

• They can be modelled as zones of brecciation sitting at the intersection of faults: • either simple pipe geometries (e,g., Oak Dam)

• more complex architectures (e.g., Olympic Dam:

• The magnetic and gravity anomalies are: • broadly coincident at surface,

• Bodies may be at different depths, e.g., Olympic Dam

• Most IOCGs outside of SA are not hematite dominated.


Magnetite Breccia Pipes

• Cause a bullseye magnetic anomaly, • Coincident gravity anomalies. • They can be modelled as: • Pipes at the intersection of faults • Within fault jogs (e.g., Ernest Henry).

• Magnetite and sulphides may be concentrically zoned.

• Sulphides are both • syngenetic with magnetite precipitation, • overprint oxides via metasomatism


Ironstone Hosted deposits

• Cause narrow elongate anomalies • Modelled as a sub-vertical sheets or

elongate elliptical prisms. • High density (4 g/cm3 at Monakoff) • too narrow to be mapped with gravity

data, because resolution is inadequate. • Magnetite and Fe-Sulphides can be: • syngenetic and/or • precipitated by replacement of Fe-oxide.

• laterally zoned: • magnetite ± sulphides in the core • hematite distal

Figure 4: Magnetic Model of the Monakoff ICOG, showing lateral zonation of magnetite. - from Austin et al, 2013b.


NORTHERN TERRITORY CASE STUDIES


Tennant Creek Fe-oxide Au-Cu-Bi deposits

• The deposits are structurally controlled and tend to align within a number of distinct mineralisation corridors, e.g., The Peko Line.

Figure 6: Total Magnetic Intensity map of an area to the east of Tennant Creek, showing the magnetic anomalies associated with several different IOCGs. The deposits sit along a number of mineralisation corridors, such as the two shown in white. data – Milligan et al., 2010.


Geochemical Variability

• Deposits have a range of oxidation states • Peko and Warrego are reduced end

members • Similar to Eloise and Mt Elliot • Pyrrhotite-magnetite-pyrite mineralogy.

• Juno is typical of an intermediate type • Similar to Ernest Henry) • Magnetite-hematite-pyrite ± pyrrhotite.

• Eldorado is an oxidised end-member • Similar to Starra, • Pyrite-hematite-magnetite mineralogy

Fig 20 - Skirrow and Walshe (2002) -> Understanding IOCG magnetic targets from the inside out

Geophysical Variability

• Variable chemistry = variable magnetic properties and anomalies.

• Reduced –intermediate Mt-rich end members: • very high susceptibilities (e.g., 0.7 SI) • sharp positive magnetic anomalies, e.g., Peko • Highly prone to self-demagnetisation effects.

• More oxidised deposits tend to have: • Much lower bulk susceptibilities (e.g, 0.2 SI) • more subtle anomalies, e.g., Juno, Eldorado

• Anomalies are also a function of depth and orientation.


Modelling Tennant Creek type IOCGs

• Their anomalies can be modelled as ellipsoidal (pod-like) bodies

• The Eldorado deposit can be modelled as: • a sheet-like body or • an ESE plunging ellipsoid

• However, in reality the deposit architecture is most likely intermediate, i.e., the anomaly is: • partly due to magnetite/hematite in

a Fe-rich host horizon/ shear zone • partly due to magnetite in an

ellipsoidal shape, plunging to ESE.


Rover 3 (former IOCG target)

• Remanence in usually minor in IOCGs • due to coarse-grained magnetite.

• Negative magnetic anomalies = remanent magnetisation

• Can’t be measured with a MagSus meter.

• Q-meter is required to measure remanent magnetisation in the field.


• Rover 3 was an IOCG target within the Rover Field, SW of Tennant Creek. • Drilled due to large negative magnetic anomaly • Drilling found intermediate volcanics with low susceptibility.

• Anomaly source is remanent magnetisation within the volcanics

The Arunta Block

• Represents a relatively new IOCG exploration frontier, containing a number of promising prospects.

• Some prospects show evidence of copper, associated with Fe-oxide veining (e.g., the Illogwa area),

• Others have strong affinities with skarn-like carbonate-magnetite alteration (e.g., Johnnies Reward).


The Illogwa area (150 km east of Alice Springs)

• Historically, surface copper shows were found (e.g., Bullhole Bore, Albarta), which were described as sediment hosted Cu.

• More recently, Mithril Resources have been exploring for IOCG-style mineralisation, across a number of prospects shown below.

Figure 9: A Total Magnetic Intensity map of the Illogwa area, showing a number of Mithil Resources prospects, and some interpreted structures. Major structures are in grey, minor structures are black. - after Hutton, 2012, data – Milligan et al., 2010.


Indications of IOCG mineralisation

• Copper ± gold anomalism, ± Fe-veining ± fluorite alteration

• Associated with altered granite

• Sit on fault zones, adjacent to a major crustal structure, • a trait typical of IOCGs.


• Significant Mt-alteration associated with the major structures, • There are a number of bullseye targets in the area, many of which

sit proximal to observed Cu-anomalism.

Photo stolen from Hutton, 2012

Bigglesworth

• Basic (very basic) Modelling of bullseye targets @ Bigglesworth • Source bodies have magnetic

susceptibilities of 0.2 – 0.5 SI • Indicative of significant magnetite.

• Most other anomalies are subtle: • hematite dominated mineralogy • consistent with alteration by relatively

oxidized fluids,


Johnnies Reward (NNE of Alice)

• Bullseye magnetic anomaly • Sits along a N-S structural corridor

which hosts similar mineralization. • Can be modelled as a pipe-like body • Modelled susceptibility is 0.15-0.3 SI

• Carbonate-rich, Au-rich, Cu-poor IOCG • Skarn-like affinities, • diopside-magnetite ± sulphide • marble ± magnetite lithologies • similar to many IOCGs in the higher grade

areas of the Cloncurry region, e.g., Brumby.


VARIABLES THAT CONTROL GEOPHYSICAL SIGNATURE

• The magnetic anomalies of “IOCG” deposits are widely variable. • Depth and orientation will always affect the amplitude and

wavelength of any given anomaly, • But there are several specific variables that control the different

geophysical signatures of IOCGs. These are: • 1. Abundances of magnetite, pyrrhotite, hematite and other Fe±Cu sulphides

• 2. Degree to which these minerals coincide spatially (i.e., zonation)

• 3. Structural controls on precipitation of Fe-oxides and Fe±Cu sulphides

• 4. Mechanisms & controls on Fe-Oxide/Sulphide destructive alteration.


Researcher in Business Grants • Enterprise Connect can provide up to 50% of salary costs, up to $50,000

to help explorers, via targeted research. • A proposed project must: • develop a new idea with commercial potential within the business • involve activities that are not currently being carried out in the business • involve activities which will create new competencies and capabilities within the

business that are aligned with the business strategy. • To be eligible to apply you must : • have an Australian Company Number (ACN) • be solvent • have turnover or expenditure of >$1 million and <$100 million • have been trading for the last three years


Visit enterpriseconnect.gov.au or call 131 791.

http://www.enterpriseconnect.gov.au/

CSIRO Earth Science and Resource Engineering James Austin Postdoctoral Fellow in Geophysics t +61 2 9490 8876 e [email protected] w www.magresearch.org

ADD BUSINESS UNIT/FLAGSHIP NAME

Thank you Palaeomagnetic sampling during a dust storm in the Musgraves

Documents

CSIRO EARTH SCIENCE AND RESOURCE ENGINEERING · Understanding IOCG magnetic targets from the inside out . ... • Juno is typical of an intermediate type ... CSIRO Earth Science and