Geophysical Signatures of IOCG and Sedex style mineralization
in the Mount Isa Eastern Succession, Queensland, Australia
Austin, J.R.[1], Patterson, B.O.1], le Gras M. [2], Birchall, R. [2] ,Walshe, J.L.[2]
_________________________
1. CSIRO Mineral Resources, 11 Julius Ave, North Ryde, NSW, 2113, Australia
2. CSIRO Mineral Resources, 26 Dick Perry Ave, Kensington, WA 6151, Australia
ABSTRACT
The Cloncurry district is renowned for Iron-Oxide Copper-Gold (IOCG) mineralization, but includes a wide spectrum of deposit styles,
including: IOCG, Iron Sulfide Copper-Gold, Broken Hill Type (BHT), Sedex, Skarn and intrusion related Cu-Mo. Measurements were
made of: density, magnetic susceptibility and Natural Remanent Magnetization (NRM), and Koenigsberger ratios calculated, on 1731
specimens from 14 different mineral deposits and prospects from the Cloncurry district. The deposits studied have a wide variety of
petrophysical properties, primarily dictated by the relative contents of magnetic minerals (e.g., magnetite, monoclinic pyrrhotite, and
hematite) and other non-magnetic minerals (e.g., hexagonal pyrrhotite, pyrite, galena, sphalerite, barite). Any combination of these
minerals can be associated with high densities. High magnetic susceptibilities (and hence high amplitude magnetic anomalies) are
invariably associated with coarse magnetite. Monoclinic pyrrhotite is associated with moderate magnetic susceptibility and high
remanence, whereas hematite is only weakly magnetic. Many deposits and prospects contain mixtures of different Fe-oxide and sulfide
phases, which can be related to redox and/or overprinting. Assemblages usually fall into one of three basic categories: 1.Oxidized
assemblages, which contain hematite, no pyrrhotite, but typically pyrite and variable magnetite, are associated with retrograde (magnetitedestructive) alteration, often in association with Cu ± Au- and/or Mo- mineralization. 2. Intermediate assemblages are typically magnetiterich, but can contain pyrrhotite and/ or pyrite. These are often only weakly mineralized, but are associated with large positive magnetic
anomalies, caused by substantial quantities of multidomain magnetite. Magnetite-rich lithologies are advantageous for further
mineralization, insomuch as they can be further oxidized or reduced. 3. Reduced assemblages are typically pyrrhotite dominant, contain no
hematite, but often do contain magnetite. Hexagonal (non-magnetic) pyrrhotite is typically associated with galena and sphalerite, whereas
magnetic pyrrhotite is more typically associated with chalcopyrite. Where monoclinic pyrrhotite is present the remanence is oriented subvertical up, and as such, is difficult to isolate from the steep, upward north oriented inducing magnetic field.
INTRODUCTION
The Cloncurry district (the Mount Isa Eastern Succession;
Fig 1) is world renowned for Iron-Oxide Copper-Gold
(IOCG) mineralization, but in reality the mineralization
present is highly diverse, spanning a number of deposit
styles, including: IOCG, Iron Sulfide Copper-Gold
(ISCG) to Broken Hill Type (BHT), Sedex, Skarn and
intrusion related Cu-Mo. Whilst some deposits are
monogenetic, many deposits formed via overprinting of
two or more different ore forming systems, which
occurred during (often interrelated) tectonic, metamorphic
magmatic and metasomatic events (Fig 2). The deposits
are both mineralogically and petrophysically diverse, and
accordingly have wide variability in both their
geochemical and geophysical expressions.
This paper presents some of the findings of the Uncover
Cloncurry initiative (Austin et al., 2016 a-h). Here we
focus on geophysical properties, but the entire report,
which includes detailed research on 14 deposits and
prospects, plus summary reports for the mineral system is
available at: http://bit.ly/2jESB74.
Figure 1: Location map, showing the position of mineral
deposits/ prospects studied with RTP magnetics underlain.
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Figure 2: Condensed tectonic, metasomatic and metallogenic history of the Cloncurry District mineral system, and the processes
that occurred at each of the deposits and prospects discussed in the study.
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PETROPHYSICS OVERVIEW
Measurements of: density, magnetic susceptibility and
Natural Remanent Magnetization (NRM) were made on
1731 specimens from 14 different mineral deposits and
prospects from the Cloncurry district. The magnetic
susceptibility and NRM of each sample were also used to
calculate Koenigsberger (Q) ratios, (i.e., the ratio of
remanent to induced magnetization).
The petrophysical properties of the ore zones have been
assessed by clipping the petrophysical data to the top 10%
of measured densities within each deposit or prospect. In
general, the top 10% of measurements are correlated with
the ore zone for each deposit. However, the extent to
which this is true is dependent on the number of samples
obtained, and whether those samples accurately reflect the
deposit and its host rocks. In cases where the ore zone has
been over-sampled the top 10% is likely to be
representative. However, where under-sampled the
inverse applies. As such, some adjustments were made for
individual deposits, to make the results more
representative.
The mineralized zones of deposits/prospects assessed by
this study have a large range of densities, between 3.1 and
4.8 g/cc (Fig 3). Elevated density can be a result of large
concentrations of magnetite, hematite, pyrrhotite, galena,
sphalerite, pyrite, and in some cases can be attributed to
barite (e.g., at Monakoff; Austin et al., 2016b).
Deposits/prospects have a large range in magnetic
susceptibility, from negligible (e.g., 1 x 10-6 SI) to 2.1 SI
at Osborne (Figs 3, 4). In many cases high densities are
correlated with high magnetic susceptibilities (e.g., Fig 3),
and in these cases the dominant dense/susceptible mineral
is magnetite. For the most part the magnetite is coarse
grained, multi-domain magnetite (Fig 4), which does not
retain significant, or stable remanence. High densities
correlated with low-moderate susceptibilities, are
typically due to pyrrhotite. High densities and negligible
susceptibilities can be related to hematite, pyrite,
hexagonal pyrrhotite, and/or practically any species of
sulfide.
The various deposits have a range of natural remanent
magnetization (NRM) intensities, from negligible up to
mean values of 450 A/m, and associated Koenigsberger
ratios of up to 130 (Fig 4). Prospects which have high
Koenigsberger ratios (e.g., Cormorant, Canteen) contain
monoclinic pyrrhotite as the dominant magnetic phase.
Figure 3: Plot of magnetic susceptibility vs density for ore samples from deposits / prospects assessed in this study.
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Figure 4: Plot of the ratio of NRM to magnetic susceptibility (Q, Koenigsberger Ratio) vs magnetic susceptibility for ore samples
from deposits / prospects assessed this study.
In terms of petrophysical properties there are several
recognized associations:
1.
2.
Deposits with high density, high susceptibility, and
low Q ratios are dominated by coarse MD magnetite,
e.g., Osborne, SWAN.
Deposits with high density, high susceptibility and
moderate Q ratios (e.g., Cormorant, Maronan) may
contain pseudo single domain (PSD) magnetite which
may be indicative of a sedimentary origin.
3.
Deposits with high density, low susceptibility, and
high Q are rich in monoclinic pyrrhotite, e.g.,
Cormorant, Canteen.
4.
Horizons with high density, low susceptibility, and
moderate Q may be indicative of metamorphosed
hematite, e.g., Monakoff West BIF.
5.
Deposits with high density low susceptibility, and
low Q may contain hexagonal pyrrhotite and /or
sphalerite, galena, pyrite and hematite, i.e., are
associated with a relative absence of magnetite.
Many specimens contain mixtures of different Fe-oxide
and sulfide phases. These different assemblages may be
related to redox gradients and /or overprinting by different
metasomatic events. However, the assemblages usually
fall into one of three basic categories, with some overlap.
1.
Oxidized assemblages contain hematite, and do not
contain pyrrhotite, but typically are associated with
pyrite ± chalcopyrite, and typically contain minimal
amounts of magnetite.
2.
Intermediate assemblages are typically magnetiterich, and may contain pyrrhotite and /or pyrite, but
typically only in small proportions. Many magnetiterich lithologies are often also apatite-rich.
3.
Reduced assemblages are typically pyrrhotite
dominant, contain no hematite, but often do contain
magnetite. In these cases, hexagonal (non-magnetic)
pyrrhotite is typically associated with galena and
sphalerite. Monoclinic (magnetic) pyrrhotite is more
typically associated with Cu prospects, however this
is only a qualitative observation.
The relationships between oxidation state and magnetic
expression are illustrated in Fig 5.
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Figure 5: Synthetic models of the El Dorado deposit, Tennant Creek (based on Austin and Foss, 2014), which contain different
proportions of Fe-oxide and Fe-sulfide minerals, based on their Redox state. All the mineral assemblages have comparable density,
but the different redox states have very different geophysical expressions.
SYN-DEPOSITIONAL SEDEX/BHT & DISTAL BIF
MINERALIZATION AT ca 1650 MA
The early BHT/ Sedex-style Pb-Zn-Ag mineralization and
the associated barren distal BIFs have a similar
geophysical expression. In both cases the mineralization
is stratiform, and therefore associated with bedding
parallel elongate magnetic anomalism. In general, BHT/
Sedex and the corresponding distal BIFs contain
significant magnetite. However, there appears to be a
redox control on the species of Fe-oxide, whereby
mineralization closer to the source is more reduced (e.g.,
magnetite ± pyrrhotite) whereas the more distal deposits
are more oxidized (e.g., hematite ± magnetite). In general
this is coupled with corresponding zonation of metals,
with greater economic minerals closer to the core of the
system, and greater sediment content distal to the vent.
In terms of geophysical response, the distal parts of these
systems generally have fairly consistent, stratiform,
laterally extensive, but low amplitude magnetic
anomalies, associated with relatively modest magnetic
susceptibility (e.g., the anomalies that sit close to the
contact between the Mt Norna Quartzite and the Toole
Creek Volcanics in the Pumpkin Gully Syncline; near
Monakoff; Austin et al., 2016b). Measured samples from
this horizon show significant variability in their
petrophysical properties, related to both oxidation state
and total Fe- oxide content. The distal BIF near Monakoff
is hematite-rich along strike to the west where it has a
mean susceptibility of ~0.03 SI, a Q ratio of ~13, and a
density of ~3.2 g/cc. Conversely, along strike to the east,
the BIF contains both hematite and magnetite, with a
mean susceptibility of ~0.3 SI, a Q ratio of 1.5, and a
density of 4.2 g/cc (see Figs 6, 14, 15). In this case the
eastern BIF has a total magnetization approximately twice
that of the hematite BIF. However, the total remanence is
only 33% greater in the east BIF, and the relative
remanence (the Q ratio) of the hematite-rich BIF is much
higher.
Parts of a Sedex system that are proximal to the vent
commonly sit along-strike of these subtle stratiform
anomalies, but they coincide with much higher amplitude
anomalies, which are a result of thicker magnetized
horizons and/ or greater magnetite content, e.g., Osborne
and Maronan (Gazley et al., 2016b; Austin et al. 2016c).
The vent-proximal mineralization at Maronan for example
has a mean susceptibility of ~0.6 SI and a Q ratio of ~5.3
and a density of ~3.7 g/cc. (Fig 7) In this case the
proximal BHT/Sedex-style mineralization has a total
magnetization approximately 25 times that of the hematite
BIF, and 7 times greater than the magnetite-hematite BIF
at Monakoff. However the main mineralized zone which
sits in-between the magnetic anomalies has a completely
different pyrrhotite-rich mineralogy (discussed shortly).
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The deposits at both the Pb-Zn and Cu-Au end-members
of this system are overprinted in a number of different
ways during subsequent metamorphism, deformation and
metasomatism, and these processes can potentially lead to
enhancement of the magnetic signal (addition of
magnetite/pyrrhotite), destruction/depletion of the
magnetic signal (i.e., magnetite destruction; including
removal of Fe, oxidation of magnetite to hematite,
reduction of magnetite to pyrrhotite), or modification of
the magnetic signal from a largely induced magnetization
(i.e., held in multi-domain (MD) magnetite) to a largely
remanent magnetic signal (i.e., held in monoclinic
pyrrhotite). Several of these scenarios are discussed
further in following sections.
form a magnetite-dolomite-chalcopyrite reaction skarn
during peak metamorphism. In other cases, e.g., Maronan,
the coarse-grained calcite-pyrrhotite-galena assemblage
sits in the core of the BHT/ Sedex system, and is
potentially just a recrystallized version of the protolith. In
terms of their geophysical response, the two types of
peak-metamorphic assemblages discussed below have a
very different response. However, in each case the
properties may not differ substantially from the
protolith(s).
Pyrrhotite ± Pb-Zn-Cu melts (skarn)
Pyrrhotite ± Pb-Zn-Cu “skarn” occurs at Artemis, Altia
and Maronan. At Maronan the “skarn” is located in the
central part of the mineralized zone, with magnetitedominated lithologies occurring to the north and south
along strike. This is consistent with the Sedex
mineralization model whereby there is a zonation within
the deposit, from reduced assemblages (e.g., pyrrhotite –
rich; Fig 8, below) in the core, to moderately oxidized
assemblages proximally (e.g., magnetite; Fig 8, top) and
more oxidized distally (e.g., hematite).
Figure 6: Comparison of the geophysical signatures and
petrophysical properties associated with the Monakoff
deposit and Maronan prospect, both of which are
interpreted to have initially formed as BHT/ Sedex systems.
PEAK METAMORPHIC PYRRHOTITE “SKARNS” AT
ca 1590 - 1570 Ma
Calcite-rich assemblages that are totally recrystallized, or
newly precipitated are often referred to as “skarns” across
the Cloncurry district (as discussed in Austin et al.,
2016a). If these are skarns, they are not typical skarns,
formed via interaction of magmatic fluids with carbonate
rocks. However, they may be reaction skarns, i.e., formed
via chemical interaction of adjacent protoliths. For
example adjacent BIF and calcsilicate rocks at Starra may
Figure 7: Comparison of the geophysical signature and
petrophysical properties associated Maronan prospect. Note
the relatively subtle magnetic signature of the core of the
anomaly, labelled “Po skarn”.
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Whilst these assemblages are sometimes referred to as
“skarns”, it is not necessary that the mineralogy is
changed substantially in the core of the system during the
formation, e.g., a calcite–pyrrhotite mineralogy may be
common to both. However, we infer that the pyrrhotite
within the system can change its crystal structure, from
monoclinic to hexagonal pyrrhotite as a result of
recrystallization during metamorphism. At Artemis (Fig
9) pyrrhotite appears to be almost entirely hexagonal
(non-magnetic) within the mineralized core of the system.
More distal to the mineralized zone, there are magnetic
rocks (inferred from modelling), particularly to the north
and above, which sit at the fault intersection. This
suggests redox zonation within the mineral system,
possibly related to near surface oxidation.
Figure 8: TIMA (Tescan integrated mineral analyzer)
imagery of: Top. Banded garnet-magnetite-calcite sample
from the distal ‘Sedex” part of Maronan, and; Below.
Relatively reduced pyrrhotite-galena-calcite rock with an
equigranular (recrystallized) texture, from the core of the
system. All TIMA scans are approx. 22mm in diameter.
By contrast, at Maronan, monoclinic (magnetic) and
hexagonal (non-magnetic) pyrrhotite are both clearly
present within the mineralized Po-“skarn” assemblages.
Where there is monoclinic (magnetic) pyrrhotite,
remanence is very strong and the remanence directions
are typically sub-vertical and upward oriented. This
suggests that they should form relatively symmetrical
positive magnetic anomalies for upright bodies (i.e.,
bodies with no significant dip). Similar calcite-pyrrhotite
mineralogies are also encountered at Canteen, and these
also have sub-vertical upward oriented magnetizations,
but are not associated with mineralization.
Figure 9: Density vs magnetic susceptibility plot for Artemis
(lower) indicates that the sulfides present within the ore zone
are generally non-magnetic. The ore zone at Artemis (in
turquoise; above) is primarily associated with hexagonal
(non-magnetic) pyrrhotite. However, more distal to the
mineralized zone, there is a zone of high susceptibility
(deduced by magnetic modelling) at the fault intersection
(purple).
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Magnetite ± Cu skarn
Magnetite ± Cu “skarn” is observed at Starra, and may
also be present at Osborne. At Osborne, we do not have
enough data to differentiate the peak-metamorphic
mineralization from the earlier sedex/BIF mineralization.
However, at Starra, the dolomite-magnetite±chalcopyrite
assemblages are distinct from other “ironstones” and
“BIFs” regionally, which tend to be associated with
quartz, apatite and sometimes feldspars (e.g., at Osborne)
rather than carbonate.
Although the radiometric signatures of different alteration
types are not well understood, there is a correlation
between U-rich radiometric signatures (e.g., Fig 12) and
the highly magnetic zone at Canteen as indicated by the
shading in Fig 12. The U-rich signature at Canteen is
correlated with magnetite-apatite alteration within that
system, and potentially at other deposits and prospects
within the district. However, U-rich radiometric
signatures are also correlated with sodic alteration at
redox boundaries (e.g., Cormorant, Fig 16). We therefore
infer that the two alteration types are related.
Dolomite-magnetite±chalcopyrite assemblages at Starra
are strongly Magnetized, with mean susceptibility of
~1.63 SI and a Q ratio of ~0.8 and a density of ~4.06 g/cc.
BIF/sedex style mineralization at Osborne has a mean
susceptibility of ~1.77 SI and a Q ratio of 0.9 and a
density of ~4.29 g/cc. The measured Q is likely to be
approximately 3times greater than the in situ Q due to
drilling induced magnetization. Hence the real Q is
probably closer to 0.3, which means that remanence is not
a significant contribution to the magnetic field expression
of the deposit. The geophysical properties are almost
identical to those at Osborne, which might be due to their
common genesis as skarn mineralization derived from a
BIF/sedex protolith. The main contrast is that the “skarn”
at Starra has a lower density, owing to the greater
proportions of dolomite compared to silicate minerals
(which are denser), and distinct lack of apatite.
POST-METAMORPHIC, PRO-GRADE
HYDROTHERMAL SYSTEMS (ca1530 - 1515 Ma)
The main types of alteration which appear to be
synchronous with D4 (ca 1530-1525 Ma) are magnetiteapatite alteration, observed at Ernest Henry at Canteen
and at E1 and the sodic (±calcic) alteration observed in
numerous deposits across the district.
Magnetite-Apatite-dominated systems
Magnetite-apatite assemblages, e.g., Fig 10 (top), are
common in IOCG-style deposits, particularly the Kirunatype sub-class (e.g., Nystroem and Henriquez, 1994).
Such alteration, is associated with extreme susceptibility,
large magnetic anomalies (e.g., Fig 11) but relatively
minor, (viscous) remanence, which together are typical of
multidomain magnetite. Sample EHM005 from Ernest
Henry had a magnetic susceptibility of ~1.8 SI and a Q
ratio of ~0.5 (NB: remanence is likely amplified by
~300% due to drilling induced magnetism (DIM), so the
in situ Q would be 0.1-0.2). Similar samples drilled from
the pit at E1 (e.g., samples E1-K and E1-M) also have
mean magnetic susceptibilities ranging from 1.5 – 2.0 SI,
and Koenigsberger ratios of 0.1 – 0.2. (NB: because these
were surface sampled, they are not affected by DIM).
Figure 10: Examples of magnetite-apatite assemblages
(sample CAN034) and sodic-calcic alteration (albiteactinolite-magnetite; sample CAN016) observed in TIMA
data from Canteen prospect.
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Figure 11: 3-D perspective view of the modelled bodies, with
a rendered version of the TMI field above. NB. The TMI
anomaly is zoned, with the highest intensity toward the
center of the magnetized zone.
Sodic Alteration (albite-magnetite-titanite)
Sodic alteration tends to be mainly manifested by albitedominated lithologies with variable amounts of magnetite
as the main magnetic phase. It can be pervasive,
particularly within calc-silicate dominated units, e.g., the
Doherty and Corella formations. It has similar
petrophysical properties to the magnetite-apatite alteration
(already discussed), sits along the same susceptibility –
density trend (e.g., Fig 17) and is possibly genetically
related. However, sodic alteration is less magnetic, due to
its lower magnetite content (as shown in Fig 10, lower).
The proportion of magnetite (and titanite) within sodic
altered lithologies is variable, but significantly less than
for magnetite-apatite altered rocks. For example, the
mean susceptibility of sodic altered samples from Ernest
Henry is 0.76 SI, which is high, but approximately 50%
of the susceptibility of magnetite-apatite altered samples
from Ernest Henry and E1. The sodic alteration also has
relatively minor remanence, with Q ratios of ~0.1 to 0.2
(after a DIM correction is applied). Therefore, the
magnetic expression of sodic alteration is variable, but is
commonly associated with moderate to high susceptibility
and low remanence, which is typical of moderate amounts
of coarse multi-domain magnetite.
Figure 12: Ternary radiometric image for the Canteen and
Artemis area in which: red = K; green = Th; and blue = U.
RTP magnetic overlain as shading. (Austin et al., 2016e).
Potassic Alteration
Potassic alteration is present throughout the Cloncurry
district, but tends to be less pervasive than sodic
alteration. Potassic alteration can take two main forms;
replacement of sodic and/or calcic feldspars by Kfeldspar, e.g. Fig 13 (top), and/or replacement of
ferromagnesian minerals by biotite, e.g. Fig 13 (lower).
Potassic alteration is typically considered to be associated
with hematite (based on the reddish color of the Kfeldspar). However, magnetite is the dominant magnetic
phase in K-feldspar altered mineralized lithologies,
assessed in this study.
Potassic alteration is associated with mineralization at a
number of deposits and is also present within nonmineralized zones of other deposits and prospects, e.g.,
Maronan (Austin et al., 2016c) where it is localized by a
number of cross-cutting faults in the core of the system. It
can be both magnetite and pyrrhotite-destructive, e.g., via
oxidation (formation of hematite ± pyrite) and/or uptake
of Fe into biotite. However, it is usually coincident with at
least some magnetite within mineralized systems. It is
associated with molybdenum at Merlin but predates
mineralization (Patterson et al., 2016). At Osborne, Cu
mineralization is associated with retrograde biotite
(Gazley et al., 2016b) and at Ernest Henry, K-feldspar
alteration is associated with relatively moderate levels of
chalcopyrite mineralization (Austin et al., 2016d).
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Magnetite-barite-fluorite-calcite alteration
Magnetite-fluorite-barite-calcite alteration assemblages
are present within Monakoff, E1 and to a lesser extent
Ernest Henry (Austin et al., 2016b, e, d). Monakoff
provides the best example, in which there is clear
zonation between the host BIF discussed previously and
the magnetite-fluorite-barite-calcite-chalcopyrite zone
that overprints it. The main high susceptibility lithologies,
and by association, the sources of the magnetic anomaly
at Monakoff, include a magnetite BIF and the magnetitebarite-fluorite-rich mineralized zone (Fig 14, upper). Both
have mean susceptibilities of ~0.5 SI and both units share
a linear susceptibility/density gradient, as shown in Figure
14 (lower).
Figure 13: Some typical potassic alteration from Ernest
Henry. Top. Pervasively altered mineralogy comprised of Kfeldspar, magnetite/hematite, and pyrite in sample EHM025.
Below. Biotite-K-feldspar partially overprinting sodic
alteration (albite-magnetite) in sample EHM004.
Assessment of the petrophysical properties of rocks from
Ernest Henry suggest that where sodic alteration is
overprinted by potassic (K-feldspar) alteration, the
magnetic susceptibility is reduced by 30 – 50% compared
to the sodic altered rocks (e.g., Fig 21). Some of this
reduction could be attributed to oxidation of magnetite to
hematite. However, the potassic altered rocks do
generally still sit on the same susceptibility/density trend
as magnetite-rich rocks, suggesting they do not contain
significant hematite. Some iron may have formed pyrite,
but again the formation of pyrite cannot be significant for
the aforementioned reason. It seems most likely that the
majority of Fe produced from the destruction of magnetite
is going into biotite and potentially chlorite (chamosite)
during potassic alteration.
Figure 14: Plot of magnetic susceptibility vs density for
samples from the Monakoff mine. The different trends
correspond to different mineralization styles.
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The magnetite BIF tends to be high susceptibility with
lower density, and this is due to magnetite being the main
magnetic phase and high SG mineral (SGMt = 5.15 g/cc).
Conversely, the mineralized horizon tends to have much
higher, which is in part due to iron being taken up in Fesulfides (SGPy = 5.02 g/cc), but mostly due to the high
volume of barite (SGBa = 4.5 g/cc) present within the
mineralized assemblage. Within the mineralized zone
there are a range of susceptibilities which occur within a
broadly comparable density range, and hence there is
significant variability in the magnetite content. The
magnetite-fluorite-barite-calcite-chalcopyrite
overprint
has very similar magnetic properties to the magnetite-BIF
protolith, but is distinguished by its strong NE-oriented
fabric, which crosscuts the E-W layering of the host BIF.
Austin et al. (2016b) demonstrated that the magnetitefluorite-barite-calcite-chalcopyrite overprinted the BIF,
and that the redox zonation from relatively oxidized to
more reduced (intermediate) assemblages were
associated, to some extent, with overprinting. However,
as discussed previously in relation to Maronan, it is quite
possible that the mineralized zone formed within a Sedex
horizon and was already barite- and magnetite-rich. So it
is possible that this alteration style is really a pre-existing
mineralized zone that was recrystallized via
metasomatism at ca 1515 Ma.
Figure 15: Transition from highly oxidized (Left) to relatively reduced assemblages (middle) within an early Sedex system at
Monakoff. The system was overprinted by a magmatic fluid leading to a Cu-, Barite and Fluorite-rich recrystallized ore.
PYRRHOTITE ASSEMBLAGES
Calcite in association with coarse pyrrhotite is present at a
number of other prospects and deposits (e.g., Canteen,
Maronan, and Artemis; Austin et al., 2016 b, e, f).
However, it is not clear that they are co-genetic. In all
cases, the textures are relatively undeformed. However,
some assemblages appear to have formed in zones of
dilation, e.g., veins and breccia matrix, others appear to
have formed by recrystallization. The former are
consistent with late brittle deformation, whereas the latter
are more consistent with high temperature metamorphic
conditions, e.g., Artemis, Maronan, discussed previously.
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Canteen
At Canteen the alteration paragenesis is fairly complex,
due to the variable origin of the protoliths, with some
clearly mafic igneous in origin and others sedimentary
(Austin et al., 2016e), leading to commensurate variations
in altered mineralogy. The early phase of alteration
consists of albite and/or andesine, in association with
actinolite, (fine) magnetite, apatite, titanite and pyrite.
This is variably overprinted by patchy potassic alteration,
which is either K-feldspar-, or biotite-rich, depending on
the protolith. The calcite-pyrrhotite phase is associated
with both quartz and dolomite at Canteen and is observed
as veining and breccia infill (Fig 16).
Figure 16: TIMA imagery from Canteen which show the
calcite-pyrrhotite assemblage overprinting albitized rocks at
Canteen, as a breccia matrix.
However, the morphology of the veining suggests that the
veins are actually a proto-breccia texture. Both clearly
overprint several earlier alteration phases, most notably:
pervasive sodic (albite) alteration; a magnetite-pyriteapatite assemblage (which appears to be common to
several other deposits); and potassic alteration. Although
the mineralogy is similar to that observed at Artemis and
Maronan (Austin et al., 2016 f, b), the textural context of
the calcite-pyrrhotite phase suggests that it is a relatively
late alteration phase.
cases albite-pyrrhotite-chalcopyrite and titanite are
intergrown (Fig 17), but in other samples the titanite,
chalcopyrite and pyrrhotite occupy cracks and grain
boundaries in massive albite. Mineralization is associated
with pyrrhotite-dominant vein phases (Fig 18), and it
appears that the fluid responsible for alteration is reduced
since it precipitated large volumes of pyrrhotite but small
amounts of chalcopyrite. Elevated Cu occurs either side
of an apparent redox boundary at approximately 450 m in
the studied drill-hole (Fig 19). Here we see a transition
down hole from magnetite dominant (400–425 m) into
magnetite-pyrrhotite (425–450 m) into pyrrhotite
dominant (450–500 m), and then titanite dominant
lithologies (500–550 m).
Figure 17: Perspective view of the Cormorant magnetic
model, in relation to a 3-D rendering of the magnetic field.
Viewed toward the NE. The graticule spacing is 1km.
Cormorant
At Cormorant textural evidence suggests that the
mineralization is associated with pyrrhotite in a series of
microbreccias and veins (Fig 17, 18; Austin et al., 2016c).
Cormorant is distinct from most of the other pyrrhotiterich deposits/prospects (e.g., Canteen and Maronan) in
that the pyrrhotite is not associated with calcite. In many
Figure 18: Two Albite-pyrrhotite-chalcopyrite-titanite
assemblages from Cormorant (COR015).
Exploration '17 Petrophysics Workshop: 08-12
POST METAMORPHIC RETROGRADE ALTERATION
AT ca 1510-1500 Ma
Quartz-calcite-chlorite-hematite alteration
Late quartz-calcite-chlorite-pyrite-hematite alteration is
present in a number of deposits and prospects in the
Cloncurry district, and is associated with copper and/ or
molybdenum mineralization at Ernest Henry, Canteen,
Kalman and Merlin (Austin et al., 2016 d, f; Gazley et al.,
2016 b; Patterson et al., 2016). This alteration is the most
oxidized style observed in the Cloncurry district, as
indicated by the presence of pyrite and hematite, rather
than pyrrhotite and magnetite (Fig 20). This is further
demonstrated by Fig 21, in which mineralized samples
(with such alteration) sit below the magnetite trend on the
density/susceptibility plot. This suggests that the iron in
magnetite is not only being converted to ferromagnesian
minerals (e.g., chamosite), but also chalcopyrite, pyrite
and/or hematite during the late alteration history.
Therefore, these samples are oxidized relative to the
system as a whole, which is magnetite dominant.
Figure 19: Comparison of assay data with down-hole
susceptibility, measured susceptibility and remanence.
Remanence in Monoclinic Pyrrhotite
Monoclinic pyrrhotite in several of these deposits (e.g.,
Canteen, Cormorant, Maronan, Mt Colin) is associated
with high Koenigsberger ratios. However, the associated
remanence directions are generally very similar, with subvertical upward oriented magnetizations. Such
magnetizations are consistent with metamorphic resetting
associated with intrusion of voluminous mafic magmas at
ca 1114 Ma, (Austin et al. 2016 b, f, g, h). The
magnetizations are sub-vertical and sit within a N-S plane
in a similar orientation to the inducing Field. Hence,
although pyrrhotite-dominated mineralization has a
characteristic remanent magnetization, it is not different
enough to the inducing field to allow remote mapping,
and therefore is not effective as an exploration tool.
Figure 20: Typical Quartz-hematite-chlorite-chalcopyrite
assemblage from the Ernest Henry ore zone.
Ernest Henry
A comparison of the different alteration types at Ernest
Henry was compiled based on key mineralogies within
the deposit (Table 1). The results reveal that sodic
alteration has no association with Cu-mineralization, and
that potassic alteration has only a minor spatial
association with mineralization as mentioned previously.
However, quartz-calcite-chlorite alteration has a major
spatial association with Cu mineralization (Table 1). So
for Ernest Henry there is an inverse redox gradient. The
most oxidized parts of the system contain the majority of
Cu mineralization, whereas the more reduced parts
Exploration '17 Petrophysics Workshop: 08-13
generally do not. Geophysically, the mineralized zone is
less magnetic than the enveloping potassic alteration,
which in turn is significantly less magnetic than the
enveloping sodic alteration (Fig 22). This complexity of
overlapping zones and cumulative magnetization
contrasts is why the ore-pipe at Ernest Henry is so
difficult to model (e.g., Austin et al. 2016d).
Figure 22: 3-D perspective of the Ernest Henry magnetic
model generated in ModelVisionProTM. Susceptibility is
indicated by the color of the bodies. The Marshall, and
Footwall shears are the most magnetic the ore zone is the
only weakly magnetic. The inverse magnetic zonation is due
to oxidation in the mineralized zone. View is toward the NE.
Figure 21: Comparison of the petrophysical properties
associated with different alteration types at Ernest Henry.
Table 1: Comparison of key minerals for different alteration types at Ernest Henry.
Alteration type
Chalcopyrit e Chamosit e
Pyrit e
Albit e
M icro cline Quart z
And Alb+Pot assic+Calcic
0.00
0.82
0.27
7.41
23.54
M agnet it e Apat it e*
3.80
1.84
1.09
1.09
1.09
1.09
Po t assic (Bt )
0.00
0.29
0.07
58.79
2.80
8.48
Po t assic (Kf )
0.25
2.27
3.38
2.52
45.91
8.22
Po t assic+ Cal Qt z Py
2.00
1.19
4.03
0.96
31.62
15.88
Qt z Cal Chl Py±Cpp±Hem
2.88
10.26
13.96
0.16
2.70
33.56
Sodic (Ab M t Ti)
0.00
0.31
0.31
54.96
3.73
2.38
Sodic + Po t assic (Bt )
0.05
1.12
0.27
20.11
20.00
10.03
Canteen
At Canteen, mineralization is also mainly associated with
late calcite-quartz-pyrite-dolomite-chlorite alteration
assemblages (Table 2). However, the overprinting
relationships are less consistent at Canteen than Ernest
Henry, and copper is also associated with magnetiteapatite assemblages and Po-Calcite veining (e.g., Austin
10.56
et al., 2016f). Mineralization associated with magnetiteapatite has high density and high magnetic susceptibility
and NRM, but a relatively low Q, all due to the presence
of coarse multi-domain magnetite. The pyrrhotite-breccias
have high density, high NRM and Q, but only moderate
susceptibility (e.g., 0.1 SI) which is comparable with most
other lithologies at Canteen. Mineralization is mainly
Exploration '17 Petrophysics Workshop: 08-14
present in fractures and veins, with pyrrhotite-calcite. The
mineralized veins have low density and magnetic
susceptibility values, but a moderate NRM and high Q,
which is consistent with the presence of monoclinic
pyrrhotite. By contrast, where mineralization is associated
with the calcite-quartz-pyrite overprint, petrophysical
values are low across the board, other than for elevated
density where pyrite content is high. This is due to a
number of factors, but mainly to the loss of magnetite
and/or pyrrhotite, and precipitation of pyrite ± chlorite.
The copper mineralization appears to be spatially
associated with a number of faults that cross-cut the main
NNW-trending magnetite-rich zone at Canteen (Fig 11),
which causes zonation whereby a relatively reduced (e.g.,
magnetite-dominated) assemblage is overprinted by a
more oxidized (e.g., pyrite-dominant) assemblage. This
results in an inverse zonation between sulfur+copper (i.e.,
a proxy for copper sulfide) with susceptibility (i.e., a
proxy for magnetite) at the deposit scale, as illustrated in
Fig 23.
Therefore, at Canteen the petrophysical zonation is related
to redox, insomuch as the earlier, more reduced
assemblages have high magnetic susceptibility, related to
their magnetite content. Conversely, the more oxidized,
more mineralized assemblages which overprint the
magnetite-rich zone are pyrite-rich, and whilst they have
comparable density, they have significantly reduced
magnetic susceptibility due to magnetite destruction
coupled with pyrite precipitation.
Table 2: Comparison of petrophysical properties and associated mineral assemblages for various alteration types at Canteen. The
numbers represent mean proportions of the mineral as a percentage of the total volume, based on estimates from TIMA analyses.
Alt erat io n
Bio t it e Andesine ovp Sodic
2.9 0.11
14
7
3
23
0
19
2
0
1
0
2
0
0
0
6
1
0.0
Bio t it e Andesine ovp M t Ap Py
3.3 0.78 237
8
2
6
2
6
2
2
0
0
25
0
5
0
7
1
0.2
0.4
Ca Py Qt z M t
3.1 0.12
12
2
2
2
1
5
13
2
0
0
6
0
15
0
25
1
Ca Po Py Qt z Chl ovp Bio t And
2.9 0.12
26
7
19
14
0
17
10
1
0
5
1
0
1
1
10
0
0.0
Ca Py Qt z ovp Sodic
2.9 0.10
10
3
28
0
0
3
14
1
1
7
3
0
10
0
13
1
0.0
Cal Py Chl ovp Pot assic ovp Sodic
2.8 0.15
16
3
24
1
0
10
8
0
6
0
4
0
2
0
5
2
0.7
Po Calcit e Bx
3.2 0.08 196
64
24
0
0
3
15
0
1
8
0
0
0
27
7
0
0.0
Po Ca Do l ovp Sodic
2.8 0.02
47
52
64
0
0
0
2
0
1
7
0
0
0
1
11
0
1.3
Pot assic ovp Sodic
2.8 0.08
25
12
28
0
0
8
6
0
1
0
1
0
0
6
9
1
0.0
Figure 23: left. Interpolated volumes calculated for S (yellow) and Cu (green), using Leapfrog TM superimposed on the 3-D magnetic
model generated using ModelvisionTM . Note the spatial coincidence of both Cu and S with the faults; right. Interpolated volumes
calculated for magnetic susceptibility (pink) using Leapfrog TM superimposed on the 3-D magnetic model. Note the inverse spatial
coincidence of susceptibility with the faults. In this case magnetic susceptibility is used as a proxy for magnetite content in the
prospect. Model is viewed looking WSW, along the strike of the interpreted faults.
Exploration '17 Petrophysics Workshop: 08-15
CONCLUSIONS
REFERENCES
This is the first time any study has brought together so
much petrophysical data from so many different styles of
mineralization across a mineral district, and the results
provide important constraints for future exploration for
various styles of mineralization undercover. The deposits
studied have a wide variety of petrophysical properties,
primarily dictated by the relative contents of magnetic
minerals (e.g., magnetite, monoclinic pyrrhotite, and
hematite) and other non-magnetic minerals (e.g.,
hexagonal pyrrhotite, pyrite, galena, sphalerite, barite).
Any combination of these minerals can be associated with
high densities. High magnetic susceptibilities (and hence
high amplitude magnetic anomalies) are invariably
associated with coarse magnetite, whereas monoclinic
pyrrhotite is associated with moderate magnetic
susceptibility, high remanence (and potentially unusual
magnetic anomalies). Hematite is only weakly magnetic.
Austin, J.R. and Foss, C.A., 2014. Understanding iron oxide
copper-gold (IOCG) magnetic targets from the inside out: case
studies from northern Australia. Annual Geoscience Exploration
Seminar (AGES), 18-19 March 2014, Alice Springs.
Many deposits and prospects contain mixtures of different
Fe-oxide and sulfide phases, which can be related to
redox and/or overprinting.
1.
2.
3.
Oxidized assemblages contain hematite, no
pyrrhotite, but typically pyrite and variable
magnetite. In IOCG systems hematite is associated
with retrograde (magnetite-destructive) alteration,
and Cu, Au and/or Mo- mineralization. In sedex
systems hematite is formed distal to the source.
Intermediate assemblages are magnetite-rich, and
often contain pyrrhotite and/or pyrite. In IOCGs
systems they are often mineralized, but always
associated with large positive magnetic anomalies,
due to multidomain magnetite. In Sedex systems
magnetite forms proximal to sources. Magnetite is
advantageous for further mineralization, because it
can be further oxidized or reduced.
Reduced assemblages are typically pyrrhotite
dominant, contain no hematite, but often do contain
magnetite. Hexagonal (non-magnetic) pyrrhotite is
typically associated with galena and sphalerite in
Sedex systems, whereas magnetic pyrrhotite is more
typically associated with chalcopyrite in ISCGs.
The study has shown that where pyrrhotite is present it is
usually subordinate to magnetite, and the magnetic field
responses from the different components interfere
(particularly at the resolution of the data) and cannot be
easily separated. In cases where pyrrhotite is the dominant
phase, it is commonly hexagonal (non-magnetic)
pyrrhotite (e.g., Artemis, Maronan). Where monoclinic
pyrrhotite is present the remanence is typically oriented
sub-vertical up (e.g., Canteen, Cormorant, Maronan, Mt
Colin), and as such, is difficult to isolate from the steep,
upward north oriented inducing magnetic field.
Austin, J.R., Gazley, M.F., Walshe, J.L., and Patterson, B.O.,
2016b. Uncover Cloncurry – Summary: Integrated structural,
metasomatic and metallogenic history of the Cloncurry District.
CSIRO, Australia, EP166451, pp. 45.
Austin, J.R., Gazley, M.F., Walshe, J.L., Godel, B., leGras, M.,
and Paterson, B.O., 2016c. Uncover Cloncurry – The Monakoff
Cu-Au-U deposit: Integrated Petrophysical and Geochemical
analyses. CSIRO, Australia, EP165042, pp. 50.
Austin, J.R., Gazley, M.F., Godel, B., Hawkins, S., and leGras,
M., 2016b. Uncover Cloncurry – The Maronan Pb-Ag deposit:
Integrated Petrophysical and Geochemical analyses. CSIRO,
Australia, EP165052, pp. 50.
Austin, J.R., Walshe, J.L., Gazley, M.F., Ibrahimi, T., Patterson,
B.O., and leGras, M., 2016d. Uncover Cloncurry – The Ernest
Henry Cu-Au deposit: Integrated Petrophysical and
Geochemical analyses. CSIRO, Australia, EP165053, pp. 56.
Austin J.R., Gazley M.F., Ibrahimi T., Walshe J.L., and
Patterson B.O., 2016e. Uncover Cloncurry – The E1 Cu-Au
deposit: Integrated Petrophysical and geochemical analyses.
CSIRO, Australia, EP165071, pp. 50.
Austin, J.R., Walshe, J.L., Gazley, M.F., Sisson, M., leGras, M.,
and Godel, B., 2016f. Uncover Cloncurry – The Canteen Cu-Au
prospect: Integrated Petrophysical and geochemical analyses.
CSIRO, Australia, EP165041, pp. 52.
Austin J.R., Gazley M.F., Patterson B.O., leGras M., and
Walshe J.L., 2016g. Uncover Cloncurry – The Cormorant CuAu Prospect: Integrated Petrophysical and Geochemical
analyses. CSIRO, Australia, pp. 44
Austin, J.R., Hawkins, S., Gazley, M.F., Patterson, B., leGras,
M., Walshe. J.L., 2016h. The Mount Colin Au-Cu deposit:
Integrated Petrophysical and geochemical analyses. CSIRO,
Australia, pp. 32.
Gazley, M.F., Patterson, B.O., Austin, J.R., and Walshe, J.L.,
2016b. Uncover Cloncurry – Osborne Cu-Au deposit: Integrated
Petrophysical and Geochemical analyses. CSIRO, Australia,
EP165511, pp. 25.
Gazley, M.F., Austin, J.R., Hawkins, S., Patterson, B.O., leGras,
M., Godel, B., and Walshe, J.L., 2016c. The Kalman Mo-ReCu-Au deposit: Integrated Petrophysical and Geochemical
analyses. CSIRO, Australia, EP165471, pp. 35.
Nystroem, J.O. and Henriquez, F., 1994. Magmatic features of
iron ores of the Kiruna type in Chile and Sweden; ore textures
and magnetite geochemistry. Economic Geology, 89(4), pp.820839.
Patterson, B.O., Gazley, M.F., Austin, J.R., and Walshe, J.L.,
2016. Uncover Cloncurry – The Merlin Mo-Re deposit:
Integrated Petrophysical and Geochemical analyses. CSIRO,
Australia, EP164990, pp. 30.
Exploration '17 Petrophysics Workshop: 08-16