Presentation from Exploration in the House 2017. New South Wales.

1. High heat flow, skarn formation and
mineralisation in the eastern Cobar
Basin: a magmatic driver for
mineralisation?
Joel Fitzherbert & Phillip Blevin

2. Tuesday 9 May 2017
Parliament House, Sydney
Cobar Type deposits – the enigma
Page 2
Cobar Basin Central NSW
– Siluro-Devonain volcanosedimentary basin
Littered with mineralisation
– MVT, SEDX/Irish type, epithermal, Intrusion
related, structural and Cobar Type Au-Cu-Ag-Zn-
Pb deposits.
Two main models for Cobar Type
– Syn-genetic exhalative and subhalative
mineralisation subsequently remobilised during
orogeny
• Probable VAMS, epithermal or magmatic
related
– Structurally controlled syn-deformational model
involving regional metamorphic derived fluids
that scavenged sulfur and metals from basin and
basement
Can metamorphism help?
– Look at burial metamorphic regime versus
Hydrothermal metamorphism/alteration
(mineralisation)

3. Tuesday 9 May 2017
Parliament House, Sydney
Burial metamorphism - CAI map
1) Virgin Hills
(0-100oC)
2) The Rookery
(100-150oC)
3) Manuka
(150-200oC)
5) Beloura Tank
(250-300oC)

4. Tuesday 9 May 2017
Parliament House, Sydney
Cobar Basin metamorphic map
• Main focus on
– Burial metamorphism
• Affects of deep burial under a sedimentary and/
or volcanic pile.
– Hydrothermal metamorphism
• Affects of interaction between cool basin rocks and
hot fluids.
• Devonian Burial metamorphism
– Dark Orange = Diagenetic Zone (<200°C)
– Pale Orange = Low Anchizone (200-250°C)
• Devonian Hydrothermal
metamorphism/alteration
– Dark Green = Epizone(300-400°C)
– Light Purple = Biotite zone (350-400°C)
– Dark Purple = Amphibolite (>400°C)
• Others
– Ordovician blues
– Contact/rift basin metamorphic
Pink, Pale brown, yellow, light green

5. Tuesday 9 May 2017
Parliament House, Sydney
Nymagee / Cobar outputs to date

6. Tuesday 9 May 2017
Parliament House, Sydney
Recent studies
Mineral deposits hosted in
the Lower Amphitheatre
Group. Package of finely
laminated turbiditic
siltstone and sandstone
and packages that contain
coarser grit stones.
Cobar

7. Tuesday 9 May 2017
Parliament House, Sydney
Hera ore body
Variable preservation of high-T
alteration. Southern garnet-rich skarn
(Main South-Main-North) moving
north to pyroxene-rich skarn (Far
west) to the northern most tremolite
rich skarn (North pod).
• Au-Ag-Zn-Pb-W mineralisation
occurs in steeply west dipping
sulfide vein breccia zones similar to
other Cobar type deposits.
• Au is associated with sulfide zones,
but not always within the sufides.
Evidence of remobilization within
foliation and late-stage veins.
• Ore body represents a single
continuous horizon that has been
repeated through late stage
thrusting.

8. Tuesday 9 May 2017
Parliament House, Sydney
High-T garnet rich skarn
Garnet-scheelite veining in high-T garnet skarn zone.
Main North lode
Garnet-scheelite-quartz breccia.
Far West lode.

9. Tuesday 9 May 2017
Parliament House, Sydney
High-T pyroxene ± garnet skarn (Far West)
Clast of High-T garnet-pyroxene-zoisite-
calcite-quartz skarn enveloped by galena and
sphalerite.
Clasts of pyroxene-zoisite ± garnet skarn
enveloped by Fe-rich sphalerite and galena.
Pelitic rock (top) is exclusive host to pyrrhotite
and chalcopyrite.

10. Tuesday 9 May 2017
Parliament House, Sydney
Carbonate clast and skarn (North Pod)
Bedded carbonate (dolomitic) clast
surrounded by intense sphalerite
replacement.
Tremolite-anorthite-zoisite skarn clast after
carbonate clast

11. Tuesday 9 May 2017
Parliament House, Sydney
Carbonate clast and skarn (Main South)
Hydrous retrograde alteration associated with massive sulfides. High-T skarn has been
completely retrogressed to a tremolite-rich assemblage during cooling and mineralisation.

12. Tuesday 9 May 2017
Parliament House, Sydney
Hera ore body
380 Ma
SHRIMP
(GA)
Tourmaline, free gold, titanite

13. Tuesday 9 May 2017
Parliament House, Sydney
Mineral chemistry
• Garnet associated with high-T skarn stage
carbonate-scheelite-pyroxene association is
grossular-rich. Strong zoning back to
spessartine. Host pelitic rock garnet alteration
spessartine-rich. Switch to lower XCO2 calcite
to zoisite-quartz through zonation profile.
• Garnet associated with hydrous retrograde
phase spessartine-rich.
• Pyroxene is all diopside with 2-3% MnO.
• Retrograde amphibole phase is Mg to Mn
actinolite to Mn-tremolite with 2-4% MnO
• Abundant near pure anorthite associated with
all carbonate poor alteration in all lodes.
• Abundant almost pure zoisite throughout the
deposit. Nymagee FeO lodes Fe-rich epidote.

14. Tuesday 9 May 2017
Parliament House, Sydney
Mineral chemistry
• Garnet associated with high-T skarn stage
carbonate-scheelite-pyroxene association is
grossular-rich. Strong zoning back to
spessartine. Host pelitic rock garnet alteration
spessartine-rich. Switch to lower XCO2 calcite
to zoisite-quartz through zonation profile.
• Garnet associated with hydrous retrograde
phase spessartine-rich.
• Pyroxene is all diopside with 2-3% MnO.
• Retrograde amphibole phase is Mg to Mn
actinolite to Mn-tremolite with 2-4% MnO
• Abundant near pure anorthite associated with
all carbonate poor alteration in all lodes.
• Abundant almost pure zoisite throughout the
deposit. Nymagee FeO lodes Fe-rich epidote.

15. Tuesday 9 May 2017
Parliament House, Sydney
Mineral chemistry
mineralising

16. Tuesday 9 May 2017
Parliament House, Sydney
Garnet chemistry. Talks like a skarn, walks like a skarn….
Ca-Mn garnet, Mg-Ca pyroxene, Mg-Mn amphibole and Mg-biotite.
All are consistent with W or Zn-skarn mineralogies.

17. Tuesday 9 May 2017
Parliament House, Sydney
Nymagee – “CSA of the south”
Nymagee garnet in pervasively chlorite
retrogressed high-T skarn.
• Mined for Cu, although abundant Zn-
Pb-Ag occur.
• Lesser-deformed equivalent of the
main Cobar field deposits.
• Also hosted in lower Amphitheater
Group. Stratigraphy and ore body dip
80 degrees west to vertical.
• Mineralisation over 750m strike length.
Eastern zone is Cu-rich and western
zone is Zn-Pb-rich.
• Strong stratigraphic/lithological control
on mineralisation.

18. Tuesday 9 May 2017
Parliament House, Sydney
Nymagee – “CSA of the south”
Blue-green ferrotschermakite, annite-epidote-
chlorite skarn associated with Fe-oxide Cu lode.

19. Tuesday 9 May 2017
Parliament House, Sydney
Happy Jacks
Laminated garnet-zoisite-quartz rich skarn. Garnet-rich quartz vein.

20. Tuesday 9 May 2017
Parliament House, Sydney
The regional connection
• Lateral continuity and stratigraphic level,
potentially basin-wide slump events and
exploration for skarn in the southern lower
Amphitheatre Group.
• Potentially laterally continuous large strike
length allochthonous package that is host to
moderate sized limestone clasts. Possibility
of limestone in the underlying shelf
sequences in the sequence to host skarn
systems
• Is there similar stuff elsewhere?
• Shume Formation in the western and
central basin.
• But need to think big picture – MVT system
scale across basin and further afield?

21. Tuesday 9 May 2017
Parliament House, Sydney
Norma Vale
Skarn development beneath an
allochthonous limestone block in the
Shume Formation.
Mallee Bull hosted within allochthonous
packages of the Shume Formation.
Skarn assemblages and
Cu mineralisation at the
contact between
granite and limestone

22. Tuesday 9 May 2017
Parliament House, Sydney
Looking to the north, is it so different?
• Same stratigraphic level.
• Early oxide phase also associated with Au-W-Sn-Bi (pre-deformation)
overprinted by sulfide (Cu-rich). CSA mine paired Pb-Zn and Cu-rich lodes, just
starting to uncover FeO rich lodes associated with Cu.
• Preliminary work suggests mostly colder system with Fe-rich stilpnomelane
during oxide phase. But some zones of coarse annite porphyroblasts - similar to
Nymagee Fe-oxide minerals.
Early-formed intensely
deformed qtz +/- mt veins Folded massive mt horizon with cpy breccia infill

23. Tuesday 9 May 2017
Parliament House, Sydney
Great Cobar
Annite chlorite alteration – Fe oxide-Cu
lode.
Magnetite - green stilpnomelane vein.

24. Tuesday 9 May 2017
Parliament House, Sydney
Tracing fluid and metal sources
(S-types)
(I-types)
δD vs δ18O - tremolite
water values (‰)
• Pb and S isotopes
• εNd - Sri for scheelite and garnet
• δ18OH2O - δDH2O on hydrous phases
to track fluid evolution history.
• δ18OWR in deposit haloes
• δ18Omineral-mineral geothermometry
• LAICPMS - TE chemistry of
scheelites
• Dating: apatite, monazite,
titanite and zircon by LAICPMS
and SHRIMP,
• Nd and Sr isochrons on gt-sch

25. Tuesday 9 May 2017
Parliament House, Sydney
Hera deposit – a magmatic connection?
Hera likely represents the
northern limb of mineral
fluids sourced from the
Hebe magnetic anomaly.
MAGNETICS
N
1km
HERA
HEBE
EAST
Grt-sch
Pyrrhotite is not magnetic (high T)

26. Tuesday 9 May 2017
Parliament House, Sydney
423 – 410 Ma I-type magmatic systems
Tollingo and
Fountaindale
Field of Cu, Zn and W skarns (Meinert)
Mineral Hill

27. Tuesday 9 May 2017
Parliament House, Sydney
423 – 410 Ma I-type magmatic systems
Actinolite - quartz veins cutting
granodiorite. Near pervasive K-feldspar
flooding and actinolite alteration of
granodiorite.
Near pervasive albite-chlorite-epidote-
titanite alt’n of granodiorite with later
carbonate flooding. Note pinking of albite
and K feldspar and bright green epidote.
Albitised plag strongly clouded with
clinozoisite. Biotites are chloritised with 2°
fine titanite and epidote. Prehnite
pseudomorphs chlorite. 1° phases replaced
by prehnite, quartz and clinozoisite.

28. Tuesday 9 May 2017
Parliament House, Sydney
Summary
• Cobar Cu-Au-Ag-Zn-Pb deposits preserve early high T hydrothermal
systems associated with mineralisation
• Deformation occurred along the retrograde path (during cooling) and is
associated with a number of subsequent mineralising episodes
• Deformation/retrogression very intense in the north masking the early
event
• At the very least the deposits require a proximal magmatic heat, fluid
and more than likely metal source – another exploration tool.
• Strong stratigraphic control (apart from obvious structural control), but
may be compositional/lithological rather than competency perse.
Closing the gap between magmatism, mineralisation, and deformation
through detailed petrogenetic studies on hydrothermal alteration coupled
with targeted dating and isotopic studies