Geology and mineralisation of the Endeavour 41 gold deposit, Cowal district, NSW, Australia

287 Pages·2012·14.32 MB·English

by  Wojtek Zukowski

Geology and Mineralisation of the
Endeavour 41 Gold Deposit, Cowal District,
NSW, Australia
by
Wojciech Zukowski
M.Sc.
Submitted in fulfi lment of the requirements for the degree of
Doctor of Philosophy
November, 2010
CODES, ARC Centre of Excellence in Ore Deposits at the University of Tasmania
Statement
This thesis contains no material which has been accepted for the award of any other
degree or diploma in any tertiary institution and, to the best of my knowledge and
belief, contains no copy or paraphrase material previously published or written by
another person, except where due reference is made in the text of the thesis.
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Abstract
Epithermal and porphyry styles of alteration and mineralisation occur at the Endeavour
41 (E41) gold deposit in the Cowal Igneous Complex, New South Wales, Australia.
E41 is one of three economically signifi cant gold centres (E46, E42 and E41) in the
Cowal district. These deposits formed within the Ordovician Macquarie island arc by
subduction-related processes, and are hosted by a subaqueous volcano-sedimentary
succession of interbedded sedimentary and resedimented volcaniclastic facies,
trachyandesite and porphyritic andesites. The volcanic facies architecture at E41 is
consistent with a distal submarine volcanic setting.
The host succession at E41 has been intruded by numerous sills, dykes and
stocks, which defi ne the E41 intrusive complex. The magmas evolved from mafi c
to more felsic and then back to mafi c compositions with time. There is evidence of
both mafi c and silicic magmatism of high-K to shoshonitic affi nity at the time of gold
mineralisation, consistent with an alkalic association for gold mineralisation.
The pre-mineralisation Muddy Lake diorite intruded the Cowal district at 461
± 5.2 Ma. The stratigraphy was then tilted prior to the emplacement of numerous
dykes and mineralised veins. A mafi c monzonite intrusion emplacement after tilting at
458.5 ± 5.2 Ma provides the upper age constraint on deformation. Magmatic activity
culminated in the emplacement of a series of post-mineralisation dioritic dykes around
450 to 447 Ma. Geochronological results have identifi ed two mineralising events in the
Cowal district: (a) calc-alkalic Cu-Au porphyry deposits formed in the southeastern
part of the district at around 463 Ma, based on Re-Os dating of molybdenite from E43,
and (b) epithermal deposits formed in the central western part of the district around
455 Ma (E41, E42 and E46).
The earliest fl uids that caused hydrothermal alteration at E41 were magmatic-
hydrothermal in origin. They produced potassic alteration (magnetite ± biotite) in clastic
units and high temperature propylitic alteration (actinolite – magnetite) in diorite. Rare
magnetite- and andradite-bearing veins formed during this early phase of magmatic-
hydrothermal activity. These early fl uids were relatively oxidised (hematite- and
andradite-stable), hot ~ >400º C (biotite- and actinolite-stable) and had near-neutral to
alkaline pH (feldspar-calcite stable).
The early high-temperature alteration assemblages and veins have been
overprinted by gold-mineralised domains associated with lower-temperature alteration
facies. Gold mineralisation at E41 formed during two veining events: (1) quartz –
pyrite ± calcite ± adularia veins (stage 3); and (2) carbonate-base metal sulphide
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veins that contains calcite, ankerite, quartz, pyrite, sphalerite, galena, chalcopyrite,
Ag-tellurides, arsenopyrite, hematite, apatite, illite ± muscovite and chlorite (stage 4).
Gold occurs principally in the crystal lattice of arsenian pyrite. Stage 4 mineralisation
produced Au-Ag-tellurides and Au inclusions in pyrite, sphalerite and chalcopyrite.
Hydrothermal alteration halos associated with stage 3 veins evolved from high
temperature epidote and K-feldspar – epidote to illite – muscovite – K-feldspar
alteration. Stage 4 mineralisation is spatially and temporally associated with illite –
muscovite – carbonate alteration assemblages. Late stage gypsum-, calcite-, epidote-,
prehnite-, hematite-, and ankerite-bearing veins are unmineralised.
Fluid inclusions from actinolite-bearing stage 1 and garnet-bearing stage 2 veins
have low (~250ºC) homogenisation temperatures, suggesting either that these fl uid
inclusions have re-equilibrated, or that signifi cant pressure corrections are required
for these temperature estimates. The salinities of stages 1 and 2 were around 11.0
and 7.0 wt. % NaCl, respectively. Main-stage quartz – pyrite veins (stage 3) trapped
vapour- and liquid-rich, moderate salinity (~9.0 wt. % NaCl) fl uid inclusions under
boiling conditions at temperatures around 310ºC. Stage 3 veins are estimated to have
formed approximately 1 km below the paleosurface at hydrostatic pressure (~90 bars).
No fl uid inclusions were found in stage 4 veins, but the presence of illite indicates
formation temperatures below ~280ºC.
Sulfur isotope analyses have provided evidence for a magmatic sulfur
component prior to and during gold mineralisation. The δ 34S values for early vein
sulfi de
stages range between -4.9 to -0.5 per mil. The stage 3 has δ34S values ranging from
sulfi de
-5.2 to +0.8 per mil with the most 34S-enriched sulfi des values deposited away from the
mineralised centre. Stage 4 sulfi des have isotopic compositions from +2.5 to -7.5 per
mil. The negative isotopic values are consistent with sulfate-predominant magmatic-
hydrothermal fl uids. Sulfur isotopic zonation patterns defi ned by stage 3 and 4 sulfi des
at E41 broadly correlate with high-grade domains.
Stage 3A-c calcite has δ13C and δ18O values that range from -5.2 to
calcite calcite
-4.6 and from +11.6 to +12.1 per mil, respectively. Calculated fl uids for these mineral
values at 300ºC (δ13C = -3 per mil; δ18O = +6 per mil) are consistent with a
fl uid fl uid
magmatic-hydrothermal source of carbon and oxygen during stage 3A-c. A component
of meteoric waters is inferred for stage 4, because δ13C and δ18O values
carbonate carbonate
range from -6.9 to -0.5 and from +10.9 to +30.1 per mil respectively, corresponding
to δ13C and δ18O values of -5 and -2 per mil at 200-250ºC. The involvement of
fl uid fl uid
external waters during stage 4 is also supported by the δD and δ18O
illite-muscovite illite-muscovite
compositions that range from -67.7 to -54.4 and +5.0 to +9.5 per mil, respectively.
These values correlate to δ18O and δD values of +2.9 and -85.4 per mil at 250ºC,
H2O H2O
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and are consistent with meteoric fl uids that have partially equilibrated with volcanic
rocks.
Gold is inferred to have been transported as a bisulfi de complex in stage 3 and
4 in weakly acidic to alkaline aqueous fl uids. Gold precipitated due to a combination of
boiling and wall rock sulfi dation. Some evidence for fl uid mixing is provided by C-O
and D-O isotopic data from stage 4, and this process may also have been important for
ore formation.
E41 records the transition from deep, porphyry-style to shallow-level epithermal
style magmatic-hydrothermal activity, and potentially implies unroofi ng of the system
synchronous with mineralisation. High-temperature propylitic actinolite and epidote,
and potassic assemblages (biotite, orthoclase, magnetite) indicate that E41 is located
proximal to an alkalic centre of magmatic – hydrothermal activity. This is the fi rst
documented occurrence of low-sulfi dation alkalic-style epithermal mineralisation in
the Macquarie Arc. Continued exploration around E41 may lead to the discovery of an
alkalic porphyry Cu-Au deposit.
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Acknowledgements
This PhD dissertation was completed as part of an MDRU-CODES Alkalic Project
investigating shallow and deep-level alkalic mineral deposits. Full sponsorship for the
project was provided by Barrick Gold Corp., Newmont Mining, Teck, and AngloGold
Ashanti, with logistical and fi eld support provided by Barrick Gold of Australia
Ltd. Additional funding was provided by Society of Economic Geologists which is
gratefully acknowledged.
I would like to thank my principal supervisor Professor David Cooke
for including me in the alkalic project and giving me the opportunity to travel the
world and work with excellent geologists. Thanks for your constant encouragement,
guidance, advice and your patience while correcting chapters written in English-ish.
I am especially grateful for keeping me motivated over the last few months when the
brain was particularly resistant.
I would also like to thank my two additional co-supervisors Tony Crawford and
Bruce Gemmell who provided advice and technical support during my time at CODES.
Cari Deyell is thanked for her time and teaching at the early stage of this project.
Anthony Harris, Sebastien Meffre, Zhaoshan Chang and Julie Hunt also provided
stimulating and insightful discussions and feedback. The fi eld advice, support and
reviews of Kirstie Simpson, Thomas Bissig and Dick Tosdal are greatly appreciated
as well. Lots of serdecznych dziekuję to Amber Henry for a great time at Cowal and
numerous chats not only about Cowal geology.
A big thank-you goes to all the geologists and fi eld assistants at Cowal. In
particular, Paul McInnes is thanked for his support, guidance, teaching and fantastic
hospitality over my fi eld seasons at Cowal. Phil Greenhill, Steve Casey, Lyndall Freer,
Paul Balind, Andrew Bywater, Reuben Morrison, Adrian Ferguson, Damien Hart,
Trent Strickland and Stuart Mathews are all gratefully acknowledged.
I am grateful to the CODES/SES support staff, which provided top quality
service in processing samples and thesis preparation. Simon Stephens is thanked for
polished slab and thin section preparation. June Pongratz is thanked for her art-eye
opinion and assistance with graphic software and printing. To Dianne Steffans, Helen
Scott and Christine Higgins, thank you for dealing with all the travel and fi nance
documents.
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To all my fellow PhD students, too many to list here, but you’ve all provided
inspirational insights into my project, much needed mental escapes, and plenty
of unforgettable moments. Thanks to you all for sharing this unique experience of
happiness and frustration that comes with the PhD.
Jacq, thanks a lot for the last two weeks with all your help while putting this
thesis together. A massive vodka and polskie ogorki session is inevitable now!

And fi nally to my family and friends in Poland: Tato i Mamo, bardzo Wam
dziekuje za wsparcie przez te wszytskie lata edukacji. Obiecuje, ze na tym koniec!
Aska, dzieki za telefony, musze przyznac, ze niejednokrotnie motywowaly mnie one
do pracy. Agnieszko, Marku, Agnieszko i Piotrku ogromnie Wam jestem wdzieczny
za widomosci, listy, rozmowy i czeste: ‘Wojtku, do domu marsz’ – bez watpienia
ukonczenie tej pracy w duzej mierze zawidzieczam wlasnie Wam moi przyjaciele.
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Table of Contents
Chapter 1. Introduction ..........................................................................................1
1.1 Preamble .............................................................................................................1
1.2 Location and environment ..................................................................................3
1.3 Exploration history ..............................................................................................5
1.4 Previous work at Cowal district .........................................................................6
1.5 Characteristics of alkalic porphyry and epithermal deposits .............................7
1.6 Thesis aims and objectives ................................................................................12
1.7 Methodology and analytical work completed during this study .......................13
1.8 Thesis organisation ...........................................................................................14
Chapter 2. Regional- and District-scale Geology ...............................................17
2.1 Introduction .......................................................................................................17
2.2 The Tasmanides .................................................................................................17
2.2.1 Benambran cycle ..................................................................................20
2.2.2 Tabberabberan cycle ..............................................................................20
2.2.3 Kanimblan cycle ....................................................................................20
2.3 Lachlan Orogen .................................................................................................21
2.4 Macquarie Arc ...................................................................................................22
2.4.1 Junee-Narromine belt ............................................................................22
2.4.2 Molong and Rockley-Gulgong belts ......................................................24
2.4.3 Magmatism ............................................................................................24
2.4.4 Metallogeny of the Macquarie Arc ........................................................26
2.5 District scale geology ........................................................................................27
2.5.1 Cowal Igneous Complex .......................................................................27
2.5.2 Cowal Volcanic Complex ......................................................................29
2.5.2.1 Facies architecture of E46 ..........................................................29
2.5.2.2 Facies architecture of E42 ..........................................................29
2.5.2.3 Facies architecture of E40 ..........................................................30
2.5.3 Intrusive rocks .......................................................................................30
2.5.4 Derriwong Group ...................................................................................31
2.5.5 Recent sediments ...................................................................................31
2.5.6 District scale structure ...........................................................................31
2.5.7 Mineral deposits of the Cowal district ...................................................33
2.6 Summary ...........................................................................................................34
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Chapter 3. Geology of the E41 deposit ................................................................35
3.1 Introduction .......................................................................................................35
3.2 Pre-mineralisation volcano – sedimentary stratigraphy ....................................35
3.2.1 Mudstone facies .....................................................................................35
3.2.2 Polymictic volcaniclastic breccia .........................................................39
3.2.3 Sandstone facies (monomictic to polymictic) .......................................41
3.3 Intrusive history ...............................................................................................42
3.3.1 Pre – mineralisation intrusions ..............................................................47
3.3.1.1 Diorite ........................................................................................47
3.3.1.2 Plagioclase-phyric dykes ............................................................48
3.3.1.3 Blocky plagioclase-phyric coherent unit (‘Robin’s lode’) .........50
3.3.1.4 Crowded plagioclase-phyric dyke ..............................................50
3.3.1.5 Trachyte dykes ...........................................................................51
3.3.1.6 Mafi c dykes ................................................................................52
3.3.1.7 Monzodiorite dyke .....................................................................53
3.3.1.8 Mafi c monzonite .........................................................................53
3.3.1.9 Hybrid monzodiorite – mafi c monzonite zone ..........................57
3.3.2 Syn-mineralisation rock units ................................................................59
3.3.2.1 Aplite dykelets ...........................................................................59
3.3.2.2 Quartz-monzonite dykelets ........................................................59
3.3.2.3 Pyroxene-phyric dykes ...............................................................61
3.3.3 Late-mineralisation rock units ...............................................................63
3.3.3.1 Syenite dykelet ...........................................................................63
3.3.3.2 Hornblende-phyric quartz-rich dyke ..........................................63
3.3.4 Post-mineralisation rock units ...............................................................65
3.3.4.1 Plagioclase-phyric diorite dyke ..................................................65
3.3.4.2 Amygdaloidal dyke ....................................................................65
3.3.4.3 Hornblende-phyric dyke (‘lamprophyre’) .................................66
3.4 Structure ............................................................................................................66
3.4.1 Bedding .................................................................................................67
3.4.2 Faults .....................................................................................................67
3.4.3 Dykes .....................................................................................................69
3.5 Discussion ........................................................................................................70
3.5.1 Volcanic facies associations, architecture and paleoenvironment
of the Cowal district ...............................................................................70
3.5.2 E41 intrusive sequence and controls on emplacement .........................73
3.5.3 Geochronology ......................................................................................76
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