The Tasmanian Geologist - Geological Society of Australia

The
Tasmanian
Geologist
NEXT MEETING:
Monday 16 th February
7.30 pm
Hotel Cecil, Zeehan
Speaker:
Prof. Ross R. Large
CODES
ARC Centre of Excellence in Ore Deposits
University of Tasmania
Pyrite:what it can tell us
about ore deposits
Abstract inside
Newsletter of the
Tasmania Division
Royal Society of Tasmania
Charles Darwin bicentenary activities
DARWIN SYMPOSIUM
7.00 pm. Wednesday 11 th February
Stanley Burbury Theatre,
University of Tasmania
Speakers are Mr Peter Stevenson, Dr David
Leaman, Dr Peter McQuillan, Prof. Michael
Roe and Dr Robert Banks.
ADMISSION FREE
Also-
Saturday 14 th February-
Excursion to Queen’s Domain (cost $10)
Sunday 15 th February-
Excursion to Mt Wellington (cost $10)
Further enquiries to
royal.society@tmag.tas.gov.au or Ph 62114177
February 2009
ADVANCE NOTICE
Thursday 19 th March
Joint meeting with the Australian Institute of
Geoscientists (AIG):
Speaker- Dr. Greg Corbett
Topic, time and venue to be announced
ALSO INSIDE
• King Island field symposium
• Proposed Twelvetrees Medal
• Geological Evolution of Tasmania
volume- progress.
The Tasmanian Geologist, 10th February 2008

King Island Field Symposium
13 th -16 th March 2009
This is now fully booked, with
arrangements made for 24 registrants and 2
reserves. Eight participants have chosen an
add-on trip to visit possible Tasmanian
correlates at Waratah Bay, Victoria on 16 th -
17 th March (organized by David Taylor,
GSV).
Further details are available on the
GSA website: www.gsa.org.au
For further inquiries contact the Secretary
andrew.mcneill@utas.edu.au
Organizing committee: Nick Direen, Clive
Calver, Andrew McNeill.
W. H. Twelvetrees Medal
The Committee has proposed that a
medal be struck in honour of the memory of
the eminent Tasmanian government
geologist, William Harper Twelvetrees
(1848 – 1919).
The suggestion is that the medal be
awarded at the discretion of the Tasmanian
Division of the Geological Society of
Australia, usually annually, to recognize
either:
• important contributions to the Earth
Sciences with Tasmania (including
Macquarie Island); or
• outstanding contributions to Earth
Sciences while resident in Tasmania.
In the spirit of W. H. Twelvetrees,
contributions shall be considered to include
meritorious feats of geological exploration
in Tasmania or elsewhere; significant
published works; or meritorious service to
the Earth Sciences.
Two or more members of the Society
can nominate a member for the award by
submitting to the Divisional Committee the
name of the candidate, biographical data,
and a citation in about 300 words describing
the candidate’s work and its significance.
Nominations will be valid for up to five
years after the nomination date. The medal
may be awarded posthumously if the
candidate was alive at the time of
nomination.
Comments and suggestions from
members are invited. The proposal will be
put to a meeting in early 2009.
“Geological Evolution of
Tasmania” Volume
The committee met with the editors
on 22 nd January to review progress. The
majority of contributions have been
received, and letters have been sent to the
remaining contributors. The following
timetable will be implemented:
30 th June: final deadline for all submissions
15 th August: deadline for return of peer
reviews
15 th September: deadline for author
corrections
15 th October: deadline for completion of
editorial content.
Dr Clive Calver has been appointed
joint editor, in addition to Dr Keith Corbett
and Prof. Patrick Quilty.
The committee has also resolved that
the book will be produced in electronic, as
well as printed form.
Membership
New applications
Jafar Taheri
Lindsay Clarke (student)
J. Moye (student)
Students remember- the Tasmanian
Division will pay your $10 membership fee
in full!
Geological Society of Australia website:
www.gsa.org.au
2009/2010 Divisional
Committee
Speakers wanted
The Annual General Meeting is planned for
Please April. contact All executive the Secretary and if you committee know of
any positions visitors will or returning fall vacant. Tasmanians To assist willing in
and planning, able to present expressions a talk at of a GSA interest meeting. from
members willing and able to serve are
sought by the current Secretary. Please
consider if you are able to assist in your
society!
The role of diagenetic and metamorphic
processes in the multi-stage concentration of
The Tasmanian Geologist, 10th February 2008

gold, arsenic and vanadium in black shale
and turbidite-hosted gold deposits.
Ross R Large
Organic-rich mudstones or shales, within thick
packages of calcareous or siliciclastic turbidites,
have long been recognised as important host rocks
for major gold deposits (e.g. Victorian Goldfield,
Carlin District, Lena Gold Province, Otago Schist
belt). Many of the deposits in these districts are
referred to as orogenic gold deposits, emphasising
the tectonic and metamorphic processes involved in
the concentration of gold (Groves et al., 2003;
Goldfarb et al., 2005). The current genetic models
propose that the organic-rich sedimentary host rocks
are the trap-rock for gold precipitated during
deformation-related fluid flow, with the gold being
transported by deep-sourced fluids of mantle or
lower crustal origin, or in some cases from felsic
magmas. Recent research at CODES (Wood and
Large, 2007; Large et al., 2007) suggests the
possibility that organic-rich black shales are
potential source rocks for gold, arsenic and
vanadium, and that the gold was originally trapped
on seafloor organic material by chemical process
during sedimentation and concentrated in arsenian
pyrite during diagenesis.
Research on the giant black shale-hosted Sukhoi
Log deposit in the Lena gold province of Siberia
(Large et al. 2007, Scott et al, 2007) has revealed a
multi-stage process of gold concentration and
release that occurred throughout the 50 million year
period of sedimentation, diagenesis and
metamorphism in the Bodaibo trough. We have used
the rapidly developing laser ablation ICP-MS
technology to study the siting and partitioning of
gold, arsenic, vanadium and other metals during
diagenesis and metamorphism of the black shales.
During periods of extreme euxinic conditions on the
seafloor, gold and a range of other trace elements
(As, Zn, Ag, V, Ni, Mo, Pb, Se, Te, Cu, Cr, U) are
trapped on organic-surfaces forming strong organometallic
ligands, leading to a concentration of metals
in the most organic-rich facies of the sediments (e.g.
Algeo and Maynard, 2004; Rimmer, 2004). During
diagenesis, pyrite framboids, and clusters of pyrite
micro-crystals of various textural forms, grow
within the organic-rich sediments, commonly
mediated by the biological reduction of marine
sulfate to H 2 S. A portion of the gold and arsenic,
plus many other trace elements (particularly Ni, Mo,
Se, Pb, Ag, Te, Cu) are partitioned into the
diagenetic pyrite. Other trace elements remain in the
organics or form insoluble oxy-hydroxides
(particularly V, U, Cr, Algeo and Maynard, 2004).
The arsenic content of the diagenetic pyrite is a
critical control on the amount of gold that is trapped
within the sediment (Reich et al., 2005). The more
arsenic-rich the black shale package, the more gold
that is likely to be trapped during early diagenesis,
thus contributing to form a more suitable gold
source rock.
With the onset of late diagenesis, the early
forms of micro-crystalline gold-bearing arsenian
pyrite, recrystallise to larger crystals ( commonly <
0.2 mm) of subhedral to euhedral pyrite, or in some
cases pyrite nodules, developed parallel to bedding.
Some gold and selected other trace elements are
released during this process forming a gold-bearing
diagenetic fluid, that may migrate and deposit free
gold in late diagenetic structures. As the sediment
passes through the oil window, mobile organics
migrate along structures and may transport further
gold, dissolved within the organic-rich fluid
(William-Jones and Migdisov, 2007). These
processes have also been observed in the Carlin
district (Emsbo and Koenig, 2005).
Deformation and metamorphism is
accompanied by further recrystallisation and growth
of pyrite in the black shales. The syn-deformation
pyrite is commonly coarser grained (0.2 to 20 mm),
with less sediment inclusions, and a significantly
lower dissolved trace element content (Large et al.,
2007). Gold originally dissolved in the diagenetic
pyrite is released during metamorphism to form free
gold, which concentrates in preferred structural sites
(anticlinal zones or shears) associated with either
metamorphic pyrite or syn-deformation quartz veins.
Not only is the gold released from the diagenetic
pyrite during metamorphism, but other trace
elements, in particular, Cu, Zn, Pb, Ag, Bi, Te, are
also released to form minor discrete sulfides
(sphalerite, chalcopyrite, galena and various goldsilver-tellurides)
which commonly form inclusions
within the metamorphic pyrites. Certain elements,
that are strongly held within the structure of pyrite
(e.g. As, Ni, Co, Se), remain to become concentrated
in the metamorphic pyrite. Vanadium which was
originally concentrated in organic matter in the
sediments, by reduction of V 5+ to V 4+ (Wood, 1996),
is subsequently incorporated into gold-bearing
diagenetic pyrite as micro-inclusions of V-bearing
illite, and finally occurs as roscolite, intergrown with
pyrite and free gold, in metamorphic assemblages.
At higher metamorphic grades, above middle
greenschist facies, much of the early diagenetic
pyrite is converted to pyrrhotite. This is
accompanied by the complete release of gold to the
metamorphic fluid, as pyrrhotite, unlike pyrite, has a
low trace element content and does not
accommodate significant gold or arsenic within its
structure (Buryak, 1982; Large et al, in press). For
this reason pyrrhotite-rich black shales are unlikely
to be good host-rocks for gold.
In conclusion, organic rich black shales,
especially those enriched in As, Ni, Mo, Pb, Zn and
V are ideal source rocks for gold. Diagenetic and
metamorphic processes are critical to releasing the
gold, originally bound in organic-matter and
dissolved within early arsenian pyrite, to become
concentrated in later stages of syn-deformation
pyrite and associated quartz veins.
Full references are available from the editor or
author on request.
Abstracts from IAVCEI meeting
The GSA (Tasmanian Division) gave financial support to
assist two students attending the IAVCEI ( International
The Tasmanian Geologist, 10th February 2008

Association of Volcanology and Chemistry of the Earth’s
Interior) meeting in Reykjavik, Iceland in 2008. In
accordance with our funding policy (see November
newsletter), we reproduce abstracts of the papers
presented by them.
Facies analysis of a partly extrusive, basaltic
submarine cryptodome, Shirahama Group,
Izu Peninsula, Japan
Sarah M. Gordee, J. McPhie & S. R. Allen
University of Tasmania
The physical evolution of modern and ancient
volcanic environments can be best understood through
the essential field technique of facies mapping. Detailed
mapping of volcanic deposits in SW Izu Peninsula, Japan
has provided information that exemplifies the importance
of this method. Two main facies associations were
identified that provide information on the eruption
history and lateral changes in eruption style of a
submarine basaltic volcanic complex
The first facies association comprises massive,
monomictic, jigsaw-fit basalt breccia that grades
vertically and laterally into thick beds of reversely
graded, clast-rotated to chaotic, monomictic basalt
breccia. Clasts are angular with curviplanar margins and
are interpreted to have formed by quench fragmentation.
Jigsaw-fit breccia and associated breccia beds are
interpreted as in situ and resedimented hyaloclastite,
respectively. The succession is overlain and partly
infilled by thin beds of pumiceous sand.
The second facies association comprises
elongate, tightly packed, coherent basalt lobes that grade
into domains of monomictic, jigsaw-fit to chaotic, clastto
matrix-supported basalt breccia with pumiceous sand
matrix. The coherent lobes cross-cut and locally disrupt
the overlying pumiceous sand beds. The clasts in the
sediment-matrix basalt breccia are texturally and
compositionally identical to the coherent lobes and the
facies is interpreted as peperite.
Collectively, the two facies associations are
interpreted to represent the margin of a partly extrusive
submarine cryptodome. In situ and resedimented
hyaloclastite breccias indicate an extrusive, subaqueous
environment and are consistent with expected facies
associations across the quench fragmented carapace and
resedimented flanks of a seafloor basaltic lava dome.
Pumiceous sand infilled the interstices within the breccia
pile to form a sediment-matrix breccia. The facies
association comprising coherent lobes and peperite
indicates an intrusive origin and is interpreted to
represent the subsurface portion of the cryptodome. The
intrusion utilized syn-depositional micro-grabbens within
the pumiceous sand that developed in response to the
expanding magma body beneath the surface, and
interaction of the magma with the wet, unconsolidated
sand produced peperitic domains. Regionally, another
similar facies association at the same stratigraphic
interval includes a thick mound of basaltic fluidal clast
breccia that may represent weakly explosive fire
fountaining at another seafloor vent.
The Luise amphitheatre, Lihir Island, Papua
New Guinea: Caldera, maar crater or
sector-collapse scar?
Jacqueline L. Blackwell 1 , Jocelyn McPhie 1 ,
David R. Cooke 1 , Kirstie A. Simpson 1 ,
Jonathon Rutter 2
1
Australian Research Council Centre for Excellence in
Ore Deposits
2
LGL, Lihir Operations, Papua New Guinea
The 40Moz. Ladolam gold deposit on Lihir
Island, Papua New Guinea occurs in a 3.5 by 4.0 km
seaward-facing, horseshoe-shaped amphitheatre.
Previous workers have postulated that the amphitheatre
is a caldera, a maar-diatreme crater or a volcanic sector
collapse amphitheatre. Correct interpretation of the
amphitheatre underpins understanding of ore formation.
Our analysis of the dimensions and morphology
of the amphitheatre, together with mapping and logging
the bedrock facies in which it occurs, strongly favours
sector collapse of the former Luise volcano. The
amphitheatre is much larger than maar-craters and lacks
the pyroclastic facies typically associated with both
maars and caldera. The bedrock mainly comprises
variable altered trachyandesitic and trachybasaltic lavas
and shallow intrusions, and volcaniclastic facies typical
of the proximal and medial parts of andesitic cone
volcanoes. Furthermore, bathymetry surrounding Lihir
Island has revealed the presence of hummocky
topography and marginal levees extending offshore from
the Luise harbour, interpreted to be a debris avalanche
deposit. A coral-limestone unit that encircles the rest of
the island is absent from the Luise harbour, having been
destroyed by a sector-collapse-generated debris
avalanche.
Sector collapse involving removal of ~ 1 km of
the former Luise volcano is consistent with porphyry –
style alteration being overprinted by epithermal-style
mineralisation and alteration.
GSA Tas. Division Committee (2008-09)
Chairman: Dr Nick Direen
Tel 0413 030612
ndireen@frogtech.com.au
Secretary: Dr Andrew McNeill
C/- CODES, University of Tasmania
Private Bag 79
Hobart 7001
Tel: 03 62262487
Fax: 03 62267662
andrew.mcneill@utas.edu.au
Treasurer: Dr Peter McGoldrick
Committee Members:
Dr Ron Berry
Dr Garry Davidson
Dr Mark Duffett
Mr John Everard (newsletter editor)
jeverard@mrt.tas.gov.au
Dr Jacqueline Halpin
Mr Michael Vicary
Ms Isabella von Lichtan
The Tasmanian Geologist, 10th February 2008