Hunting base-metal giants down-under - current research into Australian
Proterozoic Zn-Pb-Ag deposits
Garry Davidson, Peter McGoldrick and Tony Webster
CODES SRC, University of Tasmania, PO Box 252-79, Hobart, Tasmania 001,
AUSTRALIA
In 1883, a 37 year old German boundary rider, collected some heavy dark
rock samples from a low range of black, red and brown hills at the edge
of the Australian desert in far western New South Wales. The material
was subsequently assayed and identified and found to contain some Ag and
Pb. Charles Rasp had discovered the gossanous outcrop of the giant Broken
Hill Pb-Zn-Ag deposit and started a golden era of base-metal mining and
exploration that continues in Australia today. High-grade Pb-Zn-Ag
ore still issues from the headframes at Broken Hill, a powerhouse of inspiration
to miners and researchers. Charles Rasp and his partners faced heat, isolation,
and financial difficulties to make their dream a reality, but today a whole
new set of problems besets the earnest explorer. Land access has
become a highly technical issue, with the rights of many land-users now
being considered in the exploration process. Whereas in previous decades
explorers fossicked the outcropping areas, the new generation is exploring
deeper in those same terrains, and peering out with dim geophysical eyes
beneath the thin cover of surrounding undeformed sedimentary basins, a
far more expensive style of exploration. The key problem bringing explorationists
and researchers together is the need to understand the genesis of large
Proterozoic base-metals deposits in order to provide new and better exploration
models. Exploration models that are needed to increase the rate of
new discoveries in Australia, and elsewhere.
Research Framework
Australia has a long history of government and industry support for ore
deposit research. Government support has traditionally been through
the universities, AGSO (Australian Geological Survey Organisation) and
the CSIRO (Commonwealth Scientific and Industrial Research Organisation)
and more recently by providing additional funding to 'centres of excellence'
in universities under the Key Centres, Special Research Centres and Cooperative
Research Centres schemes. The government also allows industry a 125%
tax deductions for money invested in 'innovative' or 'high-risk' R&D.
While some industry research is undertaken 'in house', much is carried
out through collaborative arrangements with universities and CSIRO, often
brokered by the umbrella organisation such as AMIRA (Australian Mineral
Industry Research Association). AMIRA projects typically attract funding
of up to several hundred thousand dollars per year (A$1 = US$0.7). In these
projects, companies invest on average A$10,000-25,000 per annum each over
two or three years, with most projects succeeding only if more than 6 companies
are involved. The projects are thoroughly vetted by potential sponsors
prior to committing money. Once commenced the results of the project
remain confidential to sponsoring companies for at least the life of the
project, and regular meetings with, and reporting to, sponsors takes place
(normally every six months). The benefits to companies involved are
manifold and include: (1) the prospect of a competitive edge; (2) improvement
in the education of their geologists who attend the regular progress review
meetings and read the progress reports; and (3) the 125% tax rebate on
research funding. The Australian federal government also recognises the
worthiness of industry funded research partnerships, by making available
matching funding from the Australian Research Council (ARC) through a competitive
granting scheme. If successful, obtaining ARC funding for collaboration
with industry effectively doubles the budget available to carry out the
research.
Types of Proterozoic base metal deposits
There is a natural division of base metal deposits in Proterozoic terrains
in Australia into (1) Broken Hill-type lead-zinc-silver deposits;
(2) stratiform sediment-hosted zinc-lead silver deposits and (3) epigenetic
copper-gold deposits. The polymetallic nature and gigantic tonnages of
many of these deposits has made them popular exploration targets.
This article summarises some of the current research into, and controversies
surrounding the first two important deposit types.
Although both Zn-Pb-Ag deposit types in Australia occur in rocks of
broadly similar age (1700 to 1600 million years) they are geographically
separate (Fig. 1). The stratiform sediment-hosted deposits mainly
occur in unmetamorphosed to amphibolite facies grade sedimentary basins
in northern Australia (the 'Carpentaria Zinc Belt' of Queensland and the
Northern Territory - McGoldrick and Large, 1997). These rocks are
to the west of amphibolite to granulite grade terrains in Queensland that
contain Broken-Hill-type deposits. Laing (1996) argued that the Queensland
rocks (the 'Cloncurry Terrane') were once linked with the Georgetown Inlier
to the north and the Willyama Inlier (host to the Broken Hill deposit)
to the south in north-south orogenic belt he called the Diamantina Orogen.
Figure 1: Palaeoproterozoic to early Mesoproterozoic terranes of eastern
Australia; the black terranes are the components of the Diamantina Orogen
as defined by Laing, (1996), and the hashed (McArthur - Mt Isa) terrane
is his Carpentaria Orogen.
Broken Hill-type deposits
Broken Hill deposits were formally defined by Beeson (1990), supported
by Parr & Plimer (1993) and Walters (1996), although there has been
dispute over the validity of the classification for many years. The critical
things that separates them from all other sediment-hosted deposits is their
geochemistry: (1) strongly enriched in Mn, F, Ca and P, which results in
abundant apatite, fluorite, and manganoan varieties of many different metamorphic
minerals; (2) very high Pb/Zn and Ag/Zn ratios; (3) an unusual and characteristic
suite of lateral marker facies, including gahnite-bearing cherts, amazonite
pegmatites, garnet-rich psammopelite, and sillimanite-rich horizons, and
several other important factors reviewed by Walters (1996). A metamorphosed
VHMS or sediment-hosted deposit is not likely to drastically alter its
chemistry, so it is not valid to continue to refer to BHT deposits as metamorphosed
equivalents of other deposit types.
Broken Hill
The origin of the great Broken Hill lode continues to be debated (Pongratz
and Davidson, 1996), and, while no consensus exists on genetic models (e.g.,
Fig. 2a), empirical exploration models (e.g., Fig. 2b) have proved extremely
useful in the search for new deposits (Walters, 1996). The application
of detailed geological knowledge, stratigraphic interpretation and systematic
drilling at Broken Hill resulted in the discovery of the Potosi Orebody
which lies within a separate stratigraphic horizon to the main ore lenses.
The Potosi Orebody is now known to be the second largest zinc-lead orebody
discovered in the Willyama Inlier.
Figure 2 a) A genetic model for Broken Hill deposit (after Plimer and
Parr, 1993); b) The geological part of an empirical exploration model
for BHT deposits (after Walters, 1996).
New research work in the Willyama Inlier is leading to a re-evalution
of the accepted stratigraphic succession of the Willyama Supergroup.
The research, being undertaken by the Australian Geodynamics Cooperative
Research Centre (AGCRC), involves new structural mapping, high resolution
ion probe zircon dating, and includes information from AGSO's seismic transect
of the Broken Hill Block. The structural evolution of the Broken Hill orebody
is also being re-examined. Detailed aeromagnetic surveys are helping
define favourable areas under shallow cover at the margins of the Willyama
Inlier, and the South Australian government survey is re-mapping the Olary
Block.
Broken Hill-type deposits of the Mt Isa Inlier
Potential for Broken Hill Type deposits in the eastern Mount Isa Inlier
was recognised during the 1970's exhalative-driven exploration phase in
Australia by companies such as Amoco Exploration and Shell Metals, when
it was appreciated by Prof. Dick Stanton, and others, that prospects in
the (now) Maronan Supergroup, which forms the eastern border
of the Mt Isa Inlier,had similar features to Broken Hill, including small
BHT deposits such as Pegmont, Maramungee, Dingo, and Fairmile (Fig. 3).
The Maronan Supergroup is a deeper water facies, and much higher metamorphic
grade, compared to most units in the rift-related Mt Isa Inlier. It is
dominated by feldspathic schists with a strong felsic volcanic component,
now collectively referred to as the Fullarton River Group. This is overlain
by the Soldiers Cap Group, in which pelitic deep water turbidite units
are overlain by basalts and metadolerite sills, with thin oxide and silicate-facies
banded iron formations. The Maronan Supergroup was complexly deformed by
three major fold events, and during D1 to D2, it experienced sustained
upper amphibolite facies metamorphism, resulting in partial melting of
feldspathic units. Although the original polarity of the Maronan Supergroup
is not known, and its contacts with all other units of the Mt Isa Inlier
are major faults, its volcanic geochemistry is consistent with a rift origin.
The unusual aspect of the geology is the occurrence of a major intense
sodic-calcic alteration front along most of the western fault contacts
of the Maronan Supergroup, essentially haloing intrusives of the 1530-1500
Ma Williams Batholith. Large epigenetic copper-gold deposits such as Starra
and Ernest Henry, are spatially related to these intrusives, and at least
one example, Osborne, is partly hosted by Maronan Supergroup iron formation.
Figure 3 a) The Mount Isa Inlier and southern McArthur Basin showing
the locations of important sediment-hosted Zn-Pb-Ag deposits; b) The Cloncurry
Orogen showing the location and geological setting of Broken Hill-type
deposits and prospects (after Williams et al., 1996).
The discovery of the Cannington deposit (45.3 Mt of 11.1 % Pb, 4.4%
Zn, and 500 g/t Ag) in 1990 by BHP Minerals, vindicated the old comparisons
with Broken Hill, and injected feverish excitement into Maronan Supergroup
exploration. The discovery, coined "La Plateada" by BHP (Skrzeczynski,
1993) was essentially geophysical, based on a follow-up drilling of a small
aeromagnetic anomaly within a large magnetically quiet region under ~40
m of Great Artesian Basin (Mesozoic) sediment cover. However, BHP geologists
were targeting the area because of their strong belief in the metallogenic
comparisons with the Broken Hill district.
Current research is focused on the genesis of the Broken Hill-type
deposits in the Maronan Supergroup, with the greatest efforts at Cannington
itself. Careful documentation by Bodon (1996) has shown that the deposit
has a pre- to syn-metamorphic gangue assemblage of pyroxenoids, Mn-garnet,
olivine, graphite and base-metals, but this is severely overprinted by
a syn- to post-D2 calc-silicate metasomatic assemblage of almandine, hedenbergite
and quartz, which was associated with, at the very least, significant
base-metal remobilisation that may account for the fabulously rich silver
content.
The contrasting assemblages have fueled two very different interpretations
of the deposit genesis, with implications for Broken Hill-type
deposits in general: (1) pre-deformational
formation within an immature setting, possibly in the diagenetic environment
to account for the abundant metasediment alteration, and followed by significant
metamorphic fluid reaction in situ (Bodon 1996, Walters and Bailey,
in press), and (2) an epigenetic model, in which no pre-metamorphic component
is recognised, and mineralisation formed "entirely from alteration processes
late in the geological evolution of the Cloncurry district" (Williams et
al., 1996).
The epigenetic view is supported by very large-scales of high temperature
saline fluid flow that occurred during D2 to D3 on the western margin of
the Maronan Supergroup, and which certainly moved vast amounts of metal.
Research by workers from the AGCRC has documented terrain scale east-dipping
thrust faults below the
Maronan Supergroup that may have facilitated this fluid movement (O'Dea
et al., 1997). AMIRA project P438, lead by Peter Pollard and Pat Williams
of James Cook University, has endeavoured to characterise the types of
fluids involved, and their possible connection to granite intrusions, a
task which has long been enthusiastically supported by Lesley Wyborn of
the Australian Geological Survey Organisation.
Stratiform sediment-hosted Zn-Pb-Ag deposits
This important group of deposits (Fig. 3) accounts for the majority of
Australia's current Zn, Pb and Ag production, with operating mines at Mount
Isa, Hilton, Dugald River, and HYC (McArthur River). They will continue
their dominant position into the 21st century with mining development underway
at the Century deposit (100 million tonnes); mining of high grade parts
of the small Lady Loretta deposit due to commence in early 1998; and feasibility
studies nearing completion on the large George Fisher deposit near Mount
Isa.
With the exception of Dugald River, all these deposits are located
in the Mount Isa Inlier 'Western Succession' or the southern McArthur Basin
(Fig. 3), and these areas have been the focus of intense research activity
in recent years. Since 1992 the Centre for Ore Deposit Research (CODES
SRC) at the University of Tasmania has carried out a major multidisciplinary
research project aimed at determining the primary geological, geochemical
and structural controls on the location and timing of formation of these
deposits.
Figure 4: Schematic showing important geological components of an exploration
model for Australian stratiform sediment-hosted Zn-Pb-Ag deposits (after
McGoldrick and Large, 1997).
Finance for the multimillion dollar project has come from AMIRA (Project
P384/384A) and the ARC. The research has led to a more detailed understanding
of the genesis of the HYC and Lady Loretta deposits, and to the development
of empirical lithogeochemical and isotopic vectors for these deposits (Large
and McGoldrick, 1997). Computer modelling of brine chemistries and water-rock
interactions has yielded new insights into metal transport and deposition
processes. Textural evidence from different deposits indicates that
some form as syngenetic accumulations of base metal sulphides at the basin
floor, while others form in the unconsolidated sediment as porosity infill
and/or replacement. The Century deposit is thought to have formed
during late diagenesis at depths of up to 1000 m during the first stages
of basin inversion (Broadbent et al., 1996). Regardless of the precise
timing, these deposits have clearly formed early in the geological evolution
of their host sedimentary basins, and the CODES project has used integrated
structural, sedimentological and geophysical studies to determine key elements
of the basin architecture and evolution responsible for base metal mineralisation
(Fig. 4).
In a complementary project AGSO, through their Northern Australian
Basin Resource Evalution program (NABRE-rhymes with sabre), have begun
to apply modern sequence stratigraphic principles to the Mount Isa and
McArthur Basins. In the absence of biostratigraphic control and detailed
seismic profiles, this project has made novel, extensive use of gamma-ray
logs of surface exposures and drill-core. This approach, combined
with new, high-precision ion probe zircon dates, is yielding a much more
realistic lithostratigraphic framework for the northern Australian late
Palaeoproterozoic to early Mesoproterozoic sedimentary sequences.
Conclusions
Charles Rasp probaly would not recognise Broken Hill today, but 'the rush
that never ended' to find and exploit Australia's mineral wealth continues.
The era of stumbling over outcropping base metal 'giants' is probably over.
However, sound scientific research into known deposits and their geological
setting (in the broadest sense), will help 21st century explorers to locate
the hidden quarry they seek.
Much of the work referred to here will be appearing in two special
publications due out in 1998. The first of these will be v. 44 n.
1 of the Australian Journal of Earth Sciences which is a thematic issue
entitled 'Geology and mineralisation in the Proterozoic Carpentaria Zinc
Belt of northern Australia', and later in the year an Economic Geology
Monograph covering many aspects of the geology and mineralisation of the
McArthur Basin and Mount Isa Inlier will be released.
REFERENCES
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and Metallurgy Transactions B99:163-175.
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and rare-earth element geochemsitry at the Cannington Ag-Pb-Zn deposit,
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Broadbent, G., Myers, R., and Wright, J., 1996. Geology
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