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.


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.


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