“Australian Sedex Deposits” are code words for fine-grained sediment–hosted Pb-Zn-Ag deposits of mid-Proterozoic age which may or may not be exhalative and which are best developed in the Mt Isa Inlier and McArthur Basin regions of northern Australia. One could also include in this classification the Broken Hill Pb-Zn-Ag deposit type which is also of mid-Proterozoic age and is arguably of sedimentary-exhalative origin, but differences between the “Isan” type and “Broken Hill” type of deposits are sufficient to warrant their separate treatment. (see separate abstract, this conference).
The Mt Isa to McArthur series of deposits include Isamine, Hilton, George Fisher (all at Mt Isa – in production or development), Lady Loretta (at feasibility), Century (in production), HYC (in production), and Dugald River (undeveloped – on hold). Details of these and smaller plays such as Walford Creek and Kamarga are contained in Table 1 (from McGoldrick and Large, 1998).
By any standards, these are major accumulations of metal – average size is about 100mt @ +10% Pb+Zn; direct shipping grades of +50% Pb+Zn are present at Lady Loretta locally, and Ag credits range from 35g/t at Century, 60g/t at HYC to 100 to 150g/t at Isa-Hilton. Despite first appearances, and their low metamorphic grade, these deposits are NOT part of a geological monoculture with a single prevailing view on their origin. They are now known to extend over a time range of about 50 to 70 Ma, from 1650 to 1575Ma within a range of stratigraphic units.
Main regional geological criteria can be described as “The Fabulous Four” (Derrick, 1996) – viz:
Most settings are intracontinental rifts or rifted margins, and deposits are best developed in the youngest of sag basins overlying older rift-sag cyclic sequences. Dolomitic siltstones and shelf carbonates are common regional host lithologies, formed in both emergent and sub-wavebase settings, but generation of local reduced facies through differential and reactivated movement on orthogonal sets of growth faults is a key factor in locating the ultimate repository for Pb-Zn mineralisation.
Complacency and tradition have no role in current debate regarding origins of these deposits. Fluids may be near-neutral moderately low temperature and oxidised (120-1500C) brines, which interact with biogenic S or thermochemically reduced SO4 .pyrite and carbonaceous facies to precipitate base metal by redox reactions. Other (more controversial) research proposes all pyrite, Pb, Zn and Cu at Mt Isa for example are part of one late, post-orogenic epigenetic system some 150 Ma younger than host sequences, concepts which many find run counter to common sense and good science. Apart from HYC, direct evidence of metal-charged brines exhaling onto the seafloor is lacking, and a diagenetic timing of mineralisation is now commonly preferred, with fluids permeating reactive and permeable laminated reduced sequences. Century, the most laminated of all the ores in hand sample, is convincingly replacive in origin, at a time some 15-20Ma after sedimentation, and from processes akin to MVT formation – mediation of metal-charged brines by both gaseous and liquid hydrocarbons in a gently folded anticlinal trap.
Further understanding of these deposits and their origins involves sophisticated Pb isotope studies and Pb growth curves allied with high resolution U-Pb-Zr dating of sequences and alteration. Sequence stratigraphy and seismic imaging of concealed prospective packages are bringing new insights into basin architecture and area selection, and studies of the polar wander path, esoteric as that may seem, show that inflections in the path may coincide with continental-scale movements responsible for tectonic events, basin inversion, and triggering of fluid flow along various P and T gradients.
Even the most controversial research becomes more explicable when it is clear that a system of rift-related structures may be continuously reactivated; the Cu deposits at Mt Isa and Gunpowder are 150Ma younger than the Pb-Zn-bearing sequences, but their associated Cu-rich saline fluids at temperatures of about 3000C are focussed by older structures, and are themselves chemically capable of dissolution, recrystallisation and extensive retexturing of pre-existing Pb-Zn mineralisation, similar for example to Cambrian to Devonian events at Rosebery VMS in Tasmania.
All of the known deposits were discovered by outcrop sampling of gossans or shallow soil geochemistry. Increasingly one will require expert knowledge to explore vast areas of concealed permissive geology, including airborne and ground EM geophysics to detect pyrite and carbonaceous facies, gravity methods to detect massive sulphides and possibly thickness variation across buried faults, seismic imaging to support the view that sedex Pb-Zn deposits are mere special cases of petroleum exploration, regional geochemistry searching for fluid flow path alteration, and carbon and oxygen isotopes to detect vectors towards mineralisation. Some experience in and ability to recognise growth faults in the field may also be an exploration advantage.
Derrick, G.M. 1996 The geophysical approach to Metallogeny of the Mt Isa Inlier – What Sort of Orebody Do You Want? Proceedings, AusIMM Annual Conference, Perth. , pp. 349-366
McGoldrick, P & Large, R., 1998 Proterozoic stratiform sediment-hosted Zn-Pb-Ag deposits. AGSO Journal of Australian Geology and Geophysics v. 17 (4) – (Special edition “Exploration models for major Australian Deposit Types”)
Abstract for Mt Isa Inlier workshops and lectures at the Sullivan Workshop held at Cranbrook, BC, Canada – May 2000.
The Century Zinc Mine Atlas 2002 All the Sedex you can handle!
Lady Loretta Link
The massive Broken Hill (NSW) Pb-Zn-Ag deposit is an excellent example of a single orebody defining the BHT ore deposit model. With the passage of time and further exploration and ore deposit research, other deposits have been added to the BHT classification, including Zinkgruven (Sweden), Aggeneys-Gamsberg (South Africa), possibly Sullivan (Canada), and, most significantly, Cannington (Queensland). The latter contains the “Super Ore” of our abstract title – the Glenholme Breccia, commonly containing grades of 20%Pb, 10% Zn and 2500g/t Ag, all coarse-grained and with recoveries of 80 to 85%. The mine will also be the world’s largest Ag producer, at 750 tpa Ag.
The descriptor “Super Ore” can be applied to some of these deposits on the basis of size, grade, metallurgical characteristics, and their contributions to Gross National Product in their respective host countries. Age and pre-mining tonnes and grade of some BHT deposits are as follows, noting that both Sullivan and Broken Hill are now close to the end of their economic life.
Broken Hill (NSW): 300 mt @ 15% Pb+Zn, 150g/t Ag (1690Ma)
Cannington (Qld) 45mt @ 11.1% Pb, 4.45 Zn, 500g/t Ag (?1690Ma)
Sullivan (BC) 155mt @ 6.1%Pb, 5.9% Zn, 68g/t Ag (1400Ma
Broken Hill and Cannington are low S, pyrite-poor deposits of similar age, and which form part of the Diamantina orogen, a 2000km long depositional and orogenic belt which probably defined the eastern edge of the Australian craton in the period 1750 to 1600Ma. This orogen is thrust-faulted against the Carpentaria orogen to the west (which hosts Isa-Century type Pb-Zn-Ag deposits). The tectonic setting is rift-related intracontinental or continental margin – coarse clastic sediments (e.g. conglomerates) are rare – quartzo-feldspathic sediments (graded wackes etc) and some volcanics dominate the lower stratigraphy, with psammopelitic to pelitic sequences , perhaps locally evaporative, most common in the upper stratigraphy. The package can be considered relatively oxidising in nature, and generally lacks the 3rd order reduced packages which control Isa-type Pb-Zn-Ag deposits, although graphitic metasediments may be present locally. Basic magmatism is relatively common especially in the lower half of the column. BHT deposits are concentrated at the transition from lower to upper sequences, with associated packages of “unusual” rock types such as BIF, quartz-gahnite (Zn spinel) and Mn-garnet ‘sandstone’, thought to represent exhalitive chemical sediments. Identification of protoliths is complicated by high-grade metamorphism (high amphibolite to granulite), which reflects the relatively thin crustal setting, high heat flow and abundant high-level intrusions and tholeiitic underplating.
Mineralisation is hosted by a range of skarn-like Ca-Mn-Fe-P-F rich assemblages, containing garnets, pyroxenes and pyroxenoids (e.g. bustamite, pyroxmangite, rhodonite, spessartine); textures are typically coarse-grained and annealed, with complex ductile breccias, and evidence exists at Broken Hill for presence of sulphide melts. Structural upgrading and complex retrograde metasomatism are characteristic.
There is no obvious footwall feeder or root zone such as at Sullivan, but abundant sillimanite and garnet in gneisses define a large-scale alteration envelope. “Lode” pegmatite sweats with greenish Pb-rich microcline, garnet quartzite and quartz gahnite rocks are also considered part of the regional alteration assemblage. To this author, the presence of spessartine-quartz-apatite bedded rocks and BIF parallel to S0 and pre-D1 in low greenschist facies rocks near Cloncurry (Qld), within the Cannington package is convincing evidence that these unique chemical sediments are truly a synsedimentary to diagenetic/exhalative part of the BHT ore deposit model. It is also acknowledged that superimposed multiple metamorphic and metasomatic events may form many generations of garnets and other minerals for example.
Some researchers in the past 5 years have proposed, from structural, petrographic and geochronological studies, that deposits such as Broken Hill and Cannington are possibly skarns, or at worst ‘metamorphic’ deposits formed solely during peak to retrograde metamorphism and metasomatism at about 1600-1500Ma. The basis for the new Pb-Pb geochronology has been challenged, and proponents of skarn origins now concede the possibility of a Pb-Zn-Ag pre-metamorphic precursor. This acknowledgement is based partly on the presence of metal sulphide, apatite and fluorite inclusions in peak metamorphic minerals such as olivine, bustamite etc; recognition of highly saline, metal-rich fluid inclusions in late quartz-hedenbergite veins (Pb to 3.6%, Zn to 3.45, Cl to 33.7%), indicating interaction of late S-poor high T fluids with base metal accumulations; and positive Eu REE anomalism similar to modern ocean ridge and VHMS systems. Fluid geochemistry also suggests that hot saline fluids would be difficult to saturate with Pb and Zn under skarn-like conditions of 450 to 5000C.
Simple calculations of metal extraction rates for Super Orebodies such as Broken Hill indicate that exploration within, say, 30km of Broken Hill may not be fruitful in exposed rocks, insofar as hydrothermal systems such as Broken Hill have obtained their metals from about 8,000km3 of crust , equivalent to 30 x 30 km to 7km depth. The greatest practical aid to exploration for BHT deposits (outside the magical 30km of course!) may be the ability to recognise the unique alteration triumvirite – assemblages of quartz gahnite, Mn garnet ‘quartzite” and BIF.
Pongratz, J & Davidson, G.J, 1996 (eds) New Developments in Broken Hill Type Deposits . CODES Special Publication 1. 164pp
Walters, S., 1998 Broken Hill-type Deposits in AGSO Journal of Australian Geology and Geophysics v. 17 (4) – (Special edition “Exploration models for major Australian Deposit Types”)
Abstract for Mt Isa Inlier workshops and lectures at the Sullivan Workshop held at Cranbrook, BC, Canada – May 2000.