Origins,
Current Research, Exploration Guidelines
by
G.M.Derrick
(G.M.Derrick and Associates, May 2000)
PO Box 184 Corinda Qld
Australia 4075
Email: geoffd@powerup.com.au
“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:
·
Rifting,
·
Growth Faulting,
· 3rd
order pyritic & carbonaceous basins and
·
Basement feldspar-rich source rocks
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.
j j j j j
References:
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!
Century Link
Lady Loretta Link
Noranda-Buka Announcements
Sedex Super-Ore
like Sullivan, or Just another Skarn?
by
G.M.Derrick
(G.M.Derrick and Associates, May 2000)
PO Box 184 Corinda Qld
Australia 4075
Email: geoffd@powerup.com.au
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.
j j j j j
References:
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.
*
Competition No.3, Clue No.3:
Anagram: Non-gin, can't. In '93 Symposium.