Good Afternoon Ladies and Gentlemen.
During the 1950s, the Australian Government in conjunction with the State
Governments began a program of geological mapping of the entire Australian
Continent at 1:250,000 Scale ( 1 inch = 4 miles). This mapping was
accomplished by field traverses by geologists using aerial photographs. The
maps produced have underpinned the discovery many of the mines operating
today.
During the 1970s, following the completion of the broad scale mapping, a
program of more detailed mapping was undertaken in mineralised terrains or
in areas of more complex geology. By the late 1980s, regional geophysical
datasets, mostly collected by airborne surveys, helped to refine the earlier
mapping. Geophysical datasets used include areomagnetics and radiometrics
and ground gravity surveys. These datasets were compiled at mapping scales
and underpinned many of the advances in the maps.
This slide shows radiometrics on the left ( a proxy for lithology measuring the
uranium, thorium and potassium content of the rocks) and magnetics on the
right. The next slide shows the resultant map.
However, the true values of the geophysical datasets lie in their capabilities to
allow mapping of known geology under cover. Many of the newer discoveries
in Australia over the last ten years have come from using geophysics to define
blind targets. Aeormagnetics allow us to trace faults under cover and provide
some information on the magnetic characteristics of the rocks. These slides
shows how magnetics can show extensions under cover.
Recently geophysical techniques such as magnetotellurics, airborne
electromagnetics, and deep seismic profiling have been used to further add a
depth dimension to our knowledge of the geology. This has allowed us to
construct 3D models, particularly in data rich areas. We are also using detailed
geochronology programmes, mostly based around SHRIMP (Super High
Resolution Micro Probe) dating of zircons. This data can be constructed into
time-space plots which allow us to understand the relative timing of geological
events and allow us to construct more robust tectonic models. Here are two
time space plots for different parts of Queensland.
While not all surface discoveries have been made, it is broadly agreed that the
next generation of discoveries in Australia will be made on blind targets under
up to several hundreds of meters of cover rocks. In order to allow effective
exploration under cover, a good understanding of the mineralising system is
required. If looking for elephants – look in elephant country! There is no point
looking for elephants in Kazakhstan! The important point here is to understand
what you are looking for.
We now use a mineral systems approach, to the discovery of deposits, rather
than just simply trying to identify geophysical targets of known mineralisation
models.
The mineral system approach uses the answers to five questions:
1.
2.
3.
4.
5.
What is the geodynamic setting of the system?
What is the architecture of the system?
What are the fluids, their sources and reservoirs?
What are the fluid flow drivers and pathways?
What are the metal transport and deposition processes?
You will note that the questions relate from the regional to the small scale.
At the regional scale, the continental geodynamic setting influences what type
of mineral deposits we should be looking for.
I will now illustrate how these questions are used in the assessment of mineral
deposits in the Mount Isa Inlier in Australia. The Mount Isa Inlier is one of the
most heavily mineralised terranes on earth, with several world class deposits
including the Mount Isa lead/zinc deposit, Mount Isa copper, Century zinc
mine, Ernest Henry copper/gold mine and the Cannington lead/silver mine. I
will look at mineralisation in the Inlier using three deposit styles to illustrate
how answering the five questions can assist in exploring for new deposits. The
three deposit styles are:
1. Shale hosted lead/zinc
2. Shale hosted copper
3. Granite related copper/gold (IOCG)
Shale hosted lead/zinc: These systems occur in shale and carbonate filled
graben structures. The fluids, derived from salt-rich beds, are drawn down
deeper into the basin during active extension. The fluids liberate zinc and lead
from feldspars in sandstone beds within the basin. The fluids are then expelled
during inversion (compression) following cessation of extension. The fluids
migrate up along the graben bounding faults and are deposited either on the
sea floor or just below the sea floor in highly reducing black shale layers.
These systems are therefore entirely constrained within upper crustal
extensional sedimentary basins. Looking for new deposits relies on the
identification of these basins, and possibly inversion events in the basins, and
also location of the early-formed bounding faults.
Shale hosted copper: In contrast to the lead/zinc systems, the source for
copper (and locally gold) is not from within the upper crustal rocks, but instead
is derived from deep crustal or possibly upper mantle sources, often in
multiple stages. Models for the generation of sediment hosted copper require
a mechanism to get the fluids and copper from the deep crust into upper
crustal settings where the copper, like the lead/zinc is deposited in black shale
beds. In order to achieve this deep crustal faults are required in order to tap
the deep crustal fluids. Expelling the fluids occurs during major regional
compression which affects the whole crust – not just the local basins as was
the case in the lead/zinc system. Much larger scale orogenesis is required.
In order to find new shale hosted copper deposits, we need to identify the
major deep crustal faults which would act as conduits. Timing of the copper
mineralisation occurs during major orogenies, rather than the smaller local
events for the lead/zinc.
Granite related copper/gold (IOCG): Like the shale hosted copper, these
deposits require transport of fluids and copper/gold from the deep crust into
the upper crust. However the mechanism for transport is different. In this case,
the fluids accompany granite intrusion, rather than requiring the deep crustal
faults as in the shale hosted system.
In order to find new granite related copper/gold, we need to understand which
granites are implicated in driving the deep fluids into the upper crust.
This figure shows the copper deposits identified in the Mount Isa Inlier. In the
west, where the shale-hosted deposits occur, mineralisation occurs along
faults. In the east however, the deposits are scattered due to their relationship
to granite intrusions.
To put this is context with the techniques required to find new deposits under
cover, we are looking at what data we would require to identify possibly
mineralised regions.
In the case of shale-hosted lead/zinc, we need to identify the presence of
extensional basins with the required salt-rich layers. We would also need to
identify the graben bounding faults. Such basins can be identified during the
regional geological mapping by observing thickness changes in sedimentary
sequences, with changes in thickness related to the graben bounding faults.
We can then use geophysics (mostly magnetics) to trace these faults under
cover. Recently we are using magnetotellurics and airborne electromagnetics
to identify fertile shale basins. We also need to understand the sedimentary
and geodynamic environment – many of these inversion events are reflected in
a change to the sediments deposited. Coarse sandstone beds in a shale
sequence for example, indicate a change in geodynamics and may represent
periods of inversion and may be good areas to look for mineral deposits.
If searching for shale-hosted copper, we would need to identify deep crustal
faults which could act as conduits for the fluids. Such faults are often
associated with changes in crustal structure and can be mapped on
aeromagnetics and gravity profiles. An understanding of basin evolution and
the timing of major orogenesis is critical to identifying prospective areas.
If searching for granite-related copper gold, a different array of datasets would
be required, and different criteria for exploration targeting. We need to
identify the presence of granite, and also the presence of fertile grounds for
deposition of the metals.
All of this preceding discussion highlights several important points:
1. Reliable geological maps are the basis for understanding the geology and
are a prime requirement for finding future mineral deposits of all types.
2. In order to effectively target deposits, particularly under cover, we need
to understand the type of deposit we are looking for
3. We need to understand the mineral system (five questions), in order to
know what features are critical for the localisation of deposits.
4. If we don’t immediately find the deposit, we need to perhaps identify
the fluid pathways which can provide vectors to the ore itself.
5. Particularly if we are targeting deposits under cover, we need to acquire
the appropriate geophysical data, particularly regional datasets
Development of these models in Australia has taken place because of a close
working relationship between government, academia and the
mining/exploration industry. Over the last 10 years, I have been involved as a
researcher in a Cooperative Research Centre (CRC) where the five questions
model was developed. CRCs are projects – with well-defined structure,
timeframes, and outcomes – which are funded by contributions from
government, academia and industry. Sometimes, the contributions are ‘in kind’
where academia provide researchers as their contribution to the CRC.
This model has the strength of targeted research which is of value to the
industry (in many cases supervised by the industry) and which is therefore
more likely to be adopted by industry in their exploration programmes.
However, there needs to be attention to the needs of industry or the
programmes develop may not be adopted as exploration strategies and much
of the value of the research is lost.