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Integrated geoscience projects
Economic geology has played a critical role in some recent integrated geoscience
projects with novel sharing of data and techniques across previously disparate
geoscience fields. The Yandal Project in Western Australia used 100,000 drill
holes in one of the world's exploration success areas of the 1990s (Yandal gold
province) to provide a 3-D view of basement geology, regolith cover, groundwater
characteristics and metal redistribution, especially of gold. This flat desert
landscape is now known to be underlain by an extensive network of paleochannels
stretching hundreds of kilometers, and in places 100 meters wide and nearly
100 meters deep. The Gilmore Project, and proposed Victorian Geotraverse, both
in eastern Australia, focus on the interface between rock outcrop of the highlands
and the extensive cover of the Murray River basin.
With benefits for mineral exploration, agriculture, land and water management,
and earthquake risk, these projects bring together a range of skills, technologies
and resource opportunities.
The TEMPEST electromagnetic system is a classic example of the multiuse of technologies.
TEMPEST was originally developed to probe covered areas in the investigation
for buried conductive base metal sulfide deposits, but in some cases this interpretation
was complicated by conductive saline groundwater. The method has built on this
difficulty to now be a very useful method for mapping the subsurface distribution
of saline groundwater layers.
Some of these projects draw upon the successes of the Canadian
Lithoprobe project, which has been longer established but less focused on
economic geology.
New gravity survey technique
Since 1991, BHP Billiton has been working on developing a method to measure
slight variations in gravity from the air (Falcon Gravity Gradiometry Project)
and in December 1997 the first airborne gravity gradiometer (AGG) system for
exploration was in operation. Ground-based gravity surveys are slow and costly
and often only survey a very small area to the required density to find mineral
deposits. To be able to measure, from a plane, 1 in a 1,000,000 variations in
Earth's gravitational field, numerous obstacles needed to be overcome. This
airborne process can complete a survey in a day, which would take months to
complete using traditional ground gravity.
The Falcon system produces large amounts of data, which are later corrected using mathematical algorithms. The accuracy of the AGG system is comparable to ground surveys and can be used in exploring for a large range of deposit types. Currently, two aircraft are in operation and two more systems are being developed, one for helicopter operation. The helicopter-mounted system will allow for a lower flight height and further improve the accuracy. The AGG system is continually being improved and the processing methods are also being improved. BHP Billiton has exclusive rights for the process until 2007.
Seabed ore systems
Leg 193 of the Ocean Drilling Program (ODP) investigated sub-seafloor hydrothermal
processes at a felsic volcanic — a convergent margin setting. The findings
will be compared to basaltic mid-ocean ridge sites previously drilled on ODP
Legs 139, 158 and 169.
The main aims of Leg 193 were:
• to sample the third dimension of the dacite-hosted PACMANUS hydrothermal
system, in the Manus Backarc Basin of Papua New Guinea, Southwest Pacific Ocean,
• to better understand factors that govern the nature and location of mineral
deposition,
• to seek evidence relating to fluid and metal sources, and
• to investigate subsurface microbial life.
The PACMANUS system is a modern analog of a common geological setting for ore
bodies in ancient sequences, where subsequent deformation and metamorphism often
obscure evidence for their modes of formation. The Leg 193 strategy was to drill
deep holes into both the Snowcap area of low-temperature diffuse venting (Site
1188) and the Roman Ruins high-temperature chimney site (Site 1189). Penetration
to 387 meters below the sea floor at Snowcap, and 206 meters below the sea floor
at Roman Ruins, allowed comparison of mineralization and alteration patterns
below these different seafloor settings.
What did Leg 193 find out related to its aims?
With respect to its main aims, Leg 193 was successful. However, a number of
major outcomes of the drilling came as a surprise: (1) intensity and extent
of subsurface hydrothermal alteration; (2) predominance of clay minerals in
altered rocks; (3) high porosity of altered rocks, (4) frequency of anhydrite,
implying a major role for seawater; and (5) scarcity of sulfide mineralization
(apart from disseminated pyrite), though with the constraint of low core recovery.
Leg 193 also confirmed that microbes flourish in subsurface rocks.
Leg 193 was truly an international effort, with CSIRO Exploration and Mining's
Dr Ray Binns and his team doing the bulk of the ground work leading to the drilling
proposal being accepted, with help from Professor Steve Scott of the University
of Toronto, Canada. Dr Ray Binns was Co Chief Scientist of Leg 193 together
with Professor Fernando Barriga of Lisbon University; and Dr Jay Miller was
the ODP scientist (Binns, Barriga, Miller et al., 2002).
References.
Binns, R.A., Barriga, F.J.A.S., Miller, D.J. et al., 2002. Proc. ODP, Init.
Repts., 193 [CD-ROM]. Anatomy of an Active Felsic-Hosted Hydrothermal System,
Eastern Manus Basin Sites 118-1191. 7 November 2000-3 January 2001. Available
from: Ocean drilling Program, Texas A&M University, College Station TX 77845-9547,
USA.
A new deposit type for the future
Zinc oxide deposits are emerging as a new and important class of zinc ore because
they offer a low-cost option for the production of zinc metal without pyrometallurgical
processing. Do you want to define mine gate here, or perhaps use a more general
term? For example, the zinc oxide at the Skorpian mine in Namibia contains 21.4
million tons at 10.4 percent zinc. It is coming on-stream and is planned to
produce around 150,000 tons of zinc metal annually over 20 years at a cash cost
of around U.S. $0.20 per pound. It will be the first zinc oxide operation to
employ a solvent extraction electrowinning hydrometallurgical process, a relatively
new process for extracting the mineral from ore. Though unlikely ever to transplant
zinc sulfides as the major source for zinc, this style of mineralization warrants
further attention as it avoids some of the environmental issues of processing
sulfide ores (such as the effects of releasing sulfur gases in to the atmosphere).
It is also one of the lowest-cost zinc producers in the world.
Many minerals that comprise zinc oxide mineralization, such as zinc-rich clays
and zinc-rich hydrated phosphates, can be difficult to identify in hand specimens
and therefore can be easily overlooked.
Recent developments in geochronology using uranium-thorium/helium or uranium-helium
significantly broaden the potential application of this new technology to economic
geology. Uranium-helium dating applications in economic geology primarily involve
the determination of the sub-300 degrees Celsius thermal history of an ore deposit
or a mineral district. Time-temperature data can be applied as a direct indicator
of cooling rates in an ore genesis context or to determine the chance of preservation
of ore deposits in structurally complex terranes that have been differentially
exhumed.
Moving beyond apatite, uranium-helium radiometric dating methods have recently
been developed for fluorite, titanite, zircon and rutile with diffusion experiments
demonstrating a progressive increase in the minimum helium closure temperature
(i.e., the temperature below which a certain mineral retains its uranium-helium
signature) for these minerals from 60 degrees Celsius for fluorite to 220 degrees
Celsius for rutile (for cooling rates of 10 degrees Celsius per million years).
This means there is now a set of minerals that between them record stages of
the cooling history. Work is underway on developing uranium-helium dating methods
for scheelite, epidote, garnet, magnetite and other minerals associated with
ore deposits. Commercial helium extraction and measurement instruments employing
compact diode lasers and remote automation routines are making entrance into
this field easier and more affordable for university and government research
labs.
In Tertiary copper belts, combined radiometric studies using zircon uranium-lead
and zircon/apatite uranium-helium have been used to reveal the complete thermal
history of a porphyry deposit from the time of emplacement of the magma through
the time of cessation of copper transport in hydrothermal solutions to the time
the deposit cooled. Because zircon/apatite, (uranium-thorium)/helium dating
methods reveal the sub-300 degrees Celsius portion of the cooling history, economically
important heat transfer processes associated with emplacement depth and hydrothermal
fluid flow can now be determined.
Such applications in the classical porphyry copper belts of Indonesia, Papua
New Guinea, Chile and Iran have constrained the timing of copper deposition
from magmatic hydrothermal fluids to less than 500,000 years. Cooling rates
through the temperature interval 750 degrees Celsius to 70 degrees Celsius for
the Pliocene copper deposits at Batu Hijau, Ok Tedi, Grasberg and El Teniente
exceed 1,000 degrees Celsius per million years, in accordance with their emplacement
within the uppermost 3 kilometers of the crust. With pluvial denudation rates
of 3 to 5 millimeters per year typical for these mountain belts, these ore systems
have an expected preservation time of less than 1 million years. In contrast,
Ordovician porphyry copper-gold deposits of the Lachlan Fold Belt in eastern
Australia have been exhumed at a rate of around 0.02 millimeters per year since
the Tertiary, explaining why eastern Australia remains an important exploration
target for buried copper-gold deposits.
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