Geotimes
Highlights
Economic Geology
Neil Phillips

Economic geology is the discovery, descrip
tion, interpretation and understanding of the economic development of mineral deposits. It contributes to the social and environmental aspects of a mine's development and eventual closure. Economic geology is integrative, drawing upon all fields of science and engineering relevant to understanding ore deposits. It also covers a breadth of scale from the nanoscience of gold complexing with reduced sulfur species, to plate motions forming mountain belts and ore deposits. Some of the earliest recorded periods of Earth's history contain evidence of ore formation processes, and these processes are in many ways little changed from those we see today on parts of the ocean floor.

The current climate in economic geology is dominated by a global downturn in the level of mineral exploration activity. This deep downturn started after the peak of exploration activity in 1997 and affects a wide range of commodities. All countries have felt the effects of the downturn, but some more so than others. Lately, there has been a greater importance placed on sovereign risk and security issues when companies decide where to explore around the world, with the result that the opening of new frontiers witnessed in the early 1990s has been reversed.

Exploration success (or lack of it) has also played a role with few major discoveries over the last years, although there are some notable exceptions to these statements. Diamond exploration success has underpinned continued interest throughout Canada, gold successes in Tanzania have continued, and Peru has seen base metal and gold success. Gold in Australia has been a standout success since the early 1980s, with 8,000 tons of gold discovered over the two decades, and new discoveries through the 1990s.

At the same time, the high level of exploration success for copper in Chile of the early 1990s has not been sustained, and the promises and gold successes in Indonesia have not been fulfilled. These trends and contrasting success rates mean that exploration activity in 2003 is focussed in Canada, Australia, Peru and a small list of other countries, and that emphasis overall is still on gold.

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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Phillips is chief of CSIRO Exploration & Mining. He joined CSIRO in 2001 after six years as general manager (geology) for Great Central Mines and associated companies. Prior to that he was professor of economic geology at James Cook University and director of the National Key Centre in Economic Geology. He is an Honorary Professorial Fellow of the School of Earth Sciences at the University of Melbourne, and regional vice-president (Australasia) of the Society of Economic Geologists.

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