Geotimes
Untitled Document

Highlights
Economic Geology
Neil Phillips

The global mineral exploration industry has continued the slow recovery from the major cyclical downturn of post-1997. Prices of commodities, such as gold, iron, coal, copper, nickel and zinc, have risen significantly in U.S. dollar terms but somewhat less so in the local currency of many producers.

Total mineral exploration by country remains focussed on Canada and Australia with nearly 20 percent each, and significant rises in South American countries such as Peru. The shift of exploration away from the United States, Indonesia and Zimbabwe has continued for a combination of policy, security and sovereignty reasons.

The 2003 Fraser Institute report combines these parameters with the perceived favorability of the geologic factors for making discoveries; it highlights the long-term opportunities for Russia, and the favorable prospects for Nevada, Chile and Western Australia.

Gold

A number of issues are emerging in gold geoscience research and development as topics in the year ahead. Deep weathering effects of gold deposits and their immediate host rocks are recognized as critical in several areas including the Carlin Province, Nev., and the Yilgarn and Victorian gold provinces of Australia. The sulfide minerals in these ores have reacted with oxidizing groundwater to generate acids that, in turn, react with the surrounding carbonate halo to produce over-deep weathering at several deposits. This effect is exaggerated where any structural complexity around the ore body facilitates deep groundwater movement.

In the Carlin gold province, this weathering extends to 800 meters and possibly 1,500 to 2,000 meters in depth — as deep as the world's deepest cave systems. The next research frontier will be integrating this deep weathering into current ideas on how and when these gold deposits formed.

Another gold issue that does not go away for long is the origin of the Witwatersrand (Wits) goldfields. The origin of Earth's early atmosphere is closely linked to the Wits gold. The discovery of uraninite in the Wits in thr 1940s posed a problem for placer gold origin in an oxidizing atmosphere. Thus in 1955, based on the Wits gold, researchers proposed a reducing early atmosphere. Since then many related, and some barely related, chemical systems have been used to constrain the early atmosphere, using elements of progressively less direct connection to the atmosphere. The results from the recent Mars robotic mission are sure to help in the understanding of ore deposits, early atmosphere and the origins of life.

Publication of Yandal Gold Province during the year brings together summaries of the exploration and deposits in a province attracting great interest in recent years. The book was only possible so soon after the 1995 discoveries in the Yandal province because of the open-data policy of the companies at the time, and the full integration of geology and research as part of the exploration, development and mining process.

Base metals, gold and partial melting

Partial melting of ore deposits is an exciting area of development with studies at both gold and base metal deposits in upper amphibolite to granulite facies domains. At the Challenger gold deposit in the Gawler craton of South Australia, Andy Tomkins and John Mavrogenes have documented melting of the gold-sulfide ore and the importance of this in gold re-distribution. At the gold-lead-zinc Broken Hill deposit, also in granulite facies, Heather Sparkes and Mavrogenes have attributed silver-lead rich dropper ores to partial melting of the pre-existing massive sulfide ore with re-distribution over tens of meters. Dick White and Roger Powell using the Thermocalc approach have recently provided the thermodynamic framework to semi-quantitatively understand these melting processes.

Nickel

In the nickel world, the emphasis in the 1990s on the treatment of the vast laterite nickel deposits shifted to small, high grade nickel sulfide deposits, as the laterite processing met technical and commodity price barriers. These now seem to be being slowly resolved. Laterite nickel deposits are formed by deep weathering of nickel bearing rocks, and they need pressure leaching to remove the nickel. These ores are common in Cuba, New Caledonia and Australia.

There is a broad consensus that most of the major nickel-copper-platinum group elements sulfide deposits of the world involve some component of crustal sulfur assimilation in their genesis, but there is much debate about details and basic principals. The giant Noril'sk-Talnakh deposits have been the subject of vigorous debate over several years centered around the origin of a distinctive signature of chalcophile (particularly platinum group) element depletion in related overlying basalts, and the timing and depth of sulfur assimilation. The debate has important implications for the use of platinum depletion signatures as a potential regional targeting tool.

A similar debate surrounds the origin of komatiite-hosted deposits (the main nickel bearing rocks formed at very high temperatures from deep in Earth), along with a related controversy about whether or not komatiites could have been generated as hydrous melts in subduction zones, rather than as deep-seated anhydrous melts from mantle plumes. There is also an ongoing debate on the relationship between komatiite flows and associated thick (and sometimes mineralized) dunitic (nickel-related rock) cumulate bodies; the debate centers on whether these dunite bodies are extrusive bodies formed in vast feeder lava tubes, or whether they are younger sub volcanic intrusions. This has important implications for targeting strategies.

Back to index


Philips is chief of CSIRO Exploration and Mining at Clayton, Victoria. E-mail: Neil.Phillips@csiro.au.

Untitled Document

Geotimes Home | AGI Home | Information Services | Geoscience Education | Public Policy | Programs | Publications | Careers

© 2014 American Geological Institute. All rights reserved. Any copying, redistribution or retransmission of any of the contents of this service without the express written consent of the American Geological Institute is expressly prohibited. For all electronic copyright requests, visit: http://www.copyright.com/ccc/do/showConfigurator?WT.mc_id=PubLink