Present and Future Geophysical Methods for Ni-Cu-PGE Exploration: Lessons from McFaulds Lake, Northern Ontario
Published:January 01, 2010
Stephen J. Balch, James E. Mungall, Jeremy Niemi, 2010. "Present and Future Geophysical Methods for Ni-Cu-PGE Exploration: Lessons from McFaulds Lake, Northern Ontario", The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries, Richard J. Goldfarb, Erin E. Marsh, Thomas Monecke
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Geophysical surveys have played a defining role in the discovery and subsequent delineation of many nickel, copper, and platinum group element (Ni-Cu-PGE) deposits. The high conductivity of pyrrhotite and the robustness of the electromagnetic methods that have been developed to directly detect this mineral are responsible for the exploration success. The introduction of concentric time domain electromagnetic (EM) systems towed by helicopters (known as HTEM systems) has led to direct drilling programs, providing more timely feedback on the nature of conductive sources, and, therefore, an increased ability to test more targets in a given field season. The EM techniques have also evolved to better penetrate conductive overburden allowing for more confidence in areas with no outcrop.
In this paper, we summarize a number of geophysical surveys from two Ni-Cu-PGE occurrences in the McFaulds Lake of northern Ontario. The discovery was made during a period of time in which HTEM systems were not fully accepted for direct-drill programs. As a result, exploration began using traditional methods including ground geophysics and later migrated toward modern airborne methods.
The future of Ni-Cu-PGE exploration using geophysics will continue to be evolutionary. There will be a gradual decrease in the reliance on ground geophysics because surface methods have not kept pace with airborne methods and offer little to no additional information on the nature, position, and orientation of the target conductor. Infield interpretation with additional flight lines designed to better define discrete targets will be slowly implemented as more geophysicists become familiar with real-time profile interpretation. Multiple flights over conductive sources at different flight heights, will reduce the uncertainty between small targets near surface and deeper sources that are only partially resolved. Closer spacing of the flight lines will provide improved strike direction estimates and will help resolve the nature of the conductor (e.g., a continuous source versus a series of discrete lenses). Geophysical technology will ultimately lead the geologist in an interesting direction, one where geophysical surveys will be followed by drilling and then geological mapping methods, in an effort to develop a working exploration model for the discovery of buried mineral deposits in areas with little to no surface exposure and thus geologic information.
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The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries
VOLCANIC-ASSOCIATED and sedimentary-exhalative massive sulfide deposits on land account for more than one-half of the world's total past production and current reserves of zinc and lead, 7 percent of the copper, 18 percent of the silver, and a significant amount of gold and other by-product metals (Singer, 1995). A new source of these metals is now being considered for exploitation from deep-sea massive sulfide deposits. Because the oceans cover more than 70 percent of the Earth's surface, many expect the ocean floor to host a proportionately large number of these deposits. However, there have been few attempts to estimate the global mineral potential. Significant accumulations of metals from hydrothermal vents have been documented at some locations (e.g., 91.7 Mt of 2.06% Zn, 0.46% Cu, 58.5 g/t Co, 40.95 g/t Ag, and 0.51 g/t Au in the Atlantis II Deep of the Red Sea: Mustafa et al., 1984; Nawab, 1984; Guney et al., 1988). Even more metal is contained in deep-sea manganese nodules. Current estimates in the U.S. Geological Survey (USGS) mineral commodities summaries indicate a global resource of copper in deep-sea nodules of about 700 Mt. In the Pacific "high-grade" area, an estimated 34,000 Mt of nodules contain 7,500 Mt of Mn, 340 Mt of Ni, 265 Mt of Cu, and 78 Mt of Co (Morgan, 2000; Rona, 2003). A number of countries, including China, Japan, Korea, Russia, France, and Germany, are actively exploring this area.