首页|The direct indicator mineral concept and QEMSCAN~R applied to exploration for carbonatite and carbonatite-related ore deposits
The direct indicator mineral concept and QEMSCAN~R applied to exploration for carbonatite and carbonatite-related ore deposits
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Indicator minerals have long been used in diamond exploration and the approach has been expanded to search for other commodities. Orientation surveys using stream sediment samples collected from the drainages of three carbonatite-related deposits in the Canadian Cordillera (Aley,Lonnie,Wicheeda) establish that a number of carbonatite indicator minerals contain elevated concentrations of high field strength elements (HFSE),including rare earth elements (REE) and Nb. These elements are detectable by traditional geochemical or portable X-ray fluorescence (XRF) analyses of stream-sediment samples even if only a few indicator mineral grains are present. We refer to these minerals,which are rich in carbonatite pathfinder elements,as direct carbonatite indicator minerals (e.g.,REE-fluorocabonates,REE phosphates,pyrochlore supergroup minerals,and columbite-iron). These direct indicator minerals can also be effectively identified by QEMSCAN~R (Quantitative Evaluation of Minerals by SCANning electron microscopy),a fully automated micro-analysis system that is widely available at commercial laboratories. There is a strong link between high concentrations of direct carbonatite indicator minerals and high concentrations of HFSE in stream sediments draining known carbonatites and related deposits. This relationship is the basis of the direct indicator mineral concept; only samples identified as geochemically anomalous by traditional laboratory or portable XRF analyses need to be examined for carbonatite indicator minerals by QEMSCAN*. Advantages of QEMSCAN~R over traditional indicator mineral proceBing and analysis include the capability to: 1) identify minerals that are too small for hand picking; 2) provide automated,simultaneous indicator mineral grain counts and weight % equivalents; 3) perform detailed grain-size analysis on a mineral-by-mineral basis; 4) quantitatively aBeB abundances of free and polymineralic grains; and 5) identify mineral species within polymineralic grains. Because all samples are first geochemically analysed for carbonatite pathfinder elements,only those with concentrations above background need to be evaluated for carbonatite indicator minerals,thereby reducing costs. The direct indicator mineral concept in combination with QEMSCAN~R analysis is an effective mineral exploration approach because it combines traditional geochemical analysis with the interpretative power of modem indicator mineral surveys. This approach may have exploration applications for deposits other than those that are carbonatite-hosted.
Indicator mineralsniobiumrare earth elementscarbonatitepyrochlorecolumbiteREE-fluorocarbonatesstream sedimentsautomated scanning electron microscope analysisQEMSCAN~R
G.J. Simandl、D.A.R. Mackay、X. Ma
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School of Earth and Ocean Sciences,University of Victoria,Victoria,BC,Canada,V8P 5C2
British Columbia Geological Survey,Ministry of Energy,Mines and Petroleum Resources,Victoria,BC,Canada,V8W 9N3
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