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TitleA crystal-chemical investigation of the suitability of scheelite as a discriminator for different ore-forming settings: Initial results from cathodoluminescence studies
AuthorPoulin, R S; McDonald, A M; Kontak, D J; McClenaghan, M B
SourceGeological Association of Canada-Mineralogical Association of Canada, Joint Annual Meeting, Programs with Abstracts vol. 36, 2013 p. 165
LinksOnline - En ligne
Alt SeriesEarth Sciences Sector, Contribution Series 20120490
MeetingGAC-MAC Joint annual meeting; Fredericton, NB; CA; May 22-24, 2014
File formatpdf
Subjectsgeochemistry; scheelite; mineral deposits; arsenic; molybdenum; rare earths geochemistry
ProgramIntrusion/Porphyry Ore Systems, Targeted Geoscience Initiative (TGI-4)
AbstractScheelite (CaWO4) is a relatively common primary or accessory mineral in a large number of geologically diverse ore-deposit settings
that span a broad range of physico-chemical (PTX) conditions: vein/stockwork, skarn, porphyry and disseminated deposits.
In an effort to assess criteria that may be useful in discriminating scheelite from these diverse environments, a suite of grains from a range of environments and regions has been studied using cathodoluminescence (CL). The results represent the first in a series of analytical methods that will be used to fully document the crystalchemical features of these samples.
Alteration- and inclusion-free, subhedral scheelite grains, ranging in size from 0.5-5 mm, were selected for detailed study characterization. The initial backscattered secondary electron images of the grains indicate a lack of a pronounced zonation with regards to major and minor element chemistry. Whereas scheelite is known to exhibit a strong fluorescence under short wave UV radiation (254 nm), its fluorescence character under CL is significantly weaker.
This work indicates that extremely low beam currents (0.007-0.002 nA) are required to detect a CL-response; higher beam currents do not generate any CL-response as a result of quenching. It is also noteworthy that in cases where a CL response is evident, the beam current employed varies from grain to grain and that several beam currents must be tested in order to evaluate which is effective. These results show that scheelite grains obtained from disseminated epithermal quartz veins exhibit a long wavelength (blue-yellow) response, with grains typically showing complex oscillatory zoning patterns (on the scale of 5-100 ?m). A preliminary interpretation attributes the zonation to be a likely product of chemical changes during crystal growth. Conversely, scheelite grains from vein/stockwork and skarn deposits show almost no CL-response. In a few cases where there is a CL-response is minor and not oscillatory, the areas showing the greatest activation are located near fractures, suggesting late-stage, fluid-mediated alteration may be responsible. Previous studies investigating the CL-response of scheelite have suggested that it is directly related to REE3+ concentrations. Investigations are proceeding to study this relationship further.
The initial results of differing CL-responses of scheelite from different deposit types may be useful for screening scheelite from surficial sediment samples as a tool in determining ore genesis and drift prospecting.