Visible/Infrared Spectroscopy (VIRS) as a Research Tool in Economic Geology: Background and Pilot Studies from Newfoundland and Labrador

Visible/Infrared Reflectance Spectroscopy (VIRS) measures absorption in the 350 nm to 2500 nm region of the electromagnetic spectrum, where many minerals have distinct responses that allow their identification, even within mixed parageneses. The VIRS method characterizes a range of hydrothermal and supergene alteration minerals, and also some primary igneous and metamorphic minerals. Data acquisition is simple and rapid, but interpretation is more complex, requiring a large database of reference spectra. Automated computer programs can simplify and expedite this process, but human analysis and reasoning are also required, and VIRS data are most useful where combined with other data, notably petrography and geochemistry.

Several pilot studies, summarized here, illustrate the versatility of the VIRS method. Studies of alteration, associated with epithermal gold mineralization, easily recognized key indicator minerals such as pyrophyllite, alunite, dickite and topaz; such information may be useful in mapping zoned alteration systems, and vectoring toward the most favourable areas. A study of footwall alteration in a VMS deposit did not support previous visual identifications of pyrophyllite, but gave ambiguous results with respect to kaolinite. Modelling showed that subordinate kaolinite could be masked by interference from more abundant white mica (sericite). The VIRS data from a mesothermal vein-type gold deposit indicate subtle alteration signatures, on a scale of several metres, around auriferous veins, even though visual evidence for such effects is muted. The mineralogical causes for this effect are as yet unresolved, but this does not prevent its use as a proximity indicator in exploration. Studies of porphyry-style Mo–Cu deposits, associated with sheeted veins, document the progressive overprinting of regional chloritedominated propylitic alteration by focused potassic to phyllic alteration associated with the mineralization. A later stage of advanced argillic alteration, typified by kaolinite, was also documented in one deposit. A reconnaissance investigation of rareearth element (REE) mineralization in Labrador suggests potential for the VIRS method in studies of host rocks and mineralization. The VIRS data can distinguish Na–Fe-rich chain silicates characteristic of peralkaline igneous rocks from common pyroxenes and amphiboles, facilitating identification of potential host suites. The REE also generate unusual absorption features in the visible and near infrared, which directly indicate specific REE ions. Although there are few reference spectra for complex REE-bearing minerals, the VIRS data recognize mineralized samples, and may help to identify samples collected on the basis of other criteria, for geochemical assays. Finally, VIRS analysis of samples from central Labrador now provides the first reference spectra for the REE-enriched mineral eudialyte.


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