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Characterization of alteration minerals by portable SWIR- and XRF-Analyzer of drill cores, WOF 3, Rosh Pinah Mine, Namibia

Sebastian Heitzer

In this study, a portable short-wave infrared spectrometer (SWIR – TerraSpec) and a portable X-ray fluorescence analyzer (XRF Niton XL3) were used to characterize alteration minerals and their position in mineralized rocks from the Western Orefield 3 of the Rosh Pinah Zn-Pb (Ag) deposit, Namibia. The Rosh Pinah deposit is owned by Rosh Pinah Zinc Corporation Pty Ltd and produced 89,000 tons of zinc and 16,000 tons of copper in 2011.

Spectral analyses were performed in one centimeter distance over the length of the core sections. The spectral data were supported by comparing with X-ray fluorescence-readings on the same position where the spectrum was taken and additionally proven by petrographic thin sections and scanning electron microscope analyses (SEM, equipped by Energy-dispersive X-ray spectroscopy). All samples investigated in this study represent mineralized core sections of three boreholes of the Western Orefield 3 including ore zones and their host rocks.

Carbonate-dominated infrared spectra of the carbonate ore zone were highly influenced by sphalerite and pyrite, but a dolomitic background was still well developed and detectable by reflectance spectroscopy (Figure 1). Similar spectral properties were found also in the `massive ore` with its minor carbonatic background.

Figure 1: The spectra of sample u2707-13 are very variable, depending on their amount of dolomite and sphalerite and their interaction; Reference spectra: Sphalerite – sphaler5.spc.asd, Collection: USGS Jules Friedmann (USGS-Spectral Library).

Figure 1: The spectra of sample u2707-13 are very variable, depending on their amount of dolomite and sphalerite and their interaction; Reference spectra: Sphalerite – sphaler5.spc.asd, Collection: USGS Jules Friedmann (USGS-Spectral Library).

Figure 1: The spectra of sample u2707-13 are very variable, depending on their amount of dolomite and sphalerite and their interaction; Reference spectra: Sphalerite – sphaler5.spc.asd, Collection: USGS Jules Friedmann (USGS-Spectral Library).

SWIR-analyzes within the arkose ore zone show abundant the spectral occurrence of muscovite. Furthermore, spectra of the arkose ore zone are characterized by biotite and chlorite, both occurs above and underneath veins. Niton-readings measured over the arkose ore zone sections show a change in Ba-content from coarse grained to fine grained arkose (Figure 2).

Figure 2: Stack of arkose infrared spectra. Note the varying 1.9 µm feature of u2707-03 and its downsection measuring path. The low Ca values suggest a carbonatic matrix. Reference spectra: Muscovite – MuscON1.006f.asd, Collection: Dr. H. Kodama, Agriculture Canada (ASD-Spectral Library).

Figure 2: Stack of arkose infrared spectra. Note the varying 1.9 µm feature of u2707-03 and its downsection measuring path. The low Ca values suggest a carbonatic matrix. Reference spectra: Muscovite – MuscON1.006f.asd, Collection: Dr. H. Kodama, Agriculture Canada (ASD-Spectral Library).

Figure 2: Stack of arkose infrared spectra. Note the varying 1.9 µm feature of u2707-03 and its downsection measuring path. The low Ca values suggest a carbonatic matrix. Reference spectra: Muscovite – MuscON1.006f.asd, Collection: Dr. H. Kodama, Agriculture Canada (ASD-Spectral Library).

Dark and fine grained microquartzite (-ore zone) hosts dolomite veins and mottled replacement texture of potassium feldspars by barium bearing feldspar (Figure 3).

Figure 3: Detailed view of the very fine grained microquartzite of u2775-06. Note the brighter rims around the darker grains caused by Ba- and K-feldspars.

Figure 3: Detailed view of the very fine grained microquartzite of u2775-06. Note the brighter rims around the darker grains caused by Ba- and K-feldspars.

Figure 3: Detailed view of the very fine grained microquartzite of u2775-06. Note the brighter rims around the darker grains caused by Ba- and K-feldspars.

Both are not detectable using infrared spectroscopy, but were proven by X-ray fluorescence (increasing of potassium and barium; Figure 4), thin sections microscopy and by SEM investigations.

Figure 4: Downsection plot of the Niton values of u2806-13. The decreasing of K and Ba closer to the vein mineralization suggest a mineralization after the K-Ba-alteration.

Figure 4: Downsection plot of the Niton values of u2806-13. The decreasing of K and Ba closer to the vein mineralization suggest a mineralization after the K-Ba-alteration.

Figure 4: Downsection plot of the Niton values of u2806-13. The decreasing of K and Ba closer to the vein mineralization suggest a mineralization after the K-Ba-alteration.

An issue in detection of infrared active alteration minerals within fine grained, sedimentary rocks like the microquartzite or argillite, is a decreasing of reflectance caused by grain size effects and spectral influence of dark matter, e.g. carbonaceous material (Figure 5).

Figure 5: Spectrum of the vein mineralization. The 2.25 µm region is caused by dolomite. Note, that the characteristic hull line of sphalerite is not developed, although the vein contain up to 20 % zinc.

Figure 5: Spectrum of the vein mineralization. The 2.25 µm region is caused by dolomite. Note, that the characteristic hull line of sphalerite is not developed, although the vein contain up to 20 % zinc.

Figure 5: Spectrum of the vein mineralization. The 2.25 µm region is caused by dolomite. Note, that the characteristic hull line of sphalerite is not developed, although the vein contain up to 20 % zinc.

Investigations of the chlorite-biotite-schist show well developed, Fe2+-dominated spectra. Garnets, associated with chlorite-biotite-schist, are rich in manganese, iron and calcium (EDX-analyses) and indicate a metamorphose under upper greenschist to lower amphibolite facies conditions for the chlorite-biotite schist.

By short-wave infrared spectroscopy it is able to follow vein mineralization and their hydrothermal alteration products and it is also a method which is capable to allow statements to the related ore bodies that display the same absorption properties of infrared active alteration minerals, but showing mostly much weaker developed spectra. A portable XRF analyzer supports the SWIR spectrometer and allows an in-situ correlation of spectra and geochemical data in exploration studies (Figure 4).

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