Abstract

Hydrothermally altered rocks around volcanogenic massive sulfide (VMS) deposits can be used to define exploration vectors toward ore based on geochemical and mineralogical zonation. Near infrared spectrometry allows the proportions and compositions of white micas and chlorites in drill cores to be studied relatively quickly, in a nondestructive manner. In this study we focus on the spectral characteristics of alteration minerals in two effusive rhyolites, forming the immediate footwall and hanging wall of the Archean McLeod VMS deposit in the Matagami mining district, Québec, Canada. Mineral proportions and compositions are assessed within these individual lithostratigraphic units to avoid effects from stratigraphic position and different protoliths. Twenty-two individual drill cores taken from 0 to 1,700 m from the ore were chemically and spectrally examined to assess systematic proximal to distal variations. The most useful spectral parameter for exploration vectoring at McLeod is the average AlOH spectral absorption peak wavelength of white mica in the footwall rhyolite, which systematically decreases from >2,211 to 2,200 nm as the ore zones are approached laterally. In contrast, parameters such as the chlorite composition (FeOH wavelength) or mineral proportions are less successful at defining gradients at the McLeod deposit.

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