Mapping regional alteration patterns using hyperspectral drillcore scanner

Abstract

Hyperspectral drill core scanning undertaken using the CSIRO HyLoggerTM between 2002 and 2005 focused on accumulating spectral data from a series of signature holes from across the State of South Australia. One component of the software used to process the data provides a summary for each hole indicating the amount of each detected mineral as a percentage of the scanned hole. By converting this percentage into the number of metres of detected mineral present in the drill hole this information can be presented in a GIS. Four mineral suites are coming to the fore in alteration mineral mapping using HyLoggerTM: white micas, chlorites, carbonates and iron oxides. Each of these suites can relate to Eh-pH conditions in a mineralising system. For white mica, the transition from muscovite through phengite, as measured by the progressive change in wavelength position of the ~2200 nm absorption feature to longer wavelengths, corresponds with increasing replacement of aluminium in the crystal structure by iron or magnesium. Empirical studies show a correlation between concentration of economic metals and the presence of phengite that may also reflect local fluid pressure conditions. The ratio of Fe/Mg in chlorite has also been shown to vary in proximity to mineralisation. Calcium, iron and magnesium carbonates are a third component. Spectral studies have distinguished between hematite, Fe2+ goethite and Fe3+ goethite. HyLogger detects in wavelengths appropriate to these suites and software interprets relative abundances. Plotting the number of downhole metres of core containing these assemblages in their relative geographic locations permits interpretation of regional patterns of alteration. With some 600 holes (including 300 on open file) and 61,000 metres of core scanned across South Australia, regional patterns are starting to appear.