Hydrothermal alteration is what happens when hot, chemically active fluids pass through rock and change its mineralogy. Around many ore deposits the fluid that carried the metal also rewrote the surrounding rock over a much larger volume than the ore itself. That halo is the target of alteration mapping, because it is bigger than the deposit and far easier to see.
The assemblages
Alteration mineralogy is not random. It records the temperature, acidity and fluid chemistry at each point, so distinct assemblages form in a predictable arrangement. In the classic porphyry copper model described by Lowell and Guilbert, the zones nest around the intrusion.
Potassic alteration sits innermost and hottest, carrying potassium feldspar and biotite.
Phyllic or sericitic alteration surrounds it, dominated by fine grained white mica, illite and muscovite, with quartz and pyrite.
Argillic alteration lies outward from that, carrying clays such as kaolinite and illite smectite formed at lower temperature and moderate acidity.
Advanced argillic alteration forms where fluids were strongly acidic, producing alunite, pyrophyllite, dickite and sometimes diaspore. It can sit above the system rather than beside it.
Propylitic alteration marks the cool distal edge, with chlorite, epidote, calcite and albite.
Why hyperspectral data reads them well
Most of these minerals are hydroxyl, water or carbonate bearing, which means they have diagnostic absorption features in the shortwave infrared between roughly 2.1 and 2.4 microns. Aluminium bound hydroxyl in white mica and clays absorbs near 2.2 microns, magnesium and iron bound hydroxyl in chlorite nearer 2.33 microns, carbonate near 2.34 microns, and alunite has its own characteristic position and shoulder shape.
These are narrow features separated by tens of nanometres. Broadband multispectral sensors can flag that clay rich or iron rich ground exists. Distinguishing alunite from kaolinite from illite, which is the distinction that carries the geological meaning, requires contiguous narrow bands.
There is more still. The exact position of the aluminium hydroxyl feature in white mica shifts with its chemical composition, and that shift has been used as a vector toward higher temperature parts of a system.
What it does and does not deliver
Alteration mapping tells you where fluids went and roughly how hot and acidic they were. Read as a zoned pattern rather than a set of isolated detections, it points toward the centre of a system and narrows a large licence area to a short list of targets.
It does not tell you whether metal was deposited, at what grade, or at what depth. It sees the surface. Drilling remains the only thing that reports grade.