Porphyritic rocks can be great indicators of the intensity of alteration in a hydrothermal system and several other useful things if they have the right set of minerals. I found one with all the right stuff.

The numbering system that I use in this video is for intensity of hydrothermal alteration. In short it is:
0: No visible changes.
1. Partial replacement of one mineral.
2. Complete selective replacement of one mineral.
3. Partial replacement of all minerals.
4. complete replacement with texture preserved.
5. Original texture destroyed by alteration.
I'm slowly collecting video examples for each of the levels to make a comprehensive video guide for the system and this example added some good material for the future video.