Reddened alteration zones, Glas Allt Mr

East Meur Gorm - gulley following alteration vein

 

Decomposed granite at the head of Coire Raibert, with widespread development of haematite and chlorite amid multiple quartz veins

more on hydrothermal alteration                  

Hydrothermal alteration

Definition: the mineralogical changes resulting from the deep circulation of hot, pressured and supersaturated groundwater at a late stage in the cooling of a granite.

Alteration of the Cairngorm Granite is widespread, especially east of Gleann Einich (Harrison and Hutchinson, 1987). Tell-tale signs include a strong red colouration to the granite, due to the oxidation of disseminated magnetite to haematite, particularly in the feldspar lattice. Plagioclase is brick red and opaque where alteration is most pervasive and is replaced by sericite. Greenish hues may also be present, reflecting the presence of chlorite clays. Such weathering is known to occur sporadically at depths of up to 300 m in a borehole beneath the ski area (Webb and Brown, 1984).

Altered block from the path up the Fiacaill above Coire Cas. Similar debris litters the ground at the head of Coire Raibert and it is clear that the line of weakness provided by the alteration veins defines the location of the col here.

The alteration is often associated with quartz veins and confined to linear zones. The rock may be wholly or partially disaggregated and the hydrothermal alteration tends to predispose the granite to breakdown into grit. It is often unclear if kaolinisation is a result of hydrothermal action or subaerial weathering or some combination of the two. Although zones of hydrothermal alteration have yet to be mapped systematically, the zones of deep weathering adjacent to the floors of upper Glen Dee and middle Glen Avon suggest that hydrothermal alteration may have been a factor in determining the location and orientation of major valleys. These alteration zones are mineralised and some are associated with the development of cairngorms.