The geometry of Fe and Fe variations around fractures in weathered granite

Generally, in weathered granite, conspicuous Fe alteration occurs in ironstained rims subparallel to fracture planes. The fractures delineate core-blocks each of which has its inner envelop of Fe staining. Colour changes and magnetic susceptibility logging of granite drill cores show that, as the fracture is approached from within the core-block: 1) Fe staining begins up to a metre or more from the fracture. 2) Titanomagnetite content (a rough measure of Fe2+) drops sharply within centimetres of the fracture. Oxide loss and iron staining is usually associated with subsurface weathering since it is seen to decrease with depth. Some 1 Km deep open fractures (Canadian granites) show Fe staining and primary oxide loss comparable to near-surface zones, testifying to the considerable low-temperature alteration at depth in open fractures.