Faults are important to the economic geologist in that they can either control the formation of some classes of ore deposits or modify the position and geometry of ore bodies. Fault rock classification is an important part of understanding the structural geology of an area. In this paper practical modifications are suggested to the most commonly used classifications of fault rock, to make it more applicable for the economic geologist. The system recommended is based in the first instance on the classification according to cohesiveness of the rock. The second defining attribute is whether the matrix is foliated or has a random fabric, providing the subdivision of cohesive fault rocks into the mylonitic and cataclastic series. The third attribute used to categorize fault rocks is the proportion of matrix to clasts, and differs in detail from those previously published. The terminology relating to crush breccias used by earlier systems is considered confusing and superfluous and has been eliminated.

Although of lesser importance to the economic geologist, the terminology evolving for mylonites and mylonitc gneisses is included and is based on internal fabrics and textures.

Application of this simplified classification to geological mapping in underground exploration development and logging of faults in core will enhance the geologist’s ability to interpret the structure of ore deposits.

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