Conceptual Methods for Modelling Systems of Mineralized Echelon Veins: Examples from Southwest England and Portuga

Exploration & Mining Geology, Vol. 4, No. 3, 1995

K.A. FOXFORD*, R. NICHOLSON, Geology Department, The University of Manchester, Manchester, M13 9PL, United Kingdom. R.P.B. HEBBLETHWAITE Consulting Geologist, The Coach House, Blackwell Hall Lane, Latimer, Nr Chesham, Bucks, HP5 1TN, United Kingdom D.A. POLYA Geology Department, The University of Manchester, Manchester, M13 9PL, United Kingdo

A variety of techniques for interpreting systems of mineralized echelon veins is discussed using examples from Panasqueira and SW England. As aids for exploration and mine planning, and as a tool for ore reserve evaluation, particular emphasis is placed upon establishing vein morphology in the third dimension, and upon understanding vein dilation and propagation histories. Special attention is also placed upon investigating vein thickness and spacing relationships, and upon understanding vein segmentation. The relevant data for these techniques are all readily obtainable from stope and core sources. The vein systems described were produced in host-rock sequences that underwent incremental dilation in response to ?uid overpressures, and individual veins display evidence for mineral in?lling synchronous with their propagation. Echelon vein geometries resemble those reported from igneous dikes and sills: vein walls are stepped and veins are divided into segments arranged in echelons. Changes in vein geometry, from arrays to composite structures, re?ect both the evolution of the vein system with time, and the changes occurring between the margins (arrays) and core (composite structures) of the system. These characteristic morphologies can be used to infer vein propagation and ?uid injection trends, proximity of neighboring veins, and the shape of the system as a whole. The techniques are applicable to systems of echelon veins produced under tensile conditions in rocks where faults were absent at the time of vein formation, and where subsequent deformation has been minimal. Further work is required to assess the application to echelon vein systems formed within more complex tectonic regimes.

Mots Clés: Modelling Systems,Mineralized echelon veins, Vein systems, Vein dialation