The geology and geological development of the broken hill lode in the new broken hill consolidated mine, Australia part I: Structural geology

Abstract

Three age groups of structures are recognized regionally; the oldest of these can be subdivided into at least two age groups within the NBHC Mine. The oldest structures, which are defined as those which occur in rocks affected by the highest grade regional metamorphism, formed before the final high‐grade equilibration of the rocks. In the last phase of the oldest deformation, and in the second deformation, which was accompanied by amphibolite‐facies retrograde metamorphism, shear‐zone‐like structures formed, whose orientation and character indicate that the rocks were compressed horizontally and extended vertically. Foliation within the ‘shear zones’ is inclined to the attitude of the zone containing it; so folds within ‘shear zones’ are en‐echelon and do not plunge parallel to large folds formed on the boundaries of zones. Most of the lode and associated rocks must have existed as mechanically distinctive units at the time of the youngest phase of the oldest period of deformation, since the style of folds formed in this deformation are related to present differences in the lithology of the folded rock units. However, stratigraphic relations indicate that some of the rocks with a low Pb‐Zn content and at least some garnet quartzite formed by metasomatism which accompanied the regional metamorphism, i.e. by metasomatic interaction between adjacent rocks during metamorphism.