Structure of the Broken Hill Mine area and its significance for the genesis of the orebodies

Abstract

A comprehensive reinvestigation of the Broken Hill mine area shows that the structural evidence is consistent with, and reinforces, interpretation of the orebodies as stratiform premetamorphic deposits.The fold structure is interpreted in terms of three deformation episodes (F 1 , F 2 , F 3 ). F 1 and F 2 were accompanied by high-grade metamorphism, whereas F 3 is associated with retrograde metamorphism and with the initiation of retrograde shear zones.The principal macroscopic folds of the mine area are shown to be of F 2 age, and the earlier concept of a Broken Hill anticline is reinstated as the Broken Hill antiform of F 2 age. Folds which were thought to disprove the existence of the Broken Hill anticline are now seen as F 3 folds superposed on the F 2 antiform. The orebodies lie in the northwest limb of the antiform, the hinge of which is largely replaced by a synmetamorphic slide, previously recognized as the belt of attenuation.Evidence from metamorphosed graded bedding confirms the existence of the antiform and also shows that the macroscopic F 2 folds are superimposed on a previously inverted stratigraphic succession. This inversion is the consequence of F 1 deformation.The three-fold episodes were roughly coaxial, with southwest plunge, over much of the area but have different axial plane orientations. In the North mine area the plunge of the orebody and of the F 2 fold axes is northeasterly, but sillimanite lineations continue to plunge southwest. A new analysis of this region is given and, contrary to recent views, it is shown that the lode horizon, which contains the ore lenses, is conformable with the stratigraphic succession.These conclusions lead to the establishment of a new stratigraphic succession, in which the orebodies lie close to the interface between a lower, partly volcanogenic sequence and a higher, nonvolcanogenic sequence. The lead lodes lie stratigraphically above the zinc lodes.It is inferred, therefore, that the ore constituents were originally deposited in a zone of uncertain but probably linear shape, within a limited stratigraphic interval toward the end of a phase of volcanism represented by the Potosi gneisses. There have, however, been substantial mechanical migrations of ore, within the lode horizon, during the deformation history and it is this deformation which controls the details of orebody thickness and shape.