Broken Hill area, Australia, as a Proterozoic fold and thrust belt: implications for the Broken Hill base-metal deposit

摘要

The Broken Hill Pb-Zn-Ag deposit of New South Wales, Australia, occurs in the metamorphosed Willyama Supergroup, which was deposited in the Willyama Basin in the early middle Proterozoie and forms a part of the Broken Hill Block. Both the interpretation of the stratigraphy and models for the genesis of the mineralization have assumed that there has been no major tectonic disruption by shears. It is demonstrated that this assumption is incorrect and that the metamorphosed Willyama Supergroup in the Broken Hill area was dismembered and tectonically transported during a prolonged Olarian thrust event, also in the early middle Proterozoic. Thrusting and associated folding, which may have begun under lower-temperature pro grade conditions, occurred at peak metamorphic conditions and continued through lower-grade conditions, with increasing partitioning of deformation into the shears. Major transfer and escape structures further subdivide the Broken Hill area of the Broken Hill Block into three provinces-the Northern, Central and Southern. The Broken Hill deposit occurs in the Central Province, which is the focus of the investigation reported here. The province is an assemblage of thrust sheets, each with its characteristic combination of lithostratigraphy and fold style. The geometry of the latter can often be directly related to the kinematics of the underlying footwall thrust. The Broken Hill mineralization occurs within the Broken Hill Thrust Sheet, which, because of its original rock types and subsequent metamorphic mineral assemblages, was mechanically weaker than the adjacent thrust sheets and has been extensively sheared internally. Most of the internal shears were also thrusts. This has led to tectonic stacking of the mineralized zones and the remobilization and local juxtaposition of some of the ore lenses. The tectonic stacking has inverted the stratigraphy locally, but it is doubtful if the entire stratigraphic succession is inverted in the Broken Hill mine area. The individual thrust and transfer shears have induced localized alteration zones at both high and low metamorphic grades. Those associated with the former are often parallel to bedding in the metasediments and, particularly in the case of the high-grade alteration zones, must not be regarded as part of the original basin sequence. The effects of the thrusting must be considered in any