The lead-silver-zinc ore deposit of Broken Hill, New South Wales, is among the great ore deposits of the world because of its size, richness, and continuity. Up to the end of 1946 approximately £50,000,000 in dividends had been won from recoverable metals worth £210,500,000 gross contained in 63,800,000 tons of ore. The deposit is a hypothermal deposit of Precambrian age resulting from the selective replacement of two closely adjacent, tightly and complexly folded stratigraphic rock layers.
The original sedimentary rocks of the area now consist of tightly folded sillimanite-garnet gneisses with subordinate thin quartzite beds. These contain numerous folded sills of augen gneiss (granite), amphibolite (gabbro), and pegmatite. Post-folding peridotite, granite, and pegmatite occur. Probably following considerable uplift and erosion, thin dikes of diabase (dolerite) were intruded, followed by pegmatite dikes and silicifying solutions, and finally by ore-bearing solutions.
The folds of the region are tight, steeply inclined, and extremely complex structures resulting from plastic deformation. Individual minor folds were studied in great detail by a method of axial-plane and axial-line analysis. An angular relationship exists between minor and major folds due to strain under torsional stresses. The regional pitch is flatly south. However, sudden reversals of pitch and divergences of pitch in adjacent folds are common. “Second-order” folds or folded folds exist. Cutting and offsetting the folds are “buckles” with vertical axes and crush zones of schisted rocks resulting from post-folding but pre-ore faulting movements.
The lode occurs in a “belt of attenuation” between a wide arch on the west and wide basin on the east. It consists of massive lead-zine-sulphide replacement ore bodies forming (before erosion) a long continuous, irregularly shaped, flat, curving pencil of ore, 2000–3000 feet high and 300 feet thick. In longitudinal section it describes a flat arc pitching downward at each end.
The “lead lodes” resulted from the selective replacement of two closely adjacent highly folded favorable beds. Each lode is distinguishable by its gangue mineralogy and metal ratios. No. 3 Lens (the lower) has fluoritic-rhodonitic gangue and comparatively high Zn:Pb and Ag:Pb ratios. No. 2 Lens, the upper, has calcitic gangue and comparatively low Zn:Pb and Ag:Pb ratios. At least three “zinc lodes”, with similar mineralogy, occur at higher stratigraphic horizons at the south end of the field.
There is little observable zoning. Ore solutions are believed to have migrated up the regional pitch from the south. Intra-mineralization fracturing helped to localize ore shoots within favorable formations.