The Broken Hill Pb-Zn-Ag deposit

The massive Broken Hill Pb-Zn-Ag deposit consists of nine separate stratabound sulfide lenses enclosed within strongly deformed and metamorphosed pelitic schists and gneisses. The ore lenses are now considered to have a syngenetic exhalative origin, representing periodic outpouring of hot metal-bearing fluids in a sea-floor setting in which pelitic and psammitic sediments were deposited. The entire sequence belongs to the Willyama Supergroup, dated at around 1700 million years (Stevens et al., 2008). Peak metamorphism reached granulite facies conditions of 750-800 °C and 5-6 kilobar. It was during this episode that the sulfide lenses underwent partial melting and remobilisation, followed by recrystallisation into the characteristic assemblage of coarse-grained galena-sphalerite-manganese silicates (Frost et al., 2005). There were later phases of intrusion and faulting, which had localised effects on the ore lenses. The structural geology of that part of the sequence where mining has taken place is extremely complex and has generated considerable discussion. Several generations of folding are present, at least one involving overturning of the ore lens sequence. Longitudinally, the shape of the orebody resembles a boomerang up to 8 kilometers long and 850 meters thick. Eventually the orebody was exposed as a ragged black gossanous ridge, giving rise to its name.