Latitude -31.960974, Longitude 141.458759
Located in the Outback Region of NSW
References "Mining and Exploration at Broken Hill: A Review ". Sydney: Department of Mineral Resources, New South Wales, 1981.
The Broken Hill Ore Deposit is located underneath Broken Hill in western New South Wales, and is the namesake for the town. It is arguably the world's richest and largest zinc-lead ore deposit.
The Broken Hill ore deposit was discovered in 1883 by boundary rider Charles Rasp, who discovered the gossan (weathered sulphide outcrops of massive lead-zinc sulphides) on a feature known locally as Broken Hill. Rasp reported finding massive galena, sphalerite, cerussite and other oxide minerals, but was most concerned with the galena, a primary source of lead. His reports (believed exaggerated at the time) of masses of lead in the desert, soon proved true and sparked a 'lead rush' similar to the gold rushes in other areas of eastern Australia.
Broken Hill was initially exploited by small prospectors working the gossan for easily won galena, and it was not long before dozens of shafts were sunk. Ore from these shafts was carted to South Australia by camel trains, wagons and pack mules. A major secondary source of income was also quickly identified: extremely high silver grades (including native silver) were recovered, and rare silver minerals were present in abundance.
Since the boom-time of Broken Hill, mining has gradually moved away from the initial small prospectors (in line with the experience of all other major mineral fields), toward gradual consolidation of claims and ownership, an increase in tenure and mine size and efficiencies in operation, resulting in smaller workforces.
This has accelerated in the last part of the 20th century via the formation of the Broken Hill Proprietary Company - now BHP Billiton - and its exit from Broken Hill. There are now only two operators in this area, both utilising highly efficient bulk underground mechanised mining.
The Broken Hill ore body is hosted within the gneisses (rock formed by high-grade regional metamorphic processes) of the Willyama Supergroup. The Broken Hill ore deposit is considered to be approximately 1,800 million years old.
The geology of the Broken Hill ore body is a series of boomerang-shaped, highly sheared and disrupted, ribbon-like and poddy (elongated, lens shaped) massive sulphide lenses which outcrop in the central section (hence the name the "Broken Hills") and then plunge steeply north and moderately south.
The ore consists of massive, recrystallised sphalerite-rich (zinc rich), galena-sphalerite(leadzinc rich) and galena-rich (lead rich)sulphide lenses often consisting of up to 100% lead-zinc sulphides. The ore itself is hosted within a unit of gneiss known as the Potosi Gneiss.
The origin of the Broken Hill ore body is of great historical significance to geologists, particularly in Australia, as it is an iconic ore body and one of the most studied in the world, with over 1,500 related papers published to date. It is also of great importance, as theories about the beginning of this perplexing ore deposit and its structural and stratigraphic setting, drives local exploration for repetitions of the deposit along its structural trend. Also, these theories are helpful in finding similar ore bodies elsewhere in the world.
The origin of Broken Hill is also of interest because this topic is still fraught with controversy and conjecture; the jury is mostly still out on the matter although consensus has been reached on several key facets of the genetic processes which resulted in Broken Hill's formation. The interpretations presented below are the most likely middle view of a range of opinions.
SedEx deposits (Sedimentary exhalative deposits) are ore deposits which are usually interpreted as having been formed by the release of ore-bearing hydrothermal fluids into a water reservoir (i.e.:the ocean), resulting in the precipitation of stratiform ore (ore minerals are distributed in a manner that resembles particles in a sedimentary rock). Broken Hill is widely considered to be a sedimentary exhalative (SedEx) deposit which has been extensively reworked and modified by metamorphism and shearing. Key evidence for this over-arching theory includes the association of silver, lead and zinc, which is found in many other SedEx deposits worldwide, and the position of the bulk of mineralization at a key stratigraphic contact between psammite and psammopelite gneisses which are stratiform in composition.
The Potosi Gneiss, and the manganiferous (containing manganese) garnet horizon, are considered key indicators of original bedding orientation and are thus central exploration targets, as there is a proven association of anomalous lead and zinc within the gneissic stratigraphy within these horizons on a regional basis.
The Broken Hill ore deposit is hosted within the Proterozoic gneisses of the Broken Hill Block, adjacent to the Curnamona Craton in South Australia. The terrane (crustal material formed on, or broken off from, one tectonic plate and sutured to crust lying on another plate) in which Broken Hill is hosted has undergone a series of metamorphic deformations. This has resulted in the squeezing of the lead and zinc sulphides into the current basic boomerang shape, and resulted in the separation of the ore body into zinc-rich and lead-rich lodes and domains.
The lodes themselves show various structural facies and responses to shearing, though mostly in a ductile fashion (a solid material's ability to deform under tensile stress). Many lodes, particularly the lead lodes, have sharp contacts with gneissic host rocks, indicating they have become structurally relocated during peak metamorphism. Similarly, it is conjectured that the current position of the zinc and lead lodes at Broken Hill may not necessarily be related to their original position along the bedding planes, or vertically within the stratigraphic section.
It has taken some considerable effort to 'see through' the overwhelming structural overprint of metamorphism to infer the SedEx classification.
The influence of high-temperature metamorphic fluids on the ore deposit cannot be discounted, although it is considered less central to genetic factors than previous theories of hydrothermal origins for the deposits. The current consensus view is that evidence of the chemical alteration of rock by hydrothermal fluids (metasomatic effect) is present at Broken Hill as a result of the focusing of flow through the zones of weakness around the massive sulphides.
Metasomatic effects include re-equilibrating isotopic systematics of the lead-zinc sulphides and wall-rocks, and introduction of rare elements into the sulphide bodies forming one of the most diverse mineralogical assemblages in the Earth's crust, with 1500 or more mineral species recognized at Broken Hill, including several dozen not reported elsewhere.
A protolith is the original rock from which a given metamorphic rock is formed. For example, the protolith of marble is limestone. The association of the Broken Hill line of lode with an horizon of manganiferous garnets (silicate minerals) could be a potential protolith of exhalative manganiferous chert, metamorphically upgraded to a garnetiferous gneiss. Perhaps some reconsititution of that protolith by metasomatism is associated with the nearby massive sulphides.
Broken Hill is the archetypal locality for a class of ore deposits known as Broken Hill Type, or BHT, ore deposits. This is a classification grouping of similar deposits for use in ore genesis theories and mineral exploration methodologies.