Broken Hill Ore Deposit

The Broken Hill Ore Deposit is located underneath Broken Hill in western New South Wales, Australia, and is the namesake for the town. It is arguably the world's richest and largest zinc-lead ore deposit.

Discovery

The Broken Hill ore deposit was discovered in 1883 by boundary rider Charles Rasp, who discovered the gossan or weathered sulfide outcrop of massive lead-zinc sulfides on a feature known 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 gold rushes.

Exploitation

Broken Hill was exploited initially by small prospectors working the gossan for easily won galena cubes, and soon dozens of shafts were sunk. Ore was carted to South Australia by camel trains, wagons and pack mules. A major secondary source of income became apparent, with extremely high silver grades recovered, including native silver, and other rare silver minerals present in fantastic abundance.

Mining has gradually moved away from the initial small prospectors, in line with the experience in all other major mineral fields, toward gradual consolidation of claims and tenure, an increase in tenure and mine size and efficiencies in operations 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 Limited - and its exit from Broken Hill, toward only two operators at present, utilising highly efficient bulk underground mechanised mining.

Geology

The Broken Hill ore body is hosted within the gneisses of the Willyama Supergroup ( [http://www.pir.sa.gov.au/__data/assets/pdf_file/0014/20426/curnamona_geology_minerals.pdf Map] ), a mesoproterozoic sequence of quartz-rich feldspathic gneisses of interpreted sandstone protolith, and micaceous gneisses of siltstone protolith. The Broken Hill ore deposit is considered to be roughly 1,800 million years old.

The simplified geology of the Broken Hill ore body is a series of boomerang-shaped, highly sheared and disrupted ribbon-like and poddy massive sulfide lenses which outcropped in the central section (the old "Broken Hills" gossan hills) and plunge steeply north and moderately south.

The ore consists of massive, recrystallised sphalerite-rich, galena-sphalerite and galena-rich sulfide lenses often consisting of up to 100% lead-zinc sulfides, with little or no pyrite, chalcopyrite or gangue sulfides. The ore is hosted within a unit of gneiss known as the Potosi Gneiss.

The footwall to the mineralization is a psammopelite gneiss, consisting of feldspar, quartz, garnet, biotite and amphibole, with a pelite gneiss on the hanging wall. The footwall gneiss contains anomalous mineral chemistries including a rare lead-rich feldspar and manganese-rich garnet chemistries.

Ore is predominantly hosted at this stratigraphic break, but much of the ore body is structurally remobilised or offset into both the hangingwall and footwall, and the geometry of the ore deposit is particularly complex on the local scale.

Genesis

The genesis of the Broken Hill ore body is of great historical importance 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 papers published to date. It is also of great current importance, as conceptions of the genesis of this enigmatic ore deposit and its structural and stratigraphic setting drives exploration for repetitions of the ore deposit along strike, and in finding analogues elsewhere in the world.

The genesis of Broken Hill is also of interest as it is of enduring controversy and conjecture, with the jury still mostly 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 palatable middle view of a range of opinions.

SEDEX mineralisation

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.

The Potosi Gneiss, and the manganiferous garnet horizon, are considered key indicators of original bedding orientation (S0) and are thus key exploration targets, as there is a proven association of anomalous lead and zinc within the gneissic stratigraphy with these horizons on a regional basis.

Metamorphic overprints

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 in which Broken Hill is hosted has undergone a series of several metamorphic deformations at amphibolite facies [cite web|url=http://www.pir.sa.gov.au/minerals/geology/geological_provinces/curnamona/curnamona_geology|title=Curnamona Geology overview, PIRSA |accessdate=2007-08-14] . This has resulted in the 'squeezing" of the lead and zinc sulfides 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 show variable responses to shearing, though mostly in a ductile fashion. 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 (S0), 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.

Metasomatism

The lower part of the Willyama Supergroup has undergone intense sodium alteration, particularly the Broken Hill Block and subdomain. This has resulted in pervasive albite alteration particularly in the Olary domain adjacent to Broken Hill.

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 metasomatic overprints are present as a result of the focusing of flow through the zones of weakness around the massive sulfides, which are ductile failure loci in themselves.

Metasomatic effects include re-equilibrating isotopic systematics of the lead-zinc sulfides and wall-rocks, and introduction of rare elements into the sulfide bodies to form 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.

The association of the Broken Hill line of lode with a horizon of manganiferous garnets is considered to be partly a function of a potential protolith of exhalative manganiferous chert, metamorphically upgraded to a garnetiferous gneiss, and perhaps some reconsititution of that protolith by metasomatism associated with the nearby massive sulfides.

Broken Hill Type ore deposits

Broken Hill is the type 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.

The key criteria for BHT ore deposits are;
* Association with major sedimentary packages of sandstone protoliths underlying siltstone protolith sequences in highly disturbed metamorphic terranes
* A Proterozoic age is considered important, as no other major SEDEX lead-zinc deposits of this style are known from the Phanerozoic or Archaean
* Association with manganiferous garnets.

See also

* Sedimentary exhalative deposits
* Ore genesis
* Broken Hill, New South Wales

References

"Mining and Exploration at Broken Hill: A Review ". Sydney: Department of Mineral Resources, New South Wales, 1981.