The Pegmont lead-zinc deposit is considered to be a body of stratiform lead-zinc sulphides contained within a metamorphosed silicate facies banded iron formation (BIF). It is considered an example of a Broken Hill Type (BHT) deposit.
The Broken Hill lead-zinc-silver deposit, located in New South Wales, is the archetype for this style of lead-zinc-silver deposit. Broken Hill is classed as a “super giant” and is believed to have originally contained in excess of 280 Mt, and is the largest zinc-lead-silver accumulation on Earth (Huston, 2006). The world class Cannington silver-lead-zinc deposit, located 25 km to the east of Pegmont, is another important example of a BHT deposit. Other examples of BHT type deposits include; Zinkgruvan (Sweden), and Aggeneys-Gamsberg district deposits (South Africa).
In Australia, Broken Hill-type deposits occur in dominantly sedimentary basins that have been overprinted by high-grade (upper amphibolite to granulite) metamorphism. These basins, which include the Broken Hill block and the Eastern succession of the Mount Isa inlier, are dominated by siliciclastic sedimentary rocks, with minor but genetically important felsic volcanic rocks, granites, and tholeiitic mafic sills.
Both Broken Hill and Cannington consist of a series of stacked lenses with Ag, Sb, and F abundance increasing stratigraphically upward, and with Au, Bi, and Cu abundances decreasing. Stacking of ore lenses suggests that Broken Hill and Cannington were the sites of long-lived and episodic hydrothermal vents. A Pegmont, there is an indication of two stacked lenses.
Broken Hill type deposits exhibit post depositional modifications. High-grade metamorphism (amphibolite facies), metasomatism, and deformation have all affected the morphology and architecture of Broken Hill-typ deposits, coarsening the ores, converting original alteration assemblages to metamorphic mineral assemblages partially melting ore minerals, and remobilizing and upgrading ore into high-grade zones.
Within the Proterozoic Mount Isa-McArthur basin there are six world class lead-zinc-silver deposits (Mount Isa Hilton, George Fisher, Century, MacArthur River and Cannington). On a regional scale they are spatially related however the Broken Hill type deposits (Pegmont and Cannington) differ from the other Mount Isa type sedimentary deposits in the following ways:
- They occur in high-grade metamorphic rocks
- A lack of finely laminated, bedding parallel sulphide mineral textures
- A lack of organic-rich siltstones and a more oxidized siliciclastic sedimentary rock host
- A magnetite association, either within or along stricke from ore
- The presence of semi-regional garnet bearing, alteration halo