Overview

Map of NSW showing the Curnamona Craton and Delamerian, Lachlan, Thomson and New England Orogens

Much of the mineral wealth of New South Wales is contained in its orogenic and cratonic belts. The Proterozoic Curnamona Craton contains the giant Broken Hill silver-lead-zinc deposit. The poorly exposed Precambrian to Cambrian rocks of the Delamerian Orogen have potential for gold. The Palaeozoic Lachlan Orogen hosts significant mineralisation, including world class porphyry copper gold deposits and several gold base metal deposits. Palaeozoic rocks of the poorly exposed, but near-surface Thomson Orogen also have potential for gold and base metal mineralisation.

Curnamona Craton

The Proterozoic Curnamona Craton, in western New South Wales, is represented by the Broken Hill Block and Euriowie Block, and is bounded along its eastern margin by the Delamerian Orogen. The cratonic units consist of strongly deformed and metamorphosed sedimentary and igneous rocks of the Willyama Supergroup. These rocks represent sedimentary rocks and volcanic rocks deposited in one or more rift basins about 1700-1600 million years ago and they were subsequently strongly deformed. The rocks now at the surface at Broken Hill were originally buried to 12-20 km depth (in the middle to lower crust) and host the giant, extremely rich Broken Hill silver-lead-zinc deposit. Mining profits from Broken Hill have underpinned much of the mineral industry and other economic development in Australia since 1883.

Delamerian Orogen

The Delamerian Orogen is a major orogenic belt in the eastern half of South Australia and the western parts of Victoria and New South Wales. It includes Precambrian and early Cambrian rock sequences that experienced Cambrian deformation and metamorphism. It separates the Australian Precambrian cratons from the younger Palaeozoic to Mesozoic orogenic belts of eastern Australia.

It is poorly exposed in New South Wales, outcropping in the Koonenberry Belt and as part of the Adelaidean rift sequence flanking the Broken Hill Block and Eurowie Block. The Stawell Zone, well defined in Victoria, can be traced through into southwestern New South Wales on the high resolution magnetic data, where it comprises the easternmost structural zone of the Delamerian Orogen. On the western side of the Stawell Zone, north of Mildura, there is potential for Stawell or Magdala-type orogenic gold deposits under about 400 m of cover.

The Adelaidean sequences north of Broken Hill record the progressive rifting of continental crust and formation of the proto-Pacific Ocean as the Precambrian supercontinent Rodinia broke up. Rocks are of shallow-water origin in the west, becoming deeper water in character to the east, where deformed narrow linear belts of Precambrian and Cambrian sea-floor volcanic rocks occur. Plate convergence began about 520 million years ago, with formation of a subduction zone and development of island arc volcanism. This tectonic regime was destroyed late in the Cambrian around 500 Ma, when the arcs collided with thinned passive margin of the Australian continent. The resultant Delamerian Orogeny consisted of several phases of deformation from the late Middle Cambrian to Early Ordovician.

Lachlan Orogen

The Lachlan Orogen is a broad orogenic belt that records Cambrian to the Carboniferous convergence between the Australian craton and the proto-Pacific Ocean. It is not a collisional orogenic belt.

It underlies most of New South Wales, extending south into Victoria and eastern Tasmania. The oldest rocks are Ordovician continental margin, craton-derived turbidite packages, deposited on Cambrian oceanic crust, that are now separated from the Ordovician intra-oceanic Macquarie Arc by suture zones containing relics of Ordovician ocean crust.

The Macquarie Arc hosts world class porphyry Cu-Au deposits in the Cadia valley and Northparkes districts. Smaller deposits occur at Cargo and Copper Hill. Structurally controlled Au-Cu occurs at Cowal. Silurian volcanic-hosted, structurally controlled base metal deposits at Captains Flat, Woodlawn and Mineral Hill occur in volcanic-rich rifts. Sediment-hosted structurally controlled deposits in sediment-rich basins include those in the Hill End Trough, as well as three operating gold-base metal mines in the Cobar Basin. There are excellent opportunities for further major discoveries under shallow cover.

The Lachlan Orogen has a northwesterly to northerly structural grain. The structural style is one of thick- and thin-skinned thrusting, coupled with strike-slip faulting on major faults. Deformation was episodic, with major events in the early Silurian, mid-Devonian and in the early Carboniferous.

Main features of the Lachlan Orogen are as follows.

  • The Ordovician period was largely represented by the deposition of Early to Middle Ordovician quartz-rich deepwater turbidite deposits (and chert) that pass up into Late Ordovician black shales. The turbidites are quartz-rich and are craton-derived, sourced from the uplifted Delamerian Orogen to the west and its continuation in Antarctica, the Ross Orogen. In central and southern New South Wales, Ordovician turbidite rocks and shales are structurally faulted against Early to Late Ordovician intermediate to mafic volcanic rocks that represent the core and fringe of a 1000 km-long intraoceanic island arc that was subsequently split into four structural belts by later extension. This arc hosts major Au-Cu deposits. This stage of convergence between the Australian part of Gondwana and the proto-Pacific plate ended when the arc collided with the back-arc sedimentary rocks in the latest Ordovician to early Silurian. Chert-rich rocks around Narooma and Batemans Bay on the south coast of New South Wales were formed on the floor of the proto-Pacific ocean and represent an exotic terrane that drifted towards, and accreted with, the developing Lachlan Orogen.
  • The Silurian to Middle Devonian stage of development of the Lachlan Orogen was characterised by extension behind a new arc that developed in the northeast area of New South Wales. The orogen underwent rifting, leading to formation of mineral-rich sedimentary basins and mixed sedimentary and volcanic basins, as well as the emplacement of large amount of S- and I-type granites. These rifts were closed by a mid-Devonian deformation.
  • The final stage of development of the Lachlan Orogen was characterised by widespread mid- to Late Devonian fluviatile sedimentation after local rifting and localised emplacement of A-type igneous rocks. The rocks were deformed in the early Carboniferous and then intruded by I-type granites in the Mudgee-Oberon region.

Thomson Orogen

The Thomson Orogen lies north of the Lachlan Orogen and extends north into central Queensland. In New South Wales the orogen has an east-west orientation and, although most of it is covered by younger sedimentary sequences, about 30% of units making up the orogen are less than 300 m from the surface. Recent work on the Thomson Orogen suggests that it may have had a similar tectonic history to that of the Lachlan Orogen to its south, as it also contains Ordovician oceanic island basalt and turbidite rocks, mid Silurian to mid Devonian rock packages and possible Late Devonian basins. Recent drilling has found calc-alkaline andesitic rocks with a subduction-related geochemical signature in the Bourke region. Airborne magnetic data and ground-acquired gravity data suggest an arcuate amalgamation of these rocks that is divided into smaller, geophysically distinct domains. These are interpreted as deformed volcanic belts which are variably cross-cut by elliptical domains of younger intrusive complexes. Seismic reflection data suggest that the sutured contact between the Lachlan Orogen and Thomson Orogen extends to, and significantly offsets the Mohorovicic discontinuity and also truncates a Late Devonian basin. Hence, it can be surmised that the suture was active since the Late Devonian.

In New South Wales, the orogen has potential for structurally controlled gold and base metal deposits in Cambrian to Ordovician oceanic crust and arc rocks, craton-derived turbidites and Silurian-Devonian basins.

New England Orogen

The New England Orogen had its main growth from the mid-Devonian until the mid-Triassic. From the Devonian to Carboniferous, the orogen developed as a classical orogenic belt, with subduction complex rocks in the east and a forearc basin and Andean arc (now missing) in the west. The Peel Fault system, hosting Cambrian ultramafic rocks with asbestos, chromite and gold, separates the subduction complex rocks from the forearc basin. Multiple deformation, metamorphism, and emplacement of granites occurred east of the Peel Fault system in the late Carboniferous to early Permian. In the early Permian, convergence along this plate margin changed into extension coupled with strike-slip faulting. This led to formation of small rift basins and a major back-arc rift basin (that became the basal parts of the Sydney and Gunnedah basins) containing basal mafic and felsic volcanic rocks. Renewed plate convergence in the Late Permian to Triassic led to volcanism, formation of epithermal gold and base metal deposits and emplacement of I- and A-type granites that represent the roots of a new continental margin arc which intruded the former accretionary complex rocks.The New England Orogen is a significant mineral province, with potential for large Au/Ag systems. Major deposits include gold at Hillgrove, Uralla, Nundle and Bingara. Significant gold resources have been discovered at Tooloom. Deposit styles include mesothermal and epithermal gold, VMS, epithermal silver, and lateritic nickel. The orogen also has porphyry copper and gold potential. Other economically important commodities include tin, sapphires, diamonds, molybdenum, tungsten, magnesite, cobalt and antimony.

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