Crocodile Gold's gold deposits lie within the Archean to Paleoproterozoic Pine Creek Orogen, one of the major mineral provinces of Australia. The Precambrian rocks of the Northern Territory have been subdivided into two principal orogenic provinces: the North Australian Craton and the Central Australian Mobile Belt. Orogenic domains within the North Australian cratons include the Pine Creek Orogen, the Tanami region, the Murphy, Tennant and Arnhem Inliers and the Northern Arunta Province. These underwent extensive orogenic movement and regional metamorphism between 1880 and 1850Ma, termed the Barramundi Orogeny, followed by transitional tectonics and igneous activity from 1850Ma, in some areas continually, to 1800Ma.Pine Creek Orogen
The Pine Creek Orogen is a deformed and metamorphosed sedimentary basin up to 14 kilometers thick covering an area of approximately 66,000 square kilometres and extending from Katherine in the south to Darwin in the north. It hosts significant resources of gold, uranium and platinum group metals, as well as substantial base metals, silver, iron and tin-tantalum mineralization.
The Pine Creek Orogen comprises series of late Archean granite-gneiss basement domes, which are overlain by a fluvial to marine sedimentary sequence. Several highly reactive rock units are included within this sedimentary sequence including carbonaceous shale, ironstones, evaporite, carbonate and mafic to felsic volcanic units of the South Alligator and Finniss River Groups. This sequence has been subjected to regional greenschist facies metamorphism and multiphase deformation, which has resulted in the development of a northwest trending fabric. Subsequent widespread felsic volcanism and the intrusion of granitoids caused contact metamorphism, in aureoles between 500 metres and two kilometres wide that overprint the earlier regional metamorphism. After the granitoid intrusions, an extensive array of northeast and northwest trending dolerite dykes intruded the metasedimentary sequence during regional extensional deformation.
Gold mineralization within the Pine Creek Orogen is preferentially developed within strata of the South Alligator Group and lower parts of the Finniss River Group along anticlines, strike-slip shear zones and duplex thrusts located in proximity to the Cullen Granite Batholith. Of particular stratigraphic importance are the Wildman Siltstone, the Koolpin Formation, Gerowie Tuff, Mt Bonnie Formation and the Burrell Creek Formation. Formation of Gold Deposits at Cosmo/Howley
Gold mineralization is typically found in anticline structures along the Cosmo/Howley corridor. The steps below outline the genesis of gold mineralization in this area:
Ore Deposit Models
- Early Proterozoic (>2000-1885Ma) sedimentary rocks, including sandstones, siltstones, shales, graphitic shales, dolomite and iron formations, and volcanic (tuff) rocks were deposited on a Late Archean (>2500Ma) granitic basement
- Three major deformation events and regional metamorphism occurred, a result of multiple phases of granitic intrusions of the Cullen Batholith
- Granitic intrusions generated heat which caused circulation of mineralized hydrothermal solutions over an extended period of time
- Fluid was trapped by impermeable rocks in suitable structural sites at various levels along anticlinal crests and at margins of domal structures
- Late granitoids (eg Burnside Granite) caused local metamorphic thermal aureoles up to five kilometres wide and refocused mineralization in structural conduits within the sedimentary rocks to form economic mineral deposits higher in the stratigraphy as the hydrothermal systems cooled
- Certain sedimentary lithologies, including the Mt Bonnie Formation turbidites, Zamu Dolerite sills, and Koolpin Formation banded iron formations, provide greatest competency contrasts and also favourable chemical conditions localizing richer and more continuous ore shoots, such as those at Cosmo
A variety of genetic models, ranging from magmatic through hydrothermal to syngenetic, have been postulated for the formation of gold deposits in the region. Gold and base metal mineralization in the area is commonly associated with granite intrusions and have often been classified as high temperature contact aureole deposits. A secondary host rock control has also been suggested due to the association of gold mineralization with carbonaceous metasedimentary rocks.
However, much of the gold mineralization occurred after the main intrusive event, the intrusion of the Cullen Batholith, and the relationship of gold mineralization and carbonaceous rocks is not the most important control on mineralization. More recently, it has been suggested that gold mineralization is structurally controlled, occurring in brittle-ductile structures at the greenschist-amphibole facies boundary and hence has an epigenetic origin.
In certain locations such as the Cosmo/Howley area, duplex thrust folds with buckle folding or basin and dome structures appear to be more significantly mineralized. The presence of shear systems linking anticlines higher in the sequence also appears to have provided the ideal fluid focusing mechanisms to localize gold-bearing fluids.
Gold deposits in the Northern Territory can be divided into seven types, assuming they have a structurally controlled mesothermal setting:
- Gold-quartz veins, lodes, sheeted veins, stockworks, saddle reefs (Pine Creek Orogen)
- Gold-ironstone bodies (Tennant Inlier)
- Gold in iron rich sediments (Pine Creek Orogen, Tanami)
- Polymetallic deposits (Iron Blow, Mt Bonnie)
- Gold-PGE deposits (South Alligator River area)
- Uranium-gold deposits (Pine Creek Orogen, Murphy Inlier)
- Placer deposits
Over half of the gold occurrences are gold-quartz vein deposits.Gold Mineralization
Native gold is the main ore mineral and is commonly present as micron sized grains; coarse nuggets are rare. Gold is commonly associated with pyrite, arsenopyrite and pyrrhotite and in places with minor base metal sulphides. Quartz, chlorite, sericite and carbonates are the common gangue minerals in the gold-quartz deposits.
Common factors for most gold deposits include:
- Gold deposits are nearly all in low-grade, sub-greenschist to lower greenschist facies regionally metamorphosed sediments (commonly greywacke-siltstone-shale)
- Anticlinal hinges and shear zones are generally the most favourable location
- Subsequent to regional metamorphism and deformation, the metasediments were intruded by I-type granite and the gold mineralization is within the contact metamorphic aureole
- Fluid inclusion data suggest the involvement of moderate to high salinity fluids in temperature range from 200°C to 300°C
- Stable isotope data suggest a magmatic/metamorphic origin of these fluids
All gold deposits in the Northern Territory show some structural control at the regional and deposit scales, with most deposits within the Pine Creek Orogen trending northwest-southeast. Base metal veins in the Pine Creek Orogen strike significantly differently than the gold veins, suggesting different discrete mineralizing events.
Most deposits show a preference for competency contrast situations in dilatant or low pressure zones, such as anticlinal crests, recurrent shear zones and necking zones. Gold mineralization is invariably late, occurring after orogenic events.
Five main types of mineralization have previously been recognized within or in proximity to Crocodile Gold's projects. These are:
- Sheeted and stockwork quartz vein systems located along major anticlinal hinges in the Mt Bonnie and Burrell Creek Formations and to a lesser extent, the Gerowie Tuff. Mineralization is hosted by carbonaceous or sulphidic host rocks or along zones of competency contrast between greywacke and shale (Enterprise, Union Reefs, Howley, Fountain Head) or dolerite. Axial planar quartz veins have been identified in some deposits (Enterprise). Stratabound quartz reefs occur in most of these deposits, and may develop into saddle reefs along fold hinge zones (Enterprise, Union Reefs and Fountain Head)
- Sediment-hosted stratiform gold mineralization and quartz-sulphide-vein-hosted stratabound gold mineralization in cherty ironstone and carbonaceous mudstones of the Koolpin Formation (Tom's Gully, Cosmo) or the Gerowie Tuff (Brocks Creek)
- Stratiform, massive to banded, sulphide-silicate-carbonate mineralization in the Mt Bonnie Formation (Mt Bonnie and Iron Blow)
- Sediment-hosted stratiform and stratabound gold mineralization in cherty, dolomitic and sulphidic shales of the Mt Bonnie Formation, with sheeted quartz-sulphide veins (Mt Bundy (Rustler's Roost))
- Sheeted or stockwork quartz-feldspar-sulphide veins hosted by Zamu/Maud Creek Dolerite sills (Maud Creek, Howley)
Most gold mineralization in the Pine Creek Orogen occurs within the South Alligator Group, especially above the Middle Koolpin Formation, and in the lower parts of the Burrell Creek Formation. Most of the fold-associated deposits were probably formed during intrusion of granitoids such as the synorogenic Cullen Batholith and the Burnside Granite.Regional Geology Map
Magnetic map and structural controls of the region
Structural models in Burnside tenements