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The results are discussed of a project involving the use of fluid flow modelling of the Mount Isa and McArthur basins with the aim of developing an understanding of the Zn-Pb-Ag and U deposits based on an ore-forming, basin system. Regional geometric models were constructed to depict sediment architecture and basin shape at the time(s) of metal-bearing fluid migration The models were built using the results of chronostratigraphic basin analysis studies. Mineral paragenesis and associated geochemical studies were undertaken at the regional scale to determine the relationships between early and late fluid evolution in siliciclastic, carbonate and volcanic rocks to establish the links between the deep basinal brines and the deposits and to characterise the thermal history and organic matter evolution in sediments that host the ore deposits. The geometric basin models were digitised and imported into fluid-flow modelling software packages so that scenario-based simulations could be run to test concepts for the origin of the deposits. The results were used to constrain thermal gradients and physical properties used in the models., The results are discussed of a project involving the use of fluid flow modelling of the Mount Isa and McArthur basins with the aim of developing an understanding of the Zn-Pb-Ag and U deposits based on an ore-forming, basin system. Regional geometric models were constructed to depict sediment architecture and basin shape at the time(s) of metal-bearing fluid migration The models were built using the results of chronostratigraphic basin analysis studies. Mineral paragenesis and associated geochemical studies were undertaken at the regional scale to determine the relationships between early and late fluid evolution in siliciclastic, carbonate and volcanic rocks to establish the links between the deep basinal brines and the deposits and to characterise the thermal history and organic matter evolution in sediments that host the ore deposits. The geometric basin models were digitised and imported into fluid-flow modelling software packages so that scenario-based simulations could be run to test concepts for the origin of the deposits. The results were used to constrain thermal gradients and physical properties used in the models.

Results are presented which show that proximal fluvial sediments in the Leichhardt River fault trough evolved through burial diagenesis to become aquifers that hosted basinal brines with chemical characteristics that are indistinguishable from those which are reported in all of the Zn- Pb deposits. Illite crystallinity north of Mount Isa, where the effects of the Isan orogeny were less, shows that these fluids probably had temperatures of about 200 degrees C at the time when the Zn-Pb deposits formed but significantly higher when Cu precipitation occurred. Aquifer development and metal leaching in immature clastic sediments occurred at a depth of.5 to 10 km. The Calvert superbasin was the most likely source for metalliferous fluids in the Lawn Hill platform., Results are presented which show that proximal fluvial sediments in the Leichhardt River fault trough evolved through burial diagenesis to become aquifers that hosted basinal brines with chemical characteristics that are indistinguishable from those which are reported in all of the Zn- Pb deposits. Illite crystallinity north of Mount Isa, where the effects of the Isan orogeny were less, shows that these fluids probably had temperatures of about 200 degrees C at the time when the Zn-Pb deposits formed but significantly higher when Cu precipitation occurred. Aquifer development and metal leaching in immature clastic sediments occurred at a depth of.5 to 10 km. The Calvert superbasin was the most likely source for metalliferous fluids in the Lawn Hill platform.

Fluid inclusion and isotope analysis data are presented which show that clastic sediments dominated by poorly sorted sandstone and conglomerate facies evolved through burial diagenesis to become aquifer lithologic units that hosted basinal brines with chemical characteristics and ages similar to those reported from the Pb-Zn and U deposits of the southern McArthur basin. The brines formed mainly in the Westmoreland Conglomerate and the Warramana sandstone and Gold Creek volcanics. The composition of the brines suggests that there were capable of leaching Pb- Zn +/- Cu and U from sediments and adjacent volcanic units and transporting them to a trap site., Fluid inclusion and isotope analysis data are presented which show that clastic sediments dominated by poorly sorted sandstone and conglomerate facies evolved through burial diagenesis to become aquifer lithologic units that hosted basinal brines with chemical characteristics and ages similar to those reported from the Pb-Zn and U deposits of the southern McArthur basin. The brines formed mainly in the Westmoreland Conglomerate and the Warramana sandstone and Gold Creek volcanics. The composition of the brines suggests that there were capable of leaching Pb- Zn +/- Cu and U from sediments and adjacent volcanic units and transporting them to a trap site.

The stratiform Century Zn-Pb deposit and more than 47 younger, discordant Zn-Pb lode deposits of the Burketown mineral field are hosted in shales and siltstones belonging to the Isa superbasin and comprise sphalerite, pyrite, carbonate, quartz, galena, minor chalcopyrite, and minor illite. The Zn at Century was transported to the deposit by a homogeneous, Ca2+- and Na+-bearing brine The mineralisation probably formed at 50 to 85 Mpa, corresponding to a depth of 1 900 to 3 100 m. Diagenetic aquifers in the underlying Calvert superbasin are the most likely sources for the fluids that formed Century and the discordant lode deposits. Clear, buck, and drusy quartz in veins unrelated to any form of Pb-Zn mineralisation record the last major fluid event in the Burketown mineral field and form distinct outcrops and ridges in the district., The stratiform Century Zn-Pb deposit and more than 47 younger, discordant Zn-Pb lode deposits of the Burketown mineral field are hosted in shales and siltstones belonging to the Isa superbasin and comprise sphalerite, pyrite, carbonate, quartz, galena, minor chalcopyrite, and minor illite. The Zn at Century was transported to the deposit by a homogeneous, Ca2+- and Na+-bearing brine The mineralisation probably formed at 50 to 85 Mpa, corresponding to a depth of 1 900 to 3 100 m. Diagenetic aquifers in the underlying Calvert superbasin are the most likely sources for the fluids that formed Century and the discordant lode deposits. Clear, buck, and drusy quartz in veins unrelated to any form of Pb-Zn mineralisation record the last major fluid event in the Burketown mineral field and form distinct outcrops and ridges in the district.

The Mesoproterozoic Jameson Range intrusion as part of the Giles Complex is predominantly mafic in composition comprising olivine-bearing gabbroic lithologies with variable amounts of magnetite and ilmenite. Lithologies containing more than 50 vol% magnetite and ilmenite are classified as magnetitites. The Jameson Range hosts several of these magnetitites forming laterally extensive layers, which can be traced for at least 19 km as continuous magnetic anomalies. Similar occurrences of magnetitites are known from the upper parts of other layered intrusions, such as the Bushveld Complex. In addition, the intrusion hosts several P-rich zones, one of which is at least 59 m in thickness containing 1.0 wt% P2O5. The P-rich zones are not directly associated with the magnetitites, but mostly occur slightly above them. Magnetite compositions are characterised by low TiO2 concentrations (0.39 to 3.04 wt%) representing near end-member magnetite with up to 1.2 wt% Cr and 1.3 wt% V, respectively. The basal magnetite layer reaches up to 68.8 wt% Fe2O3(t) and 24.2 wt% TiO2, and it is also markedly enriched in Cu (up to 0.3 wt% Cu), V (up to 1.05 wt% V2O5) and platinum-group elements (up to 2 ppm Pt + Pd). Sulphide minerals comprising bornite, chalcopyrite and minor pentlandite occur finely disseminated in the magnetitite and account for the elevated base metal and PGE concentrations. Several lines of evidence suggest that the magnetite layers formed in response to primarily density-controlled mineral sorting within crystal slurries, although the grain size also affects the sorting process., The Mesoproterozoic Jameson Range intrusion as part of the Giles Complex is predominantly mafic in composition comprising olivine-bearing gabbroic lithologies with variable amounts of magnetite and ilmenite. Lithologies containing more than 50 vol% magnetite and ilmenite are classified as magnetitites. The Jameson Range hosts several of these magnetitites forming laterally extensive layers, which can be traced for at least 19 km as continuous magnetic anomalies. Similar occurrences of magnetitites are known from the upper parts of other layered intrusions, such as the Bushveld Complex. In addition, the intrusion hosts several P-rich zones, one of which is at least 59 m in thickness containing 1.0 wt% P2O5. The P-rich zones are not directly associated with the magnetitites, but mostly occur slightly above them. Magnetite compositions are characterised by low TiO2 concentrations (0.39 to 3.04 wt%) representing near end-member magnetite with up to 1.2 wt% Cr and 1.3 wt% V, respectively. The basal magnetite layer reaches up to 68.8 wt% Fe2O3(t) and 24.2 wt% TiO2, and it is also markedly enriched in Cu (up to 0.3 wt% Cu), V (up to 1.05 wt% V2O5) and platinum-group elements (up to 2 ppm Pt + Pd). Sulphide minerals comprising bornite, chalcopyrite and minor pentlandite occur finely disseminated in the magnetitite and account for the elevated base metal and PGE concentrations. Several lines of evidence suggest that the magnetite layers formed in response to primarily density-controlled mineral sorting within crystal slurries, although the grain size also affects the sorting process.

The deposit, hosted by Palaeoproterozoic metamorphic rocks, has high-grade ore confined to a reverse fault/shear zone that crosscuts a series of interbedded schists and amphibolites. Petrographic studies, fluid-inclusion and isotope analysis and Ar-Ar dating have identified up to eight significant fluid events. Uranium mineralisation is associated with an inner and outer alteration halo that extends as far as 1 km from the fault. U-Pb and Pb-Pb dating reveals that massive uraninite precipitated with illite and haematite at around 1640 Ma at about 200 degrees C., The deposit, hosted by Palaeoproterozoic metamorphic rocks, has high-grade ore confined to a reverse fault/shear zone that crosscuts a series of interbedded schists and amphibolites. Petrographic studies, fluid-inclusion and isotope analysis and Ar-Ar dating have identified up to eight significant fluid events. Uranium mineralisation is associated with an inner and outer alteration halo that extends as far as 1 km from the fault. U-Pb and Pb-Pb dating reveals that massive uraninite precipitated with illite and haematite at around 1640 Ma at about 200 degrees C.

Geological, thermometric and Raman data have identified three different vein systems associated with distinct periods of geological time, hosting four fluid types represented by a variety of fluid inclusions. Mo-type quartz veins were precipitated by a mixture of hot, highly saline and cooler, less saline fluids before the development of the Junction shear zone. Syn-gold quartz veins associated with extensive alteration were precipitated from a hot, moderately saline, H2O-CO2 (+/-CH4) gold-bearing fluid introduced during shear-zone development and post-gold quartz veins were precipitated from a hot, CO2-rich fluid that penetrated normal fault sets after the gold-mineralising event ended. Many of the syn-gold veins are zoned with respect to CO2:CH4 ratios, CH4 occurring near the margins and pure CO2 towards the centre. The gold-bearing fluid probably contained only trace CH4 and the zoning may have arisen when CO2 was reduced to CH4., Geological, thermometric and Raman data have identified three different vein systems associated with distinct periods of geological time, hosting four fluid types represented by a variety of fluid inclusions. Mo-type quartz veins were precipitated by a mixture of hot, highly saline and cooler, less saline fluids before the development of the Junction shear zone. Syn-gold quartz veins associated with extensive alteration were precipitated from a hot, moderately saline, H2O-CO2 (+/-CH4) gold-bearing fluid introduced during shear-zone development and post-gold quartz veins were precipitated from a hot, CO2-rich fluid that penetrated normal fault sets after the gold-mineralising event ended. Many of the syn-gold veins are zoned with respect to CO2:CH4 ratios, CH4 occurring near the margins and pure CO2 towards the centre. The gold-bearing fluid probably contained only trace CH4 and the zoning may have arisen when CO2 was reduced to CH4.

This paper describes uranium deposits at the contact between the Westmoreland Conglomerate and the Seigal Volcanoes. The mineralogy, paragenesis and geochemistry of the Redtree and Junnagunna uranium deposits are largely indistinguishable from the basement-hosted Nabarlek and Jabiluka uranium deposits in the northern McArthur basin. This suggests that the Westmoreland uranium field might also be prospective for basement-hosted uranium deposits where suitable structural and chemical traps can be found., This paper describes uranium deposits at the contact between the Westmoreland Conglomerate and the Seigal Volcanoes. The mineralogy, paragenesis and geochemistry of the Redtree and Junnagunna uranium deposits are largely indistinguishable from the basement-hosted Nabarlek and Jabiluka uranium deposits in the northern McArthur basin. This suggests that the Westmoreland uranium field might also be prospective for basement-hosted uranium deposits where suitable structural and chemical traps can be found.

The late Mesoproterozoic Giles Complex of the West Musgrave province hosts one of the largest concentrations of mafic-ultramafic layered intrusions on Earth. Therefore, the area is highly prospective for hosting significant Ni-Cu and platinum-group element (PGE) mineralisation, especially following the discovery of a large magmatic sulphide deposit at Nebo-Babel in 2000. More recently, a significant occurrence of massive to disseminated sulphide mineralisation reaching up to 0.62 wt% Cu, 0.47 wt% Ni, and 1 ppm Pt + Pd was identified at the Manchego prospect. The magmatic sulphide mineralisation at Manchego is hosted by a range of gabbronoritic rock types. The principal ore minerals comprise massive to disseminated pyrrhotite, chalcopyrite, pentlandite, magnetite, and ilmenite. The presence of several geochemically distinct gabbronoritic lithotypes and an abundance of xenoliths strongly indicate a dynamic magmatic plumbing system, such as a conduit-type environment. Whole-rock samples of sulphide-rich gabbronoritic lithologies have d34S values ranging from -11.8 to -8.4 per mille, which clearly demonstrates the addition of crustal sulphur at Manchego and therefore the availability of crustal S in the West Musgrave province. In comparison to Nebo-Babel, the Manchego prospect provides evidence for the prospectivity of intrusions derived from high Ti basalts in the area, such as the Alcurra Dolerite., The late Mesoproterozoic Giles Complex of the West Musgrave province hosts one of the largest concentrations of mafic-ultramafic layered intrusions on Earth. Therefore, the area is highly prospective for hosting significant Ni-Cu and platinum-group element (PGE) mineralisation, especially following the discovery of a large magmatic sulphide deposit at Nebo-Babel in 2000. More recently, a significant occurrence of massive to disseminated sulphide mineralisation reaching up to 0.62 wt% Cu, 0.47 wt% Ni, and 1 ppm Pt + Pd was identified at the Manchego prospect. The magmatic sulphide mineralisation at Manchego is hosted by a range of gabbronoritic rock types. The principal ore minerals comprise massive to disseminated pyrrhotite, chalcopyrite, pentlandite, magnetite, and ilmenite. The presence of several geochemically distinct gabbronoritic lithotypes and an abundance of xenoliths strongly indicate a dynamic magmatic plumbing system, such as a conduit-type environment. Whole-rock samples of sulphide-rich gabbronoritic lithologies have d34S values ranging from -11.8 to -8.4 per mille, which clearly demonstrates the addition of crustal sulphur at Manchego and therefore the availability of crustal S in the West Musgrave province. In comparison to Nebo-Babel, the Manchego prospect provides evidence for the prospectivity of intrusions derived from high Ti basalts in the area, such as the Alcurra Dolerite.

The portion of the prospect examined is most similar to a basement-hosted or simple unconformity-related deposit but is unique in that basement-hosted sulphide minerals, rather than graphite or Fe chlorite, played a major role in reducing U. Alteration is also unusual in being restricted to a narrow zone of muscovite, lacking chlorite and haematite. It is concluded that thick and largely unconfined diagenetic aquifers of the Indicator Formation conducted basinal brines with a composition and temperatures appropriate for leaching U from sediments of the Otish basin. U mineralisation dated at 1721 +/-20 Ma was related to a phase of the Otish gabbro intrusion at c.1730 Ma, which promoted circulation of U-bearing brines and triggered mineralisation at several locations.The preservation of dates from 1720 to 1410 Ma suggests minimal interaction of fluids with uraninite and muscovite during the later Grenville orogenesis and that the aquifers were probably indurated by 1200 Ma. Fault zones, fractures and depositional sequence boundaries, which were preferential conduits for post-depositional dispersion of radiogenic Pb and trace Mo+W+Nb from the U mineralisation, can be used to explore for further deposits., The portion of the prospect examined is most similar to a basement-hosted or simple unconformity-related deposit but is unique in that basement-hosted sulphide minerals, rather than graphite or Fe chlorite, played a major role in reducing U. Alteration is also unusual in being restricted to a narrow zone of muscovite, lacking chlorite and haematite. It is concluded that thick and largely unconfined diagenetic aquifers of the Indicator Formation conducted basinal brines with a composition and temperatures appropriate for leaching U from sediments of the Otish basin. U mineralisation dated at 1721 +/-20 Ma was related to a phase of the Otish gabbro intrusion at c.1730 Ma, which promoted circulation of U-bearing brines and triggered mineralisation at several locations.The preservation of dates from 1720 to 1410 Ma suggests minimal interaction of fluids with uraninite and muscovite during the later Grenville orogenesis and that the aquifers were probably indurated by 1200 Ma. Fault zones, fractures and depositional sequence boundaries, which were preferential conduits for post-depositional dispersion of radiogenic Pb and trace Mo+W+Nb from the U mineralisation, can be used to explore for further deposits.