Isan Orogeny Research Articles

Hot lithosphere at Mount Isa: implications for Proterozoic tectonics and mineralisation

Conductive heat transfer in the lithosphere at Mount Isa has been modelled based on published data on heat flow, thermal rock properties, and lithosphere architecture. Modelling results suggest that the high heat flow at Mount Isa is due to a combination of a thicker than normal crust and high concentrations of radioactive elements. During the last major tectonic activity in the Proterozoic, higher concentration of parent isotopes and the additional sedimentary overburden have increased lithosphere temperatures further. These results confirm that heat-producing elements play a key role in the thermal history of the Mount Isa Inlier and in other Australian Proterozoic terrains. The elevated lithosphere temperatures have significant implications for understanding Proterozoic tectonics and mineralisation, in particular the potential for shallow crustal melting, thermal weakening of the lithosphere, and for the evolution of deformation-related permeability. In general, high temperature gradients in continental crust cause a tectonic style dominated by pervasive viscous flow in partially molten crust, which in Mount Isa is evident from the formation of metamorphic core complexes during extension, and the absence of alpine-type nappes during contraction. In contrast, steep shear zones and fabrics formed in the upper crust during shortening of the rift complex in the Mid-Proterozoic Isan Orogeny. It is suggested that these steep structures formed where lateral strength contrasts existed in the lithosphere, caused by the differences in burial depths between cooler, tectonically denuded basement, and hotter basement covered by sedimentary basins. Increased permeability along steeply dipping faults and shear zones enables fluids to flow at higher rates across steep gradients of pressure, temperature and chemical concentration between magmatic, metamorphic and surface reservoirs. It is concluded that the link between thermal and mechanical processes in hot continental lithosphere greatly enhances the likelihood of hydrothermal mineralisation.

Paleomagnetism of the Mt Isa Zn–Pb–Cu–Ag and George Fisher Zn–Pb–Ag deposits, Australia

Paleomagnetic results are reported for the Mt Isa Zn–Pb–Cu–Ag and George Fisher Zn–Pb–Ag black-shale-hosted deposits in northwestern Queensland, Australia. Both deposits are hosted in greenschist facies carbonaceous and dolomitic siltstones, mudstones and shales of the ca 1655 Ma Urquhart Shale Formation of the Mt Isa Group. Lead model ages for both deposits are ca 1655 Ma, and an Ar–Ar biotite age for Cu mineralisation at Mt Isa is ca 1523 Ma. Paleomagnetic and rock magnetic analyses of 333 specimens (28 sites) isolate, excluding modern hematite, a stable characteristic remanent magnetisation (ChRM) carried mainly by single- or pseudosingle-domain pyrrhotite for Zn–Pb–Ag ore specimens and pyrrhotite and/or titanomagnetite for Cu ore specimens. A paleomagnetic fold test shows that the ChRM postdates D3 deformation of the ca 1595 to 1500 Ma Isan orogeny, yielding a Mesoproterozoic paleopole at ca 1505 Ma on the northern Australian apparent polar wander path, thereby providing a minimum age for the ores and an age for greenschist metamorphism during the last major metamorphic peak of the Isan orogeny in the Mt Isa inlier.

A New Geochronological Framework for Mineralization and Alteration in the Selwyn-Mount Dore Corridor, Eastern Fold Belt, Mount Isa Inlier, Australia: Genetic Implications for Iron Oxide Copper-Gold Deposits

New Re-Os in molybdenite and U-Pb in titanite and zircon age data have been used to discern several discrete alteration, mineralization, and igneous events that occurred in iron oxide-copper-gold (IOCG) deposits in the Selwyn-Mount Dore corridor of the Proterozoic Mount Isa inlier. Two distinct sodic-calcic alteration events that occurred prior to mineralization have been recognized. This suggests that sodic-calcic alteration may be a fundamental precursor to IOCG mineralization as it mobilizes large amounts of metals, including Fe, K, and Cu. Sodic-calcic alteration at Starra was early synmetamorphic (Isan orogeny) at 1594 ± 8 Ma. Some ironstone-hosted mineralization at Starra may have formed during this event, but an Re-Os molybdenite age of 1568 ± 7 Ma suggests that mineralization also occurred during a (late) metamorphic tectonic event. These ages demonstrate that the Starra system is unrelated to igneous activity; a metamorphic fluid source is proposed. Titanite from a preore alteration assemblage at Mount Elliott yields a U-Pb age of 1530 ± 11 Ma which is within error of the ~1515 Ma Re-Os molybdenite ages from both Mount Elliott and the adjacent SWAN deposit. A magmatic-related origin for the Mount Elliott and SWAN deposits is favored given that the titanite and main-stage molybdenite ages are similar to the emplacement ages for the nearby Squirrel Hills Granite. Molybdenite-bearing calcite veins that crosscut main-stage IOCG mineralization at SWAN have been dated at ~1355 Ma, indicating that this paragenetic stage is unrelated to the Squirrel Hills Granite. U-Pb SHRIMP zircon analyses of volumetrically minor trachyandesitic dikes from Mount Elliott and SWAN reveal relatively young crystallization ages of 1119 ± 15 and 1096 ± 10 Ma, therefore they are not a suitable fluid or metal source for these deposits. Two Re-Os molybdenite analyses from Mount Dore (1503 ± 5 and 1508 ± 5 Ma) are similar to the ~1515 Ma ages from Mount Elliott and SWAN emplacement age of and the Mount Dore Granite. However, two other Re-Os dates (1497 ± 6 and 1501 ± 5 Ma) demonstrate that parts of the Mount Dore system may be slightly younger than Mount Elliott and SWAN. This younger timing is consistent with relative timing criteria that demonstrate mineralization postdates the crystallization of the Mount Dore Granite. Molybdenite from Lady Ella gives an age of 1487 ± 5 Ma. Mineralization at Mount Dore and Lady Ella is probably related to a late-stage, evolved magmatic fluid that generated more potassic alteration. The revised geochronological framework for alteration and mineralization in the Cloncurry district partially explains the fundamental differences in geologic characteristics between IOCG deposits in the district and clearly demonstrates that not all of the IOCG occurrences are related to the intrusion of the voluminous Williams-Naraku batholith. Instead metal-rich reservoirs were generated under the Mount Isa inlier by subduction along the southern margin of the North Australia craton; these reservoirs have been periodically tapped during tectonothermal events, including metamorphism during the Isan orogeny and hot spot activity that generated extensive A-type magmatism that have contributed to the significant metal enrichment across the Mount Isa inlier.

Paleomagnetism of the world-class Century Zn–Pb–Ag deposits, Australia

Paleomagnetic results are reported from the Century Zn–Pb–Ag SEDEX deposits in northwestern Queensland, Australia. The stratiform mineralization occurs in fine parallel lamellae in ∼ 1595 Ma siltstones and black shales of the Lawn Hill Formation in the Proterozoic McNamara Group. Galena has given a Pb/Pb model age of ∼ 1570 Ma. Paleomagnetic analysis of 333 specimens from ore zones (15 sites), hanging wall sandstones or siltstone (4 sites), and footwall siltstones (5 sites) using mostly thermal and then alternating field step demagnetization, isolates a stable characteristic remanent magnetization (ChRM) for ore sites only. Step demagnetization and rock magnetic analyses of ore, Zn and Pb concentrates and tailings show that the main remanence carriers are single- or pseudosingle-domain inclusions of titanomagnetite in sphalerite and gangue, and pyrrhotite in galena with modern goethite and/or hematite from the weathering of siderite. A paleomagnetic fold test using the ChRM directions of the ore sites is positive, showing that the ore ChRM predates the D 2 deformation of the ∼ 1595 to 1500 Ma Isan orogeny. The orogeny folded the main-stage mineralization, indicating that the ore retains a primary magnetization. The optimum 80% tilt-corrected unit mean ChRM direction for the ore yields a Mesoproterozoic paleopole at 1558 ± 4 Ma on the northern Australian apparent polar wander path. Thus this result both constrains the timing of mineralization and provides an upper age limit for D 2 deformation in the orogeny.

Preface: thematic issue on uranium deposits

This thematic issue comprises five papers covering various aspects of the genesis of different types of uranium deposits from Australia, Canada, India, and central Europe. Three papers are on Mesoproterozoic uranium deposits: Valhalla from Queensland, Australia, related to sodium metasomatism, by Polito et al., the unconformity related deposits of the Athabasca Basin, Saskatchewan, by Alexandre et al., and an iron oxide–copper–gold (IOCG) type of uranium mineralization from the Singhbhum Shear Zone, India, by Pal et al. Two other papers concern hydrothermal uranium veins hosted by Early Paleozoic metamorphic rocks of the Variscan Bohemian Massif (Dolnicek et al.; Kribek at al.). There are numerous uranium deposits related to Na metasomatism, but the two largest and presently mined ones are those of the Central Ukrainian Province (Belevtsev and Koval 1968) and the Lagoa Real district in Brazil. The geotectonic setting of this type of mineralization and the origin of the mineralizing fluids is debated (Lobato et al. 1983; Turpin et al. 1988). The paper by Polito et al., besides a detailed mineralogical description of the alteration and ore minerals, provides U–Pb isotopic ages on the uranium minerals and Ar/Ar ages on alteration minerals, allowing to link these processes to a very-low-grade metamorphic event at about 1,550 Ma, the Isan orogeny. The temperature of alteration and mineralization (340–380°C) at Valhalla is, however, significantly lower than in the Ukrainian and Brazilian occurrences (>500°C to 300°C). Fluids associated with the Valhalla deposit would be either basinal brines expulsed during the metamorphic event or magmatic fluids, whereas the fluids associated with the Lagao Real deposits have a seawater composition (Lobato et al. 1983). The age of the primary uranium deposition in unconformity-related uranium deposits has been continuously revised since the first determinations during the 1970s. With improvement of the dating techniques, older and older ages have been obtained from about 900 Ma to nearly 1,600 Ma for the latest ages obtained by Kurt Kyser’s group at Queen’s University and reviewed by Alexandre et al. in the present volume. Uranium oxides are easily remobilized by highto low-temperature fluid percolations and tend to lose their radiogenic lead because of the much larger diameter of Pb compared to U. The use of in situ dating techniques by secondary ion mass spectroscopy or laser ablation inductively coupled plasma mass spectroscopy allows analyzing the less disturbed domains of uranium oxide minerals. Alexandre et al. also associate, the first time for these deposits, in situ laser step heating Ar/Ar dating of metamorphic host rock minerals and syn-ore alteration clay minerals. The events younger than 1.6 to 1.5 Ga are interpreted as remobilization of the primary uranium mineralization in response to far-field tectonic or magmatic events. The possibility that some of these events may represent new uranium deposition events, upgrading the uranium deposits and thus explaining their extremely high grade, is excluded. With a 1.6 Ga deposition age, the timing of the formation of the Athabasca deposits becomes very close to the age of the uranium deposits of the Kombolgie Basin, Northern Territory, Australia, which formed in the same P-T-X conditions (Derome et al. 2003, 2005). Miner Deposita (2009) 44:1–2 DOI 10.1007/s00126-008-0218-y

Syn- and post-extensional tectonic activity in the Palaeoproterozoic sequences of Broken Hill and Mount Isa and its bearing on reconstructions of Rodinia

Palaeoproterozoic sequences deformed during the 1600 Ma Olary and Isa orogenies in the Broken Hill and Mount Isa terranes preserve an earlier record of syn-extensional magmatism, deformation and low-pressure/high-temperature metamorphism linked to basin formation and normal faulting at upper crustal levels. Crustal extension occurred nearly continuously from 1800 to 1640 Ma and produced several stacked sedimentary basins, the content of which was mainly controlled by growth faulting along the margins of NNW- and NW-trending half-graben. Mid-crustal magmatic rocks intruded and unroofed during the course of extension are commonly mylonitised and were carried to higher structural levels on extensional shear zones active at the time of sedimentation. Metamorphic monazite ages from deformed host rocks of the syn-extensional Sybella Granite at Mount Isa support this interpretation and include a 1675 Ma population that is much too old to be related to either later crustal shortening or the Isan orogeny. This record of near continuous extension at upper and mid-crustal levels is difficult to reconcile with existing reconstructions of Rodinia in which the Broken Hill and Mount Isa terranes are juxtaposed against rocks of similar age in southwest Laurentia that preserve a history of contractional deformation related to terrane accretion, amalgamation and collision. Equally difficult to reconcile are palaeogeographical reconstructions of Australia which place the Broken Hill and Mount Isa terranes adjacent to each other in a back-arc position while maintaining an along-strike continuity with the rocks of southwest Laurentia. In such a configuration, the Mojave, Yavapai, and Mazatzal provinces lie south of the proposed common Palaeoproterozoic suture (Cheyenne Belt) whereas their inferred age equivalents in the Broken Hill and Mount Isa terranes lie north of this former convergent plate margin. An alternative reconstruction of Proterozoic eastern Australia is proposed in which back-arc extension in the Broken Hill and Mount Isa terranes was linked to retreat of a west-dipping (present-day coordinates) subduction zone and associated magmatic arc that now resides partially or wholly within North America. Eastern Australia and Laurentia ceased to be part of a single continental landmass soon after 1800 Ma and thereafter followed increasingly divergent tectonic paths until re-amalgamated during collision at ca. 1600 Ma. Extension in the Broken Hill and Mount Isa terranes produced a North American Basin-and-Range-style crustal architecture that has no obvious counterpart in southwest Laurentia.

The crustal scale architecture of the Eastern Succession, Mount Isa: The influence of inversion

The three-dimensional crustal architecture of the eastern part of the Mount Isa Inlier is investigated from serial cross-sections constructed using geological map data, revised chronostratigraphy, gravity, magnetics, worms (multiscale wavelet edges of potential field data) and seismic data. The top part of the crust consists of rift and platform type metasediments that were deposited in three cover sequences from 1850 to 1610 Ma. These rocks constitute the Mount Isa Eastern Succession, and they were intruded by mafic–felsic plutons, dykes and sills of various ages before and during the Isan Orogeny ( ca. 1.6–1.5 Ga). The Eastern Succession overlies a felsic metamorphic basement, which in turn sits on a tonalitic–gabbroic lower crust. The depositional basin architecture for the Eastern Succession was controlled by major N–S trending structures that penetrated the lower crust, and accommodated E–W extension. These structures also underlie major upper crustal structures such as the Mitakoodi Culmination and Snake Creek Anticline that were formed by contraction in the Isan Orogeny. Positive inversion may therefore have been a key process in the evolution of the eastern part of the inlier, and governs its architecture at the crustal scale. Inversion involved reactivation of basement-penetrating structures, which localised contractional structures in the cover sequences above, as well as influencing pluton emplacement. The felsic metamorphic basement may have been penetratively deformed during inversion. The spatial association between the basin-controlling and contractional structures suggests that either early extensional displacements were completely reversed by later contraction, or that much of the Eastern Succession has remained essentially parauthochthonous relative to the basement.

Age constraints on the tectonothermal evolution of the Selwyn Zone, Eastern Fold Belt, Mount Isa Inlier

The Selwyn Zone is characterized by multiple ductile deformation and metamorphic events. There is general consensus that early in the Isan Orogeny there were both extensional and thrust events that produced bedding-parallel foliation (S bp). We propose that these were followed by the D 1 event that resulted in a regional steep east–west foliation, and that the Snake Creek Anticline and the dominant S 2a regional foliation formed as upright north–south structures. Local overturning of D 2 structures was due to subsequent localized deformation (D 2b) that resulted in structures with flat-lying axial planes. D 3 and D 4 produced steep upright folds and crenulations with north–northwest and northeast trending axial surfaces. Metamorphism, largely controlled by the emplacement of mafic rocks, is dominantly low pressure, but the P–T–t path is complex. Early growth of cordierite (M 1) followed by andalusite, was succeeded by a clockwise medium pressure loop of ca. 500–600 MPa in the northern Snake Creek Anticline, as indicated by early D 2a kyanite growth. The metamorphic peak during D 2 was around 580 °C and 400 MPa at the sillimanite isograd and reached a maximum of around 670 °C and 400–500 MPa in the migmatitic sillimanite K-feldspar zone at the Cannington and Osborne mines. Previous age determinations for the metamorphic peak of ∼1580–1600 Ma are confirmed with new electron microprobe dating of monazite, whereas an age grouping of 1600–1630 Ma may represent the D 1 (M 2) event. Additional ages in the range 1640–1680 Ma are recorded in albitites from the Soldiers Cap Group, and it is suggested that they formed by infiltration of hydrothermal fluids during extension and the emplacement of mafic sills. This is supported by SHRIMP dating of zircon from a trondhjemite lens in a largely mafic sill complex in the Snake Creek Anticline that provided an age of 1686 ± 8 Ma, also indicating that at least part of the Soldiers Cap Group is older than previously indicated. A zircon SHRIMP age of 1527 ± 4 Ma was obtained for a sample of albitized Saxby Granite. This age also represents the D 4 (M 6) event, and is the timing of the extensive brecciation and albitization of calc-silicate rocks of the Corella Formation in the Selwyn Zone.

Discrete element modelling applied to mineral prospectivity analysis in the eastern Mount Isa Inlier

Numerical modelling using a discrete element technique is employed here to examine the response of a fracture system in the eastern Mount Isa Inlier to an applied stress regime. Model scale, parameters and boundary conditions were varied to test which combinations of geometry and material properties produce the best correspondence between the known mineral-deposit distribution and zones of anomalous model stress. Modelled areas of combined low-minimum principal stress ( σ 3) and low-mean stress ( σ m) show the best correlation with deposits, but these areas do not clearly correspond to specific fault orientations or configurations. Rather, the models produce complex zoning of stress anomalies in response to the partitioning of stress across complex fault blocks, and the interaction between more competent granitoid bodies, less competent meta-sedimentary rocks, and the fault and rock boundary complexities. Adding fluid to the models, and identifying an optimum orientation of the stress field (with σ 1 in an ESE direction), produces the highest degree of visual correspondence with the known mineral-deposit distribution and the previous empirical prospectivity analysis, and also identifies several potentially prospective areas that may not have been previously tested by explorers. The models are consistent with mineralisation occurring (or being remobilised from earlier concentrations) during a major phase of regional fluid flow facilitated by a complex fault array, late during the evolution of the Isan Orogeny and synchronous with the waning stages of the emplacement of the Williams Batholith.

Structure of the Isan Orogeny under cover to the east of the Mount Isa Inlier revealed by multiscale edge analysis and forward and inverse modelling of aeromagnetic data

Aeromagnetic data reveals the presence of three north–south orientated, tens of kilometre scale, elliptical structures under cover to the east of the Mount Isa Inlier in Queensland, Australia. The Holy Joe Creek, Boot Creek and Nora Creek structures are typically defined by three narrow, continuous magnetic ridges. Highly magnetically susceptible amphibolite bodies, similar to those exposed in the Inlier, are probably the source of most of these magnetic anomalies. Multiscale edge analysis, 2D forward modelling and 3D inverse modelling methods have been used to resolve the geometry and near surface dip of the structures. These techniques indicate that the structures dip moderately to steeply around their edges. The Holy Joe Creek Structure exhibits sub-vertical to steeply west-dipping sides and a moderate plunge around the northern closure. The Boot Creek and Nora Creek structures are more complex magnetically, but their eastern edges also have sub-vertical to steep westerly dips. Diapirism and domes in an egg-box style (type 1) fold interference pattern are considered as possible origins for the elliptical structures. However, disparity between the shape of the structures and possible diapiric bodies and the lack of complementary basin structures to the domes, makes these two explanations unlikely. A more plausible alternative is that the structures are the result of deformation in a single phase, producing folds with a variable hinge-line plunge. These results indicate that east–west shortening during the D 2 event of the Isan Orogeny was the major deformation in the buried rocks of the study area, suggesting a shared structural history with the Mount Isa Inlier to the west.

The Proterozoic, albitite-hosted, Valhalla uranium deposit, Queensland, Australia: a description of the alteration assemblage associated with uranium mineralisation in diamond drill hole V39

The Valhalla uranium deposit, located 40 km north of Mount Isa, Queensland, Australia, is an albitite-hosted, Mesoproterozoic U deposit similar to albitite-hosted uranium deposits in the Ukraine, Sweden, Brazil and Guyana. Uranium mineralisation is hosted by a thick package of interbedded fine-grained sandstones, arkoses and gritty siltstones that are bound by metabasalts belonging to the ca. 1,780 Ma Eastern Creek Volcanics in the Western Succession of the Mount Isa basin. Alteration associated with U mineralisation can be divided into an early, main and late stage. The early stage is dominated by laminated and intensely altered rock comprising albite, reibeckite, calcite, (titano)magnetite ± brannerite. The main stage of mineralisation is dominated by brecciated and intensely altered rocks that comprise laminated and intensely altered rock cemented by brannerite, apatite, (uranoan)-zircon, uraninite, anatase, albite, reibeckite, calcite and hematite. The late stage of mineralisation comprises uraninite, red hematite, dolomite, calcite, chlorite, quartz and Pb-, Fe-, Cu-sulfides. Brannerite has U–Pb and Pb–Pb ages that indicate formation between 1,555 and 1,510 Ma, with significant Pb loss evident at ca. 1,200 Ma, coincident with the assemblage of Rodinia. The oldest ages of the brannerite overlap with 40Ar/39Ar ages of 1,533 ± 9 Ma and 1,551 ± 7 Ma from early and main-stage reibeckite and are interpreted to represent the timing of formation of the deposit. These ages coincide with the timing of peak metamorphism in the Mount Isa area during the Isan Orogeny. Lithogeochemical assessment of whole rock data that includes mineralised and unmineralised samples from the greater Mount Isa district reveals that mineralisation involved the removal of K, Ba and Si and the addition of Na, Ca, U, V, Zr, P, Sr, F and Y. U/Th ratios indicate that the ore-forming fluid was oxidised, whereas the crystal chemistry of apatite and reibeckite within the ore zone suggests that F− and \( {\text{PO}}^{{3 - }}_{4} \) were important ore-transporting complexes. δ18O values of co-existing calcite and reibeckite indicate that mineralisation occurred between 340 and 380°C and involved a fluid having δ18Ofluid values between 6.5 and 8.6‰. Reibeckite δD values reveal that the ore fluid had a δDfluid value between −98 and −54‰. The mineral assemblages associated with early and main stages of alteration, plus δ18Ofluid and δDfluid values, and timing of the U mineralisation are all very similar to those associated with Na–Ca alteration in the Eastern Succession of the Mount Isa basin, where a magmatic fluid is favoured for this style of alteration. However, isotopic data from Valhalla is also consistent with that from the nearby Mount Isa Cu deposit where a basinal brine is proposed for the transport of metals to the deposit. Based on the evidence to hand, the source fluids could have been derived from either or both the metasediments that underlie the Eastern Creek Volcanics or magmatism that is manifest in the Mount Isa area as small pegmatite dykes that intruded during the Isan Orogeny.

Numerical Modeling of the Coupled Mechanical and Hydrological Processes during Deformation and Mineralization in the Mount Isa Block, Australia

AbstractMount Isa is a major Australian and world Pb‐Zn‐Ag mineral province. The wide varieties of mineralization in the province are believed to be closely related to the geodynamic processes of Isan Orogeny, which occurred between ca 1500 and 1620 Ma. In order to understand the geodynamic processes associated with the Isan Orogeny and the giant mineralization systems in the Mount Isa district, a series of numerical models has been constructed to simulate coupled mechanical–hydrological processes, using Fast Lagrangian Analysis of Continua (FLAC), a finite difference computer code. The numerical modeling results have demonstrated that the most probable far‐field stress orientation during the Isan Orogeny is the asymmetrical E–W shortening, which led to greater easternward tectonic movement at the west boundary of the district in comparison with westward movement at the east boundary. During the initial and early stage of the Isan Orogeny, the mechanical and hydrological conditions in the Leichardt Fault Trough of the West Fold Belt are much more favorable for fluid accumulation and mineralization than in the East Fold Belt. The Mount Isan fault zone developed as a high dilation shear zone where the fluids were focused. As the asymmetrical shortening progressed, shortening deformation and shear strain localization became intensified in the eastern part of the orogenic district. The eastern region therefore became a more favorable locality for hydrothermal mineralization. This structural development feature seems to explain why mineralization in the East Fold Belt is generally later than in the West Fold Belt. Fluid production from the Williams–Naraku granites could result in fluid over‐pressuring, and this probably contributed to the extensive brecciation and related mineralization in the East Fold Belt.

Inversion in the Eastern Succession of the Mt Isa inlier: A key to crustal architecture

New insights into the crustal structure and evolution of the Mt Isa Eastern Succession have come from constructing a crustal scale 3D model. A revised chronostratigraphy, detailed aeromagnetic data, multiscale wavelet edges of potential field data, and reprocessing and reinterpretation of seismic data suggest hitherto unsuspected links between upper and lower crustal structures. Three major approximately NS structures are interpreted to underlie the Eastern Succession, respectively along the edge of the Wonga belt, under the Mitakoodi anticline and at the western margin of the contiguous Soldiers Cap outcrop. Major contrasts in potential field data occur along these structures; they also correspond to major changes in stratigraphy. They are interpreted to have originated as extensional faults during basin formation, which were positively inverted during the Isan orogeny from 1.6 to 1.5 Ga. Inversion apparently involved both the cover sequences and basement rocks, although net extension is preserved on some parts of the major structures. Reactivation of these structures in the Isan orogeny may explain some of the complexity of structure within the Eastern Succession, which has hitherto been interpreted as evidence for multiple overprinting deformation. Furthermore, the localisation of contractional features over extensional structures suggests that the Eastern Succession is parautochthonous. The two largest mineral deposits in the Eastern Succession (Cannington and Ernest Henry) are proximal to the easternmost major crustal structure, suggesting the importance of these structures to mineralization.

A Fluid Inclusion and Isotopic Study of the Mineralization in the Zinc-Rich Burketown Mineral Field, Northern Australia

The ca. 1575 Ma Century Zn-Pb deposit and the 47 near-by discordant Zn-Pb lode deposits of the Burketown Mineral Field, northern Australia host ore and gangue minerals that have not been overprinted by deformation associated with the the 1610?1520 Ma Isan Orogeny (Page and Bell, 1986; Blake, 1987), which obliterated many of the primary textural features at the Mt Isa, Hilton and George Fisher deposits. The Burketown Mineral Field deposits, which are hosted in shales and siltstones belonging to the Isa Superbasin, comprise sphalerite, pyrite, carbonate, quartz, galena, chalcopyrite and minor illite that lend themselves to isotopic and fluid inclusion studies that cannot be performed on the deposits to the south. For instance, there are no published O-H isotopic data from any of the known Zn-Pb deposits in the Mt Isa Basin and very limited, pre-Isan 40Ar/39Ar and fluid inclusion data (Rohrlach et al., 1998). A paragenetic and isotopic study of Century and 14 of the discordant lode deposits combined with published Pb/Pb model ages reveal that multiple fluid events spanning a period lasting ca. 400 million years since the initial formation of the Century deposit are preserved in this area.

Thermal and fluid flow history of the Lawn Hill Platform, northern Australia: implications for the formation of Zn-Pb deposits

Isa Superbasin strata in the Lawn Hill Platform host major base metal sulfide mineralization including the giant Century Zn-Pb deposit. Mineral paragenesis, organic maturation, K-Ar dating and stable isotope studies demonstrate that long-lived structures such as the Termite Range Fault acted as hot fluid conduits several times during the Paleoproterozoic and Mesoproterozoic. K-Ar dating of illites, in combination with other datasets, may identify three thermal events in the Lawn Hill Platform at 1500, 1440-1400 and 1250-1150 Ma. 1500 Ma is a Late Isan Orogeny age recorded only in the south that may reflect exhumation of a provenance region. The 1440-1300 Ma ages are related to fault reactivation and a thermal/?fluid pulse at ~1440-1400 Ma, with subsequent enhanced cooling. The youngest thermal/fluid flow event at 1250-1150 Ma is recorded mainly in the northeast and is probably related to the assembly of Rodinia. An extensive carbonate alteration halo in the footwall of the circa 1575 Ma Century Zn-Pb deposit extends some 15 km along strike. Calculations of fluid oxygen and carbon isotope composition based on model temperatures of 120°C for the ore zone siderites and 180°C for siderites and ankerites in the regional carbonate alteration halo indicate similar ore and alteration fluid compositions (d18O = 3 to 10 per mil; d13C = -7 to -3 per mil). This fluid isotope composition is consistent with highly evolved basinal brines and mixed inorganic and organic carbon sources. The good agreement between maximum temperature estimates from illite crystallinity and organic reflectance and inverse correlation with carbonate oxygen isotopes in northeast Lawn Hill platform boreholes indicate thermal maturation and carbonate formation resulted from interaction with the 1250-1150 Ma fluids. The calculated fluid isotope compositions are consistent with an evolved basinal brine (d18O = 5.1 to 9.4 per mil V- SMOW; d13C = -13.2 to -3.7 per mil V-PDB) that contained a significant organic carbon component. Differences in carbon and oxygen isotope systematics of alteration carbonate are significant to their use with lithogeochemical vectors in mineral exploration in the northern Australian basins.

Noble gas and halogen constraints on mineralizing fluids of metamorphic versus surficial origin: Mt Isa, Australia

The origin of hydrothermal fluids responsible for silica–dolomite alteration associated with Cu ore in the giant Mt Isa deposit of Queensland, north east Australia has been investigated. Main-stage quartz and dolomite veins, and late- to post-ore quartz from the Buck Quartz Fault in the orebody footwall, plus the regional Mt Isa and Railway Faults contain two phase liquid–vapour fluid inclusions with variable salinity. Fluid inclusion molar Br/Cl values are mostly in the range 1.5–12 × 10 − 3 and molar I/Cl values are in the range 9–186 × 10 − 6 . Greater than seawater Br/Cl and I/Cl values can characterise either sedimentary formation waters with a bittern brine origin by the sub-aerial evaporation of surface water beyond the point of halite saturation or some metamorphic dehydration fluids with a deep crustal source. We propose that these alternative origins can be distinguished in our samples based on the relationship between 40Ar/ 36Ar and 36Ar concentration. Metamorphic fluids formed by dehydration of crystalline rock are predicted to have low 36Ar concentrations with the salinity, 40Ar/ 36Ar and 40Ar E/Cl values determined by the whole rock concentration of Cl, 36Ar, OH −, K and age. The main-stage quartz and Buck Quartz Fault samples host fluid inclusions with < 20 wt.% NaCl eq., 1–3 ppb 36Ar, 40Ar/ 36Ar in the range ∼ 10,000–28,000 and high 40Ar E/Cl values of ∼ 3–8 × 10 − 4 . These values are consistent with metamorphic fluids sourced by devolatilisation of either the underlying Eastern Creek Volcanics or the Barramundi Basement. In contrast, surface-derived fluids acquire elevated 40Ar/ 36Ar values (> 296) through fluid–rock interaction which leads to a simultaneous increase in 36Ar concentration. The main-stage Ca-rich dolomite fluid inclusions have high salinities close to the point of halite saturation (∼ 26 wt.%), 1.7–5.5 ppb 36Ar, 40Ar/ 36Ar in the range 1600–8400 and 40Ar E/Cl of ∼ 0.2–1 × 10 − 4 . These values are most similar to those expected in an evolved bittern brine; however, the 36Ar concentration and 40Ar/ 36Ar value exhibit a weakly negative correlation, which is the opposite to that expected in surface-derived fluids. Only fluid inclusions in samples from the Railway Fault and some fluid inclusions in samples from the Buck Quartz Fault have 36Ar concentrations of greater than Air Saturated Water (ASW = 1.3–2.7 ppb 36Ar), up to 27 ppb, and 40Ar/ 36Ar values of ∼ 1000–2500 that are expected for fluids with an exclusively sedimentary formation water origin. Fluid inclusions in the Mt Isa Fault samples have the highest 36Ar concentrations of up to ∼ 100 ppb, some of the lowest 40Ar/ 36Ar values and the highest Br/Cl value of up to ∼ 30 × 10 − 3 . These features distinguish it from all other sample groups in the mine area. Silica–dolomite alteration (and Cu mineralization) are interpreted to have resulted from mixing of the deeply derived metamorphic fluid, best preserved in quartz fluid inclusions, and a surface-derived bittern brine, partially preserved within the main-stage dolomite fluid inclusions. Inflow of a second surficial fluid, along regional faults and into the mine through the late- to post-D 3 Buck Quartz Fault may have terminated mineralization. The involvement of metamorphic fluids is compatible with the syn-D 3 (∼ 1530 Ma) timing of mineralization during the final stages of the Isan orogeny.

Sandstone Diagenesis in the Mount Isa Basin: An Isotopic and Fluid Inclusion Perspective in Relationship to District-Wide Zn, Pb, and Cu Mineralization

Sandstone Diagenesis in the Mount Isa Basin: An Isotopic and Fluid Inclusion Perspective in Relationship to District-Wide Zn, Pb, and Cu Mineralization

Zinc Deposits and Related Mineralization of the Burketown Mineral Field, Including the World-Class Century Deposit, Northern Australia: Fluid Inclusion and Stable Isotope Evidence for Basin Fluid Sources

The stratiform Century Zn-Pb deposit and the discordant Zn-Pb lode deposits of the Burketown mineral field, northern Australia, host ore and gangue minerals with primary fluid inclusions that have not been affected by the Isan orogeny, thus providing a unique opportunity to investigate the nature of the ore-forming brines. All of the deposits are hosted in shales and siltstones belonging to the Isa superbasin and comprise sphalerite, pyrite, carbonate, quartz, galena, minor chalcopyrite, and minor illite. According to Pb model ages, the main ore stage of mineralization at Century formed at 1575 Ma, some 20 m.y. after deposition of the host shale sequence. Microthermometry on undeformed, primary fluid inclusions hosted in porous sphalerite shows that the Zn at Century was transported to the deposit by a homogeneous, Ca 2+ - and Na + -bearing brine with a salinity of 21.6 wt percent NaCl equiv. δD fluid of the fluid inclusion water ranges from −89 to −83 per mil, consistent with a basinal brine that evolved from meteoric water. Fluid inclusion homogenization temperatures range between 74° and 125°C, which are lower than the 120° to 160°C range calculated from vitrinite reflectance and illite crystallinity data from the deposit. This discrepancy indicates that mineralization likely formed at 50 to 85 Mpa, corresponding to a depth of 1,900 to 3,100 m. Transgressive galena-sphalerite veins that cut stratiform mineralization at Century and breccia-filled quartz-dolomite-sphalerite-galena veins in the discordant Zn-Pb lodes have Pb model ages between 1575 and 1485 Ma. Raman spectroscopy and microthermometry reveal that the primary fluid inclusions in these veins contain Ca 2+ , Na + , but they have lower salinities between 23 and 10 wt percent NaCl equiv and higher δD fluid values ranging from −89 to −61 per mil than fluid inclusions in porous sphalerite from Century. Fluid inclusion water from sphalerite in one of the lode deposits has δ 18 O fluid values of 1.6 and 2.4 per mil, indistinguishable from δ 18 O fluid values between −0.3 to +7.4 per mil calculated from the isotopic composition of co-existing quartz, dolomite, and illite. The trend toward lower salinities and higher δD fluid values relative to the earlier mineralizing fluids is attributed to mixing between the fluid that formed Century and a seawater-derived fluid from a different source. Based on seismic data from the Lawn Hill platform and paragenetic and geochemical results from the Leichhardt River fault trough to the south, diagenetic aquifers in the underlying Calvert superbasin appear to have been the most likely sources for the fluids that formed Century and the discordant lode deposits. Paragenetically late sphalerite and calcite cut sphalerite, quartz, and dolomite in the lode deposits and contain Na + -dominated fluid inclusions with much lower salinities than their older counterparts. The isotopic composition of calcite also indicates δ 18 O fluid from 3.3 to 10.7 per mil, which is larger than the range obtained from synmineralization minerals, supporting the idea that a unique fluid source was involved. The absolute timing of this event is unclear, but a plethora of Pb model, K-Ar, and 40 Ar/ 39 Ar ages between 1440 and 1300 Ma indicate that a significant volume of fluid was mobilized at this time. The deposition of the Roper superbasin from ca. 1492 ± 4 Ma suggests that these late veins formed from fluids that may have been derived from aquifers in overlying sediments of the Roper superbasin. Clear, buck, and drusy quartz in veins unrelated to any form of Pb-Zn mineralization record the last major fluid event in the Burketown mineral field and form distinct outcrops and ridges in the district. Fluid inclusions in these veins indicate formation from a low-salinity, 300° ± 80°C fluid. Temperatures approaching 300°C recorded in organic matter adjacent to faults and at sequence boundaries correspond to K-Ar ages spanning 1300 to 1100 Ma, which coincides with regional hydrothermal activity in the northern Lawn Hill platform and the emplacement of the Lakeview Dolerite at the time of assemblage of the Rodinia supercontinent.

Timing and Chemistry of Fluid-Flow Events in the Lawn Hill Platform, Northern Australia

Mudrocks and carbonates of the Isa superbasin in the Lawn Hill platform in northern Australia host major base metal sulfide mineralization, including the giant strata-bound Century Zn-Pb deposit. Mineral paragenesis, stable isotope, and K-Ar dating studies demonstrate that long-lived structures such as the Termite Range fault acted as hot fluid conduits several times during the Paleoproterozoic and Mesoproterozoic in response to major tectonic events. Illite and chlorite crystallinity studies suggest the southern part of the platform has experienced higher temperatures (up to 300 degrees C) than similar stratigraphic horizons in the north. The irregular downhole variation of illite crystallinity values provides further information oil the thermal regime in the basin and shows that clay formation was controlled not only by temperature increase with depth but also by high water/rock ratios along relatively permeable zones. K-Ar dating of illite, in combination with other data, may indicate three major thermal events in the central and northern Lawn Hill platform Lit 1500, 1440 to 1400, and 1250 to 1150 Ma. This study did not detect the earlier Century base metal mineralizing event at 1575 Ma. 1500 Ma ages are recorded only in the south and correspond to the age of the Late Isan orogeny and deposition of the Lower Roper superbasin. They may reflect exhumation of a provenance region. The 1440 to 1300 Ma ages are related to fault reactivation and a thermal pulse at similar to 1440 to 1400 Ma possibly accompanied by fluid flow, with subsequent enhanced cooling possibly due to thermal relaxation or further crustal exhumation. The youngest thermal and/or fluid-flow event at 1250 to 1150 Ma is recorded mainly to the cast of the Tern-lite Range fault and may be related to the assembly of the Rodinian supercontinent. Fluids in equilibrium with illite that formed over a range of temperatures, at different times in different parts of the platform. have relatively uniform oxygen isotope compositions and more variable hydrogen isotope compositions (delta O-18 = 3.5-9.7 parts per thousand V-SMOW; delta D = -94 to -36 parts per thousand V-SMOW). The extent of the 180 enrichment and the variably depleted hydrogen isotope compositions suggest the illite interacted with deep-basin hypersaline brines that were composed of evaporated seawater and/or highly evolved meteoric water. Siderite is the most abundant iron-rich gangue phase in the Century Zn-Pb deposit, which is surrounded by all extensive ferroan carbonate alteration halo. Modeling suggests that the ore siderite formed at temperatures of 120 degrees to 150 degrees C, whereas siderite and ankerite in the alteration halo formed at temperatures of 150 degrees to 180 degrees C. The calculated isotopic compositions of the fluids are consistent with O-18-rich basinal brines and mixed inorganic and organic carbon Sources (6180 = 3-10 parts per thousand V-SMOW, delta C-13 = -7 to -3 parts per thousand V-PDB). in the northeast Lawn Hill platform carbonate-rich rocks preserve marine to early diagenetic carbon and oxygen isotope compositions, whereas ferroan carbonate cements in siltstones and shales in the Desert Creek borehole are O-18 and C-13 depleted relative to the sedimentary carbonates. The good agreement between temperature estimates from illite crystallinity and organic reflectance (160 degrees-270 degrees C) and inverse correlation with carbonate delta O-18 values indicates that organic maturation and carbonate precipitation in the northeast Lawn Hill platform resulted from interaction with the 1250 to 1150 Ma fluids. The calculated isotopic compositions of the fluid are consistent with evolved basinal brine (delta O-18 = 5.1-9.4 parts per thousand V-SMOW; delta C-13 = -13.2 to -3.7 parts per thousand V-PDB) that contained a variable organic carbon component from the oxidation and/or hydrolysis of organic matter in the host sequence. The occurrence of extensive O-18- and C-13-depleted ankerite and siderite alteration in Desert Creek is related to the high temperature of the 1250 to 1150 Ma fluid-flow event in the northeast Lawn Hill platform, in contrast to the lower temperature fluids associated with the earlier Century Zn-Pb deposit in the central Lawn Hill platform.

Crustal architecture of basin inversion during the Proterozoic Isan Orogeny, Eastern Mount Isa Inlier, Australia

The folded and metamorphosed supracrustal rocks of the eastern margin of the Mount Isa Inlier, NW Queensland, Australia, provide a record of basin inversion processes at high temperatures, from the foreland toward the core of a developing orogen. Deep water, turbiditic metasedimentary rocks of the lower Maronan Supergroup (ca. 1.71–1.66 Ga) occupy the hanging wall of a crustal-scale thrust, the Cloncurry Overthrust, which separates them from calc-silicate and meta-evaporitic rocks of the Doherty Formation (ca. 1.74–1.72 Ga) in the footwall. During the early stages of the Isan Orogeny (ca. 1.60–1.58 Ga) the rocks of the Maronan Supergroup were expelled from their basin and thrust to the north-northwest. Shortening was restricted to the hanging wall and produced a 10 km scale north to northwest verging nappe with a crumpled frontal zone of upright east–west trending folds. This morphology is consistent with a push-from-the-rear mechanism of nappe emplacement with the simple shear component of deformation increasingly accommodated within the hanging wall, down dip toward the core of the orogen. Metamorphism associated with this folding reached upper amphibolite facies temperatures at moderate to low pressures (4–6 kbar). Metamorphic zones in the hanging wall of the Cloncurry Overthrust were antiformally folded with continuing deformation. The relationship between faulting, folding and metamorphic zoning across the Cloncurry Overthrust are consistent with a thrust system, despite that fact that younger rocks are juxtaposed over older. Basin inversion provides the best explanation for this phenomenon. Within this context metamorphism during the early stages of orogenesis can be explained by the inheritance of elevated geothermal gradients produced by lithospheric thinning and sediment loading during formation of the basin. The overall orogenic architecture was controlled by a combination of thermal softening, which focussed early deformation within the basin, and weak zones at the base of the basin and the base of the crust. This resulted in crustal-scale thrusts of opposite vergence that partitioned upper crustal and lower crustal deformation.