Mount Isa Block Research Articles

Rejuvenation vs. recycling of Archean crust in the Gawler Craton, South Australia: Evidence from U–Pb and Hf isotopes in detrital zircon

Zircon grains from modern and ancient drainages of the Gawler Craton, South Australia have been analysed to define the relative contributions of juvenile sources and recycled crust to magmatic activity as a function of time, and to constrain the role of mantle inputs during the Proterozoic rejuvenation of the Archean Gawler Craton. The integration of Hf-isotope data with U–Pb age spectra shows that the crustal evolution was dominated by long periods of crustal reworking, and that crust generated in the Paleoarchean (3.2–3.5 Ga) was largely reworked during Proterozoic time. Four periods of juvenile mantle input can be recognised at ca 2540 Ma, 1853 Ma, 1595 Ma and 1169 Ma. The 1169 Ma event is not previously recognised in the Gawler Craton, but the absence of other important age peaks argues against the transport of these zircons from the Musgrave Block. Only minor proportions of the magmatic zircons with Paleoproterozoic to Mesoproterozoic ages have strongly juvenile signatures (εHf≥4), indicating that crustal reworking was the dominant process, with relatively minor mantle input. Comparison of the crustal evolution with that of nearby Australian terranes suggests that although the Gawler Craton shows some similarities with the Mount Isa Block and the Georgetown Inlier in Neoarchean–Paleoproterozoic time, it evolved independently between ca 1800 and 1550 Ma.

The protracted hydrothermal evolution of the Mount Isa Eastern Succession: A review and tectonic implications

Protracted metal and sulfur contributions to the Eastern Succession iron-oxide–Cu–Au (IOCG) province of the Proterozoic Mount Isa Block occurred primarily as a consequence of long-lived fluid fluxes, stimulated by repeated emplacement of voluminous magmas during rifting and thin-skinned convergence cycles. Although there is a direct role for felsic intrusions of the ca. 1530 Ma Williams–Naraku Batholith in hydrothermal ore genesis, these intrusions came at the culmination of protracted metal reorganization in the crust, not as the sole cause, as indicated by geochronology, mineral paragenesis, and the shapes of some orebodies relative to pre-1530 Ma structures. Spatial and geochemical data on mafic rocks suggests that the concentration of copper and gold into some of the mineral deposits involved a significant component of m- to 1000 m-scale remobilization and reworking of early enrichments, formed during basin evolution and initial inversion, by later regional metamorphic and magmatic–hydrothermal fluids. Osborne (eastern domain) and Eloise-type ores (or ore precursors) initially formed during or before the 1600 Ma regional metamorphic peak, by interaction of basinal or early metamorphic fluids with mafic rocks and ironstones, whereas younger oxidised brines released by the Williams/Naraku intrusions at ∼1530 Ma overprinted magnetite ± sulfides at Osborne (western domain) and Starra to produce the presently mined hematite–chalcopyrite ores. CO 2 with mantle-like stable isotope character is abundant at all stages of the hydrothermal evolution and is present in high concentrations even in felsic magmas. We thus infer that CO 2 was released directly from enriched mantle, or indirectly from mafic magmas, contaminating the process of volatile release from the top of felsic magma chambers and contributing to production of carbonate gangue in orebodies. Ernest Henry, the largest IOCG deposit in the district, remains the best candidate for a true syn-granite magmatic–hydrothermal orebody. We infer that ore deposition occurred when mantle- or mafic-derived H–C–O–S fluid mixed with saline, oxidised brine derived from the Williams/Naraku Batholith, stripping some ore components (Fe, Sr, Cu) from the local wallrocks, in particular mafic rocks. The protracted hydrothermal evolution is reminiscent of modern back-arcs but the position of the arc during the post-1800 Ma history was hundreds of kilometres east. We propose that mantle enrichment in volatiles occurred around a pre-1840 Ma plate boundary leaving the Kalkadoon–Leichhardt belt as a magmatic arc remnant. This metasomatised mantle was subsequently re-tapped during prolonged distal back-arc spreading and periodic shortening accompanying ongoing magmatism.

Noble gas and halogen constraints on regionally extensive mid-crustal Na–Ca metasomatism, the Proterozoic Eastern Mount Isa Block, Australia

Fluid inclusions in late-Isan quartz veins associated with regional Na–Ca alteration (albitisation), in the Mary Kathleen Fold Belt and the Cloncurry District of the Eastern Mt Isa Block, have been analysed for naturally occurring and neutron produced isotopes of Ar, Kr and Xe. The noble gases have been extracted using a thermal decrepitation procedure that enables partial deconvolution of the different fluid inclusion types, including variably saline aqueous, liquid carbon-dioxide and mixed aqueous-carbonic varieties. The variably saline (<5–65 wt%) aqueous fluid inclusions dominate and have 40Ar/ 36Ar values of less than 2700 in most of the samples from across the region. These fluid inclusions have extremely variable molar Br/Cl values of 0.3–4 × 10 −3 and I/Cl values of 0.2–35 × 10 −6, and the fluids are interpreted to represent sedimentary formation waters derived from the upper crust that have dissolved variable quantities of halite (or scapolite) to achieve their ultra-high salinity. Fluid inclusions in a sample from the Snake Creek anticline in the Cloncurry District have the highest 40Ar/ 36Ar value of ∼25,000 demonstrating a deep magmatic or metamorphic fluid origin in this case. A magmatic origin is favoured because this fluid is very similar to a fluid involved in Iron-Oxide-Copper-Gold (IOCG) mineralization at Ernest Henry that was also interpreted to have a magmatic origin, and because the aqueous fluid inclusions have Br/Cl of ∼1–2 × 10 −3 and I/Cl of ∼10 × 10 −6 that are similar to mantle-derived igneous rocks. Carbon-dioxide fluid inclusions dominate samples from the Knobby Quarry in the Mary Kathleen Fold Belt and have a maximum 40Ar/ 36Ar value of 6000–7000. These fluid inclusions are estimated to have a 36Ar concentration of 1–4 ppb, that is similar to the range determined for aqueous fluid inclusions in all the other samples. In addition, aqueous and carbonic fluid inclusions in all the samples have similar 129Xe/ 36Ar plus 84Kr/ 36Ar values that are unfractionated, and close to the air—Air Saturated Water (ASW) range. These data are interpreted to indicate an independent and dominantly metamorphic origin for CO 2, and the presence of mixed aqueous-carbonic fluid inclusions are attributed to fluid mixing and/or mingling, rather than fluid unmixing. However, the data do not preclude the presence of a minor magmatic CO 2 component in the samples from the Knobby Quarry. Fluid inclusions in most of the samples from the Mary Kathleen Fold Belt have higher Br/Cl and I/Cl values, higher 36Ar concentrations and lower maximum 40Ar/ 36Ar values than fluid inclusions in samples from the Cloncurry District. This suggests Na–Ca alteration in these different parts of the Eastern Mt Isa Block occurred independently. However, fluid inclusions associated with Na–Ca alteration in the Cloncurry District have a very similar composition to fluid inclusions in IOCG mineralization-stage quartz veins from Ernest Henry. These data are therefore compatible with a genetic relationship between regional Na–Ca alteration and IOCG mineralization in the Cloncurry District. In both cases the ultra-saline hydrothermal fluids had a dominant origin from sedimentary formation water, but are interpreted to contain a magmatic component sourced from the late-Isan Williams-Naraku Batholiths which may have driven fluid convection.

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.

A Basin System and Fluid-Flow Analysis of the Zn-Pb-Ag Mount Isa-Type Deposits of Northern Australia: Identifying Metal Source, Basinal Brine Reservoirs, Times of Fluid Expulsion, and Organic Matter Reactions

Paleoproterozoic rocks of northern Australia host one of the world’s largest base metal repositories and are the world’s most important zinc repository. The McArthur-Mount Isa-Cloncurry mineral belt contains several world-class Zn-Pb-Ag, U, Cu, and Cu-Au deposits (Ewers and Fergusson, 1980; Williams, 1998; Betts et al., 2003; Large et al., 2005; Fig. 1⇓). The province has the potential to host additional base metal and uranium reserves. Advances in exploration techniques in the 1980s led to the discovery of several major new Zn and Cu-Au deposits, including Cannington, Century, Ernest Henry, and Osborne. However, recent exploration results have been disappointing and new exploration strategies are required if the region is to further its growth potential and if new resources are to be realized beneath shallow cover. Fig. 1. Map of northeastern Australia, showing the principal tectonostratigraphic subdivisions for the region. The map highlights the areas of study covered by the thematic papers in this volume and the location of the principal Zn-Pb-Ag, Cu, and Cu-Au deposits. The locations of profiles P1 and P2, the Comalco seismic grid, and deep boreholes including Desert Creek 1 are shown in the inset map (bottom left). Between 1975 to 1995 geoscientists from Geoscience Australia, the Geological Survey of Queensland, and the Northern Territory Geological Survey mapped the Paleoproterozoic outcrop belt of northern Australia at a scale of 1:100,000. Subsequently, researchers at Monash University undertook detailed studies of the deformation history of the Mount Isa block, placing the ore deposits within a tectonic context (e.g., O’Dea et al., 1997; Betts et al., 1998, 2003; Betts and Lister, 2002). Researchers at James Cook University carried out structural, metamorphic and mineralization studies across the Mount Isa block, with their principal focus concentrating on the deposits and their immediate environs (Bell et al., 1988 …

Modeling the Role of Sodic Alteration in the Genesis of Iron Oxide-Copper-Gold Deposits, Eastern Mount Isa Block, Australia

Liberation of iron and potassium by widespread postmetamorphic albitization of country rocks was one of the likely contributing processes in the formation of both barren and mineralized magnetite ± chalcopyrite + biotite + gold + hematite + clinopyroxene + actinolite + apatite ironstones in the Cloncurry district of the Proterozoic Mount Isa block. Whole-rock geochemical data indicate nearly immobile Al, Ga, ±Ti, Zr during transformation of a variety of least altered rocks toward albitite. The data indicate that the addition of Na from a brine to the rock accompanied the loss of Fe, K, Ba, Rb ± Ca, Sr, Co, V, Mn, Pb, and Zn from those altered rocks and enrichment in the brine, but that Cu was not systematically stripped from a variety of wall rocks during albitization. Conversely, the formation of metasomatic ironstones, the immediate hosts to some Cu-Au ores, involves addition of most of the same elements that were lost during albitization. The correlation between intensity of alteration, its distribution and timing (e.g., in breccias cored by ca. 1530–1500 Ma granitoids), and convergence of all rock types toward magmatic stable isotope values provides strong evidence for a substantial component of igneous-derived fluid. Simulations of the albitization process were carried out isothermally at 550°C and 350 MPa and polythermally from these conditions down to 400°C and 200 MPa, using the Gibbs minimization method with HCh software and the UNITHERM database. Both simple fluid-rock mixing models and more complicated reactor-style algorithms used a range of input fluids (from fluids equilibrated fluid with two-feldspar granite through to Na/K ratios consistent with fluid inclusion results) and geochemical data for initially unaltered wall rocks. The apparent paradox of widespread albitization resulting from fluid released by two-feldspar granites can be explained by relatively small shifts away from the K-feldspar-albite equilibrium curve, and even isothermal model fluids derived from two-feldspar granites produce albitites in calc-silicate rocks outboard of granite-proximal K-feldspar-clinopyroxene skarns, matching field patterns. Those models with fluid Na/K ratios similar to those of fluid inclusions produced the most realistic alteration assemblages, dominated by albite, for both isothermal conditions and decreasing temperature, which approximate those observed in the field. PIXE and microthermometric data on fluid inclusions from quartz in two-feldspar quartz monzonite and pegmatite at the top of the Mount Angelay pluton indicate bulk Na/K molar ratios in the fluid of between 10 and 20, considerably higher than our thermodynamically calculated values for fluid in equilibrium with two-feldspar granite of around 3 (at 550°C, 350 MPa). Such shifts may have been attained by admixture of magmatic-hydrothermal fluid with small amounts of NaCl brines trapped along grain boundaries in scapolite-bearing calc-silicate wall rocks, by fluid immiscibility due to high initial CO2 contents in the felsic intrusions, by contributions from mafic magmas, or from dissolution of salt-rich layers into the intrusions prior to crystallization and fluid release. With increasing amounts of fluid-rock interaction in the models, the fluids were enriched in K, Fe, and Ca, approaching compositions observed in fluid inclusions in the ore deposits. These fluids, reacted with pelitic rocks (which are common ore hosts), would produce magnetite-clinopyroxene ± biotite-actinolite alteration at high temperature, similar to the proximal alteration around ore deposits. We infer that precipitation of sulfides in the Cu-Au deposits was the result of mixing of Cu-bearing brine, of ultimately magmatic origin, but modified extensively via albitization, with sulfur-bearing fluids or reaction of the brine with sulfur-bearing rocks. When Cu was absent from the initial magmatic fluid, barren ironstones may have been the result.

Modeling the Role of Sodic Alteration in the Genesis of Iron Oxide-Copper-Gold Deposits, Eastern Mount Isa Block, Australia

Modeling the Role of Sodic Alteration in the Genesis of Iron Oxide-Copper-Gold Deposits, Eastern Mount Isa Block, Australia

Low-latitude meteoric fluid flow along the Cloncurry Fault, Cloncurry district, NW Queensland, Australia: geodynamic and metallogenic implications

Hydrothermal alteration within the Mount Isa Block (MIB) is typically largely to have ceased at the end of the Isan Orogeny (ca. 1.50 Ga). However, late, large brittle quartz-rich vein systems, including those of the Cloncurry Fault, cut Mesoproterozoic tectonic fabrics, intrusions and products of regional Na–Ca hydrothermal alteration. Associated features display affinities to iron oxide–Cu–Au ore deposits, including specular hematite-bearing veins and Cu–(Au–U–LREE–Ba) mineralization. Stable isotope data suggest late veins in the vicinity of the Cloncurry Fault, Eastern Fold Belt (EFB), formed from fluids with δ 18O H 2O and δD H 2O (250 °C) of −4‰ to +3‰ and −22‰ to −4‰, respectively. The high δD H 2O , moderate δ 18O H 2O and CaCl 2–NaCl-rich character of the saline fluids indicates they were mixtures of low-latitude meteoric fluids and evaporite-derived basinal brines, which, with time, became progressively dominated by meteoric water, deposited barite and cooled. The low-latitude meteoric signature demonstrates that the fault-fracture systems were active when in equatorial regions. Paleomagnetic data for the MIB, and the Australian protocontinent, together with structural and thermal considerations, and geochronological arguments suggest the veins probably formed sometime after 1.10 Ga, and potentially as late as during the Alice Springs Orogeny (ca. 0.34 Ga). The late quartz veining was unrelated to two phases of regional Na–Ca alteration and economically significant Fe oxide–Cu–Au mineralization at 1.60–1.58 and 1.53–1.50 Ga.

1.8–1.5-Ga links between the North and South Australian Cratons and the Early–Middle Proterozoic configuration of Australia

The Archaean and Early–Middle Proterozoic (1.8–1.5 Ga) terranes of the North Australian Craton and the South Australian Craton are separated by ∼400 km of ca. 1.33–1.10-Ga orogenic belts and Phanerozoic sediments. However, there is a diverse range of geological phenomena that correlate between the component terranes of the two cratons and provide evidence for a shared tectonic evolution between approximately 1.8 and 1.5 Ga. In order to honour these correlations, we propose a reconstruction in which the South Australian Craton is rotated ∼52° counterclockwise about a pole located at ∼136°E and ∼25°S (present-day coordinates), relative to its current position. This reconstruction aligns the ca. 1.8–1.6-Ga orogenic belts preserved in the Arunta Inlier and the Gawler Craton and the ca. 1.6–1.5-Ga orogenic belts preserved in the Mount Isa Block and the Curnamona Province. Before 1.5 Ga, the South Australian Craton was not a separate entity but part of a greater proto-Australian continent which was characterised by accretion along a southward-migrating convergent margin (ca. 1.8–1.6 Ga) followed by convergence along the eastern margin (ca. 1.6–1.5 Ga). After 1.5 Ga, the South Australian Craton broke away from the North Australian Craton only to be reattached in its current position during the ca. 1.33–1.10 Ga-Albany–Fraser and Musgrave orogenies.

Waldemar Lindgren Award for 2002 Citation of Tim Baker

Ladies and gentleman, members, executive, family, friends and guests of the Society of Economic Geologists: It’s a pleasure to be here today to introduce you to Tim Baker, this year’s recipient of the Lindgren Award. I first met Tim 10 years ago as he was starting his Ph.D., at which time I was running an international field trip and conference centered on fluids, metamorphism, and mineralization in the Mount Isa block. At that time, Tim was amongst graduate students from James Cook University, a group of perhaps half-a-dozen students who comprised a quarter or a third of the students present on the trip. Lots of interesting things happened on that trip, most focused on the blend of excellent geology and various degrees of slight misbehavior that characterize most geology field trips. One thing particularly notable amongst the students, however, was that the JCU students were unusually well behaved. Other students (and indeed some other field trip participants and/or their leaders) were engaged in minor vehicle abuse, non-invasive pyromania, and statistical, university-approved testing of alcoholic substances. I couldn’t understand why …

Geochemistry and origin of Proterozoic skarns at the Mount Elliott Cu-Au(-Co-Ni) deposit, Cloncurry district, NW Queensland, Australia

Diopside-rich, skarn-hosted, copper–gold ore derived primarily from carbonaceous metapelites at Mount Elliott forms a distinctive member of the spectrum of Cu–Au–(Fe oxide) deposit styles in the Cloncurry district of the Paleoproterozoic to Mesoproterozoic Mount Isa Block. The mine sequence is a package of carbonaceous metapelites and metagreywackes containing amphibolites derived from tholeiitic basic rocks. A 40Ar–39Ar age spectrum with an extensive plateau-like segment at 1,510 ± 3 Ma from an actinolite associated with sulfides is taken to represent the age of mineralization and is identical within error to the ages of most of the nearby batholithic granitoids. The mine sequence is locally intruded by 1- to 10-m-thick late- to post-tectonic trachyandesite dykes, which were emplaced during the hydrothermal activity that created the orebodies and have affinities with the regional high potassium “Eureka” supersuite granitoids. Stable isotope data are consistent with dominantly magmatic fluids during mineralization and the regionally distinctive skarn (Ca–Mg) and Cu–Au–Ni–Co–Te–Se (low Pb–Zn–Ag–Sb) chalcophile element associations may reflect a primitive magmatic fluid source and/or leaching of these elements from country rocks. Mount Elliott is an unusual skarn deposit characterized by pronounced early albitization (K–Fe–Mg depletion) of the host rocks succeeded by predominantly open-space deposition of sodic diopside ± actinolite ± scapolite ± andradite ± magnetite ± sulfides ± apatite ± allanite ± tourmaline ± calcite. The Ca–Fe–Mg(–Na)-rich (manganese-poor) chemistry was imposed from the fluid phase in the absence of carbonate-rich protoliths. Immobile trace element (Ti, Zr, Nb) geochemistry shows that Mount Elliott skarns formed in both metasedimentary and mafic metavolcanic host rocks, but the former are the main hosts of ore in upper and lower ore zones that represent most of the resource. Banded skarns derived from a distinct calc-silicate/marble package at the nearby SWAN prospect have higher Nb/TiO2 and Zr/TiO2 ratios than the Mount Elliott metasediment-derived skarns, consistent with different provenance of the detrital components in the two sequences. Medium- to coarse-grained massive skarn and skarn breccia in the Mount Elliott lower ore zone formed in pelites and the trachyandesite dykes are the only intrusive rocks that could be genetically related to the mineralization in the immediate vicinity of the orebodies.

The paragenesis and spatial distribution of secondary copper minerals in the Eastern Mount Isa Block, Queensland

The paragenesis and spatial distribution of secondary copper minerals in the Eastern Mount Isa Block, Queensland

Stratigraphy and depositional setting of the upper McNamara Group, Lawn Hills region, Northwest Queensland

The Mesoproterozoic Upper McNamara Group forms the uppermost division of the Lawn Hill platform in the western Mount Isa block. Several zinc and lead occurrences, including the Century deposit, are hosted by clastic and carbonate sediments within this interval. The Upper McNamara Group is subdivided into four formations: the Shady Bore Quartzite, Riversleigh Siltstone, Termite Range Formation, and Lawn Hill Formation.The Shady Bore Quartzite comprises up to 460 m of sandstone and dolomitic siltstone, deposited in a marginal to shallow subaqueous environment. The Riversleigh Siltstone is up to 2,900 m thick in the southern Lawn Hill region, where four members, R 1 to R 4 , are recognized. Member R 1 to R 3 sandstone, carbonate, and shale were mostly deposited in mid- and outer-shelf environments, indicating a regional transgression above the Shady Bore Quartzite. Sediment transport during the R 1 to R 3 interval was mostly from the south. Member R 4 shale and sandstone represent deeper subaqueous deposits. Riversleigh Siltstone sedimentation focused on a depocenter in the southern Lawn Hill region. All members display an abrupt decrease in thickness and facies changes to the north of the Termite Range and Barramundi faults, which are interpreted as active structures during deposition.The Termite Range Formation is up to 1,300 m thick and comprises two fining upward sequences, T 1 and T 2 , both dominated by sandstone in the lower part and by shale in the upper part. The Termite Range Formation is sandiest in the north and thins to the south and east. Sandstone beds are interpreted as high-concentration turbidites, deposited in a deep subaqueous environment. T 1 and T 2 are interpreted as two major episodes of turbidite fan sedimentation, sourced from the northwest. Fault activity to the west of the Lawn Hill region at about 1640 Ma is inferred to account for deposition of sandstone and the major shift in provenance from the south to the northwest.The Lawn Hill Formation is 2,200 m thick in the southern Lawn Hill region where six members, H 1 to H 6 , are recognized. Member H 1 shale and sandstone were deposited in outer shelf and deeper environments. Member H 2 and H 3 tuffaceous sediments were deposited in a mid-shelf environment, indicating a regional regression. Member H 1 to H 3 sediments were sourced from the west. Member H 4 shale and siltstone were deposited at outer shelf and greater depths, indicating a regional transgression above member H 3 . Member H 5 sandstone beds are interpreted as high-concentration turbidites, deposited in prograding turbidite fans. Member H 1 to H 3 sedimentation occurred throughout the Lawn Hill region, but a major depocenter was located in the northeast at this time. Member H 1 H 2 , and H 3 sediments rapidly thicken to the north of the Termite Range fault, which is interpreted as an active structure during deposition of this interval. The northward shift of the depocenter with respect to the Riversleigh Siltstone can be attributed to a reversal in movement along the Termite Range fault. Member H 5 sandstone beds are widespread throughout the northern Lawn Hill platform, and their deposition indicates another significant change in the distribution and style of sedimentation. Deposition in response to uplift in the south can be interpreted by the age of the sandstone (1595 Ma), which may coincide with the onset of the D 1 phase of the Isan orogeny.This study provides a framework for the identification of prospective areas for base metal mineralization within the Upper McNamara Group, based on a knowledge of sediment architectures and their controls.

Late Archean and Early Proterozoic crustal evolution of the Mount Isa block, northwest Queensland, Australia

Recognition of Late Archean and Early Proterozoic tonalite-trondhjemite-granodiorite crust in the Mount Isa block, northwest Queensland, Australia, has important implications for Proterozoic crustal evolutionary models in the North Australian craton. Protoliths of the Black Angel Gneiss Complex formed 2.42–2.50 Ga (Black Angel Gneisses) and 1.97–2.00 Ga (Pothole Creek Gneisses). Both groups have Nd model ages of 2.42 to 2.58 Ga. Supracrustal components within the gneiss complex have Nd model ages of 2.75 to 2.83 Ga. These data, and inherited components in zircons, indicate the presence of even older Archean crust. Crystallization of 1.84–1.87 Ga rims record a thermal event during a subsequent period of granitic (Kalkadoon-Leichhardt) magmatism. Both the Black Angel Gneiss Complex and Kalkadoon-Leichhardt magmatic suite have strong arc geochemical signatures and have components that are compositionally similar to Archean tonalite-trondhjemite-granodiorite lithologies. A seismically defined, crustal-scale, west-dipping feature in the Mount Isa block may be a Proterozoic cryptic suture. Subduction-zone magmatism may be related to this feature. Late Archean and Early Proterozoic crust formation in this segment of the North Australian craton was subduction driven, involving assembly of microcontinental terranes similar to other Early Proterozoic belts. This observation is significant, because paleomagnetic data indicate that the North Australian craton and Laurentia were juxtaposed during Early to Middle Proterozoic time.

Multiple foliation development during thrusting and synchronous formation of vertical shear zones

Three orthogonal foliations developed during large-scale N-S thrusting in the central portion of the Mount Isa Block, Australia. Two of these foliations, which are well preserved in their original orientations in porphyroblasts of pelitic schists but considerably affected and commonly destroyed in the schist matrix due to the effects of subsequent deformation and metamorphism, formed synchronously and have a common stretching lineation, which is near-horizontal and trends N-S. One foliation formed near-horizontal and the other near-vertical, striking N-S. Both developed during N-S-directed thrusting, with the N-S vertical foliation accommodating sinistral differential displacement between the thrust belt to the west and that to the east. The third foliation which dips steeply to the north and strikes E-W, formed during N-S shortening of the thrust belt and tends to overprint the other foliations formed in the same orogeny. The N-S vertical foliation is well preserved within a narrow belt of gneissic rocks (called the Wonga-Duchess Belt) that also locally contains the subsequently folded remains of the foliation that formed near-horizontal. This belt separates multiply thrust-repeated beds to the west from large-scale, apparently less repetitive thrusts to the east. The direction of thrusting appears to change across this belt from N to S on the west to S to N on the east. The differential deformation to either side of the belt was taken up by sinistral shear along this belt. Numerous related, but smaller-scale vertical shear zones occur up to 3 km away to either side of this narrow zone. The primary vertical foliation within the Wonga-Duchess Belt appears to have formed during transform-like faulting by progressive inhomogeneous simple shear with no component of orthogonal shortening.

Regional syndeformational copper mineralization in the western Mount Isa Block, Australia

The recently proposed syndeformational model for copper mineralization at Mount Isa has tremendous implications for metallogenesis and exploration, and hence, a regional assessment of its validity was made. Detailed studies on the structural setting, timing, and microstructural development of four brecciated sediment-hosted copper deposits situated northwest of Mount Isa, plus the assessment of many small prospects in the same area, reveal that all primary copper mineralization is strikingly similar and involves syndeformational (syn-D 3 ) veining and alteration. Variation in and between deposits can be attributed to their host-rock lithologies and local structural relationships. Silica replacement as recognized at Mount Isa mine is associated with all copper mineralization in dolomitic host rock. Sulfide (+ chlorite) replacement in arenaceous host rock, for example at the Mammoth deposit, occurs concomitantly with silica dolomite alteration in other deposits that comprise both host-rock types. The matching alteration history of the deposits indicates a common origin for the mineralization, and together with a large number of similar small-sized copper occurrences, suggests that a major regional copper mineralizing event occurred during the third deformation phase throughout the western Mount Isa block. Local structural relationships, common to several deposits, such as the presence and degree of bedding discordance across low-angle (D 1 ) faults together with later (D 2 ) folding, may constitute suitable structural traps for subsequent (D 3 ) mineralization. Therefore, the structural geometry, together with the alteration features, is essential in exploration and assessment of prospects.