Eastern Fold Belt Research Articles

P–T conditions of metamorphic and hydrothermal events at Tick Hill gold deposit, Mount Isa, Queensland, Australia

Tick Hill gold deposit is a unique gold mineralisation style in the Mary Kathleen Fold Belt in the Mount Isa Inlier. The gold at Tick Hill is generally pure without silver and was formed during two discrete metamorphic-deformation events (D1 and D3). Early gold was observed as inclusions or coarse grains hosted within D1 peak-metamorphic diopside, scapolite and hornblende from the garnet–biotite–hornblende (tschemakite)–plagioclase (andesine)–quartz assemblage. Late gold is closely associated with bismuth selenide, chlorite, albite, sericite and K-feldspar, and formed during D3. The syn-D1 garnet–plagioclase–hornblende–quartz–biotite assemblage was used to constrain the pressure and temperature (P–T) conditions of metamorphism and mineralisation using the garnet–plagioclase–hornblende–quartz barometer and the hornblende–plagioclase, garnet–biotite and garnet–hornblende thermometers. The results show that peak metamorphism at Tick Hill reached P–T conditions of 6.0–7.6 kbar and 720–760 °C. These P–T conditions together with gneissic and migmatisation textures recorded in different rock types at the Tick Hill area indicate that the peak metamorphism preserved in the area occurred at the amphibolite–granulite facies, compared with the amphibolite facies at the southern Mary Kathleen and Eastern Fold Belt. The D3 chlorite, which formed during stage 2 to stage 4 mineralisation events, displays a wide range of compositions reflecting a gradual retrograde temperature change from ∼380 °C to ∼130 °C. The pressures during D3 could not be reliably determined, but the presence of various Bi-selenides suggests that towards the waning stages of D3, the rocks may have been exhumed to a pressure less than 1 kbar.

Genesis of the Mount Colin Cu-Au deposit, NW Queensland: Evidence from geology, multi-mineral U–Pb geochronology, S-, O- and C-isotope constraints

The Mount Colin Cu-Au deposit is located in the Mary Kathleen Domain of the Mount Isa Inlier, NW Queensland. The deposit is hosted in the Corella Formation, which transitions into the Burstall Granite at a depth of ~400 m. The host rocks of the deposit are overprinted by Na-Ca alteration and a Ca-Mg-Fe skarn assemblage. Na-Ca alteration yields a titanite 207Pb/206Pb weighted mean age of 1717 ± 8 Ma (MSWD = 0.32) and is composed of albite, actinolite, zircon, titanite and scapolite with retrograde calcite, prehnite and sericite. The Ca-Mg-Fe skarn yields a titanite 207Pb/206Pb weighted mean age of 1712 ± 7 Ma (MSWD = 1.2) and is composed of hastingsite, K-feldspar, clinopyroxene, albite, titanite, calcite, biotite, and zircon with retrograde quartz and chlorite. Based on temporal relationships, Na-Ca alteration and the Ca-Mg-Fe skarn is interpreted to have formed from a hydrothermal magmatic fluid during late-stage magmatism of the (1740–1710 Ma) Wonga-Burstall plutons. A second generation of titanite (denoted titanite II) is present within Na-Ca alteration and the Ca-Mg-Fe skarn assemblage. Titanite II yields a 207Pb/206Pb weighted mean age of 1576 ± 7 Ma (MSWD = 0.37), and is interpreted to have formed from an external metamorphic hydrothermal fluid during widespread deformation associated with the (1610–1500 Ma) Isan Orogeny. The Mount Colin Cu-Au deposit consists of a predominantly coarse-grained, mineralogically zoned assemblage that infills along the NW-striking Mount Colin Fault. The centre of the deposit comprises a largely unmineralized core composed of calcite and/or quartz ± apatite ± microcline. Cu-Au mineralization is positioned along the outer margins of the deposit and consists of biotite, actinolite, chalcopyrite and pyrrhotite with trace abundances of magnetite, pyrite, galena, melonite and arsenopyrite. Apatite from the core of the deposit yields a U–Pb age of 1523 ± 16 Ma (MSWD = 1.2). The δ34SCDT values of chalcopyrite and pyrrhotite range from 0.1–2.1 ‰ and 0.9–1.5 ‰ respectively. The δ13CVPDB values of calcite range from −5.54 ‰ to −4.54 ‰ with corresponding δ18OVSMOW values ranging from 8.16–11.26 ‰. Based on the temporal and isotopic constraints presented in this study, a hydrothermal magmatic origin for Cu-Au mineralization is favoured. The timing of Cu-Au mineralization is coeval with a regional-scale metamorphic/hydrothermal event throughout the Mary Kathleen Domain and widespread magmatism throughout the Eastern Fold Belt of the Mount Isa Inlier.

A Pb isotope model for the Proterozoic of northern Australia with a focus on the McArthur Basin

The Pb isotopic composition of mineral exploration samples can be used to help determine their metallogenic association and hence to assess prospectivity in geological provinces where there is an established Pb isotope database. We have developed such a database, available online, and models that relate Pb isotopic signatures to known metallogenic events in Proterozoic terrains in northern Australia. Two terrain-specific Pb isotope model curves, the Western Fold Belt curve and the Eastern Fold Belt curve, were developed based on major ore systems in the Mount Isa and McArthur terrains. The Western Fold Belt curve was optimised to give an age of 1640 Ma for the McArthur Formation HYC deposit. This is the well-constrained age of the Barney Creek Formation, host of the mineralisation. To validate the accuracy of the models, we attempted to determine the initial Pb isotope ratios of granitic rocks from across the Proterozoic terrains with ages varying from 1870 to 1550 Ma. The curves give model ages for major ore systems including Mount Isa, George Fisher Zn–Pb, Century and Cannington that are consistent with their stratigraphic locations and accepted metallogenesis. Focusing on the McArthur Basin, a database of 59 deposits and prospects was developed. In addition to the 1640 Ma HYC event, an older event at ca 1700 Ma has been fingerprinted, a younger Upper McArthur event at ca 1570 Ma, an event coeval with the Mount Isa Orogeny at ca 1550–1500 Ma and a late Roper event younger than 1500 Ma. There is also evidence of hydrothermal activity of Cambrian age and recent weathering-related metal concentration.

National-Scale Geochemical Baseline and Anomalies of Chromium in Papua New Guinea

Papua New Guinea (PNG) is located at the convergence edge of the Pacific Plate and the Indo-Australian Plate, consisting of three units. There are three chromium mineralization types in PNG. Based on national-scale geochemical mapping in PNG during 2015–2018, 1399 samples of stream sediments were collected from Highland Region, Papua Peninsula, and New Guinea Islands. This paper preliminarily studied chromium’s geochemical background, spatial distribution characteristics, and geochemical anomalies. The chromium concentration ranged from 3 ppm to 74,600 ppm, with a median value of 145 ppm, which was higher than the upper crustal abundance of chromium and the chromium geochemical baseline of Europe, Australia, North America, and China. In terms of stream sediment samples in different tectonic units, as mafic–ultramafic magmatic rocks are widely developed, the median chromium values of the New Guinea Orogen, including the Papuan Fold Belt, the New Guinea Thrust Belt, the Finisterre Terrane, the Aure Fold Belt, the Eastern Fold Belt, and the Eastern Papuan Composite Terrane, were higher than the value of the Melanesian Arc. The ophiolitic complexes, such as the April ophiolite, the Marum ophiolite, and the Papua ultramafic belt, significantly correlated with the higher chromium concentration. Eleven chromium high anomalies with mineralization potential were delineated, including three laterite and podiform prospecting areas and eight placer prospecting areas. Based on the chromium–nickel integrated anomaly map, comprehensive exploration and exploitation of nickel and chromium can be carried out in 1 and 11 high anomaly areas related to lateritic mineralization.

Assessment of prospective gas resources and develop a production view of eastern foldbelt (block 18), Bangladesh using Monte Carlo simulation method

In Bangladesh, 20 gas fields out of 28 produced 709.11 billion cubic feet (Bcf) through 112 wells against a vast amount of demand in the financial year 2021–22. Therefore, more gas fields need to be explored in the most prospective offshore part of Bangladesh. In this study, we have attempted to estimate a prospecting natural gas reserve in the Eastern Fold Belt (Block 18) consisting of Teknaf, Coral Dip, and St Martin Island as well as to develop a production view for the future. Based on the prerequisite research data collected from Petrobangla, Monte Carlo simulations were used to estimate the reserve values as Gas Initially In Place (GIIP) and Recoverable Gas Initially In Place (RGIIP) by Petroleum Engineering (PE) Essential software (version 2017.4). The results show that the estimated GIIP and RGIIP are 6145.88 billion standard cubic feet (Bscf) and 4745.5 Bscf, respectively which are quite close to the previous study. The total estimated proven natural gas of the study is 2064.4 Bcf.

Diagenetic history of the Miocene Surma Group sandstones from the Eastern Fold Belt of the Bengal Basin

The Eastern Fold Belt (EFB) of the Bengal Basin remains an under-explored petroleum province in Bangladesh. Interbedded sandstones and shales of the Miocene Surma Group are thought to host hydrocarbon accumulations in EFB. The diagenetic history and their controls on sandstone reservoir quality and distribution are largely unknown in this region. Therefore, this paper aims to improve the understanding of diagenetic processes and their impacts on reservoir quality in the Surma Group sandstones. Thin section petrography, Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) analysis have all been used to this end. The results of this study show that the sandstones are very fine- to medium- grained, moderately to moderately well-sorted, matrix- rich and dominantly sublitharenites. The principle diagenetic minerals/cements are calcite, authigenic chlorite, illite/illite–smectite, minor quartz cement, and minor kaolinite. Poikilotopic calcite cement drastically reduces the porosity and permeability down to 0%. Authigenic chlorite rims along quartz grain prevent quartz overgrowth, which acts to preserve porosity. The primary porosity (0–22%, average 9%) is by far the most dominant porosity in the Miocene Surma Group reservoirs, with burial enhanced secondary porosity caused by dissolution. Reservoir quality is mainly controlled by mechanical and ductile grain compaction and calcite cement.

Plio-Pleistocene to Recent tectonostratigraphic evolution of the Lalmai anticline in the western Indo-Burman Range (Bangladesh): insights from lithofacies analysis and structural synthesis

The Lalmai anticline is one of the westernmost structures within the Eastern Fold Belt of the Bengal Basin that has surficial expression. The tectonic evolution of the Eastern Fold Belt is directly linked with the development of the Indo-Burman Range in response to the late Cenozoic convergence between the Indian and Burmese plates. However, very little is known about the Plio-Pleistocene to Recent tectonic evolution of the Lalmai anticline in the context of regional tectonics. Therefore, this research is aimed at lucid understand the Plio-Pleistocene to Recent tectonostratigraphic evolution of the Lalmai anticline based on detailed lithofacies and structural observations. Results show that Plio-Pleistocene Dupitila and Pleistocene Madhupur Clay formations are exposed in the Lalmai area. Based on sedimentary texture and structures, a total of six prominent lithofacies types have been identified. These are trough cross-stratified sandstone (St), planar cross-stratified sandstone (Sp), parallel laminated sandstone (Sl), massive sandstone (Sm), laminated shale (Fl), and mudstone (Fm) facies. The facies characteristics indicate a possible fluvial depositional environment for both the formations. Structural mapping shows that the bedding surfaces are nearly horizontal. Based on the missing of strata (both the Dupitila and Madhupur Clay formations) and presence of a fault escarpment, a N-S trending east-dipping reverse fault (namely, the Mainamati Fault) has been identified on the western margin of the Lalmai anticline. The tectonostratigraphic evolutionary models show that the last major structural activation along this Mainamati Fault can be linked to late Pleistocene to Recent time. This structural activation was possibly controlled by the latest ongoing E-W crustal shortening in response to the Indo-Burman subduction processes to the far east.

1750–1710 Ma deformation along the eastern margin of the North Australia Craton

In this contribution, we present field-based evidence that in the Mary Kathleen Domain from the Eastern Fold Belt of the Mount Isa Inlier the 1790–1750 Ma metasediments of the Leichhardt Superbasin were sheared and folded between 1750 and 1710 Ma. The Mary Kathleen Domain consists of a series of anastomosing high and low strain domains intruded syn- to late-tectonically by a series of 1740–1710 Ma felsic and mafic plutons and dykes. The timing relationships in the high strain domains are unclear due to mylonitisation and transposition but in the lower strain domains are well-preserved. The earliest deformation, D1, consists of a series of low angle truncation surfaces and shear zones that are folded around large scale N-S trending upright folds during D2. The D2 folds were refolded during D3 along NW to EW trending fold axes. An undeformed granitic pluton from the Mt Godkin area dated at ~1710 Ma that crosscuts the D2-D3 folds and the youngest detrital zircon population of ~1750 Ma constrains the timing of D1-D3 deformation between 1750 and 1710 Ma. Later, N-S (D4) and N-E (D5) trending brittle-ductile shears are interpreted to indicate a late Isan Orogeny overprint. The occurrence of contemporaneous folding in the Western Fold Belt suggests that between 1750 and 1710 Ma the eastern margin of the North Australia Craton was experiencing a significant deformation event that can be correlated with similarly aged deformation events from other parts of the North Australia Craton, from South Australia Craton and from East Antarctica. Following the assembly of most of the Nuna Supercontinent by 1.8 Ga, we postulate that the 1750–1710 Ma deformation and plutonism observed in Mount Isa Inlier is part of a series of worldwide orogenic events that contributed to the final configuration of the Nuna Supercontinent.

Switching bulk horizontal shortening directions and regional-scale partitioning of deformation during the Isan Orogeny in the Eastern Fold Belt, Mount Isa Inlier, Australia

Prolonged deformation for ca 150 Ma along the Eastern Fold Belt, Mount Isa Inlier, differentially partitioned into three distinct Mesoproterozoic tectonic domains. NW–SE-trending structures dominate the northern domain, whereas E–W- and N–S-trending structures dominate the central and southern domains, respectively.Changing the direction of bulk horizontal shortening from NE–SW to N–S to E–W shifted the locus of maximum tectono-metamorphic effect. This accounts for the different generations of structures preserved in these three domains. Overprinting relationships and geochronological data reveal a component of deformation partitioning in time as well as space.Rheological contrasts in the Soldiers Cap Group between a thick interlayered pelitic, psammitic and volcanic units on the one hand, and ca 1686 Ma, competent mafic intrusives and genetically related metasomatic albitite bodies present in its lower part, on the other, enhanced strain localisation during the long-lived Isan Orogeny (ca 1670–1500 Ma).

Structural and Lithological Controls on Iron Oxide Copper-Gold Deposits of the Southern Selwyn-Mount Dore Corridor, Eastern Fold Belt, Queensland, Australia

The Eastern fold belt of the Proterozoic Mount Isa inlier has undergone numerous episodes of deformation, hydrothermal alteration, and iron oxide copper-gold mineralization. The structural and lithological controls on the location and geometry of five iron oxide copper-gold deposits (Starra, Mount Elliott, SWAN, Mount Dore, and Lady Ella) in the Selwyn-Mount Dore corridor were studied through geologic mapping, core logging, structural analysis, and examination of 3-D grade distribution. All the deposits in this area occur in close proximity to the Selwyn shear, a long-lived, regional structure which developed at the contact between carbonate-rich and siliciclastic-dominated rock packages. This fault and a series of subparallel structures have been the focus of multiple reactivation events and have controlled the distribution of alteration and mineralization, which occurred in this area at ~1595, 1570, 1530, 1515, 1500, and 1485 Ma. Ironstone-hosted mineralization at Starra developed in a preexisting zone of sodic and iron oxide alteration along the N-striking Starra shear. Kinematic indicators along the Starra shear reveal that it has been reused numerous times; early D2 thrusting, post-D2 sinistral shearing, and pre-D4 (syn-D3?) dextral brittle faulting were all associated with sodic, sodic-calcic, and iron oxide alteration. Either the post-D2 or pre-D4 strike-slip faulting events can explain the steeply plunging nature of the orebodies at Starra. However, the dextral pre-D4 event produces dilational structures in ironstones and is associated with regionally important sodic and iron oxide alteration along NE- to ENE-trending faults, which intersect the Starra shear close to the main orebodies; we consider this the more likely main mineralizing event at Starra. Both strike-slip faulting events correspond with a change from E-W–oriented compression during the main phase of the Isan orogeny to transpression that may have been driven by crustal overthickening during orogenesis. At Mount Elliott and SWAN, mineralization is associated with post-D4 dextral and reverse movement across a sinistral fault jog along the NNW-striking Mount Elliott fault during transpression. Mineralization, alteration, and veins at these deposits clearly overprint all ductile structural features. Overall, the orebody geometry is controlled by the orientation of the Mount Elliott fault, but high-grade ore shoots are controlled by breccia formation in elongate pods which are parallel to preexisting F2 fold axes. In contrast, gently plunging grade shells at the slightly younger Mount Dore and Lady Ella deposits formed in response to renewed E-W compression and reverse displacement along N- to NE-striking fault segments of the Mount Elliott fault and newly formed subparallel faults. Examination of the structural setting of Mount Dore and Lady Ella also demonstrates that strain partitioning between mechanically heterogeneous rock sequences (metasedimentary rocks, metabasalts, granite, and strongly silicified rock types) exerted strong controls on the location of later mineralized systems. The important episodes of iron oxide copper-gold mineralization in the Eastern fold belt between 1595 and 1485 Ma appear to be associated temporally with compressive or transpressive structural regimes. These events developed in response to far-field tectonic stresses that evolved in response to the collision of Laurentia with the proto-Australian continent during the Isan orogeny. Late in the orogenic history, the emplacement of voluminous A-type batholiths associated with hot-spot plume activity likely generated localized transient shortening events, which are also associated with iron oxide copper-gold mineralization. This unique geodynamic evolution was responsible for the large-scale metal enrichment and remobilization into suitable chemical and structural traps over significant time periods in the Eastern fold belt iron oxide copper-gold district.

Late paleozoic gabbroids of the Lesser Khingan terrane of the eastern Central-Asian fold belt: Age, geochemistry, and tectonic setting

This paper considers the results of mineralogical, geochemical, and geochronological studies of leucogabbroids of the Karagai Massif located within the Lesser Khingan superterrane of the Central Asian fold belt. The main features of the rock-forming minerals of the gabbroids are the high calcium content of the clinopyroxene corresponding in composition to diopside-salite (Wo48–51En33–39Fs11–16) or augite (Wo22–35En46–47Fs18–33), the high basicity of the plagioclase (An90–92), and the presence of primary magmatic magnesian hornblende. The age of the massif was determined by U-Pb zircon dating as 257 ± 1 Ma. The compositional peculiarities of the gabbroids indicate that they were most probably formed in island-arc or active continental margin settings. With allowance for the existing geodynamic reconstructions and the data obtained in the present study, the formation of the Late Paleozoic gabbroids of the Karagai Massif was presumably related to the final stages of the subduction processes that preceded the collapse of the terranes of the eastern Central Asian fold belt.

Depositional systems in the Mt Isa Inlier from 1800 Ma to 1640 Ma: Implications for Zn–Pb–Ag mineralisation

Two facies models are proposed to explain siliciclastic and carbonate depositional systems of 1800 Ma to 1640 Ma age in the Western Fold Belt of the Mt Isa Inlier. Both models record the response of depositional systems to storm-driven processes of sediment transport, dispersal and deposition on a shallow water shelf. The same suite of facies belts can also be identified in sedimentary successions of the Eastern Fold Belt. Slope driven processes of sediment transport and dispersal characterise turbidite and debrite deposits of the Soldiers Cap Group and Kuridala Formation and provide evidence for significantly greater water depths in this part of the basin from ca 1685 Ma. Through the recognition of unconformity surfaces, their correlative conformities, maximum flooding and ravinement surfaces, the facies belts are packaged into seven supersequences for the interval ca 1800–1640 Ma. The new correlations are shown in an event chart that correlates linked depositional systems across the entire Mt Isa Inlier. Thick successions of turbidite and debrite deposits are restricted to the eastern parts of the Mt Isa Inlier and do not occur in the Western Fold Belt. A major phase of extension and rifting commenced at ca 1740 Ma and by ca 1690 Ma led to significant crustal thinning and increased rates of accommodation over an area east of the Mount Dore Fault and Burke River Structural Belt. In the Mitakoodi Anticline and Kuridala-Selwyn Block, the rapid transition from shallow-water shelf depositional systems of the Prize Supersequence to significantly deeper-water slope environments of the Gun Supersequence coincided with the development of a platform margin, the deposition of turbidite and debrite deposits in deep water on the continental slope and the intrusion of mafic sills and dykes. Turbidite and debrite depositional systems of the Soldiers Cap Group and Kuridala Formations are restricted to a lowstand wedge of siliciclastic facies deposited basinward of a platform margin. Basin geometries and sediment architectures associated with this extensional event and recorded in the Gun Supersequence (ca 1685 Ma to 1650 Ma) provide an explanation for the geographic separation and fluid evolution pathways responsible for the Mt Isa Type and Broken Hill Type Zn–Pb–Ag deposits.

Polymetamorphism accompanied switching in horizontal shortening during Isan Orogeny: Example from the Eastern Fold Belt, Mount Isa Inlier, Australia

Mesoproterozoic low-P/high-T volcano-sedimentary rocks of the Soldiers Cap Group, the southeastern corner of Mount Isa Inlier, record a complex polymetamorphic history that accompanied four periods of bulk horizontal shortening directed NE–SW, N–S, W–E and NW–SE during the long-lived Isan Orogeny (~1650–1500Ma). Low-P/high-T metamorphism (M1) prevailed during a period of NE–SW bulk horizontal shortening as indicated by the early growth of cordierite porphyroblasts, which entrap monazite grains that gave an average age of 1649±12Ma. This was followed by medium-P/high-T (M2) and high-P/high-T (M3) metamorphisms that accompanied periods of N–S and W–E bulk horizontal shortening, respectively. Growth of first generation garnet, andalusite and staurolite porphyroblasts having an average age of 1645±7Ma identify the former, whereas growth of the 1591±10Ma second generation garnet, staurolite and andalusite porphyroblasts together with fewer kyanite porphyroblasts characterizes the latter.The sediments of the Soldiers Cap Group were deposited in a tectonic setting that has the characteristics of an intra-continental rift basin. The upper limit of the age of sedimentation is constrained by detrital zircons at 1654±4Ma suggesting that tectonism and metamorphism were either active during the final stage of the basin filling or immediate after deposition.Introduction of mafic dykes and sills at different stratigraphic levels over a wide time span was the major source of heat.

Foliation intersection axes (FIAs) preserved within porphyroblasts: resolving plan view orthogonal refolding, Eastern Fold Belt, Mt Isa Inlier

The relative timing of the sequence of events is problematic where deformation partitioning strongly separates zones dominated by folds that formed in different periods of orogeny. This is particularly the case where they form orthogonal to one another in plan view. A succession of foliation inflection/intersection axes (FIAs) preserved within porphyroblasts readily resolves the sequence. In situ monazite age dating of porphyroblasts preserving W–E and N–S-trending FIAs yielded ages of 1645 ± 7 and 1591 ± 10 Ma, respectively. Peaks in the distribution of the two FIA sets coincide with the trends of the axial planes of the regional folds. The succession of FIAs, as well as foliations in the matrix in the Soldiers Cap Group of the eastern Mount Isa Inlier, supported with geochronological data reveal W–E directed bulk horizontal shortening followed N–S bulk horizontal shortening. This is despite the apparent regional lack of curvature of the W–E-trending folds around those trending N–S. Furthermore, N–S and W–E-directed bulk shortening resulted in at least 2 cycles of subvertical and subhorizontal foliation development that define the W–E and N–S-trending FIAs.

Mineral Systems in the Mount Isa Inlier

Northwest Queensland contains several world class mineral deposits, being one of the worlds leading producers of Zn, Pb, Cu and Ag. Rather than focus on mineral deposit models, as has been done in the past, we are using the mineral system approach (Barnicoat, 2008), where the whole system is studied at a variety of scales and a variety of processes, which culminate in the deposition of mineralisation. Seven Mineral Systems are identified, namely: 1. Shale/siltstone/dolomite hosted Zn-Pb-Ag systems - Western Fold Belt. 2. Ag-Pb-Zn in high-grade metamorphic terrains - Eastern Fold Belt Province. 3. Structurally-controlled epigenetic iron oxide-Cu-Au - Eastern Fold Belt and Kalkadoon-Ewen Provinces. 4. Structurally-controlled epigenetic Cu±Au mineralising system - Western Fold Belt Province. 5. Phosphate Mineralisation in the basal Georgina Basin sequence. 6. U and Rare Earth element (REE) mineralisation. 7. Fe Ore - South Nicholson Group.

Mineral system analysis of the Mt Isa–McArthur River region, Northern Australia

The Mt Isa–McArthur region is renowned for a range of commodities and deposit types of world-class proportions. The region is described here in the context of a ‘mineral system,’ through consideration of processes that operate across a range of scales, from geodynamics and crustal architecture, to fluid sources, pathways, drivers and depositional processes. The objective is to improve targeting of Pb–Zn, Cu and Cu–Au deposits. Repeated extension and high heat flow characterise much of the history prior to 1640 Ma. The pre-Barramundi Orogeny (pre-1.87 Ga) metamorphic basement was the substrate on which a volcanic arc developed, focussed along the Kalkadoon-Leichhardt Belt. This is related to an inferred east-directed subduction between 1870 and 1850 Ma. From 1755 to 1640 Ma, three successive volcano-sedimentary basins developed, the Leichhardt, Calvert and Isa Superbasins, in an interpreted distal back-arc environment. The Isan Orogeny, from 1640 to 1490 Ma, overlapped with Isa Superbasin sedimentation, suggesting a transition from back-arc to a foreland basin setting. Most crustal thickening occurred in the Eastern Fold Belt, an area earlier characterised by thinned crust and deep marine environments. This region was deformed into nappe-like structures with high-temperature–low-pressure regional metamorphism and associated granites; the latter are absent from the Western Fold Belt. Metal deposition mainly occurred late in the history, with all known (and preserved) major base metal occurrences either hosted by Isa Superbasin rocks or formed during the Isan Orogeny. Earlier superbasins were potential fluid source regions. Sedimentary formation waters, metamorphic and magmatic fluids were present at prospect scale, while meteoric and possibly mantle sources are also implicated. The spatial distribution of metallogenic associations (i.e. iron oxide–copper–gold, Pb–Zn–Ag, U, Au) across the inlier may result from differences in the geodynamic make-up and evolution of the pre-1.87 Ga tectonic elements. Penetrative faults are interpreted as predominantly steeply dipping and to have acted as pathways for fluids, both in extension and compression. Fluid mixing was a potentially significant ore deposit control. Examples are drawn from the Ernest Henry iron oxide–copper–gold-related hydrothermal breccias in the east and from the Mt Isa Copper deposit in the west. Stress switching during late-stage deformation appears to have triggered a fluid mixing event that led to formation of the major copper deposits.

Hydrothermal mineral alteration patterns in the Mount Isa Inlier revealed by airborne hyperspectral data

High-resolution mineral maps derived from hyperspectral imaging (4.5 m pixel) enable the recognition of various types of hydrothermal alteration and the identification of fluid pathways. Airborne hyperspectral images from the Eastern Fold Belt of the Mount Isa Inlier were tested as a new tool for the detection of Fe-oxide Cu–Au (IOCG) related alteration. Four different types of hydrothermal alteration were identified with the hyperspectral mineral maps: (1) Metasomatic 1: white mica mineral maps show the spatial distribution of regional sodic–calcic alteration in metasedimentary successions of the Soldiers Cap Group in the Snake Creek Anticline. (2) Metasomatic 2: alteration zonation is evident from albitised granites assigned to the Williams–Naraku Suite along the Cloncurry Fault. These show characteristic absorption features in the shortwave infrared range (SWIR) which are depicted on the white mica mineral maps (white mica composition, white mica content, white mica crystallinity index). Alteration zonation in gabbros of the Cloncurry District was detected by a combination of MgOH and Fe2+ mineral maps (MgOH content, MgOH composition, amphibole/chlorite and Fe2+ and MgOH) combined with white mica mineral maps (white mica composition and white mica content). (3) Fluid channels 1: major fault zones, such as the Mt Dore fault zone in the Selwyn Corridor, are interpreted as important fluid pathways, where gradual changes in the mineral chemistry are highlighted with mineral maps (e.g. white mica content, white mica composition, white mica crystallinity index). (4) Fluid channels 2: MgOH and Fe2+ mineral maps were used to map breccia pipes in the northern Cloncurry District north of the Saxby Granite (Suicide Ridge). The MgOH and Fe2+ mineral maps were also used to distinguish various mafic rocks from amphibolites, which are host rocks for some of the IOCG deposits in the Eastern Fold Belt (e.g. Mount Elliott), and calcsilicate breccias pipes (e.g. Suicide Ridge).

Pre-1.8 Ga tectono-magmatic evolution of the Kalkadoon–Leichhardt Belt: implications for the crustal architecture and metallogeny of the Mount Isa Inlier, northwest Queensland, Australia

New geochemical, U–Pb zircon and Nd–Hf isotope data for felsic and mafic components of the Kalkadoon–Leichardt Belt (KLB) in the Mount Isa Inlier of northwest Queensland confirm that the evolution and tectonic make-up of this belt prior to 1.8 Ga was closely aligned with that of the Western Fold Belt. Like pre-1.8 Ga magmatic rocks of the Western Fold Belt, those in the KLB are characterised by late Archean to Paleoproterozoic crustal residence ages (TDM ca 2.6–2.3 Ga) but the geochemical data suggest stronger within-plate affinities, implying that the KLB intrusions underwent a greater degree of crustal assimilation and/or were emplaced further inboard of the active subduction margin. A new SHRIMP U–Pb zircon age of ca 1.86 Ga from a felsic intrusion in the northern KLB confirms earlier age constraints for the main phase of the Kalkadoon Granite. Nd isotope patterns for intrusions in the Western Fold Belt and the KLB are distinct from those of ca 1.7–1.5 Ga intrusions in the Eastern Fold Belt. This isotopic discrepancy suggests the Eastern Fold Belt underwent a separate evolutionary history and was accreted to the WFB/KLB at some stage prior to, or during, the ca 1.86 Ga Barramundi Orogeny. The occurrence of widespread, isotopically homogeneous ca 1.72 Ga magmatism (Argylla Event) across the entire Mount Isa Inlier implies that amalgamation of the allochthonous Eastern Fold Belt with the North Australian Craton was completed by that time. The significantly elevated metallogenic potential of the Eastern Fold Belt for ca 1540–1500 Ma Fe-oxide–copper–gold, relatively low endowment of the Kalkadoon–Leichhardt Belt, and predominance of ca 1650–1550 Ma base metal occurrences in the Western Fold Belt, are all consequences of the tectonic interplay and lithospheric processes that controlled the evolution of the Mt Isa Inlier prior to ca 1.8 Ga.

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.

Airborne hyperspectral imaging of hydrothermal alteration zones in granitoids of the Eastern Fold Belt, Mount Isa Inlier, Australia

ABSTRACT Hyperspectral remote sensing data from the Eastern Fold Belt, Mount Isa Inlier, Australia were compared with petrographic and geochemical studies to map the spatial extension and compositional variations of Proterozoic granitoids and endoskarns as well as hydrothermal alteration patterns in adjoining metasedimentary successions. Detailed spatial analysis of spectral remote sensing data shows an almost circular alteration zoning in the Mallee Gap Granite, which was emplaced during a late phase of the Mesoproterozoic Williams event. A combination of hyperspectral images, such as white mica, kaolin and MgOH products, were used to map the alteration zoning. The formation of the endoskarn is presumably related to autometasomatism and interaction with fluids released from the country rocks during a late phase of the emplacement. The intrusion of the Mallee Gap Granite has only a local control on the hydrothermal alteration, but high potassic granites of the southern Mount Angelay Granite might have expelled oxidized mineralizing fluids and possibly had a major impact on regional scale alteration. Hyperspectral remote sensing data may be used to estimate the imprint of single igneous bodies on the Mesoproterozoic hydrothermal evolution of the Eastern Fold Belt and are important for the study of ore-forming hydrothermal processes in general.