A tale of two synclines: Rifting,inversion and transpressional popouts at Lake Julius,northwestern Mt Isa terrane,Queensland

This study reviews the origin of two approximately east‐west‐trending synclines in the Lake Julius area at the eastern edge of the Leichhardt Rift. The genesis of one of these structures can be found in a north‐south shortening event (D₁) that occurred at the beginning of the compressional Isan Orogeny (at ca 1600 Ma). Metasediments in a cross‐rift were rammed against a competent buttress defined by the pre‐existing rift architecture, producing the approximately east‐west‐trending Somaia Syncline and its associated axial‐plane slaty cleavage. In contrast, the Lake Julius Syncline was produced by reorientation of an originally approximately north‐south‐trending (D₂) fold, in a transpressional zone adjacent to a strike‐slip fault, at the end of the Isan orogeny. The effects of late fault movement can be partially reconstructed, based on correlations assuming that regionally developed trains of upright folds formed during the peak of the Isan Orogeny (D₂). These folds have been offset, as well as having been tightened and disrupted at the same time as fault movements took place. The overall pattern of movement in the Lake Julius region can be explained as the result of an ‘indentor’ ramming into the ancient edge of the Leichhardt Rift, which acted as a buttress.     

Palaeoproterozoic mid‐basin inversion in the northern Mt Isa terrane,Queensland

The Mellish Park Syncline is located in the northern part of the Mt Isa terrane. It has an axial trace that transects the remnants of the unconformity‐bounded Palaeoproterozoic Leichhardt and Isa Superbasins. The syncline is separated into a lower and upper component based upon variation in fold geometry across the basin‐bounding unconformity. The lower syncline, in the Leichhardt Superbasin, is tight and has an inclined west‐dipping axial plane. The upper syncline, in the Isa Superbasin, is open and upright. The geometry of the lower syncline is a consequence of a period of shortening and basin inversion which post‐dated the Leichhardt Rift Event (ca 1780–1740 Ma) and pre‐dated the Mt Isa Rift Event (ca 1710–1655 Ma), forming an open and upright north‐oriented syncline. Subsequent southeast tilting and half‐graben development during the Mt Isa Rift Event resulted in the lower syncline being tilted into its inclined geometry. Sequences of the Isa Superbasin were then deposited onto the eroded syncline. The geometry of the upper syncline reflects regional east‐west shortening during the Isan Orogeny (ca 1590–1500 Ma). The position of the upper syncline was largely controlled by the pre‐existing lower syncline. At this time the lower syncline was reactivated and tightened by flexural slip folding.     

Trace‐element signatures of apatites in granitoids from the Mt Isa Inlier,northwestern Queensland

The concentrations of trace elements in apatite from granitoid rocks of the Mt Isa Inlier have been investigated using the laser‐ablation inductively coupled plasma‐mass spectrometry (ICP‐MS) microprobe. The results indicate that the distribution of trace elements (especially rare‐earth elements (REE), Sr, Y, Mn and Th) in apatite strongly reflects the chemical characteristics of the parental rock. The variations in the trace‐element concentrations of apatite are correlated with parameters such as the SiO₂ content, oxidation state of iron, total alkalis and the aluminium saturation index (ASI). The relative enrichment of Y, HREE and Mn and the relative depletion of Sr in the apatites studied reflect the degree of fractionation of the host granite. Apatites from strongly oxidised plutons tend to have higher concentrations of LREE relative to MREE. Manganese concentrations are higher in apatite from reduced granitoids because Mn²⁺substitutes directly for Ca²⁺. The La/Ce ratio of apatite is well‐correlated with the whole‐rock K²O and Na²O contents, as well as with the oxidation state and ASI. Because apatite trace‐element composition reflects the chemistry of the whole rock, it can be a useful indicator mineral for the recognition of mineralised granite suites, where particular mineralisation styles are associated with granitoids that have specific geochemical fingerprints.     

Post-orogenic (<1500 Ma) thermal history of the Palaeo-Mesoproterozoic, Mt. Isa province, NE Australia

We present hornblende, white mica, biotite and alkali feldspar ⁴⁰Ar/³⁹Ar data from Paleo-Mesoproterozoic rocks of the Mt. Isa Inlier, Australia, which reveal a previously unrecognised post-orogenic, non-linear cooling history of part of the Northern Australian Craton. Plateau and total fusion ⁴⁰Ar/³⁹Ar ages range between 1500 and 767 Ma and record increases in regional cooling rates of up to 4 °C/Ma during 1440–1390 and 1260–1000 Ma. Forward modelling of the alkali feldspar ⁴⁰Ar/³⁹Ar Arrhenius parameters reveals subsequent increases in cooling rates during 600–400 Ma. The cooling episodes were driven by both erosional exhumation at average rates of 0.25 km/Ma and thermal relaxation following crustal heating and magmatic events. Early Mesoproterozoic cooling is synchronous with exhumation and shearing in the Arunta Block and Gawler Craton. Late Mesoproterozoic cooling could have either been driven by increased rates of exhumation, or a result of thermal relaxation following a heat pulse that was synchronous with dyke emplacement in the Arunta, Musgrave and Mt. Isa province, as well as Grenville-aged orogenesis in the Albany–Fraser Belt. Latest Neoproterozoic–Cambrian cooling and exhumation was probably driven by the convergence of part of the East Antarctic Shield with the Musgrave Block and Western Australia (Petermann Ranges Orogeny), as well as collisional tectonics that produced the Delamerian–Ross Orogen. Major changes in the stress field and geothermal gradients of the Australian plate that are synchronous with the assembly and break-up of parts of Rodinia and Gondwana resulted in shearing and repeated brittle reactivation of the Mt. Isa Inlier, probably via the displacement of long-lived basement faults within the Northern Australian Craton.     

SHRIMP U–Pb monazite dating of 1600–1580 Ma amphibolite facies metamorphism in the southeastern Mt Isa Block,Australia

SHRIMP U–Pb monazite dates of ca 1600–1580 Ma are reported from three samples taken from the southeastern margin of the Proterozoic Mt Isa Block. The samples include an upper amphibolite facies paragneiss and a pegmatite from the host sequence of the Cannington Ag–Pb–Zn deposit and a middle amphibolite facies metasediment from the Soldiers Cap Group near Maronan station. These dates are interpreted to represent the timing of amphibolite facies metamorphism at the southeastern margin of the Mt Isa Block. They are in accordance with the results of earlier SHRIMP U–Pb zircon and ⁴⁰Ar/³⁹Ar dating, which suggested that metamorphism in the southeastern Mt Isa Block occurred approximately 50 million years earlier than metamorphism in the western Mt Isa Block. This challenges the common perception of orogeny in the Mt Isa Block in which ‘peak metamorphism’, and the deformation events associated with it, can be correlated across the entire terrane.     

Reactive transport model of silicification at the Mount Isa copper deposit, Australia

A distinct feature of the world-class Mount Isa copper system is the presence of a massive silica body enveloping the chalcopyrite ore body in the Urquhart shale. Because silicification may be genetically linked to the copper mineralization, it is important to understand its formation. It can be shown by numerical simulations of reactive transport processes that both hydraulic head and buoyancy driven flow are capable of generating silicification patterns similar to the field example. Considering the physical conditions required to form the observed patterns we suggest, however, that head-driven flow rather than free thermal convection produced the silica body. Free convection requires very special conditions and only one of several possible scenarios reproduces the observed silicification whereas any of the tested forced flux scenario is capable of doing so.     

Magmatic history of the Eastern Creek Volcanics,Mt Isa,Australia: insights from trace-element and platinum-group-element geochemistry

The Paleoproterozoic basalts of the Eastern Creek Volcanics are a series of continental flood basalts that form a significant part of the Western Fold Belt of the Mt Isa Inlier, Queensland. New trace-element geochemical data, including the platinum-group elements (PGE), have allowed the delineation of the magmatic history of these volcanic rocks. The two members of the Eastern Creek Volcanics, the Cromwell and Pickwick Metabasalt Members, are formed from the same parental magma. The initial magma was contaminated by continental crust and erupted to form the lower Cromwell Metabasalt Member. The staging chamber was continuously replenished by parental material resulting in the gradual return of the magma composition to more primitive trends in the upper Cromwell Metabasalt Member, and finally the Pickwick Metabasalt Member formed from magma dominated by the parental melt. The Pickwick Metabasalt Member of the Eastern Creek Volcanics has elevated PGE concentrations (including up to 18 ppb Pd and 12 ppb Pt) with palladium behaving incompatibly during magmatic fractionation. This trend is the result of fractionation under sulfide-undersaturated conditions. Conversely, in the basal Cromwell Metabasalt Member the PGE display compatible behaviour during magmatic fractionation, which is interpreted to be the result of fractionation of a sulfide-saturated magma. However, Cu remains incompatible during fractionation, building up to high concentrations in the magma, which is found to be the result of the very small volume of magmatic sulfide formation (0.025%). Geochemical trends in the upper Cromwell Metabasalt Member represent mixing between the contaminated Cromwell Metabasalt magmas and the PGE-undepleted parental melt. Trace-element geochemical trends in both members of the Eastern Creek Volcanics can be explained by the partial melting of a subduction-modified mantle source. The generation of PGE- and copper-rich magmas is attributed to melting of a source in the subcontinental lithospheric mantle below the Mt Isa Inlier which had undergone previous melt extraction during an older subduction event. The previous melt extraction resulted in a sulfur-poor, metal-rich metasomatised mantle source which was subsequently remelted in the Eastern Creek Volcanic continental rift event. The proposed model accounts for the extreme copper enrichment in the Eastern Creek Volcanics, from which the copper has been mobilised by hydrothermal fluids to form the Mt Isa copper deposit. There is also the potential for a small volume of PGE-enriched magmatic sulfide in the plumbing system to the volcanic sequence.     

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 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 δ¹⁸O_fluid values between 6.5 and 8.6‰. Reibeckite δD values reveal that the ore fluid had a δD_fluid value between −98 and −54‰. The mineral assemblages associated with early and main stages of alteration, plus δ¹⁸O_fluid and δD_fluid 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.     

U‐Pb zircon age for the Etheridge Group,Georgetown region,north Queensland: Implications for relationship with the Broken Hill and Mt Isa sequences*

The Einasleigh Metamorphics are the lowest exposed component of the Etheridge Group, part of the Etheridge Province, in the Georgetown region of north Queensland. Previous dating of granites has imposed a younger age limit of about 1550 Ma for the depositional age of the Etheridge Group. Based on SHRIMP U‐Pb analyses of zircon from mafic (1674.9 ±3.3 Ma and 1655.9 ±2.2 Ma) and felsic (1695.8 ± 1.5 Ma and 1684.2 ± 2.1 Ma) intrusive rocks in the Einasleigh Metamorphics, it can now be demonstrated that deposition commenced at about 1700 Ma, and continued for an estimated 100 million years. The stratabound, base‐metal deposits in the Mt Isa Inlier, Broken Hill Block and McArthur Basin formed in the early part of this time span, thereby suggesting potential for these styles of mineralisation in the Georgetown region.     

Reflectance spectroscopic characterisation of mineral alteration footprints associated with sediment-hosted gold mineralisation at Mt Olympus (Ashburton Basin,Western Australia)

Hydrothermal ore deposits are typically characterised by footprints of zoned mineral assemblages that extend far beyond the size of the orebody. Understanding the mineral assemblages and spatial extent of these hydrothermal footprints is crucial for successful exploration, but is commonly hindered by the impact of regolith processes on the Earth's surface. Hyperspectral drill core (HyLogger?-3) data were used to characterise alteration mineralogy at the Mt Olympus gold deposit located 35 km southeast of Paraburdoo along the Nanjilgardy Fault within the northern margin of the Ashburton Basin in Western Australia. Mineralogy interpreted from hyperspectral data over the visible to shortwave (400–2500 nm) and thermal (6000–14500 nm) infrared wavelength ranges was validated with X-ray diffraction and geochemical analyses. Spaceborne multispectral (ASTER) and airborne geophysical (airborne electromagnetic, AEM) data were evaluated for mapping mineral footprints at the surface and sub-surface. At the deposit scale, mineral alteration patterns were identified by comparing the most abundant mineral groups detected in the HyLogger data against lithology logging and gold assays. Potential hydrothermal alteration phases included Na/K-alunite, kaolin phases (kaolinite, dickite), pyrophyllite, white mica, chlorite and quartz, representing low-T alteration of earlier greenschist metamorphosed sediments. The respective zoned mineral footprints varied depending on the type of sedimentary host rock. Siltstones were mainly characterised by widespread white-mica alteration with proximal kaolinite alteration or quartz veining. Sandstones showed (1) distal white mica, intermediate dickite, and proximal alunite + kaolinite or (2) widespread white-mica alteration with associated intervals of kaolinite. In both, sandstones and siltstones, chlorite was distal to gold mineralisation. Conglomerates showed distal kaolinite/dickite and proximal white-mica/dickite alteration. Three-dimensional visualisation of the gold distribution and spatially associated alteration patterns around Mt Olympus revealed three distinct categories: (1) several irregular, poddy, SE-plunging zones of >0.5 ppm gold intersected by the Zoe Fault; (2) sulfate alteration proximal to mineralisation, particularly on the northern side of the Mt Olympus open pit; and (3) varying Al^IVAl^VISi^IV_–1(Mg,Fe)^VI_–1 composition of white micas with proximity to gold mineralisation. Chlorite that developed during regional metamorphic or later hydrothermal alteration occurs distal to gold mineralisation. ASTER mineral mapping products, such as the MgOH Group Content used to map chlorite (±white mica) assemblages, showed evidence of correlation to mapped, local structural features and unknown structural or lithological contacts as indicated by inversion modelling of AEM data.     

Combining fractal analysis of mineral deposit clustering with weights of evidence to evaluate patterns of mineralization: Application to copper deposits of the Mount Isa Inlier, NW Queensland, Australia

The clustering of mineral occurrences and their spatial associations with particular geological features are critical aspects of mineral distributions for exploration and understanding ore genesis. Variations in the degree of clustering of mineral occurrences or geological features can be measured by fractal dimensions, obtained from a shifting box counting method. Spatial associations between mineral occurrences and geological features can be quantified by the weights of evidence (WofE) method using the contrast value, which increases with the strength of the spatial relationship. A new method is proposed to evaluate mineral occurrence distributions by combining the power of fractal analysis of clustering with the WofE approach. The method compares the correlation between the variation in degree of clustering of mineral occurrences and a geological feature in a study area, with the contrast value of the same feature. The possible outcomes can be simplified into four scenarios, depending on whether the correlation in variation of clustering and the contrast are high or low, respectively. Each outcome has specific exploration implications. If either a high correlation in variation of clustering or a high contrast value is obtained, the geological feature can be used for exploration targeting.The integrated fractal and WofE approach is applied to copper occurrences in the Proterozoic Mount Isa Inlier, NW Queensland, Australia, which hosts large numbers of copper deposits (1,869 occurrences), including the world class Mount Isa copper deposit. Variation in clustering of copper occurrences has a positive correlation with variation in clustering of fault bends (R = 0.823), fault intersections (R = 0.862) and mafic rocks (R = 0.885). WofE results indicate that the copper occurrences are spatially associated with fault intersections and bends and with mafic rocks. Analyses were carried out separately for the two major lithostratigraphic sequences in the Inlier, the Eastern and Western Successions. The Western Succession copper occurrences are apparently more clustered than those of the Eastern Succession, which may reflect a lower degree of exploration and/or geological factors. The association of copper occurrences with mafic rocks compared with fault bends and intersections is greater in the Eastern Succession, which may reflect genetic factors. Correlations in the variation of clustering of mineral occurrences and geological features have a linear relationship with the contrast values, and the spatial association between all geological features and copper occurrences constitute high correlation/high contrast cases. The linear relationship suggests that the geological features that control the clustering of the copper occurrences could be the same features that control their localization.     

胶东三山岛北海域金矿床热液蚀变作用研究

三山岛北海域金矿床位于胶东金矿省的西北缘,是2015年新发现的超大型金矿床（储量470 t、Au品位4.30 g/t）,金矿体赋存于中生代玲珑式花岗岩中,主矿体受三山岛-仓上断裂带控制。中生代含矿的玲珑式花岗岩显示了复杂的蚀变、矿化共生组合关系。三山岛-仓上断裂的活动使热液流体发生渗透,导致断裂带两侧发生广泛的钾化蚀变。随后,大规模的绢云母化沿主断裂两侧形成。随着断层泥的形成,其作为"阻挡层"使含矿流体不能运移到断裂带上盘,成矿流体在下盘发生强烈的绢云母-石英-黄铁矿蚀变并伴有金的析出。最后石英-碳酸盐脉的形成标志着与金成矿相关热液活动减弱。钾化和绢英岩化岩石的平衡计算揭示了SiO2、MgO和CaO带入,TiO2、K2O基本不变,而Na2O表现为带出;大多数主量元素受强烈的矿物反应影响。Au、Ag、Bi、As、Pb、Zn等相关成矿元素呈带入特征,它们之间多呈正相关关系且与黄铁绢英岩化有密切关系,显示出在水岩反应过程中不同类型的元素具有复杂的地球化学行为。蚀变组合和流体包裹体研究表明,成矿流体以中低温（126~351℃）、中低盐度（1.02%~10.48% NaCleqv）为特征,属于CO2-H2O-NaCl±CH4体系。在热液流体中,金可能主要以Au（HS）2-络合物的形式运移;黄铁绢英岩化过程中,硫化作用使Au（HS）2-络合物失稳分解导致Au沉淀富集成矿。华北克拉通的重新活化导致软流圈上涌和大量火成岩的形成,也为胶东发生大规模金成矿作用提供了充足的的热能和流体输入。     

贵州盘县地区峨眉山玄武岩铜矿的成矿地质条件

以峨眉山玄武岩底部与中二叠统茅口组灰岩接触界面上赋存的黄见坑-哈树富铜矿带为例,论述了这一新类型玄武岩铜矿的成矿地质条件及峨眉山玄武岩浆喷-溢对Cu(Au、Pb、zn、Pt、Pd、Sb、F等)元素的富集和后期热液改造成矿作用,以扩大找矿思路。     

Application of short-wave infrared spectroscopy to define alteration zones associated with the Elura zinc–lead–silver deposit, NSW, Australia

Short-wave infrared reflectance spectra obtained from a Portable Infrared Mineral Analyser (PIMA) have been used to define alteration zones adjacent to base metal sulfide ore bodies at the Elura Mine, Cobar, Australia. The spectroscopic work identified white mica (sericite), chlorite and carbonates of hydrothermal origin in the alteration zones associated with the ore bodies. Weathering, alteration and ore zones can be discriminated by variations in the intensity and wavelength of relevant absorption features. Hydrothermal alteration is classified into four principal types, namely sericitic, silicic, chloritic and carbonate alteration. The first three types comprise overprinting hydrothermal assemblages of quartz, sericite, chlorite, ankerite, siderite, calcite and sulfides developed in strongly altered metasiltstone and slate of Early Devonian age, adjacent to the zinc–lead–silver mineralisation. An extensive zone of carbonate alteration, manifested as porphyroblasts of siderite in the host metasediments, is recognised beyond the zones of strong alteration. Spectral analysis is consistent with the mineralogical data obtained from XRD and XRF analyses and defines the limits of the alteration zones to distances of about 80 m around the ore bodies. The study demonstrates the potential for spectral analysis to assist with drill hole logging and the identification of alteration zones as part of mineral exploration and development programs.