Monazite chemistry as an exploration tool for Cloncurry-style iron oxide-copper-gold deposits

A fluid inclusion study of the Suicide Ridge Breccia Pipe, Cloncurry district, Australia: Implication for Breccia Genesis and IOCG mineralization

Mineralogy as a proxy to characterise geochemical dispersion processes: A study from the Eromanga Basin over the Prominent Hill IOCG deposit, South Australia

A magmatic source of hydrothermal sulfur for the Prominent Hill deposit and associated prospects in the Olympic iron oxide copper-gold (IOCG) province of South Australia

The Prominent Hill iron oxide-copper-gold (IOCG) deposit, located in the Gawler craton of South Australia, contains ca. 278 million metric tons (Mt) of ore at 0.98% Cu, 0.75 g/t Au, and 2.5 g/t Ag. In contrast to the predominantly granite-hosted Olympic Dam IOCG deposit, Prominent Hill is mainly within unmetamorphosed sedimentary rocks comprising coarse clastic to laminated argillaceous lithologies with some volcaniclastic components and variable carbonate, including local massive dolomite. Essentially unmetamorphosed sedimentary rocks and structurally underlying mafic to intermediate-composition lavas, inferred to be members of the lower Gawler Range Volcanics, host the economically mineralized hematite breccias. The volcanic-sedimentary package was downfaulted and tilted along a major east-west fault, north of which similar but regionally low-grade metamorphosed rocks were affected by subeconomic skarn mineralization, and (on a more regional scale of the Mount Woods domain) intruded by granitic and gabbroic bodies. Hydrothermal alteration and mineralization at Prominent Hill involved pervasive and texturally destructive replacement of formerly calcareous, dolomitic, and siliciclastic breccia components. Hydrothermal alteration minerals comprise hematite, magnetite, siderite, ankerite, quartz, sericite, chlorite, kaolinite, fluorapatite, fluorite, barite, REE-U minerals (including monazite), uraninite, and coffinite, together with Cu sulfides including chalcopyrite, bornite, and chalcocite in the highest grade ore. Brecciation and replacement caused mechanical mixing as well as chemical alteration of primary lithologies, such that sedimentary contacts became obscured. Mass-balance calculations identify Al, Ti, Si, and Zr as least mobile components during hematite-chlorite-sericite to weak hematite-quartz alteration. Because Zr was not regularly assayed in drill cores, we use concentration ratios of Ti, Al, and Si from the deposit-scale assay database to delineate the distribution of lithochemical units prior to hydrothermal alteration and Cu mineralization. The resulting lithochemical model, based on one horizontal and five vertical cross sections, is used as a basis for mapping alteration patterns calculated from molar (Fe + Si)/(Fe + Si + Al), K/Na, and K/Al ratios. These chemical patterns, in conjunction with mineral stoichiometry, indicate that the spatial distribution of hematite, chlorite, variably phengitic sericite (and/or illite) ± kaolinite ± quartz-bearing alteration is superimposed on the pattern of interpreted lithologic contacts. The alteration patterns confirm visual logging results, showing that hematite enrichment correlates only partially with the distribution of Cu grades of >0.25 wt %. A subvertical body of complete replacement by hematite and quartz with consistent but subeconomic gold enrichment forms a Cu-barren core in the central and eastern parts of the deposit. Zones of increasing K/Al and K/Na ratios extend upward and westward from this Cu-barren core, transgressively overprinting lithologic contacts. The degree of hematite-quartz replacement can be measured by a hematite-quartz alteration index, here termed the HMSI value [(Fe + Si)/(Fe +Si + Al)], which inversely correlates with the normal probability for Cu grade. Areas of highest Cu grade (>1 wt %) spatially correlate with irregular zones having intermediate molar alteration indices: 0.34 < K/Al < 0.40, 20 < K/Na < 36, and HMSI < 0.98. Hematite breccias and Cu ore deposition developed after tilting of the host sequence into its present steep orientation, as indicated by geopetal structures within the breccia matrix. Thus, the economic mineralization occurred late in the deformational history of the region and after extrusion of the lower Gawler Range Volcanics. The formation of the Prominent Hill orebodies occurred during or after upthrusting of deeper seated rocks containing subeconomic Cu in skarns north of the fault. Faulting as well as ore formation may be related to orogenic processes in the central and northern part of the Mount Woods domain. Iron oxide introduction was decoupled from, and at least partly preceded, hydrothermal deposition of high-grade Cu. Geochemical and petrographic data indicate that economic Cu mineralization occurred together with mildly acidic hematite-chlorite-sericite ± siderite alteration of originally carbonate-, illite-, and feldspar-bearing sedimentary rocks. The presence of copper enrichment with an intermediate degree of cation leaching from the host rocks indicates that pH neutralization of initially highly acidic metal-transporting fluids was an essential factor causing Cu sulfide deposition. Distinct ranges in Na-K-Al ratios and low HMSI values offer potential as exploration indicators pointing toward higher ore grades. These results from Prominent Hill are consistent with recently published mineralogical studies at the giant Olympic Dam deposit, indicating similar ore depositional controls despite lithologically different host rocks.

LA-ICP-MS U–Pb geochronological data from metamorphic monazite in granulite-facies metapelites in the Barossa Complex, southern Australia, yield ages in the range 1580–1550 Ma. Metapelitic rocks from the Myponga and Houghton Inliers contain early biotite–sillimanite-bearing assemblages that underwent partial melting to produce peak metamorphic garnet–sillimanite-bearing anatectic assemblages. Phase equilibrium modelling suggests a clockwise P–T evolution with peak temperatures between 800 and 870°C and peak pressures of 8–9 kbar, followed by decompression to pressures of ∼6 kbar. In combination with existing age data, the monazite U–Pb ages indicate that the early Mesoproterozoic evolution of the Barossa Complex is contemporaneous with other high geothermal gradient metamorphic terranes in eastern Proterozoic Australia. The areal extent of early Mesoproterozoic metamorphism in eastern Australia suggests that any proposed continental reconstructions involving eastern Proterozoic Australia should share a similar tectonothermal history.

Fluorite as indicator mineral in iron oxide-copper-gold systems: explaining the IOCG deposit diversity

The Mesoproterozoic Prominent Hill iron-oxide copper–gold deposit lies on the fault-bound southern edge of the Mt Woods Domain, Gawler Craton, South Australia. Chalcocite–bornite–chalcopyrite ores occur in a hematitic breccia complex that has similarities to the Olympic Dam deposit, but were emplaced in a shallow water clastic–carbonate package overlying a thick andesite–dacite pile. The sequence has been overturned against the major, steep, east–west, Hangingwall Fault, beyond which lies the clastic to potentially evaporitic Blue Duck Metasediments. Immediately north of the deposit, these metasediments have been intruded by dacite porphyry and granitoid and metasomatised to form magnetite–calc–silicate skarn ± pyrite–chalcopyrite. The hematitic breccia complex is strongly sericitised and silicified, has a large sericite ± chlorite halo, and was intruded by dykes during and after sericitisation. This paper evaluates the age of sericite formation in the mineralised breccias and provides constraints on the timing of granitoid intrusion and skarn formation in the terrain adjoining the mineralisation. The breccia complex contains fragments of granitoid and porphyry that are found here to be part of the Gawler Range Volcanics/Hiltaba Suite magmatic event at 1600–1570 Ma. This indicates that some breccia formation post-dated granitoid intrusion. Monazite and apatite in Fe-P-REE-albite metasomatised granitoid, paragenetically linked with magnetite skarn formation north of the Hangingwall Fault, grew soon after granitoid intrusion, although the apatite experienced U–Pb–LREE loss during later fluid–mineral interaction; this accounts for its calculated age of 1544 ± 39 Ma. To the south of the fault, within the breccia, 40Ar–39Ar ages yield a minimum age of sericitisation (+Cu+Fe+REE) of dykes and volcanics of ∼1575 Ma, firmly placing Prominent Hill ore formation as part of the Gawler Range Volcanics/Hiltaba Suite magmatic event within the Olympic Cu–Au province of the Gawler Craton.

Evaluation of cover sequence geochemical exploration sample media through assessment of element migration processes

The chemistry of hydrothermal monazite from the Carrapateena and Prominent Hill iron oxide-copper-gold (IOCG) deposits in the IOCG-rich Gawler Craton, South Australia, is used here to define geochemical criteria for IOCG exploration in the Gawler Craton as follows: Monazite associated with IOCG mineralisation: La + Ce &gt; 63 wt% (where La &gt; 22.5 wt% and Ce &gt; 37 wt%), Y and/or Th &lt; 1 wt% and Nd &lt; 12.5 wt%; Intermediate composition monazite (between background and ore-related compositions): 45 wt% &lt; La + Ce &lt; 63 wt%, Y and/or Th &lt; 1 wt%. Intermediate monazite compositions preserving Nd &gt; 12.5 wt% are considered indicative of Carrapateena-style mineralisation; Background compositions: La + Ce &lt; 45 wt% or Y or Th &gt; 1 wt%. Mineralisation-related monazite compositions are recognised within monazite hosted within cover sequence materials that directly overly IOCG mineralisation at Carrapateena. Similar observations have been made at Prominent Hill. Recognition of these signatures within cover sequence materials demonstrates that the geochemical signatures can survive processes of weathering, erosion, transport and redeposition into younger cover sequence materials that overlie older, mineralised basement rocks. The monazite geochemical signatures therefore have the potential to be dispersed within the cover sequence, effectively increasing the geochemical footprint of mineralisation.

Significant population structure in Australian Cryptolestes ferrugineus and interpreting the potential spread of phosphine resistance

Geochemical footprints of IOA and IOCG deposits in Northern Norrbotten, Sweden, and Cloncurry District, Australia

In the wheatbelt of eastern Australia, rainfall shifts from winter dominated in the south (South Australia, Victoria) to summer dominated in the north (northern New South Wales, southern Queensland). The seasonality of rainfall, together with frost risk, drives the choice of cultivar and sowing date, resulting in a flowering time between October in the south and August in the north. In eastern Australia, crops are therefore exposed to contrasting climatic conditions during the critical period around flowering, which may affect yield potential, and the efficiency in the use of water (WUE) and radiation (RUE). In this work we analysed empirical and simulated data, to identify key climatic drivers of potential water- and radiation-use efficiency, derive a simple climatic index of environmental potentiality, and provide an example of how a simple climatic index could be used to quantify the spatial and temporal variability in resource-use efficiency and potential yield in eastern Australia. Around anthesis, from Horsham to Emerald, median vapour pressure deficit (VPD) increased from 0.92 to 1.28 kPa, average temperature increased from 12.9 to 15.2°C, and the fraction of diffuse radiation (FDR) decreased from 0.61 to 0.41. These spatial gradients in climatic drivers accounted for significant gradients in modelled efficiencies: median transpiration WUE (WUEB/T) increased southwards at a rate of 2.6% per degree latitude and median RUE increased southwards at a rate of 1.1% per degree latitude. Modelled and empirical data confirmed previously established relationships between WUEB/T and VPD, and between RUE and photosynthetically active radiation (PAR) and FDR. Our analysis also revealed a non-causal inverse relationship between VPD and radiation-use efficiency, and a previously unnoticed causal positive relationship between FDR and water-use efficiency. Grain yield (range 1–7 t/ha) measured in field experiments across South Australia, New South Wales, and Queensland (n = 55) was unrelated to the photothermal quotient (Pq = PAR/T) around anthesis, but was significantly associated (r2 = 0.41, P &amp;lt; 0.0001) with newly developed climatic index: a normalised photothermal quotient (NPq = Pq . FDR/VPD). This highlights the importance of diffuse radiation and vapour pressure deficit as sources of variation in yield in eastern Australia. Specific experiments designed to uncouple VPD and FDR and more mechanistic crop models might be required to further disentangle the relationships between efficiencies and climate drivers.

The Lightning Creek Cu-Au prospect is hosted by a cogenetic suite of plutonic, I-type granitoids. The dominant rock type is a porphyritic quartz monzodiorite that is intruded by more fractionated rocks, including monzogranite and alkali feldspar granite. A series of flat-lying sills are interpreted to be late-stage differentiates, based on their timing, mineralogy, and chemistry. In parts of the prospect there is pervasive sodic-calcic alteration (pyroxene after amphibole, albite after K feldspar and oligoclase) of the plutonic rocks. This alteration predates sill emplacement and is unrelated to veining or fracturing of any kind. The presence of small amounts of carbonate in the altered rocks suggests that the fluids were CO2 bearing. Quartz and feldspar separates from these altered rocks have oxygen isotope compositions similar to those from fresh quartz-monzodiorite, suggesting that the fluids were hot and of magmatic composition. Sodium and Ca were added and K, Fe, Cl, and Cu were stripped during what is interpreted as an autometasomatic event. The sills display considerable textural and mineralogical complexity and evolved from equigranular, quartzofeldspathic rocks (aplites), with magmatic chemistry, to unusual Fe-rich rocks (albite-magnetite-quartz) that exhibit a range of bizarre spherulitic textures. Some of the albite and magnetite in the sills is secondary. Albite forms pseudomorphs after K feldspar (Na-Fe ± Ca alteration) along sill margins and within sills, at the contacts between different textural zones. Halos of disseminated magnetite + clinopyroxene (Fe-Ca ± Na alteration) are developed adjacent to early magnetite veins. Fluid inclusion studies indicate that these rocks crystallized at temperatures in excess of 500°C and at pressures in excess of 1.5 kbar. The range of spherulitic textures is taken to indicate crystallization under hydrous conditions with the episodic release of a fluid phase. This magmatic fluid phase was dominated by H2O, CO2, and chlorine and underwent phase separation into a CO2-rich vapor and a hypersaline brine (33–55 wt % NaCl equiv). The hypersaline fluid was enriched in Fe (~10 wt %) and Cu (~1 wt %, PIXE analysis), in addition to Na, K, and Ca. Where this fluid was retained within Fe-rich portions of the sills, it caused Ca-Fe ± Na alteration (pyroxene-albite ± magnetite growth at the expense of quartz). Where the fluid was expelled from the sills, it produced quartz-magnetite ± clinopyroxene ± albite veins (broadly coeval with the early magnetite veins). Although rich in Cu, these granitoid-derived magmatic fluids did not generate significant Cu(-Au) mineralization, perhaps because of the high temperatures involved and/or a lack of reduced sulfur in the fluids or host rock. However, the amount of iron present is estimated (from the aeromagnetic anomaly) to be in excess of 2,000 million tonnes (Mt). A later generation of calcite ± chlorite ± pyrite ± chalcopyrite veins contain traces of Cu-Au mineralization. Fluid inclusion and stable isotope work indicate that these veins probably crystallized from cooler (<200°C), more dilute (15–28 wt % NaCl equiv) fluids, perhaps generated by the admixture of a meteoric component. The conclusions reached in this study have implications for understanding the genesis of Fe oxide Cu-Au deposits and related sodic-calcic alteration. The study indicates the potential for CO2-rich granitoid magmas to evolve hypersaline, Fe- and Cu-rich fluids capable of causing intense magnetite veining and Cu(-Au?) mineralization. Autometasomatic sodic-calcic alteration of the granitoids may be an important precursor to mineralization, contributing Fe, K, Cu, and Cl to the magmatic fluids.

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).

GSQ - characterising signatures and footprints of IOCG deposits in the Cloncurry District