The unique Wolverine HREE deposit, Browns Range area, Western Australia

The Wolverine deposit is the largest of a number of REE ore bodies located in the Browns Range area of the Tanami region, Western Australia. These deposits collectively represent one of the worlds’ richest sources of dysprosium and other critical HREE. The Wolverine deposit consists of xenotime [(Y,REE)PO4] and minor florencite [(REEAl3(PO4)2(OH)6] mineralisation in hydrothermal lodes within massive arkosic sandstones. Small alkali granite and pegmatite bodies also intruded the sandstone in the region. Steeply dipping mineralisation is associated with silicification at major fault junctions, and occurs mostly as; 1) high grade, low tonnage lodes with large (>10m long and 1m wide) veins and chaotic breccias of massive, anhedral xenotime (±quartz, ±hematite, ±sericite), and; 2) low grade, probably higher tonnage disseminated mm-scale xenotime-quartz veins and crackle breccias in which xenotime grains occur in a number of morphological types, mainly blade-like and pyramidal overgrowth on pre-existing xenotime grains. U-Pb dating and isotopic analysis of detrital zircon grains from arkose samples from across the district yielded a single age population of ~3.1 (±~0.1) Ga (corrected for lead loss), which is interpreted to be the maximum depositional age of the sandstones. This age is significantly older than the granitic rocks in the region ( ca. 1.8 to 2.5 Ga), indicating that there is (previously unknown) Mesoarchean basement within the North Australian Craton. Highly unradiogenic Hf isotope data for these zircons combined with unradiogenic Nd isotope values for ore xenotime indicate that old (Early Archean or Hadean?) crustal components contributed to the formation of ~3.1 Ga basement rocks and potentially the xenotime ore bodies. Work is ongoing to understand the temporal evolution of the deposit, the source of the REE (i.e., mantle versus old crustal) and the processes of transport and precipitation of HREE to form the deposit

Le magmatisme de la région de Kwyjibo, Province\ud du Grenville (Canada) : intérêt pour les\ud minéralisations de type fer-oxydes associées

The granitic plutons located north of the Kwyjibo property in Quebec’s Grenville Province are of\ud Mesoproterozoic age and belong to the granitic Canatiche Complex . The rocks in these plutons are calc-alkalic, K-rich,\ud and meta- to peraluminous. They belong to the magnetite series and their trace element characteristics link them to\ud intraplate granites. They were emplaced in an anorogenic, subvolcanic environment, but they subsequently underwent\ud significant ductile deformation. The magnetite, copper, and fluorite showings on the Kwyjibo property are polyphased\ud and premetamorphic; their formation began with the emplacement of hydraulic, magnetite-bearing breccias, followed by\ud impregnations and veins of chalcopyrite, pyrite, and fluorite, and ended with a late phase of mineralization, during\ud which uraninite, rare earths, and hematite were emplaced along brittle structures. The plutons belong to two families:\ud biotite-amphibole granites and leucogranites. The biotite-amphibole granites are rich in iron and represent a potential\ud heat and metal source for the first, iron oxide phase of mineralization. The leucogranites show a primary enrichment in\ud REE (rare-earth elements), F, and U, carried mainly in Y-, U-, and REE-bearing niobotitanates. They are metamict and\ud underwent a postmagmatic alteration that remobilized the uranium and the rare earths. The leucogranites could also be\ud a source of rare earths and uranium for the latest mineralizing events

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

Numerical modelling using a discrete element technique is employed here to examine the response of a fracture system in the eastern Mount Isa Inlier to an applied stress regime. Model scale, parameters and boundary conditions were varied to test which combinations of geometry and material properties produce the best correspondence between the known mineral-deposit distribution and zones of anomalous model stress. Modelled areas of combined low-minimum principal stress (σ3) and low-mean stress (σm) show the best correlation with deposits, but these areas do not clearly correspond to specific fault orientations or configurations. Rather, the models produce complex zoning of stress anomalies in response to the partitioning of stress across complex fault blocks, and the interaction between more competent granitoid bodies, less competent meta-sedimentary rocks, and the fault and rock boundary complexities. Adding fluid to the models, and identifying an optimum orientation of the stress field (with σ1 in an ESE direction), produces the highest degree of visual correspondence with the known mineral-deposit distribution and the previous empirical prospectivity analysis, and also identifies several potentially prospective areas that may not have been previously tested by explorers. The models are consistent with mineralisation occurring (or being remobilised from earlier concentrations) during a major phase of regional fluid flow facilitated by a complex fault array, late during the evolution of the Isan Orogeny and synchronous with the waning stages of the emplacement of the Williams Batholith

Metamorphic fluids and their relationship to the formation of metamorphosed and metamorphogenic ore deposits

Metamorphic rocks produce fluids as devolatilization occurs during prograde metamorphism or as melts (which act as temporary repositories for fluids) crystallize during the early stages (>650°C) of cooling in high-grade metamorphic terranes. Metamorphosed shales and graywackes, which make up much of the sedimentary component of the upper crust, initially contain approximately 4 wt percent H20, which may be liberated during the metamorphic cycle. These fluids may combine with others derived from external sources (e.g., synmetamorphic igneous intrusions or surface-derived fluids), and have the potential to transport heat, cause metasomatism, alter the rheology of the rocks, or form ore deposits. Metamorphic fluid flow in the crust is probably initially widespread, as fluids are derived from much of the rock mass, and then becomes increasingly channeled as fluids are focused along higher-permeability layers or along structures such as faults or shear zones. This type of flow path promotes ore genesis as metals can be scavenged from a large volume of rocks, with deposition occurring where fluids are focused and flowing down temperature. Most metamorphic fluids are dominated by H20, with variable C02 and minor amounts of other species (e.g., F, Cl, B, and S). At high to moderate metamorphic grades, H20and C02 are miscible at all Xco2 values unless significant salt is present. Such fluids transport some metals (e.g., Cu, Au, Ag) relatively efficiently but not base metals. Thus, the variety of metamorphogenic ore deposits will be limited unless input of saline fluid from other sources (e.g., igneous bodies) occurs, or the terrane is composed of significant volumes of meta-evaporites. However, remobilization of preexisting orebodies may occur during metamorphism and deformation

Early tectonic dewatering and brecciation on the overturned sequence at Marble Bar, Pilbara Craton, Western Australia: dome-related or not?

Cataclastic breccias and hydrothermal fault arrays of likely c. 3400 Ma timing are well developed and exceptionally well exposed in the Marble Bar Chert Member of the Pilbara Craton. Brecciation involved centimetre- to metre-scale clast transport distances, in breccia zones up to 5 m wide, cutting the c. 60 m thick chert in a series of right-lateral fault zones. Our observations of downward facing pillow basalts, the geometry of the breccias, and oxygen isotope data for rocks and the breccia matrix suggest the rocks were at least steeply overturned on this flank of the Mt Edgar Dome prior to brecciation. The breccias are inferred to represent steep conjugate fault zones developed by local transtension. The history of overturning and brecciation predates the formation of dome-related regional foliation and metamorphism, and therefore occurred between 3460 and 3320 Ma, the established ages for deposition of the underlying Duffer Formation and intrusion of the Mt Edgar Batholith respectively. Local overturning of the Marble Bar sequence prior to both brecciation. and the main phase of dome formation suggests a protracted deformation history for this segment of the Pilbara Craton. The transtensional movement along the breccias may be representative of strain accommodation accompanying an early doming phase, or could be a deformation event that developed independently of doming. Fluids involved in brecciation were most likely formation waters expelled from the cherts and basalts in response to overpressuring induced by the overturning and progressive burial

Comparing closed system, flow-through and fluid infiltration geochemical modelling: examples from K-alteration in the Ernest Henry Fe-oxide–Cu–Au system

Potassic alteration of rocks adjacent to, and within the Ernest Henry Fe-oxide–Cu–Au deposit is used here as a test case to investigate fluid–rock interactions using various equilibrium dynamic geochemical modelling approaches available in the HCh code. Reaction of a simple K–Fe–(Na,Ca) brine (constrained by published fluid inclusion analysis) with an albite-bearing felsic volcanic rock, resulted in predicted assemblages defined by (i) K-feldspar–muscovite–magnetite, (ii) biotite–K-feldspar–magnetite, (iii) biotite–quartz–albite and (iv) albite–biotite–actinolite–pyroxene with increasing rock buffering (decreasing log w/r). Models for isothermal–isobaric conditions (450°C and 2500 bars) were compared with models run over a T–P gradient (450 to 200°C and 2500 to 500 bars). Three principal equilibrium dynamic simulation methods have been used: (i) static closed system, where individual steps are independent of all others, (ii) flow-through and flush, where a part of the result is passed as input further along the flow line, and (iii) fluid infiltration models that simulate fluid moving through a rock column. Each type is best suited to a specific geological fluid–rock scenario, with increasing complexity, computation requirements and approximation to different parts of the natural system. Static closed system models can be used to quickly ascertain the broad alteration assemblages related to changes in the water/rock ratio, while flow-through models are better suited to simulating outflow of reacted fluid into fresh rock. The fluid infiltration model can be used to simulate spatially controlled fluid metasomatism of rock, and we show that, given assumptions of porosity relationships and spatial dimensions, this model is a first-order approximation to full reactive transport, without requiring significant computational time. This work presents an overview of the current state of equilibrium dynamic modelling technology using the HCh code with a view to applying these techniques to predictive modelling in exploration for mineral deposits. Application to the Ernest Henry Fe-oxide–Cu–Au deposit demonstrates that isothermal fluid–rock reaction can account for some of the alteration zonation around the deposit

Modelling hydrothermal systems: a future for exploration geochemistry

Recent advances in geochemical modelling may yet breathe life into mineral exploration. Empirically based exploration geochemistry has seen few improvements in the last decade and its use as a mainstream exploration tool is in decline. The inherent complexities of geochemical thermodynamics and geochemical modelling in general have also hindered progress in the educational and academic sectors. The Predictive Mineral Discovery CRC (pmd*CRC) has formed a core geochemical modelling group as the hub of a broader Australian collaboration, and significant progress has been made on many of the problem areas. Here we demonstrate some applications of this new technology to mineral exploration in greenstone terranes, with examples of fluid-rock reaction, the role of intrusions, outflow above buried deposits, dispersion halos and forward modelling of geophysical signatures

Modelling the giant, Zn–Pb–Ag Century deposit, Queensland, Australia

This paper presents a combination of geometric reconstructions of the Century Zn–Pb–Ag deposit, and finite-difference modelling of coupled deformation and fluid flow. Our intention is to demonstrate that these computer-based applications represent a new approach in testing ore genesis models. We use a “visiometric” approach, utilising GoCad 3D structural and property modelling. Computer visualisation is applied to reveal metal zonations, fault distributions and timing, stratigraphic influence on zoning, and the nature and extent of metal redistribution during basin evolution and deformation. We also examine possible links between fluid flow, deformation, and mass transfer using the numerical code FLAC3D. Numerical modelling results suggest that subsurface fluid flow during basin inversion is compartmentalised, being focussed within more permeable fault zones, thus accounting for the secondary redistribution of base metals identified using the 3D reconstructions. However, the results do not explain the broad metal zonation observed. Both the spatial and numerical models suggest that Century is syngenetic, with further diagenesis and deformation producing 1–100 m-scale (re)mobilisation

Reconstructing the architecture of highly deformed and metamorphosed Zn-Cu massive sulphide deposits in the Vihanti-Pyhäsalmi district, central Finland

Highly deformed and metamorphosed Zn-Cu deposits in the Vihanti-Pyhäsalmi district of central Finland preserve the geochemical signature of pre-metamorphic seafloor alteration and are thus considered as volcanogenic massive sulphide (VMS) deposits. The distribution and chemistry of alteration associated with several deposits (Kangasjärvi, Ruostesuo) constrains an overall stratigraphic-hydrothermal\ud framework. Massive sulphide deposits are hosted by chlorite-sericite altered rhyolites (biotite-garnet-orthoamphibole\ud assemblages) associated with ca. 1.93-1.91 Ga bimodal volcanic sequences. On the mine-scale, alteration of these rocks is contiguous with chloritic stockwork zones (orthoamphibole-cordierite assemblages; depleted δ18O values) developed in adjacent mafic volcanic rocks which define hydrothermal upflow zones within the stratigraphic footwall. Local epidotization and metal leaching of basal basalts and andesites suggests that these rocks were located within high-temperature (>400°C) hydrothermal reaction zones, likely in the immediate hanging wall of felsic subvolcanic intrusions. On a regional scale, low-temperature, semi-conformable alteration of felsic volcanic rocks (alkali exchange; enriched δ18O values) reflects broad areas\ud of hydrothermal recharge and indicate the upper levels of stratigraphy. The above stratigraphic model can\ud be used to guide lithogeochemistry-based exploration strategies in the district and also provides a framework\ud to gauge and test the significance of components within the stratigraphy (e.g., felsic intrusions, high-silica\ud rhyolites) and their fundamental relationship with mineralization

Fluid flow in extensional environments; numerical modelling with an application to Hamersley iron ores

The mechanical feasibility of focusing both surface- and basinal-derived fluids towards sites of iron ore genesis during Proterozoic deformation in the Hamersley Province is tested here by computer simulation. Finite difference modelling of porous media flow during extensional deformation of a mountain range shows that surface fluids are drawn towards areas of failure and focus into the centre of the mountain. The addition of permeable structures such as a normal fault provides focused fluid pathways in which mechanical and geological conditions are particularly conducive to both upward and downward flow. Upward flow from the base of the fault within the model overall is favoured by low permeability basement materials and supra-hydrostatic pore pressures. Downward migration of fluids becomes more prominent as extension progresses and upward fluid flow from the base diminishes. The introduction of sedimentary layering into the models allows lateral fluid flow, such that sites of potential fluid mixing may then occur within permeable iron formation units close to the fault zone. Allowing parts of the stratigraphy to become more permeable as a function of high fluid flux simulates permeability enhancement by silica dissolution as a mechanism for iron ore genesis. The involvement of both basinal and surficial fluids in the genesis of the ore deposits is supported by the mechanical models and in addition provides an explanation for a progression from relatively reduced to oxidised conditions at the Mt Tom Price deposit (and possibly other large deposits) with time