Timing and origin of megabreccia and folds along the Early Middle Cambrian margin of the Georgina Basin, Australia

Megabreccia and related folds are two of the most interesting features of the Lawn Hill Outlier, a small carbonate platform situated in the northeastern part of the Georgina Basin in Queensland, Australia. Field studies and stable isotopic data were used to assess the timing and origin of folds and megabreccias in this carbonate plateau, and understand its possible relationship to an asteroid impact. Together with field and isotope data, the reconstruction of the sequence of events that led to the cratonization of the Centralian Superbasin supports a synsedimentary timing of formation for the folds and breccias. Some of the brittle faulting and veining accompanying strain localisation within the Thorntonia Limestones may represent, however, post-sedimentary, syntectonic deformation, possibly linked to the Late Devonian Alice Springs Orogeny. An origin for the folding and megabreccias linked to an asteroid impact cannot be completely discounted. Nevertheless, observed field relationships concerning the spatial distribution and typology of breccias occurring in basement and cover agree with stable isotopic signatures, suggesting that multiple intrabasinal processes contributed to platform destabilisation. Processes such as karstification, solution collapse, and fault reactivation, were the most likely mechanisms responsible for the formation of intrastratal breccias and slump folds in the Lawn Hill Outlier

Application of numerical modeling to extension, heat, and fluid flow in the genesis of giant banded iron formation-hosted hematite ore deposits

Finite difference modeling of fluid flow in response to topography, extensional collapse, and thermal structure\ud has been employed to simulate processes leading to the genesis of Proterozoic iron ores, using input data\ud appropriate to the Hamersley district of Western Australia and other iron ore districts. The geologic history and\ud questions that provide the motivation for the modeling include the presence of a mountain range formed by\ud pre-ore genesis convergent deformation, extensional collapse of that mountain range, and evidence at the deposits for two or more different fluid types, including a deep-seated (reduced) and a surface-derived (oxidized\ud and 18O-depleted) fluid. In terms of fluid-flow rates, topographically driven downward fluid flow is seen to be\ud comparable to both deformation-driven flow and also to heating and/or basal overpressures for comparable\ud permeability structures and mountains with elevations in excess of 1 km. During extensional deformation at geologically realistic strain rates, downward flow is created by the combination of dilation produced by deformation with the inability of the fluid always to flow quickly enough to account for the dilatant volume change, producing areas of fluid under pressure, particularly across permeability interfaces. This effect is most pronounced where extensional faults cut through low-permeability basement. Upward fluid flow of heated fluids, as has been proposed to initiate genesis of these giant iron ore deposits, can be achieved at the start of extensional deformation if the deep fluid is initially overpressured, for example, due to input of fluids derived from magmas or to heating and/or devolatilization deep in the system. This initial upward flow can produce substantial temperature anomalies at relatively shallow depths, particularly in the hanging wall of dipping faults. However, with time, the extension and topography drives cooler meteoric fluids downward, which competes with and\ud then eventually swamps the initial upflow. This scenario matches the envisaged sequence of events at the major\ud deposits of the Hamersley district and also explains how different deposits record different degrees of preservation\ud of the early-formed high-temperature assemblages, depending on the extent to which later surface-derived fluids have utilized the same structures as the initial upflowing fluid. Questions remaining from this modeling, and in consideration of the geochemical and stable isotope data, relate to which of the fluids (or both) was largely responsible for silica dissolution and whether both deep-seated and shallow fluids are prerequisite ingredients for genesis of this ore type

Mechanisms of fluid flow and fluid–rock interaction in fossil metamorphic hydrothermal systems inferred from vein–wallrock patterns, geometry and microstructure

Comparison of mass transfer patterns, geometry and microstructures developed within and around veins allows the interpretation of processes of fluid flow during deformation, metamorphism and mineralization. A classification of vein types based on the degree of interaction with wallrock (using petrological, geochemical or isotopic indicators) can be used to identify a range of processes, from closed system behaviour in which the vein mass is derived from local wallrock, through to open system behaviour in which the vein mass is derived externally. Microstructural characteristics, such as wallrock selvages, multiple growth events recorded by vein seams and vein crystal morphology, also help to constrain mass transfer patterns during vein formation. We present a range of processes for vein formation, including: (i) the formation of closed system fibrous veins by dissolution–precipitation creep, including varieties in which tensile failure is not required; (ii) pressure- or kinetically dependent closed system segregation veins in which transfer of soluble components from wallrock to vein leaves behind a residual selvage; (iii) similar vein–selvage patterning, but with mass imbalances between vein and wallrock requiring fluid advection through both interconnected fracture networks and in the surrounding permeable rock; and (iv) the proposed formation of veins by fluid ascent in mobile hydrofractures, in which isotopic or chemical disequilibrium within and around the vein suggests that the crack and fluid within it moved essentially as one. The postulate of rapid fluid and mass transfer via such mobile hydrofractures has implications for the release of volatiles from metamorphic terrains. Also, consideration of a broad range of possible vein-forming mechanisms is highly desirable when dealing with mineral deposits found in deformed, metamorphosed rocks, because closed system veining may produce patterns that, if erroneously recognized as being open systems, could lead to false interpretations of the role of tectonic fracturing in ore genesis

Distinguishing basinal- and magmatic-hydrothermal IOCG deposits, Cloncurry District, Northern Australia

We recognise two distinctive styles of IOCG deposits in the Cloncurry District of the Proterozoic Mount Isa Block. The earliest (Osborne) type (1680 to 1600 Ma) is dominated by basinal and/or metamorphic fluids, deformed ores in shear zones, sodic-calcic alteration, intra-basinal metal and sulphur sources, and chemically favourable ore hosts. Fluid circulation was likely driven by extensional shearing or convection and magmatic heat inputs, and there a genetic connection to the region's large Pb-Zn ± Ag deposits (Cannington, Mt Isa Pb-Zn) is possible. The later (Ernest Henry) type (c. 1530 Ma) shows potassic alteration, and co-precipitated magnetite and calcite in milled breccia. Geochemical data indicate mixtures of mantle or magmatic fluids with sedimentary formation waters (or their metamorphosed equivalents), and an abundance of CO₂₋ and Cl+F-bearing fluids. These formed primarily by release of fluids, S and probably metals from crystallizing A-type granitoids and tholeiitic gabbros via fluidised breccia pipes, and have likely parallels with Olympic Dam and other large to giant IOCGs worldwide hosted in milled hydrothermal breccias. Fluidisation occurred by violent release of overpressured fluids across permeability barriers near intrusions, carrying volatiles and cm- to m-scale rock fragments upwards, and forming ore deposits in cooling and de-pressurising dilatant structural traps

Constraints on hydrothermal fluid pathways within Mary Kathleen Group stratigraphy of the Cloncurry iron-oxide-copper-gold District, Australia

Widespread brecciation and Na-(Ca) alteration in the metacarbonate-dominated Mary Kathleen Group characterise some of the district-scale host-rocks to iron-oxide–copper–gold (IOCG) mineralisation in the Proterozoic Cloncurry District of north-eastern Australia. Structural and mineralogical observations combined with stable isotope data indicate that within the Mary Kathleen Group, late- to post-metamorphism brecciation was preferentially developed in calcite-poor lithologies, while marbles and to a lesser degree calc-silicate rocks were not prone to fracturing and acted as impermeable barriers to fluid flow. These impermeable layers helped maintain high fluid pressures promoting brecciation in adjacent lithologies. Only in areas of localised high shear strain did carbonate-rich lithologies allow for appreciable fluid flow. Such carbonate-rich rocks contain isotopically and mineralogically distinct skarn-like assemblages, the recognition of which may be important in identifying high fluid flux corridors in the vicinity of major IOCG deposits

Timing and duration of syn-magmatic deformation in the Mabel Downs Tonalite, northern Australia

Detailed outcrop mapping combined with microstructural and U–Pb SHRIMP zircon data indicate that emplacement of the Mabel Downs Tonalite spanned progressive regional D₃ deformation in the Palaeoproterozoic Halls Creek Orogen of northern Australia, and that the duration of magmatism exceeded the crystallisation time of the pluton had its entire volume been emplaced instantaneously (∼10⁵ y). The pluton comprises several compositionally distinct phases, which show (i) a regional solid-state S3a foliation-forming event, predated by a strongly deformed porphyritic monzogranite with a U–Pb SHRIMP zircon age of 1837.3±6.0 Ma (95% confidence level); and (ii) overprinting by the localised S3b Ord River Shear Zone, which crosscuts a 1831.9±3.3 Ma foliated granodiorite and contains 1826.6±7.3 Ma undeformed felsic veins, providing a younger age limit for D3 deformation.\ud \ud The protracted nature of deformation and magmatism during regional D3 deformation is significant in the context of the evolution of Halls Creek Orogen, which is characterised by a prolonged thermal event spanning three regional deformation events (D2, D3 and D4) within a 30–40 million-year interval. The accumulated finite strain is more probably the product of relatively long-lived events (of the order of several millions of years) with low average crustal strain rates, rather than high crustal strain rates during short-lived deformation episodes (=10⁵ y). Thus the partitioning of strain accumulation into discrete deformation events during the rapid development of the Halls Creek Orogen was probably not as pronounced as in orogenic belts characterised by higher accumulated strain or longer intervals between deformation events

Geology, lithogeochemistry and paleotectonic setting of the host sequence to the Kangasjärvi Zn-Cu deposit, central Finland: implications for volcanogenic massive sulphide exploration in the Vihanti–Pyhäsalmi district

The Kangasjärvi Zn-Cu deposit is a highly deformed and metamorphosed Paleoproterozoic volcanogenic massive sulphide (VMS) deposit located in the Vihanti-Pyhäsalmi\ud base metal mining district of central Finland. The host sequence to the deposit, referred to as the Inner Volcanic Sequence (IVS), is comprised of a bimodal suite of metavolcanic rocks and a regionally extensive tonalite-trondhjemite gneiss (sub-volcanic intrusions?).\ud A separate and perhaps younger sequence of mafi c volcanic rocks, with irregular intervals of undifferentiated intermediate to felsic schists and metalimestones, referred\ud to as the Outer Volcanic Sequence (OVS), are separated from the IVS sequence by intervals of metagreywacke and U-P-bearing graphitic schists. A stratigraphic scheme for rocks within the IVS is proposed based on outcrop observations, locally preserved volcanic textures, aspects of seafl oor-related hydrothermal alteration and lithogeochemistry. In this scheme, rare andesites form the lowermost volcanic stratigraphy and are overlain by typical island-arc basalts that were erupted in a subaqueous setting. Tonalite-trondhjemite subvolcanic intrusions were locally emplaced within andesites and coeval rhyolites were extruded on the basaltic substrate. The extrusion of rhyolites, including high-silica rhyolites, was coeval with regional-scale, pre-metamorphic seafloor hydrothermal alteration and local sulphide mineralization. Extensively altered rhyolites envelope massive sulphides and are underlain by altered basalts. The latter rocks are now characterized by a variety of low-variance metamorphic\ud mineral assemblages (e.g. orthoamphibole-cordierite rocks) and define a domain of intense pre-metamorphic chlorite ± sericite alteration in the stratigraphic footwall of the deposit. The altered nature of these rocks is attributed to reaction with seawater-related hydrothermal fluids within a zone of upflow at or near the seafloor. The fundamental controls on convective hydrothermal circulation and subsequent alteration and massive sulphide mineralization at Kangasjärvi, and possibly elsewhere in the district, share many characteristics with other well-described, ancient VMS deposits (e.g. massive sulphide deposits in the Flin Flon Belt, Manitoba, Canada). These characteristics\ud include: 1) an association with bimodal volcanism developed in extensional settings; 2) a close spatial association with regionally extensive felsic subvolcanic intrusions;\ud and 3) petrogenesis of ore-associated volcanic rocks (e.g. high-silica rhyolites, felsic subvolcanic intrusions) indicative of substantial heat transfer from the mantle to\ud the upper crust and the development of anomalous thermal corridors. These features translate into geochemically distinctive rock types that, when combined with aspects of\ud stratigraphy and pre-metamorphic alteration, may be used to develop regional exploration strategies in the Vihanti-Pyhäsalmi district

Carbon and oxygen isotope constraints on fluid sources and fluid–wallrock interaction in regional alteration and iron-oxide–copper–gold mineralisation, eastern Mt Isa Block, Australia

The source of metasomatic fluids in iron-oxide–copper–gold districts is contentious with models for magmatic and other fluid sources having been proposed. For this study, δ 18O and δ 13C ratios were measured from carbonate mineral separates in the Proterozoic eastern Mt Isa Block of Northwest Queensland, Australia. Isotopic analyses are supported by petrography, mineral chemistry and cathodoluminescence imagery. Marine meta-carbonate rocks (ca. 20.5‰ δ 18O and 0.5‰ δ 13C calcite) and graphitic meta-sedimentary rocks (ca. 14‰ δ 18O and −18‰ δ 13C calcite) are the main supracrustal reservoirs of carbon and oxygen in the district. The isotopic ratios for calcite from the cores of Na–(Ca) alteration systems strongly cluster around 11‰ δ 18O and −7‰ δ 13C, with shifts towards higher δ 18O values and higher and lower δ 13C values, reflecting interaction with different hostrocks. Na–(Ca)-rich assemblages are out of isotopic equilibrium with their metamorphic hostrocks, and isotopic values are consistent with fluids derived from or equilibrated with igneous rocks. However, igneous rocks in the eastern Mt Isa Block contain negligible carbon and are incapable of buffering the δ 13C signatures of CO2-rich metasomatic fluids associated with Na–(Ca) alteration. In contrast, plutons in the eastern Mt Isa Block have been documented as having exsolved saline CO2-rich fluids and represent the most probable fluid source for Na–(Ca) alteration. Intrusion-proximal, skarn-like Cu–Au orebodies that lack significant K and Fe enrichment (e.g. Mt Elliott) display isotopic ratios that cluster around values of 11‰ δ 18O and −7‰ δ 13C (calcite), indicating an isotopically similar fluid source as for Na–(Ca) alteration and that significant fluid–wallrock interaction was not required in the genesis of these deposits. In contrast, K- and Fe-rich, intrusion-distal deposits (e.g. Ernest Henry) record significant shifts in δ 18O and δ 13C towards values characteristic of the broader hostrocks to the deposits, reflecting fluid–wallrock equilibration before mineralisation. Low temperature, low salinity, low δ 18O (<10‰ calcite) and CO2-poor fluids are documented in retrograde metasomatic assemblages, but these fluids are paragenetically late and have not contributed significantly to the mass budgets of Cu–Au mineralisation

The relative effects of deformation and thermal advection on fluid pathways in basin-related mineralization

The migration of basinal brines into basement material has been proposed as a means of scouring or leaching metals for subsequent ore deposition. Here we address this issue by numerically examining competing processes, namely deformation, fluid flow and thermal gradients, to describe potential fluid pathways leading to enrichment of metals and ore deposition. Stable convective fluid patterns may be established across the cover/basement interfaces if permeability contrasts are minimized, however, at the onset of extensional deformation these convective patterns quickly collapse. On cessation of the deformation, convection cells again develop, which are oscillatory with time. Input to the thermal budget from a radiogenic heat source suggests that basinal fluids can be drawn down around the margins of granite intrusions and fluid mixing processes may take place due to small and localised convective patterns. Fluid migration from basin into basement and back is highly likely given the right conditions, however, the rate and extent of fluid flow are determined by thermal and deformation processes

Veins and hydrothermal fluid flow in the Mt. Whaleback Iron Ore District, eastern Hamersley Province, Western Australia

With the advent of recent hydrothermal models for genesis of giant microplaty hematite ore deposits of the Hamersley Province, we have undertaken regional scale analysis of veins and rocks surrounding the Mt. Whaleback deposit with an aim to characterize the nature of fluid flow before, during and after ore formation. At least five groups of veins formed in the Brockman Iron Formation in deformational events associated with the Paleoproterozoic interaction between the Yilgarn and Pilbara cratons. In a ~600 km2 area around the Mt. Whaleback microplaty hematite orebody, the five main veining events most likely correspond to the collisional (D2) and extensional collapse (D3) phases of the ca. ~2300–2200 Ma Ophthalmian Orogeny, with one or two later phases probably associated with the waning stages of the ca. 1700–1650 Ma Capricorn Orogeny (D4) and/or later Proterozoic events. The abundance of veins associated with D2 contraction and D3 extensional fabrics, and the composition (up to 15 wt.% NaCl equivalent) and pressure-corrected homogenisation temperatures of primary fluid inclusions (200–400 °C), indicate that the temperature, salinity and likely volume of fluid reached a peak during the last stages of the Ophthalmian Orogeny and the first stages of post-Ophthalmian extension. Paragenesis of veins relative to the deformation history, and localization of highest vein densities in the Dales Gorge Member, provides circumstantial evidence that the peak of hydrothermal activity in the district was synchronous with the main phase of microplaty hematite ore formation at Mt. Whaleback. We propose that the veins record part of the fluid flow system related to the genesis of this giant ore deposit