Partitioning of trace elements between plagioclase, clinopyroxene and melt

This study focusses on the partitioning of trace elements between plagioclase, clinopyroxene and equilibrium melt. Such mineral/melt partition coefficients are widely used to model petrogenetic processes in igneous systems. However, theoretical considerations lead us to expect that the values of partition coefficients will change with many variables, including both mineral and melt compositions, as well as temperature and pressure. Plagioclase and clinopyroxene are two of the most common minerals in the Earth’s crust and if we can understand what controls the partitioning of trace elements into common rock forming phases, we can more accurately model these processes. To examine the major controls of partitioning in plagioclase and clinopyroxene, these two phases were grown experimentally at controlled pressure, temperature and oxygen fugacity. These minerals were grown from a wide range of simple, synthetic systems, mostly focussing on variations in CaO-MgO-Al2O3-SiO2-Na2O±Fe2O3 (CMASN±F) compositional space. 102 successful experiments are included in this thesis, 57 of these contain clinopyroxene and melt, 76 contain plagioclase and melt and 34 of these experiments contain both plagioclase and clinopyroxene and melt. This allows for the partitioning of each phase with their equilibrium melt to be well constrained before comparing the partitioning between the solid phases. Melt composition is shown to play a significant role in the partitioning of trace elements in both phases, especially when the substituting trace element has a different charge to the element it replaces. Even-though melt composition plays a key role in the partitioning of trace elements in both phases, if the substitution and charge balancing mechanisms are the same in both minerals, the effect of melt composition will be cancelled out. Such is the case for the partitioning of the rare earth elements (REEs) between plagioclase and clinopyroxene. This is advantageous as in natural samples, the equilibrium melt is rarely preserved, so partitioning between the solid phases is much easier to measure than the mineral/melt partition coefficients. The partitioning of the REEs between plagioclase and clinopyroxene has been used to calibrate a geothermometer. The geothermometer has been applied to a selection of natural coexisting plagioclase and clinopyroxene pairs, with ambiguous results

Microscale data to macroscale processes: A review of microcharacterization applied to mineral systems

Microanalysis can provide rapid, quantitative characterization of mineral systems that complements the field- and core-scale observations traditionally made in ore deposits. We review recent innovations in microanalytical procedures and their application to studies of ore deposits. Case studies are presented examining how microanalysis can provide constraints on macroscopic processes within mineral systems. Synchrotron X-ray fluorescence shows centimetre-scale chemical variations associated with proximity to mineralization in samples from Sunrise Dam Gold Mine, Western Australia. Pseudomorphs of igneous plagioclase and chemically driven recrystallization interpreted from electron backscatter diffraction suggest that the system was dominated by fluid-driven brecciation with very little shearing. Both the fluid chemistry and fluid pressure evolved during a protracted sequence of vein formation and alteration accompanying gold mineralization. A second case study of sulphide mineralogy at the Mt Keith nickel sulphide deposit, Western Australia demonstrates how X-ray computed tomography combined with trace element mapping can constrain the chemistry and dynamics of magmatic systems. Large-scale interaction between silicate and sulphide melts, shown by homogenous palladium enrichment in pentlandite, leads to a large proportion of globular ores with a high nickel content. Increasing use of microanalysis in ore deposit geology is resulting in the constant reassessment of established models for ore genesis though a combination of micro- and macroscale datasets.This research was undertaken on the X-ray fluorescence microscopy beamline at the Australian Synchrotron, Victoria, Australia and was funded by AngloGold Ashanti. LS acknowledges support from a CSIRO Mineral Resources Flagship Internship to support this work

Evolution of the Munali Intrusive Complex: host to a carbonate-rich Ni-(Cu-PGE) sulfide deposit

The Munali Intrusive Complex is hosted within supracrustal metasedimentary rocks located along a major structural lineament within the Zambezi Belt in southern Zambia. The complex comprises unmineralised gabbro surrounded by a marginal heterogeneous mafic–ultramafic breccia unit that is host to Ni-Fe sulfide. This marginal unit comprises a range of variably evolved brecciated mafic–ultramafic rocks that include gabbro, olivine-gabbro and dolerite, alongside younger, pegmatitic, apatite-magnetite-bearing clinopyroxenite, wehrlite and dunite. The magmatic evolution is most consistent with a model whereby early mafic rocks interact with hot, MgO- and volatile-rich melts along gabbro contacts, causing localised metasomatism of gabbro and pyroxenites, and progressively replacing pyroxene-rich rocks with olivine, forming pegmatitic ‘replacive dunites’. Sulfide mineralisation is characterised by a carbonate-rich apatite-magnetite-bearing assemblage predominately present as lenses of semi-massive to massive sulfide ore. The complex is enveloped almost entirely within a unit of marble, yet C and O isotope signatures of carbonate at Munali have revealed a clear mantle signature for some of the carbonate associated with sulfide, alongside a more dominant, crustally derived component. The carbonate occurring alongside sulfide displays micro to macro textures signifying the presence of carbonate melts formed from anatectic melting of the country rocks. The presence of fracture sets that define coarse breccia clasts (>1 m) indicate that the host rock was significantly crystallised and brittly deformed prior to carbonate and sulfide melt infiltration. Both carbonate and sulfide melts appear to have independently utilised these pre-existing weaknesses producing a pseudobreccia, and accounting for the seemingly chaotic nature of the orebody. The indication of sulfide being a significantly later phase suggests that the sulfide did not form in situ and was mobilised from elsewhere to be subsequently emplaced late within the Munali system

The influence of melt composition on the partitioning of trace elements between anorthite and silicate melt

The effect of melt composition on the partitioning of trace elements between anorthite and silicate melts has been studied experimentally in five compositions in the system CaO–Al2O3–SiO2 (CAS) at ~ 1400 °C and four compositions in the system CaO–MgO–Al2O3–SiO2 (CMAS) at 1332 °C. Melt composition has a significant impact on the substitution of trace elements into anorthite, particularly if the trace-element substitution is aliovalent and requires a charge balance for substitution. Melt composition strongly influences the partitioning of the trivalent rare earth element (REE) cations into the large-cation site (M) of anorthite. Due to charge balance requirements, the activity of alumina in the melt is the most important compositional variable for the REE partitioning in anorthite. Scandium, another trivalent cation, is much more compatible than is predicted for trivalent cations partitioning on the M-site. Therefore, scandium is likely partitioning onto the tetrahedral site in place of aluminium, which requires no charge balance and therefore is not affected strongly by melt composition. Similarly, the partitioning of the small divalent cations (Be and Mg) show a stronger relationship with changing melt composition than the large divalent cations (Ca, Sr, and Ba) and therefore are likely to partition on the tetrahedral site (T) of plagioclase rather than the large-cation site (M). Detailed thermodynamic modelling of the effects of melt composition is required for an adequate parameterization of trace-element mineral/melt partition coefficients, in addition to models of the effects of mineral composition.LS was funded by an Australian Government Research Training Program (RTP) Scholarship and a scholarship from families of Bruce Chappell and Allan White. Analytical costs were funded by ARC grant FL130100066 to HON

Genesis of the central zone of the Nolans Bore rare earth element deposit, Northern Territory, Australia

The Nolans Bore rare earth element (REE) deposit consists of a network of fluorapatite-bearing veins and breccias hosted within Proterozoic granulites of the Reynolds Range, Central Australia. Mineralisation is divided into three zones (north, central, and south-east), with the north and south-east zones consisting of massive REE-bearing fluorapatite veins, with minor brecciation and carbonate infill. The central zone is distinctively different in mineralogy and structure; it features extensive brecciation, a high allanite content, and a large, epidote-rich enveloping alteration zone. The central zone is a reworking of the original solid apatite veins that formed during the Chewings Orogeny at ca. 1525 Ma. These original apatite veins are thought to derive from phosphate-rich magmatic–hydrothermal fluid exsolved from as-yet unrecognised alkaline magmatic bodies at depth. We define four ore breccia types (BX1–4) in the central zone on the basis of detailed petrological and geochemical analysis of drillcore and thin sections. BX1 ore comprises fluorapatite with minor crackle brecciation with carbonate infill and resembles ore of the north and south-east zones. Breccia types BX2, BX3, and BX4 represent progressive stages of ore brecciation and development of calc-silicate mineral (amphibole, epidote, allanite, calcite) infill. Comparison of bulk ore sample geochemistry between breccia types indicates that REEs were not mobilised more than a few centimetres during hydrothermal alteration and brecciation. Instead, most of the REEs were partitioned from the original REE fluorapatite into newly formed allanite, REE-poor fluorapatite and minor REE carbonate in the breccias. Negative europium (Eu) anomalies in the breccia minerals are accounted for by a large positive Eu anomaly in epidote from the alteration zones surrounding the ore breccias. This observation provides a direct link between ore recrystallisation and brecciation, and the formation of the alteration halo in the surrounding host rocks. Where allanite and fluorapatite are texturally related, the fluorapatite is relatively depleted in the light rare earth elements (LREEs), whereas allanite is relatively LREE enriched, suggesting co-crystallisation. We tentatively date the BX1 ore stage to 1440 ± 80 Ma based on U–Pb dating of thorianite. Sm–Nd isotope isochrons derived from in situ isotope analysis of cognate apatite and allanite date the BX2 and BX3 events to ca. 400 Ma, while U–Pb dating of late-stage monazite from the BX4 ore stage returned an age of ca. 350 Ma. Therefore, formation of the central zone at Nolans Bore involved multiple alteration/brecciation events that collectively span over 1 billion years in duration. We suggest that the BX1-type veins and breccias were formed from REE-rich, saline (F- and Cl-bearing) fluids that infiltrated the granulite-grade host rocks in association with either shear activation events of the Redbank Shear Zone (1500–1400 Ma) or intrusion of late-stage pegmatites of the Mt Boothby area. BX2, BX3, and BX4 events record deformation and hydrothermal alteration associated with the Alice Springs Orogeny (400–350 Ma). These hydrothermal events occurred at temperatures of 450 to ~600 °C, due to inflow of highly acidic hydrous fluids derived from a magmatic source, or from mixing of meteoric and metamorphic fluids. Our data testify to the long and complex geological history of not only the Nolans Bore REE deposit, but also of the rocks of the eastern Reynolds Range, and demonstrate the great utility of using hydrothermally derived REE minerals to trace the timing of crustal deformation events and source of associated hydrothermal fluids

Microscale data to macroscale processes: a review of microcharacterization applied to mineral systems

Microanalysis can provide rapid, quantitative characterization of mineral systems that complements the field- and core-scale observations traditionally made in ore deposits. We review recent innovations in microanalytical procedures and their application to studies of ore deposits. Case studies are presented examining how microanalysis can provide constraints on macroscopic processes within mineral systems. Synchrotron X-ray fluorescence shows centimetre-scale chemical variations associated with proximity to mineralization in samples from Sunrise Dam Gold Mine, Western Australia. Pseudomorphs of igneous plagioclase and chemically driven recrystallization interpreted from electron backscatter diffraction suggest that the system was dominated by fluid-driven brecciation with very little shearing. Both the fluid chemistry and fluid pressure evolved during a protracted sequence of vein formation and alteration accompanying gold mineralization. A second case study of sulphide mineralogy at the Mt Keith nickel sulphide deposit, Western Australia demonstrates how X-ray computed tomography combined with trace element mapping can constrain the chemistry and dynamics of magmatic systems. Large-scale interaction between silicate and sulphide melts, shown by homogenous palladium enrichment in pentlandite, leads to a large proportion of globular ores with a high nickel content. Increasing use of microanalysis in ore deposit geology is resulting in the constant reassessment of established models for ore genesis though a combination of micro- and macroscale datasets

Role of volatiles in intrusion emplacement and sulfide deposition in the supergiant Norilsk-Talnakh Ni-Cu-PGE ore deposits

International audienceThe Norilsk-Talnakh orebodies in Siberia are some of the largest examples on Earth of magmatic Ni−Cu−platinum group element (PGE) deposits, formed by segregation of immiscible sulfide melts from silicate magmas. They show distinctive features attributable to degassing of a magmatic vapor phase during ore formation, including: vesiculation of the host intrusions, widespread intrusion breccias, and extensive hydrofracturing, skarns, and metasomatic replacement in the country rocks. Much of the magmatic sulfide was generated by assimilation of anhydrite and carbonaceous material, leading to injection of a suspension of fine sulfide droplets attached to gas bubbles into propagating tube-like host sills ("chonoliths"). Catastrophic vapor phase exsolution associated with a drop in magma overpressure at the transition from vertical to horizontal magma flow enabled explosive propagation of chonoliths, rapid "harvesting" and gravity deposition of the characteristic coarse sulfide globules that form much of the ore, and extensive magmatic fluid interaction with country rocks

Microscale data to macroscale processes: a review of microcharacterization applied to mineral systems

Microanalysis can provide rapid, quantitative characterization of mineral systems that complements the field- and core-scale observations traditionally made in ore deposits. We review recent innovations in microanalytical procedures and their application to studies of ore deposits. Case studies are presented examining how microanalysis can provide constraints on macroscopic processes within mineral systems. Synchrotron X-ray fluorescence shows centimetre-scale chemical variations associated with proximity to mineralization in samples from Sunrise Dam Gold Mine, Western Australia. Pseudomorphs of igneous plagioclase and chemically driven recrystallization interpreted from electron backscatter diffraction suggest that the system was dominated by fluid-driven brecciation with very little shearing. Both the fluid chemistry and fluid pressure evolved during a protracted sequence of vein formation and alteration accompanying gold mineralization. A second case study of sulphide mineralogy at the Mt Keith nickel sulphide deposit, Western Australia demonstrates how X-ray computed tomography combined with trace element mapping can constrain the chemistry and dynamics of magmatic systems. Large-scale interaction between silicate and sulphide melts, shown by homogenous palladium enrichment in pentlandite, leads to a large proportion of globular ores with a high nickel content. Increasing use of microanalysis in ore deposit geology is resulting in the constant reassessment of established models for ore genesis though a combination of micro- and macroscale datasets

Imaging trace-element zoning in pyroxenes using synchrotron XRF mapping with the Maia detector array: benefit of low-incident energy

Trace-element zoning in igneous phenocrysts and cumulus phases is an informative record of magmatic evolution. The advent of microbeam X-ray fluorescence (XRF) mapping has allowed rapid chemical imaging of samples at thin section to decimeter scale, revealing such zoning patterns. Mapping with synchrotron radiation using multidetector arrays has proved especially effective, allowing entire thin sections to be imaged at micrometer-scale resolution in a matter of hours. The resolution of subtle minor element zoning, particularly in first-row transition metals, is greatly enhanced in synchrotron X-ray fluorescence microscopy (XFM) images by scanning with input beam energy below the FeK alpha line. In the examples shown here, from a phenocryst rich trachybasalt from Mt Etna (Italy) and from a Ni-Cu-PGE ore-bearing intrusion at Norilsk (Siberia), the zoning patterns revealed in this way record aspects of the crystallization history that are not readily evident from XFM images collected using higher incident energies and that cannot be obtained at comparable spatial resolutions by any other methods within reasonable scan times. This approach has considerable potential as a geochemical tool for investigating magmatic processes and is also likely to be applicable in a wide variety of other fields

Synthesis of large and homogeneous single crystals of water-bearing minerals by slow cooling at deep-mantle pressures

The presence of water in the Earth's deep mantle is an issue of increasing interest in the field of high-pressure mineralogy. An important task for further advancing research in the field is to create homogeneous single crystals of candidate deep-mantle water-bearing minerals of 1 mm or larger in size, which is required for applying them for the time-of-flight (TOF) single-crystal Laue diffraction method with a third-generation neutron instrument. In this study, we perform several experiments to demonstrate an improved methodology for growing hydrous crystals of such large sizes at relevant transition zone and lower-mantle conditions via very slow cooling over a maximum period of 1 day. Successfully synthesized crystals using this methodology include dense hydrous magnesium silicate (DHMS) phase E, hydrous wadsleyite, hydrous ringwoodite, and bridgmanite (silicate perovskite). It is also demonstrated that these hydrous crystals can be grown from deuterium enriched starting materials in addition to those having a natural hydrogen isotope ratio. Magnitudes of chemical and crystallographic heterogeneities of the product crystals were characterized by comprehensive analysis of X-ray precession photography, single-crystal X-ray diffraction (SCXRD), field-emission scanning electron microscope (FE-SEM), electron probe microanalyzer (EPMA), secondary ion mass spectroscopy (SIMS), powder X-ray diffraction (PXRD), and TOF neutron powder diffraction (TOF-NPD). The product crystals were confirmed to be inclusion free and crystallographically homogeneous. Compositional and isotopic differences of major elements and hydrogen isotope abundances were lower than 1 and 3%, respectively, among intracrystals and intercrystals within each recovered sample capsule. Phase E crystals up to 600 μm in the largest dimension were grown at a constant temperature of 1100 °C kept for 3 h. Using a lattice parameter-to-temperature relation of phase E, the thermal gradient in the sample capsules for the phase E synthesis has been evaluated to be 20 °C/mm. Hydrous wadsleyite crystals up to 1100 μm in the largest dimension were grown at 1390 °C with a temperature reduction of 70 °C during heating for 10 h. Hydrous ringwoodite crystals up to 1000 μm in the largest dimension were grown at around 1400 °C with a temperature reduction of 110 °C during heating for 12 h. Bridgmanite crystals up to 600 μm in the largest dimension were grown at 1700 °C with a temperature reduction of 30 °C during heating for 12 h. A TOF single-crystal diffraction instrument has been successfully used for analyzing one of the hydrous wadsleyite crystals, which demonstrated that single crystals appropriate for their expected usage are created using the method proposed in the present study