142 results found
Davey J, Roberts S, Wilkinson J, 2021, Copper- and cobalt-rich, ultrapotassic bittern brines responsible for the formation of the Nkana-Mindola deposits, Zambian Copperbelt, Geology (Boulder), Vol: 49, Pages: 341-345, ISSN: 0091-7613
The Central African Copperbelt (CACB) is Earth’s largest repository of sediment-hosted copper and cobalt. The criticality of these elements in battery technology and electricity transmission establishes them as fundamental components of the carbon-free energy revolution, yet the nature and origin of the hydrothermal fluids responsible for ore formation in the CACB remain controversial. Here, we present microthermometric, scanning electron microscopy and laser ablation–inductively coupled plasma–mass spectrometry analyses of fluid inclusions from the Nkana-Mindola deposits in Zambia. We find that base metal concentrations vary by one to two orders of magnitude between “barren” and “ore” fluids, with concomitant distinctions in major salt chemistry. Primary fluid inclusions, hosted by pre- to synkinematic mineralized quartz veins, are characterized by high homogenization temperatures (~200–300 °C) and salinities, with K/Na >0.8 and elevated metal concentrations (102 to 103 ppm Cu and Co). Conversely, barren, post-kinematic vein quartz contains lower homogenization temperature (~110–210 °C) and lower-salinity primary inclusions, characterized by K/Na <0.8 with low metal contents (<102 ppm Cu and Co). We propose a model in which high-temperature, sulfate-deficient, metalliferous, potassic residual brines, formed during advanced evaporation of CaCl2-rich, mid-Neoproterozoic seawater, were responsible for ore formation. During basin closure, lower-temperature, halite-undersaturated fluids interacted with evaporites and formed structurally controlled, sodic metasomatism. Reconciliation of these fluid chemistries and base metal concentrations with reported alteration assemblages from a majority of Zambian Copperbelt deposits suggests highly evolved, residual brines were critical to the formation of this unique metallogenic province.
Nathwani CL, Simmons AT, Large SJE, et al., 2021, From long-lived batholith construction to giant porphyry copper deposit formation: petrological and zircon chemical evolution of the Quellaveco District, Southern Peru, Contributions to Mineralogy and Petrology, Vol: 176, ISSN: 0010-7999
Porphyry Cu ore deposits are a rare product of arc magmatism that often form spatiotemporal clusters in magmatic arcs. The petrogenetic evolution of igneous rocks that cover the temporal window prior to and during porphyry Cu deposit formation may provide critical insights into magmatic processes that are key in generating these systems. This study documents the magmatic evolution of the Palaeocene–Eocene Yarabamba Batholith, Southern Peru, that was incrementally assembled between ~ 67 and ~ 59 Ma and hosts three, nearly contemporaneous, giant porphyry Cu–Mo deposits that formed at 57–54 Ma (Quellaveco, Toquepala and Cuajone). Whole-rock geochemistry, U–Pb geochronology and zircon trace element chemistry are reported from Yarabamba rocks that span the duration of plutonic activity, and from six porphyry intrusions at Quellaveco that bracket mineralisation. A change in whole-rock chemistry in Yarabamba intrusive rocks to high Sr/Y, high La/Yb and high Eu/Eu* is observed at ~ 60 Ma which is broadly coincident with a change in vector of the converging Nazca plate and the onset of regional compression and crustal thickening during the first stage of the Incaic orogeny. The geochemical changes are interpreted to reflect a deepening of the locus of lower crustal magma evolution in which amphibole ± garnet are stabilised as early and abundant fractionating phases and plagioclase is suppressed. Zircons in these rocks show a marked change towards higher Eu/Eu* (> 0.3) and lower Ti (< 9 ppm) compositions after ~ 60 Ma. Numerical modelling of melt Eu systematics and zircon-melt partitioning indicates that the time series of zircon Eu/Eu* in these rocks can be explained by a transition from shallower, plagioclase-dominated fractionation to high-pressure amphibole-dominated fractionation at deep crustal levels from ~ 60 Ma. Our modelling suggests
Hart-Madigan L, Wilkinson JJ, Lasalle S, et al., 2020, U-Pb DATING OF HYDROTHERMAL TITANITE RESOLVES MULTIPLE PHASES OF PROPYLITIC ALTERATION IN THE OYU TOLGOI PORPHYRY DISTRICT, MONGOLIA, ECONOMIC GEOLOGY, Vol: 115, Pages: 1605-1618, ISSN: 0361-0128
Cooper GF, Macpherson CG, Blundy JD, et al., 2020, Variable water input controls evolution of the Lesser Antilles volcanic arc (vol 87, pg 931, 2020), Nature, Vol: 584, Pages: E36-E36, ISSN: 0028-0836
Loader MA, Wilkinson JJ, Armstrong RN, 2020, The effect of titanite crystallisation on Eu and Ce anomalies in zircon and its implications for the assessment of porphyry Cu deposit fertility, 14th SGA Biennial Meeting on Mineral Resources to Discover, Publisher: SGA, Pages: 1091-1094
The redox sensitivity of Ce and Eu anomalies in zircon has been well demonstrated by experimental studies and these parameters may represent important tools in the exploration of porphyry Cu systems which are thought to be derived from oxidised magmas. However, Ce and Eu anomalies in zircon are also affected by the co-crystallisation of REE-bearing phases such as titanite. Here, we report the trace element chemistry of zircons from titanite-bearing intrusions associated with mineralisation at the world class Oyu Tolgoi porphyry Cu-Au deposit (Mongolia). Based on these data, we suggest that neither Eu/Eu*, nor Ce4+/Ce3+ are robust proxies for melt redox conditions, because they are both too strongly dependent on melt REE concentrations, which are usually poorly constrained and controlled by the crystallisation of titanite and other REE-bearing phases. In spite of this, Eu/Eu* can broadly distinguish between fertile and barren systems, so may still be a useful indicator of porphyry magma fertility.
Cooper GF, Macpherson CG, Blundy JD, et al., 2020, Variable water input controls evolution of the Lesser Antilles volcanic arc, Nature, Vol: 582, Pages: 525-529, ISSN: 0028-0836
Oceanic lithosphere carries volatiles, notably water, into the mantle through subduction at convergent plate boundaries. This subducted water exercises control on the production of magma, earthquakes, formation of continental crust and mineral resources. Identifying different potential fluid sources (sediments, crust and mantle lithosphere) and tracing fluids from their release to the surface has proved challenging1. Atlantic subduction zones are a valuable endmember when studying this deep water cycle because hydration in Atlantic lithosphere, produced by slow spreading, is expected to be highly non-uniform2. Here, as part of a multi-disciplinary project in the Lesser Antilles volcanic arc3, we studied boron trace element and isotopic fingerprints of melt inclusions. These reveal that serpentine—that is, hydrated mantle rather than crust or sediments—is a dominant supplier of subducted water to the central arc. This serpentine is most likely to reside in a set of major fracture zones subducted beneath the central arc over approximately the past ten million years. The current dehydration of these fracture zones coincides with the current locations of the highest rates of earthquakes and prominent low shear velocities, whereas the preceding history of dehydration is consistent with the locations of higher volcanic productivity and thicker arc crust. These combined geochemical and geophysical data indicate that the structure and hydration of the subducted plate are directly connected to the evolution of the arc and its associated seismic and volcanic hazards.
Davy R, Collier JS, Henstock TJ, et al., 2020, Wide‐angle seismic imaging of two modes of crustal accretion in mature Atlantic Ocean crust, Journal of Geophysical Research: Solid Earth, Vol: 125, Pages: 1-21, ISSN: 2169-9313
We present a high‐resolution 2‐D P‐wave velocity model from a 225 km long active seismic profile, collected over ~60‐75 Ma central Atlantic crust. The profile crosses five ridge segments separated by a transform and three non‐transform offsets. All ridge discontinuities share similar primary characteristics, independent of the offset. We identify two types of crustal segment. The first displays a classic two‐layer velocity structure with a high gradient layer 2 (~0.9 s‐1) above a lower gradient layer 3 (0.2 s‐1). Here PmP coincides with the 7.5 km s‐1 contour and velocity increases to >7.8 km s‐1 within 1 km below. We interpret these segments as magmatically‐robust, with PmP representing a petrological boundary between crust and mantle. The second has a reduced contrast in velocity gradient between upper and lower crust, and PmP shallower than the 7.5 km s‐1 contour. We interpret these segments as tectonically dominated, with PmP representing a serpentinized (alteration) front. Whilst velocity depth profiles fit within previous envelopes for slow‐spreading crust, our results suggest that such generalizations give a misleading impression of uniformity. We estimate that the two crustal styles are present in equal proportions on the floor of the Atlantic. Within two tectonically dominated segments we make the first wide‐angle seismic identifications of buried oceanic core complexes in mature (> 20 Ma) Atlantic Ocean crust. They have a ~20 km wide “domal” morphology with shallow basement and increased upper‐crustal velocities. We interpret their mid‐crustal seismic velocity inversions as alteration and rock‐type assemblage contrasts across crustal‐scale detachment faults.
Baker MJ, Wilkinson JJ, Wilkinson CC, et al., 2020, Epidote trace element chemistry as an exploration tool in the collahuasi district, Northern Chile, Economic Geology, Vol: 115, Pages: 749-770, ISSN: 0013-0109
The Collahuasi district of northern Chile hosts several late Eocene-early Oligocene world-class porphyry Cu-Mo deposits, including Rosario, Ujina, and Quebrada Blanca deposits, and associated high-sulfidation epithermal mineralization at La Grande. Mineralization is hosted by intermediate to felsic intrusive and volcanic rocks of the upper Paleozoic to Lower Triassic Collahuasi Group, which experienced lower greenschist facies regional metamorphism prior to mineralization. Extensive hydrothermal alteration zones surround the porphyry and epithermal deposits, associated with hypogene ore-forming processes. However, outside of the observed sulfide halo the limits of geochemical anomalism associated with mineralization are difficult to define due to mineralogical similarities between weak, distal propylitic alteration and regional metamorphism affecting the host rocks.Recent advancements in laser ablation-inductively coupled plasma-mass spectrometry analysis of epidote from hydrothermal alteration zones around porphyry and skarn deposits have shown that low-level hypogene geochemical anomalies can be detected at distances farther from the center of mineralization than by conventional rock chip sampling. Selective analysis of propylitic epidote from the Collahuasi district indicates that anomalous concentrations of distal pathfinder elements in epidote, including As (>50 ppm), Sb (>25 ppm), Pb (>100 ppm), and Mn (>5,000 ppm), were detectable 1.5 to 4.0 km from deposit centers. Significantly, the concentrations of these trace elements in epidote were obtained from samples that contained whole-rock concentrations of <25 ppm As, <2 ppm Sb, <100 ppm Pb, and <5,000 ppm Mn. Systematic increases in Cu, Mo, and Sn concentrations in epidote near deposit centers, and corresponding decreasing As, Sb and Pb concentrations, also provide effective tools for assessing the fertility and locating the centers of porphyry mineralization. In addition, anomalous conc
Pacey, Adam, Wilkinson J, et al., 2020, Magmatic fluids implicated in the formation of propylitic alteration: oxygen, hydrogen and strontium isotopic constraints from the Northparkes Porphyry Cu-Au District, NSW, Australia, Economic Geology and the Bulletin of the Society of Economic Geologists, Vol: 115, Pages: 729-748, ISSN: 0361-0128
In porphyry ore deposit models, the propylitic facies is widely interpreted to be caused by convective 16circulation ofmeteoric waters. However, recent field-‐based and geochemical data suggest that magmatic-‐derived fluids are likely to contribute to development of the propylitic assemblage. In order to test this hypothesis, we etermined the oxygenand hydrogen isotope compositions of propylitic mineral separates 19(epidote, chlorite and quartz), selected potassic mineral separates (quartz and magnetite) and quartz-‐hosted fluid inclusionsfrom around the E48 and E26 deposits in the Northparkes porphyry Cu-‐Au district, NSW,Australia. In addition, the strontiumisotope composition of epidote was determined to test for the potential contribution of seawater in the Northparkes system given the postulated island arc setting and submarine character of some country rocks.
Pacey A, Wilkinson JJ, Cooke DR, 2020, Chlorite and epidote mineral chemistry in porphyry ore systems: a case study of the Northparkes district, NSW, Australia, Economic Geology, Vol: 115, Pages: 701-727, ISSN: 0361-0128
Propylitic alteration, characterized by the occurrence of chlorite and epidote, is typically the most extensive and peripheral alteration facies developed around porphyry ore deposits. However, exploration within this alteration domain is particularly challenging, commonly owing to weak or nonexistent whole-rock geochemical gradients and the fact that similar assemblages can be developed in other geologic settings, particularly during low-grade metamorphism. We document and interpret systematic spatial trends in the chemistry of chlorite and epidote from propylitic alteration around the E48 and E26 porphyry Cu-Au deposits of the Northparkes district, New South Wales, Australia. These trends vary as a function of both distance from hydrothermal centers and alteration paragenesis.The spatial trends identified in porphyry-related chlorite and epidote at Northparkes include (1) a deposit-proximal increase in Ti, As, Sb, and V in epidote and Ti in chlorite, (2) a deposit-distal increase in Co and Li in chlorite and Ba in epidote, and (3) a pronounced halo around deposits in which Mn and Zn in chlorite, as well as Mn, Zn, Pb, and Mg in epidote, are elevated. Chlorite Al/Si ratios and epidote Al/Fe ratios may show behavior similar to that of Mn-Zn or may simply decrease outward, and V and Ni concentrations in chlorite are lowest in the peak Mn-Zn zone. In comparison to porphyry-related samples, chlorite from the regional metamorphic assemblage in the district contains far higher concentrations of Li, Ca, Ba, Pb, and Cu but much less Ti. Similarly, metamorphic epidote contains higher concentrations of Sr, Pb, As, and Sb but less Bi and Ti.These chlorite and epidote compositional trends are the net result of fluid-mineral partitioning under variable physicochemical conditions within a porphyry magmatic-hydrothermal system. They are most easily explained by the contribution of spent magmatic-derived ore fluid(s) into the propylitic domain. It is envisaged that such fluids exp
Wilkinson JJ, Baker MJ, Cooke DR, et al., 2020, Exploration targeting in porphyry Cu systems using propylitic mineral chemistry: a case study of the El Teniente deposit, Chile, Economic Geology, Vol: 115, Pages: 771-791, ISSN: 0013-0109
The mineral chemistry of epidote and chlorite from the propylitic halo at El Teniente, in samples collected at distances up to 6.6 km from the deposit center, was determined by microprobe and laser ablation-inductively coupled plasma-mass spectrometry. Results show that both minerals systematically incorporated a range of trace elements that define a much larger footprint to the system than is easily recognized using conventional means such as whole-rock geochemistry. Apart from Fe and Mg in chlorite, there is no significant control of mineral chemistry by bulk-rock composition. For chlorite, geothermometry temperatures and Ti and V concentrations are high proximal, whereas Li, As, Co, Sr, Ca, and Y are low proximal and elevated in distal positions. Ratios of these elements define gradients toward ore varying over three to five orders of magnitude. The proximal-high Ti content is thought to reflect crystallization temperature, whereas proximal-low signatures are believed to characterize elements that are relatively fluid mobile in the inner parts of the propylitic halo in the presence of mildly alkaline to mildly acidic and oxidized fluids so that they are not incorporated into crystallizing chlorite, despite being generally compatible within the mineral structure. These elements begin to substitute into chlorite in the distal parts of the propylitic halo where fluids are largely rock buffered in terms of major element chemistry. In epidote, As defines a broad proximal low and is generally elevated at distances of at least 3 km from the edge of the ore shell. Zinc, La, Yb, Y, and Zr in epidote, among others, appear to define a geochemical shoulder that surrounds the deposit. These patterns are broadly similar to those observed in previous work at Batu Hijau and in the Baguio district, suggesting that these minerals behave consistently in porphyry systems and can therefore provide useful exploration tools within propylitic green rocks.
Cooke DR, Wilkinson JJ, Baker M, et al., 2020, Using mineral chemistry to aid exploration: a case study from the resolution porphyry Cu-Mo deposit, Arizona, Economic Geology, Vol: 115, Pages: 813-840, ISSN: 0013-0109
The giant, high-grade Resolution porphyry Cu-Mo deposit in the Superior district of Arizona is hosted in Proterozoic and Paleozoic basement and in an overlying Cretaceous volcaniclastic breccia and sandstone package. Resolution has a central domain of potassic alteration that extends more than 1 km outboard of the ore zone, overlapping with a propylitic halo characterized by epidote, chlorite, and pyrite that is particularly well developed in the Laramide volcaniclastic rocks and Proterozoic dolerite sills. The potassic and propylitic assemblages were overprinted in the upper parts of the deposit by intense phyllic and advanced argillic alteration. The district was disrupted by Tertiary Basin and Range extension, and the fault block containing Resolution and its Cretaceous host succession was buried under thick mid-Miocene dacitic volcanic cover, obscuring the geologic, geophysical, and geochemical footprint of the deposit.To test the potential of propylitic mineral chemistry analyses to aid in the detection of concealed porphyry deposits, a blind test was conducted using a suite of epidote-chlorite ± pyrite-altered Laramide volcaniclastic rocks and Proterozoic dolerites collected from the propylitic halo, with samples taken from two domains located to the north and south and above the Resolution ore zone. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data of epidote provided indications of deposit fertility and proximity. Competition for chalcophile elements (As, Sb, Pb) between coexisting pyrite and epidote grains led to a subdued As-Sb fertility response in epidote, consistent with epidote collected between 0.7 and 1.5 km from the center of a large porphyry deposit. Temperature-sensitive trace elements in chlorite provided coherent spatial zonation patterns, implying a heat source centered at depth between the two sample clusters, and application of chlorite proximitor calculations based on LA-ICP-MS analyses provided a precisely
Sievwright RH, O'Neill HSC, Tolley J, et al., 2020, Diffusion and partition coefficients of minor and trace elements in magnetite as a function of oxygen fugacity at 1150 oC, Contributions to Mineralogy and Petrology, Vol: 175, Pages: 1-21, ISSN: 0010-7999
Lattice diffusion coefficients and partition coefficients have been determined for Li, Mg, Al, Sc, Ti, Cr, V, Mn, Co, Ni, Cu, Zn, Ga, Y, Zr, Nb, Mo, In, Lu, Hf, Ta and U in single crystals of natural magnetite as a function of oxygen fugacity (fO2) at 1150 °C and 1 bar by equilibration with a synthetic silicate melt reservoir. Most experiments were run for twelve hours, which was sufficient to generate diffusion profiles from 25 to > 1000 µm in length. The results were checked at one condition with two additional experiments at 66.9 and 161 h. The profiles were analysed using scanning laser-ablation inductively-coupled-plasma mass-spectrometry. Diffusion coefficients (D) were calculated by fitting data from individual element diffusion profiles to the conventional diffusion equation for one-dimensional diffusion into a semi−infinite slab with constant composition maintained in the melt at the interface. Equilibrium magnetite/melt partition coefficients are given by the ratio of the interface concentrations to those in the melt. Plots of log D as a function of log fO2 produce V-shaped trends for all the investigated elements, representing two different mechanisms of diffusion that depend on (fO2)−2/3 and (fO2)2/3. Diffusion coefficients at a given fO2 generally increase in the order: Cr < Mo ≈ Ta < V < Ti < Al < Hf ≈ Nb < Sc ≈ Zr ≈ Ga < In < Lu ≈ Y < Ni < U ≈ Zn < Mn ≈ Mg < Co < Li < Cu. Thus, Cu contents of magnetites are most susceptible to diffusive reequilibration, whereas the original content of Cr should be best preserved.
Cooke DR, Agnew P, Hollings P, et al., 2020, Recent advances in the application of mineral chemistry to exploration for porphyry copper–gold–molybdenum deposits: detecting the geochemical fingerprints and footprints of hypogene mineralization and alteration, Geochemistry: Exploration, Environment, Analysis, Vol: 20, Pages: 176-188, ISSN: 1467-7873
In the past decade, significant research efforts have been devoted to mineral chemistrystudies to assist porphyry exploration. These activities can be divided into two majorfields of research: (1) porphyry indicator minerals (PIMs), which are used to identify thepresence of, or potential for, porphyry-style mineralization based on the chemistry ofmagmatic minerals such as zircon, plagioclase and apatite, or resistate hydrothermalminerals such as magnetite; and (2) porphyry vectoring and fertility tools (PVFTs),which use the chemical compositions of hydrothermal minerals such as epidote,chlorite and alunite to predict the likely direction and distance to mineralized centers,and the potential metal endowment of a mineral district. This new generation ofexploration tools has been enabled by advances in and increased access to laserablation-inductively coupled plasma mass spectrometry (LA-ICP-MS), short wavelength infrared (SWIR), visible near-infrared (VNIR) and hyperspectral technologies.PIMs and PVFTs show considerable promise for exploration and are starting to beapplied to the diversity of environments that host porphyry and epithermal depositsglobally. Industry has consistently supported development of these tools, in the case ofPVFTs encouraged by several successful blind tests where deposit centers havesuccessfully been predicted from distal propylitic settings. Industry adoption is steadilyincreasing but is restrained by a lack of the necessary analytical equipment andexpertise in commercial laboratories, and also by the on-going reliance on well-established geochemical exploration techniques (e.g., sediment, soil and rock-chipsampling) that have aided the discovery of near-surface resources over many decades, are now proving less effective in the search for deeply buried mineral resources, and for those concealed under cover.
Hnatyshin D, Creaser RA, Meffre S, et al., 2020, Understanding the microscale spatial distribution and mineralogical residency of Re in pyrite: Examples from carbonate-hosted Zn-Pb ores and implications for pyrite Re-Os geochronology, Chemical Geology, Vol: 533, Pages: 1-21, ISSN: 0009-2541
Accurate and precise geochronology using the Re-Os isotopic system in pyrite is an invaluable tool for developing and confirming genetic models of ore systems. However, as a bulk method, the results produced by pyrite Re-Os geochronology are commonly complex, and many imprecise isochrons exist in the literature. Using LA-ICPMS methods it is now possible to map and quantify Re distribution at the ppb level, allowing an unprecedented look into the Re-Os systematics of pyrite-bearing ore.Two samples from the Lisheen Zn-Pb ore deposit in Ireland showing disparate Re-Os isotopic behavior were investigated. In-situ sulfur isotope measurements using SIMS, an analytical technique not previously attempted on the Irish deposits, was used to supplement the Re-Os dataset. A massive pyrite sample from the Main Zone produced a precise, low-scatter isochron (346.6 ± 3.0 Ma, MSWD = 1.6). The Re distribution in this sample is relatively homogeneous, with the Re budget dominated by pyrite containing 1–5 ppb Re, but the δ34S varies significantly from −45.2‰ to 8.2‰. A second, more paragenetically complex, sample from the Derryville Zone produced a younger age with high scatter (322 ± 11 Ma, MSWD = 206) and this also displays a large variation in δ34S (−53‰ to +4‰). The cores of grains of main-stage iron sulfide are depleted in trace elements and show low Re abundances (<10 ppb) but have been altered in an irregular fashion leading to Re-enriched domains that exceed 100 ppb. Additionally, micron-scale molybdenite crystals, found in close association with altered sulfides, contain Re at levels that locally exceed 10 ppm. The highly scattered (MSWD = 206) and younger age (322 Ma), produced by the Derryville Zone sample are interpreted to result from mixing of different generations of sulfide, potentially involving fluids associated with Variscan deformation (<310 Ma). Therefore, the Re-Os data produced from the
Nathwani C, Loader M, Wilkinson J, et al., 2020, Multi-stage arc magma evolution recorded by apatite in volcanic rocks, Geology (Boulder), Vol: 48, Pages: 323-327, ISSN: 0091-7613
Protracted magma storage in the deep crust is a key stage in the formation of evolved, hydrous arc magmas that can result in explosive volcanism and the formation of economically valuable magmatic13 hydrothermal ore deposits. High magmatic water content in the deep crust results in extensive amphibole ± garnet fractionation and the suppression of plagioclase crystallization as recorded by elevated Sr/Y ratios and high Eu (high Eu/Eu*) in the melt. Here, we use a novel approach to track the petrogenesis of arc magmas using apatite trace element chemistry in volcanic formations from the Cenozoic arc of Central Chile. These rocks formed in a magmatic cycle that culminated in high Sr/Y magmatism and porphyry ore deposit formation in the Miocene. We use Sr/Y, Eu/Eu* and Mg in apatite to track discrete stages of arc magma evolution. We apply fractional crystallization modeling to show that early crystallizing apatite inherits a high Sr/Y and Eu/Eu* melt chemistry signature that is predetermined by amphibole-dominated fractional crystallization in the lower crust. Our modeling shows that crystallization of the in-situ host rock mineral assemblage in the shallow crust causes competition for trace elements in the melt that leads to apatite compositions diverging from bulk magma chemistry. Understanding this decoupling behavior is important for the use of apatite as an indicator of metallogenic fertility in arcs and for interpretation of provenance in detrital studies.
Bie L, Rietbrock A, Hicks S, et al., 2020, Along‐arc heterogeneity in local seismicity across the lesser antilles subduction zone from a dense ocean‐bottom seismometer network, Seismological Research Letters, Vol: 91, Pages: 237-247, ISSN: 0895-0695
The Lesser Antilles arc is only one of two subduction zones where slow‐spreading Atlantic lithosphere is consumed. Slow‐spreading may result in the Atlantic lithosphere being more pervasively and heterogeneously hydrated than fast‐spreading Pacific lithosphere, thus affecting the flux of fluids into the deep mantle. Understanding the distribution of seismicity can help unravel the effect of fluids on geodynamic and seismogenic processes. However, a detailed view of local seismicity across the whole Lesser Antilles subduction zone is lacking. Using a temporary ocean‐bottom seismic network we invert for hypocenters and 1D velocity model. A systematic search yields a 27 km thick crust, reflecting average arc and back‐arc structures. We find abundant intraslab seismicity beneath Martinique and Dominica, which may relate to the subducted Marathon and/or Mercurius Fracture Zones. Pervasive seismicity in the cold mantle wedge corner and thrust seismicity deep on the subducting plate interface suggest an unusually wide megathrust seismogenic zone reaching ∼65km∼65 km depth. Our results provide an excellent framework for future understanding of regional seismic hazard in eastern Caribbean and the volatile cycling beneath the Lesser Antilles arc.
Estimating the location of geologic and tectonic features on a subducting plate is important for interpreting their spatial relationships with other observables including seismicity, seismic velocity and attenuation anomalies, and the location of ore deposits and arc volcanism in the over-riding plate. Here we present two methods for estimating the location of predictable features such as seamounts, ridges and fracture zones on the slab. One uses kinematic reconstructions of plate motions, and the other uses multidimensional scaling to flatten the slab onto the surface of the Earth. We demonstrate the methods using synthetic examples and also using the test case of fracture zones entering the Lesser Antilles subduction zone. The two methods produce results that are in good agreement with each other in both the synthetic and real examples. In the Lesser Antilles, the subducted fracture zones trend northwards of the surface projections. The two methods begin to diverge in regions where the multidimensional scaling method has its greatest likely error. Wider application of these methods may help to establish spatial correlations globally.
Allen R, Collier J, Stewart A, et al., 2019, The role of arc migration in the development of the Lesser Antilles: a new tectonic model for the Cenozoic evolution of the eastern Caribbean, Geology, Vol: 47, Pages: 891-895, ISSN: 0091-7613
Continental arc systems often show evidence of large-scale migration both towards and away from the incoming plate. In oceanic arc systems however, whilst slab roll-back and the associated processes of back-arc spreading and arc migration towards the incoming plate are commonplace, arc migration away from the incoming plate is rarely observed. We present a new compilation of marine magnetic anomaly and seismic data in order to propose a new tectonic model for the eastern Caribbean region that includes arc migration in both directions. We synthesise new evidence to show two phases of back-arc spreading and eastward arc migration towards the incoming Atlantic. A third and final phase of arc migration to the west subdivided the earlier back-arc basin on either side of the present-day Lesser Antilles Arc. This is the first example of regional multi-directional arc migration in an intra-oceanic setting and has implications for along-arc structural and geochemical variations. The back and forth arc migrations are probably due to the constraints the neighbouring American plates impose on this isolated subduction system rather than variations in subducting slab buoyancy.
Pacey A, Wilkinson JJ, Owens J, et al., 2019, The anatomy of an alkalic Porphyry Cu-Au system: geology and alteration at Northparkes Mines, New South Wales, Australia, Economic Geology, Vol: 114, Pages: 441-472, ISSN: 0013-0109
The Late Ordovician-early Silurian (~455–435 Ma) Northparkes system is a group of silica-saturated, alkalic porphyry deposits and prospects that developed within the Macquarie island arc. The system is host to a spectacular and diverse range of rocks and alteration-mineralization textures that facilitate a detailed understanding of its evolution, in particular the nature and controls of porphyry-related propylitic alteration.The first intrusive phase at Northparkes is a pre- to early-mineralization pluton that underlies all the deposits and varies in composition from a biotite quartz monzonite to alkali feldspar granite. Prior to total crystallization, this pluton was intruded by a more primitive quartz monzonite that marks the onset of a fertile fractionation series. Toward its upper levels, the quartz monzonite is porphyritic and locally rich in Cu sulfides. Subsequently, a complex series of synmineralization quartz monzonite porphyries was emplaced. The quartz monzonite porphyry intrusions have a distinct pipe-like morphology and are ubiquitously K-feldspar–altered with a crystal-crowded porphyritic texture. The textures of the quartz monzonite porphyries and common occurrence of porphyry-cemented contact breccias indicate they were forcibly emplaced and of relatively low viscosity. The quartz monzonite porphyries are therefore interpreted as crystal-bearing, silicate melt-aqueous fluid slurries that represent the conduits through which deep-seated magmatic-derived ore fluid was discharged into the shallow crust (1–2 km depth).Each deposit is centered on a multiphase cluster of quartz monzonite porphyry intrusions that drove discrete hydrothermal systems. Initial fluid evolution was similar in all the deposits, with three major alteration facies developed as largely concentric zones around the quartz monzonite porphyry complexes. The innermost zone is host to Cu sulfide ore and dominated by K-feldspar alteration. This transitions outward through
Goes S, Collier J, Blundy J, et al., 2019, Project VoiLA: Volatile Recycling in the Lesser Antilles, Eos, Vol: 100, ISSN: 2324-9250
Deep water cycle studies have largely focused on subduction of lithosphere formed at fast spreading ridges. However, oceanic plates are more likely to become hydrated as spreading rate decreases.
Matthewst T, Wilkinson J, Loader M, 2019, Titanite petrology and chemistry from the Strontian Igneous Complex, Scotland, 15th SGA Biennial Meeting on Life with Ore Deposits on Earth, Publisher: SOC GEOLOGY APPLIED MINERAL DEPOSITS-SGA, Pages: 991-994
Hart L, Wilkinson JJ, Armstrong R, 2019, Titanite geochronology and chlorite chemistry resolve multiple phases of propylitic alteration in the Oyu Tolgoi district, Mongolia, 15th SGA Biennial Meeting on Life with Ore Deposits on Earth, Publisher: SOC GEOLOGY APPLIED MINERAL DEPOSITS-SGA, Pages: 1317-1320
Kocher S, Wilkinson JJ, Armstrong RN, et al., 2019, Trace element chemistry, polytypes, isotopic composition and Re-Os dates of molybdenite from the Bingham Canyon Cu-Au-Mo porphyry deposit, Utah, 15th SGA Biennial Meeting on Life with Ore Deposits on Earth, Publisher: SOC GEOLOGY APPLIED MINERAL DEPOSITS-SGA, Pages: 1120-1123
Brugge E, Wilkinson JJ, Buret Y, et al., 2019, Zircon-hosted apatite inclusions at La Granja Cu-Mo Porphyry, Peru: Implications for the use of apatite as a probe of magma petrogenesis, 15th SGA Biennial Meeting on Life with Ore Deposits on Earth, Publisher: SOC GEOLOGY APPLIED MINERAL DEPOSITS-SGA, Pages: 983-986
Davey J, Roberts S, Wilkinson JJ, 2019, Anomalous metal enrichment of basin brines in the Zambian Copperbelt, 15th SGA Biennial Meeting on Life with Ore Deposits on Earth, Publisher: SOC GEOLOGY APPLIED MINERAL DEPOSITS-SGA, Pages: 299-302
Fitzpayne A, Prytulak J, Wilkinson J, et al., 2018, Assessing thallium elemental systematics and isotope ratio variations in porphyry ore systems: A case study of the Bingham Canyon district, Minerals, Vol: 8, ISSN: 2075-163X
The Bingham Canyon porphyry deposit is one of the world’s largest Cu-Mo-Au resources. Elevated concentrations of thallium (Tl) compared to average continental crust have been found in some brecciated and igneous samples in this area, which likely result from mobilization of Tl by relatively low temperature hydrothermal fluids. The Tl-enrichment at Bingham Canyon therefore provides an opportunity to investigate if Tl isotope ratios reflect hydrothermal enrichment and whether there are systematic Tl isotope fractionations that could provide an exploration tool. We present a reconnaissance study of nineteen samples spanning a range of lithologies from the Bingham district which were analysed for their Tl content and Tl isotope ratios, reported as parts per ten thousand (ε205Tl) relative to the NIST SRM997 international standard. The range of ε205Tl reported in this study (−16.4 to +7.2) is the largest observed in a hydrothermal ore deposit to date. Unbrecciated samples collected relatively proximal to the Bingham Canyon porphyry system have ε205Tl of −4.2 to +0.9, similar to observations in a previous study of porphyry deposits. This relatively narrow range suggests that high-temperature (>300 °C) hydrothermal alteration does not result in significant Tl isotope fractionation. However, two samples ~3–4 km away from Bingham Canyon have higher ε205Tl values (+1.3 and +7.2), and samples from more distal (~7 km) disseminated gold deposits at Melco and Barneys Canyon display an even wider range in ε205Tl (−16.4 to +6.0). The observation of large positive and negative excursions in ε205Tl relative to the mantle value (ε205Tl = −2.0 ± 1.0) contrasts with previous investigations of hydrothermal systems. Samples displaying the most extreme positive and negative ε205Tl values also contain elevated concentrations of Tl-Sb-As. Furthermore, with the exception of one
Mondillo N, Wilkinson JJ, Boni M, et al., 2018, A global assessment of Zn isotope fractionation in secondary Zn minerals from sulfide and non-sulfide ore deposits and model for fractionation control, Chemical Geology, Vol: 500, Pages: 182-193, ISSN: 0009-2541
We investigated extent and direction of Zn isotope fractionation in secondary zinc minerals formed during low temperature hydrothermal and/or supergene oxidation of primary sulfide deposits. Zinc isotope data have been obtained from non-sulfide zinc mineral separates (willemite - Zn2SiO4, smithsonite - ZnCO3, hemimorphite - Zn4(Si2O7)(OH)2·H2O, hydrozincite - Zn5(CO3)2(OH)6, and sauconite - Na0.3Zn3(Si,Al)4O10(OH)2·4H2O) collected from several Zn deposits in Ireland, Belgium, Poland, Namibia, Peru, Yemen and Zambia. The data are compared with Zn isotope compositions measured on Zn sulfides collected in the same areas and/or derived from the existing literature, to establish the controls of direction and likely extent of any fractionations. We find that willemite has the greatest compositional variability, with measured δ66ZnJCM-Lyonvalues ranging from −0.42 to 1.39‰, spanning the entire range of terrestrial variation in Zn isotopes recorded to date. Overall, significant fractionations in positive and negative directions are recorded relative to the precursor phase (primary sphalerite or an earlier secondary phase), with primary sphalerite falling in a relatively narrow range of isotopic values (approximately −0.1 to +0.4‰). Most of the data observed on willemite, hemimorphite and hydrozincite can be explained with a model of isotopic fractionation, in which partial dissolution of primary sphalerite is followed by precipitation of an initial secondary phase that preferentially incorporates heavy Zn isotopes. Smithsonite, instead, preferentially incorporates light Zn isotopes. This reflects the variation in the Zn-x bond strengths of these secondary phases with respect to the original sulfides. We also observed that isotope compositions do not depend only on the difference between the fractionation factors of the involved phases but also on the amount of the secondary mineral precipitated after dissolution of primary sulfi
Brugge E, Wilkinson J, Miles A, 2017, Habit and Chemistry of Apatite at Chuquicamata, Chile, 14th Biennial SGA Meeting: Mineral Resources to Discover, Publisher: SGA
Apatite is well suited to the study of porphyry systems. It occurs in both the magmatic and hydrothermal domains and the large range in possible trace element substitutions can reveal crystallisation conditions that reflect key events during porphyry formation. This project aims to use the trace element chemistry of apatite as a probe of porphyry development. A global study of apatite composition will aim to identify fingerprints which may be indicative of a fertile porphyry environment. Initial results from the Chuquicamata Cu-Mo porphyry, Chile reveal limited compositional differences in major and trace element compositions (Ca, P, Cl, Na, Mn, S, Se, Fe, Mg, Al, Si, La, Ce, Nd) between apatites of different habits that are interpreted to be magmatic in origin. However, vein-hosted and hydrothermally-altered apatite are compositionally distinct, probably controlled by strong fractionation of elements during separation of magmatic-hydrothermal fluids. This suggests potential for the future study of potassic alteration in porphyry systems using apatite.
Wilkinson JJ, Cooke D, Baker M, et al., 2017, Porphyry indicator minerals and their mineral chemistry as vectoring and fertility tools, Application of indicator mineral methods to bedrock andsediments, Editors: McClenaghan, Layton-Matthews, Ottawa, Publisher: Geological Survey of Canada, Pages: 67-77
Intrusion-centred mineral districts host a diversity of ore deposits of variable metal associations, alteration assemblages and genesis. Porphyry systems represent particularly important exploration targets but the prioritization of conventional geochemical or geophysical anomalies that might represent a deposit, particularly when systems are buried under cover, is extremely difficult. Three key questions arise: (1) is the alteration (particularly when only a propylitic type is observed) related to a porphyry system? (2) how can the fertility of a system be assessed at an early stage of exploration in order to reduce exploration risk? and (3) how can the centre of the system (in 3 dimensions) be predicted ahead of extensive, potentially deep, drilling? These fertility and vectoring challenges have been the subject of recent work, primarily based on mineral chemistry, in a series of AMIRA projects based out of the University of Tasmania, now also being continued at the Natural History Museum in London.The approach to assessing the presence of a possible porphyry system has been to establish mineral chemical criteria that discriminate between porphyry and non-porphyry environments based on: (1) the composition of igneous minerals (e.g. plagioclase, zircon, apatite, magnetite); and (2) the composition of hydrothermal alteration phases, particularly those developed in the propylitic domain (epidote, chlorite, magnetite, calcite, quartz). Many of these phases may be reworked via erosion into paleo or modern sediment transport systems and are thus available for assessment of catchment area fertility. Some of the characteristics of these minerals may allow the distinction between extensively mineralized and ostensibly barren environments (the system “fertility”); clearly these features are of significant exploration utility.The vectoring challenge has been addressed by the completion of numerous orientation studies on known porphyry systems to establish any sys
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