Publications
157 results found
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
Wilkinson JJ, Pacey A, Hart-Madigan LA, et al., 2019, A new paradigm for the origin of propylitic alteration in porphyry ore systems, Applied Earth Science, Vol: 128, Pages: 64-65, ISSN: 2572-6838
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.
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
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
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
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
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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
Wilkinson JJ, 2018, Epigenesis, ENCYCLOPEDIA OF GEOCHEMISTRY: A COMPREHENSIVE REFERENCE SOURCE ON THE CHEMISTRY OF THE EARTH, Editors: White, Publisher: SPRINGER INTERNATIONAL PUBLISHING AG, Pages: 453-455, ISBN: 978-3-319-39311-7
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
Neal LC, Wilkinson JJ, Mason PJ, et al., 2017, Spectral characteristics of propylitic alteration minerals as a vectoring tool for porphyry copper deposits, Journal of Geochemical Exploration, Vol: 184, Pages: 179-198, ISSN: 0375-6742
Short-wave infrared (SWIR) reflectance spectroscopy is a quick and effective method of detecting and characterising hydrothermal alteration associated with ore deposits, and can identify not only mineral species but also changes in the major element composition of minerals. Porphyry deposits represent large accumulations of valuable metal in the Earth's crust and have extensive alteration signatures making them an attractive target for exploration, particularly by remote sensing which can cover large areas quickly. Reflectance spectroscopy has been widely applied in sericitic (phyllic), argillic and advanced argillic alteration domains because it is particularly effective in discriminating bright clay minerals. However, the propylitic domain has remained relatively unexplored because propylitic rocks are typically dark and produce relatively poorly-defined spectra.This study utilised an ASD TerraSpec 4 handheld spectrometer to collect SWIR spectra from rocks surrounding the Batu Hijau Cu-Au porphyry deposit in Indonesia, where previous work has identified systematic spatial variations in the chemistry of chlorite, a common propylitic alteration mineral. Spectra were collected from 90 samples and processed using The Spectral Geologist (TSG) software as well as the Halo mineral identifier to characterise mineralogy and extract the positions and depths of spectral absorption features, which were then correlated with major element geochemistry. Two diagnostic chlorite absorption features located at around 2250 nm and 2340 nm correlate with the Mg# (Mg/[Mg + Fe]) of chlorite, both in terms of wavelength position and depth. As the Mg# increases, the wavelengths of both features increase from 2249 nm to 2254 nm and from 2332 nm to 2343 nm respectively, and absorption depths also increase significantly. In the spatial dimension, these feature variations act as reasonably strong vectors to the orebody, showing systematic increases over a transect away from the porphyry centr
Cooke D, Agnew P, Hollings P, et al., 2017, Porphyry Indicator Minerals (PIMS) and Porphyry Vectoring and Fertility Tools (PVFTS) – Indicators of Mineralization Styles and Recorders of Hypogene Geochemical Dispersion Halos, Exploration 17: Sixth Decennial International Conference on Mineral Exploration, Pages: 457-470
In the past decade, significant research efforts have been devoted to mineral chemistry studies to assist porphyry exploration. Theseactivities can be divided into two major fields of research: (1) porphyry indicator minerals (PIMS), which aims to identify the presence of,or potential for, porphyry-style mineralization based on the chemistry of magmatic minerals such as plagioclase, zircon and apatite, orresistate hydrothermal minerals such as magnetite; and (2) porphyry vectoring and fertility tools (PVFTS), which use the chemicalcompositions of hydrothermal minerals such as epidote, chlorite and alunite to predict the likely direction and distance to mineralizedcentres, and the potential metal endowment of a mineral district. This new generation of exploration tools has been enabled by advances inlaser ablation-inductively coupled plasma mass spectrometry, short wave length infrared data acquisition and data processing, and theincreased availability of microanalytical techniques such as cathodoluminescence. PVFTS and PIMS show considerable promise forporphyry exploration, and are starting to be applied to the diversity of environments that host porphyry and epithermal deposits around thecircum-Pacific region. Industry has consistently supported development of these tools, in the case of PVFTS encouraged by severalsuccessful “blind tests” where deposit centres have successfully been predicted from distal propylitic settings. Industry adoption is steadilyincreasing but is restrained by a lack of the necessary analytical equipment and expertise in commercial laboratories.
Loader MA, Wilkinson JJ, Armstrong RN, 2017, The effect of titanite crystallisation on Eu and Ce anomalies inzircon and its implications for the assessment of porphyry Cu deposit fertility, Earth and Planetary Science Letters, Vol: 472, Pages: 107-119, ISSN: 0012-821X
The redox sensitivity of Ce and Eu anomalies in zircon has been clearly demonstrated by experimental studies, and these may represent an important tool in the exploration for porphyry Cu deposits which are thought to be derived from oxidised magmas. These deposits are significant because they are the source of much of the world's copper and almost all of the molybdenum and rhenium, key elements in many modern technologies. 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 zircon 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 an indicator of porphyry magma fertility, and a useful tool for exploration.
Sievwright RH, Wilkinson JJ, O'Neill HSC, et al., 2017, Thermodynamic controls on element partitioning between titanomagnetite and andesitic–dacitic silicate melts, Contributions To Mineralogy and Petrology, Vol: 172, ISSN: 1432-0967
Titanomagnetite–melt partitioning of Mg, Mn, Al, Ti, Sc, V, Co, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Hf and Ta was investigated experimentally as a function of oxygen fugacity (fO2) and temperature (T) in an andesitic–dacitic bulk-chemical compositional range. In these bulk systems, at constant T, there are strong increases in the titanomagnetite–melt partitioning of the divalent cations (Mg2+, Mn2+, Co2+, Ni2+, Zn2+) and Cu2+/Cu+ with increasing fO2 between 0.2 and 3.7 log units above the fayalite–magnetite–quartz buffer. This is attributed to a coupling between magnetite crystallisation and melt composition. Although melt structure has been invoked to explain the patterns of mineral–melt partitioning of divalent cations, a more rigorous justification of magnetite–melt partitioning can be derived from thermodynamic principles, which accounts for much of the supposed influence ascribed to melt structure. The presence of magnetite-rich spinel in equilibrium with melt over a range of fO2 implies a reciprocal relationship between a(Fe2+O) and a(Fe3+O1.5) in the melt. We show that this relationship accounts for the observed dependence of titanomagnetite–melt partitioning of divalent cations with fO2 in magnetite-rich spinel. As a result of this, titanomagnetite–melt partitioning of divalent cations is indirectly sensitive to changes in fO2 in silicic, but less so in mafic bulk systems.
Wilkinson JJ, Vowles K, Muxworthy A, et al., 2017, Regional remagnetization of Irish Carboniferous carbonates dates Variscan orogenesis, not Zn-Pb mineralization, Geology, Vol: 45, Pages: 747-750, ISSN: 1943-2682
Paleomagnetic methods have been used in economic geology to date mineralization in sediment-hosted ore deposits and thereby help to develop ore deposit models and understand the geodynamic settings in which mineralization can occur. However, paleomagnetic ages are sometimes inconsistent with other geochronological techniques and with geological observations. Here we test the veracity of paleomagnetic ages for sediment-hosted ores through a study of the Irish Midlands ore field. We find that unaltered rocks distal to mineralization that are of equivalent age to the ore host sequence have comparable characteristic remanent magnetic directions to those previously derived from the ores. This indicates that remagnetization of the rocks was probably independent of the ore-forming process. Comparison with the apparent polar wander path for Europe suggests an age of ca. 310 Ma for this event, consistent with the timing of the Variscan orogeny. Fold test results support this, indicating the signal was acquired after tilting and/or folding of the host rocks. Petrology and magnetic data suggest that nanometric magnetite particles are the remanence carrier. Based on independent geochronological and geological constraints, we conclude that mineralization formed in Ireland in the early Carboniferous coincident with basin development and that paleomagnetic dates were reset during the later orogenic overprint. Caution is therefore warranted in the interpretation of paleomagnetic dates for ore systems, and geodynamic models for mineral systems based on these may be erroneous.
Wilkinson JJ, 2017, Epigenesis, Encyclopedia of Geochemistry: A Comprehensive Reference Source on the Chemistry of the Earth, Editors: White, Publisher: Springer, ISBN: 9783319391939
Hollis S, Menuge J, Dennis P, et al., 2017, Clumped C–O isotope temperature constraints for carbonate precipitation associated with the Irish-type Lisheen and Navan Zn–Pb orebodies, Mineral Deposits Studies Group AGM, Pages: 1-1, ISSN: 0371-7453
Wilkinson JJ, 2017, Metastable freezing: A new method for the estimation of salinity in aqueous fluid inclusions, Economic Geology, Vol: 112, Pages: 185-193, ISSN: 0013-0109
On cooling during microthermometry, fluid inclusions invariably supercool before freezingunder disequilibrium (metastable) conditions to form ice and hydrates. Measurements of fluidinclusions from the Irish Zn-Pb hydrothermal system reveal a strong linear correlation (R2 =0.968) between final ice melting temperature (TmI) and metastable freezing temperature (Tmf)of the form:TmI = 0.563 Tmf + 22.7 (+1.5/-3.5)The relationship is shown to be independent of heating-freezing stage model, host mineral, andlargely of inclusion size, but is affected by the presence of CO2 and by cooling rate. Thecorrelation shows that metastable freezing is predictable and in fact, in small droplets of puresolution, occurs at a well-defined, salinity-dependent temperature referred to as thehomogeneous freezing point. This relationship allows salinity to be estimated in fluid inclusionswhere the optical recognition of final ice melting is not possible due to small inclusion size orcloudy samples, or where inclusions go into a metastable, vapor-absent, state because ofcollapse of the bubble on freezing. Using a cooling rate of ~50°C/min, inclusion salinity is givenby:Salinity (wt% NaCl equivalent) = -69.7 - 2.617Tmf - 0.02603Tmf2 - 0.0000994Tmf3The homogeneous freezing point is controlled by an equilibrium thermodynamic propertyrelated to the activity of water. In small droplets of pure solution, as approximated by fluidinclusions, freezing will occur when the water activity is 0.305 above that of the stable icemelting condition at the same temperature, independent of solute type. “Early” metastablefreezing, at a temperature above the homogeneous freezing point may occur in very largeinclusions, or those containing “seed” particles or CO2. In such cases, the salinity will beunderestimated by the equation above.
Wilkinson JJ, Baker MJ, Cooke DR, et al., 2017, Exploration targeting in porphyry Cu systems using propylitic mineral chemistry: a case study of the El Teniente deposit, Chile, 14th SGA Biennial Meeting on Mineral Resources to Discover, Publisher: SOC GEOLOGY APPLIED MINERAL DEPOSITS-SGA, Pages: 1111-1114
Hollis SP, Menuge JF, Doran A, et al., 2017, Clumped C-O isotope temperature constraints for carbonate precipitation associated with Irish-type Zn-Pb orebodies, 14th SGA Biennial Meeting on Mineral Resources to Discover, Publisher: SOC GEOLOGY APPLIED MINERAL DEPOSITS-SGA, Pages: 609-612
Hart L, Wilkinson JJ, Armstrong R, et al., 2017, Classifying propylitic alteration in complex porphyry systems; a case study of the Oyu Tolgoi Cu-Au porphyry deposits, Southern Mongolia, 14th SGA Biennial Meeting on Mineral Resources to Discover, Publisher: SOC GEOLOGY APPLIED MINERAL DEPOSITS-SGA, Pages: 1081-1084
Mac Niocaill C, Muxworthy AR, Vowles K, et al., 2016, Palaeomagnetic dates of Irish Zn-Pb deposits record regional remagnetization related to Variscan fluid flow, Magnetic Interactions 2016
Chamberlain CM, Wilkinson JJ, Herrington RJ, 2015, Hydrothermal alteration of footwall basalts to the Archaean Bulyanhulu gold deposit, Tanzania, Mineral Deposit Research: Meeting the Challenge, 8th Biennial Meeting of the Society for Geology Applied to Mineral Deposits (SGA), Pages: 60-63
Spencer ET, Wilkinson JJ, Nolan J, et al., 2015, The controls of post-entrapment diffusion on the solubility of chalcopyrite daughter crystals in natural quartz-hosted fluid inclusions, Chemical Geology, Vol: 412, Pages: 15-25, ISSN: 1872-6836
The presence of chalcopyrite daughter crystals in natural quartz-hosted fluid inclusions that do not dissolve when heated to trapping conditions suggests that inclusions are subject to post-entrapment modifications that affect chalcopyrite solubility. Previous double capsule experiments conducted by Mavrogenes and Bodnar (1994) concluded that the post-entrapment outward diffusion of H2 is responsible for the presence of non-dissolvable chalcopyrite crystals in natural, quartz-hosted brine inclusions. However, recent studies have shown that quartz-hosted inclusions can also be modified by diffusional H+ loss and Cu+ gain. This means that multiple factors may influence chalcopyrite solubility in different fluid inclusion types. In this study, the experimental procedure of Mavrogenes and Bodnar (1994) was recreated in order to rehydrogenate quartz-hosted, chalcopyrite-bearing fluid inclusions from the El Teniente Cu–Mo porphyry deposit, Chile. These inclusions had a range of salinities and densities. Results show that the experimental technique is successful for fluid inclusions that contain relatively small chalcopyrite daughter crystals and have moderate salinities (> 5 wt.% NaCleq). In contrast, chalcopyrite crystals do not dissolve in low density vapor inclusions even after rehydrogenation. The failure of chalcopyrite crystals to dissolve in these inclusions is attributed to their lower initial pH and higher sulfide concentrations, which led to greater post-entrapment H+ loss and Cu+ gain. This considered, Cu concentrations in moderate to high salinity inclusions are likely to reflect those present at trapping, suggesting that H2 loss is the primary control on the failed dissolution of chalcopyrite. By contrast, Cu concentrations in S-rich vapor inclusions can increase considerably via inward Cu+ diffusion in the presence of an external Cu-bearing fluid and a pH gradient between the inclusion and this fluid (Lerchbaumer and Audétat, 2012; Seo and He
Wilkinson JJ, Chang Z, Cooke DR, et al., 2015, The chlorite proximitor: A new tool for detecting porphyry ore deposits, Journal of Geochemical Exploration, Vol: 152, Pages: 10-26, ISSN: 0375-6742
The major, minor and trace element chemistry of chlorite were evaluated as a tool for mineral exploration in the propylitic environment of porphyry ore deposits. Chlorite from eighty propylitically altered samples, located up to 5 km from the Batu Hijau Cu–Au porphyry deposit in Indonesia, was analyzed using electron microprobe and laser ablation inductively-coupled plasma mass spectrometry. The results show that a variety of elements, including K, Li, Mg, Ca, Sr, Ba, Ti, V, Mn, Co, Ni, Zn and Pb, are probably incorporated in the chlorite lattice and display systematic spatial variations relative to the porphyry center. Ti, V and Mg decrease exponentially in concentration with increasing distance, whereas the others increase. Ratioing the former to the latter provides a variety of ratios that vary up to four orders of magnitude, providing sensitive vectoring parameters. Chlorite geothermometry suggests that Ti is substituted into chlorite as a function of crystallization temperature and thus maps out the thermal anomaly associated with the mineralized center. By contrast, Mn and Zn display a maximum in chlorite at a distance of ~ 1.3 km that mirrors the whole rock anomaly for these metals, reflecting their lateral advection into the wall rocks by magmatic-hydrothermal fluids. The recognizable footprint defined by chlorite compositions extends to at least 4.5 km, significantly beyond the whole rock anomalism (≤ 1.5 km) and thus represents a powerful new exploration tool for detecting porphyry systems. Variations in chlorite chemistry are very systematic in the inner propylitic zone (to distances of ~ 2.5 km), thereby providing a precise vectoring tool in a domain where other tools are typically ineffective. In this zone, equations of the form:x=ln{R/a} / bcan be formulated, where the distance to center, x, is predicted based on a variety of element ratios in chlorite R, and where a and b are exponential fit parameters. Importantly, distal chlorite compositio
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