Scanning Electron Microscope Cathodoluminescence Imaging Research Articles

Cathodoluminescence as a tracing technique for quartz precipitation in low velocity shear experiments

Two simulated gouges (a pure quartz and a quartz-muscovite mixture) were experimentally deformed in a ring shear apparatus at a constant low velocity under hydrothermal conditions favourable for dissolution–precipitation processes. Microstructural analysis using scanning electron microscope cathodoluminescence imaging and cathodoluminescence spectroscopy combined with chemical analysis showed that quartz dissolution and precipitation occurred in both experiments. The starting materials and deformation conditions were chosen so that dissolution–precipitation microstructures could be unambiguously identified from their cathodoluminescence signal. Precipitated quartz was observed as blue luminescent fracture fills and overgrowths with increased Al content relative to the original quartz. In the pure quartz gouge, most of the shear deformation was localized on a boundary-parallel slip surface. Sealing of fractures in a pulverized zone directly adjacent to the slip surface may have helped keeping the deformation localized. In the quartz-muscovite mixture, some evidence was observed of shear-accommodating precipitation of quartz in strain shadows, but predominantly in fractures, elongating the original grains. Precipitation of quartz in fractures implies that the length scale of diffusive mass transfer in frictional-viscous flow is shorter than the length of the quartz domains. Additionally, fracturing might play a more important role than generally assumed. Our results show that cathodoluminescence, especially combined with chemical analysis, is a powerful tool in microstructural analyses of experimentally deformed quartz-bearing material and visualizing quartz precipitation.

Whether short-lived or prolonged duration of multistage combined magmatic and hydrothermal events in the giant Chalukou porphyry Mo deposit, China

The duration (or time scales) of multistage combined magmatic and hydrothermal events provides key information constraining the metallogenic history of porphyry deposits, but the duration (time scale) of the mineralising hydrothermal process remains uncertain. Here, we report zircon U–Pb isotopic age, fluorite Sm–Nd isochron ages, and Ti-in-quartz diffusion chronometry for the giant Chalukou porphyry Mo deposit with reserves of 2.46 Mt @ 0.087 wt% Mo in NE China. The deposit is genetically related to hydrothermal fluids associated with a Late Jurassic (148–147 Ma) porphyritic magmatic event coincident with our new fluorite Sm–Nd isochron age of 148 Ma. Porphyry Mo mineralisation is characterised by the superposition of multiple generations of crosscutting quartz-bearing veins including: quartz veins with minor magnetite and hematite associated with K-silicate alteration (A-veins), quartz-molybdenite veins with K-feldspar alteration halos that host the bulk of the Mo mineralisation (B-veins), and late base-metal mineralisation with phyllic alteration (D-veins), as well as post-mineralisation veins with argillic alteration. Hydrothermal veins from the Chalukou porphyry Mo deposit contain multiple generations of quartz that are distinct in their Ti concentrations and scanning electron microscope-cathodoluminescence images. The A-veins preserve subhedral to euhedral characteristics with alternating CL bright and dark growth zoning. The A-veins probably formed at a near-lithostatic pressure of ∼ 1.35 kbar and contain CL-bright quartz assaying 31–71 ppm Ti with Ti-in-quartz temperatures averaging 603 ± 30 °C, and CL-dark quartz assaying 6–25 ppm Ti with Ti-in-quartz temperatures averaging 496 ± 36 °C. The mineralised B-veins also CL-dark quartz, although they display less growth zoning with lower Ti assays of 0.4–27.9 ppm. These veins have near 0.7 kbar pressure and Titanium-in-quartz temperature averaging 390 ± 79 °C. The D-veins are dominated by CL-dark quartz and systematically lower Ti concentrations of 0.4–8 ppm, being lower than the relatively latter vein generations, formed at a temperature of 348 ± 58 °C and pressure of 0.4 kbars. The Ti diffusion in quartz has become a successful method constraining time scales and duration of magmatic and hydrothermal processes. Ti diffusion modelling indicates that the A-veins were deposited and cooled in a period between 2500 and 50,000 years. This shows that the earliest magma-related hydrothermal fluids activities formed within tens of thousands of years.

Hydrothermal evolution and ore precipitation of the No. 2 porphyry Cu–Au deposit in the Xiongcun district, Tibet: Evidence from cathodoluminescence, fluid inclusions, and isotopes

The No. 2 porphyry Cu–Au deposit in the Xiongcun district forms part of the Gangdese porphyry copper belt of Tibet. This study clarifies the fluid evolution and ore precipitation mechanisms of the magmatic-hydrothermal system that formed the deposit by combining the results of fluid inclusion petrography, microthermometry, scanning electron microscope-cathodoluminescence imaging, and stable isotope analyses of multiple quartz-dominated veins. Four distinct sets of quartz-dominated veins and eight generations of quartz (Q1–Q8) within the veins were identified, namely from early to late, early barren quartz veins (V1) comprised of equigranular CL-bright Q1 quartz, quartz–pyrite–chalcopyrite ± magnetite veins (V2) containing early equigranular CL-bright Q1 quartz and later CL-gray or CL-dark Q2–3 (Q2 + Q3) quartz grains intergrown with Cu–Fe sulfides, quartz–molybdenite ± pyrite ± chalcopyrite veins (V3) comprised of quartz grains (Q4–Q6) characterized by subhedral comb-like textures with well-developed oscillatory growth zoning, and late quartz veins (V4) containing CL-gray Q7 quartz with euhedral growth zones and later CL-dark Q8 quartz. Fluid inclusions in these veins can be classified into six types: B40, B40H, B80, B15H, B15H+, and B15. The B40 and B40H inclusions; B80 inclusions; and B15, B15H, and B15H+ inclusions contain vapor bubbles occupying 30–60 vol%, >70 vol%, and 5–25 vol%, respectively; in addition, B15H, B15H+, and B40H inclusions contain transparent daughter minerals (halite, and in some cases, sylvite). Raman spectra showed that CO2 is only present in B40 and B80 inclusions. In the Q1 quartz in V1 and V2 veins, the dominant type of inclusions is B40, which have homogenization temperatures (Th) of 278 °C–389 °C (peaking at 310 °C–330 °C) and salinities of 2.0–12.8 wt% NaCl equiv. In Q2–3 quartz in V2 veins, all inclusion types were observed. These were trapped at temperatures of approximately 325 °C–340 °C and had salinities of 1.8–69.7 wt% NaCl equiv. In V3 veins, only B15H and B15 inclusions were observed, which had Th values of 171 °C–320 °C (peaking at 240 °C–260 °C) and 173 °C–317 °C (peaking at 210 °C–230 °C) and salinities of 28.7–33.9 wt% NaCl equiv and 5.6–23.1 wt% NaCl equiv, respectively. In V4 veins, only B15 inclusions were observed. Primary B15 inclusions in V4 veins yielded Th values of 166 °C–229 °C (peaking at 170 °C–200 °C) and salinities of 5.1–17.1 wt% NaCl equiv. The 3He/4He and 40Ar/36Ar ratios of the fluid inclusions exhibited the ranges of 0.08–0.84 Ra and 451.3–1567.8, respectively, and the δ18Ofluid and δDfluid values varied from −3.2‰ to 5.8‰ and −92.8‰ to −80.3‰, respectively. By integrating all results from the fluid inclusion, cathodoluminescence, and isotopic analyses, we conclude that the initial ore-forming fluids of the No. 2 deposit were low-salinity, CO2-rich single-phase fluids of magmatic origin. Subsequently, fluid immiscibility developed in the initial ore-forming fluids, generating hypersaline liquid and low-salinity vapor phases and leading to the separation of a CO2 phase plus chalcopyrite precipitation from the fluids. As meteoric water was injected into the hydrothermal system, the ore-forming fluids gradually evolved to become meteoric water-dominated, low temperature, low-salinity, and CO2-poor; in addition, fluid-cooling due to the meteoric water input resulted in molybdenite precipitation.

The fingerprint of imperial topaz from Ouro Preto region (Minas Gerais state, Brazil) based on cathodoluminescence properties and composition

ABSTRACTA study of the cathodoluminescence (CL) properties of imperial topaz from Ouro Preto region (Minas Gerais state, Brazil) and its relation with trace-element composition was conducted, using scanning electron microscope cathodoluminescence (SEM-CL), optical microscope cathodoluminescence (OM-CL), cathodoluminescence-spectrometry (CL-spectrometry), electron microprobe analysis (EMPA), laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) and Raman spectrometry. Each analytical technique allowed characterization of the imperial topaz fingerprint. SEM-CL panchromatic images show different crystal growth and resorption events in imperial topaz crystals. Colour CL images indicate only blue to violet emissions. The CL-spectra indicate a broad emission band with low intensity peak at ~417 nm and a broad emission band with high intensity and major peaks at 685, 698, 711 and 733 nm. The EMPA indicates high OH content, in which the OH/(OH + F) ratio ranges between 0.35–0.43 (0.72 ≤ OH ≤ 0.86 apfu). High Cu and Zn concentrations (LA-ICP-MS) were measured in the high luminescence areas of SEM-CL images, suggesting both elements as CL-activators in imperial topaz. Raman and CL-spectra indicate high Cr concentrations, corroborated by EMPA and LA-ICP-MS results. The high Cr caused strong luminescence intensities that enabled their superimposition over the OH stretching mode (~3650 cm–1) of topaz in all Raman spectra. Among trace elements, the concentrations of Ti, V, Cr, Mn, Fe, Cu, Zn, Ga and Ge provide the fingerprint of imperial topaz.

Nondestructive measurement of homoepitaxially grown GaN film thickness with Fourier transform infrared spectroscopy

In vertical devices containing GaN homoepitaxial layers on GaN substrates, the layer thickness is a key parameter that needs to be clarified before starting the device process. We applied Fourier transform infrared spectroscopy (FT-IR) to a homoepitaxially grown GaN film that consisted of an n−-GaN layer. The estimated film thickness from the FT-IR spectrum agreed well with the results of cross-sectional scanning electron microscope cathodoluminescence images. This is the first report of nondestructive film thickness measurements for homoepitaxially grown GaN and indicates the applicability of FT-IR to the nondestructive inspection of vertical GaN power devices.

Scanning electron microscope cathodoluminescence imaging of subgrain boundaries, twins and planar deformation features in quartz

The study of deformation features has been of great importance to determine deformation mechanisms in quartz. Relevant microstructures in both growth and deformation processes include dislocations, subgrains, subgrain boundaries, Brazil and Dauphiné twins and planar deformation features (PDFs). Dislocations and twin boundaries are most commonly imaged using a transmission electron microscope (TEM), because these cannot directly be observed using light microscopy, in contrast to PDFs. Here, we show that red-filtered cathodoluminescence imaging in a scanning electron microscope (SEM) is a useful method to visualise subgrain boundaries, Brazil and Dauphiné twin boundaries. Because standard petrographic thin sections can be studied in the SEM, the observed structures can be directly and easily correlated to light microscopy studies. In contrast to TEM preparation methods, SEM techniques are non-destructive to the area of interest on a petrographic thin section.

A CO2-rich porphyry ore-forming fluid system constrained from a combined cathodoluminescence imaging and fluid inclusion studies of quartz veins from the Tongcun Mo deposit, South China

The Tongcun deposit is a medium-sized porphyry Mo deposit in the eastern section of the Qin-Hang metallogenic belt, South China. Fluid inclusions and scanning electron microscope -cathodoluminescence (SEM-CL) of different generations of quartz from multi-stage veins were studied to reconstruct the evolution of the porphyry hydrothermal system at the Tongcun Mo deposit. Four stages of hydrothermal veins were recognized at Tongcun, including the early barren quartz vein, the main ore-stage quartz-molybdenite vein and quartz-polymetallic vein, and the late quartz-calcite (±pyrite) vein. SEM-CL imaging revealed that these veins were composed of at least four successive quartz generations (A, B, C and D) in which only the quartz B and C are in close association with Mo mineralization. Fluid inclusions from the Tongcun deposit can be classified into four types, namely three- or four-phase CO2-rich (C), two-phase vapor-rich (V), two-phase liquid-rich (L) and three-phase liquid-rich solid-bearing (S) type. The C type fluid inclusions were further divided into C1 subtype with higher carbonic proportion (50–70% vol%), C2 subtype with lower carbonic proportion (25–40% vol%) and molybdenite daughter mineral, and C3 subtype with lower carbonic proportion (25–40% vol%) but no daughter mineral. The V type fluid inclusions were further divided into V1 subtype with higher filling degree (0.2 to 0.3) and V2 subtype with lower filling degree (<0.1). The C1 and V1 subtypes were secondary inclusions in quartz phenocrysts (P) and primary inclusions in quartz A, and show homogenization temperatures (Th) of 257–378°C, with salinities of 0.6–9.6wt% NaCl equivalent. The immiscibility assemblage of coexisting C2 and V2 subtype inclusions in quartz B exhibit a Th of 237–285°C and salinities of 1.2–4.4wt% NaCl equivalent. C3 subtype inclusions in quartz C display Th of 191–267°C and salinities of 0.2–4.6wt% NaCl equivalent. Finally, the L and S type inclusions in quartz D display Th of 133–245°C and salinities of 0.4–7.0wt% NaCl equivalent. The fluid immiscibility is restricted in quartz B and only generated low salinity end member inclusions, similar to the immiscible behavior commonly observed in orogenic gold deposits. The likely molybdenite daughter minerals were only observed in C2 subtype inclusions in quartz B, indicating enrichment of Mo had occurred during fluid immiscibility. Combined with SEM-CL texture and Raman analysis, we proposed that a rapid Mo precipitation occurred in the narrow interval during or immediately after fluid immiscibility and thus formed the Tongcun Mo deposit. The CO2-rich ore-forming fluid and continental arc setting of the Tongcun deposit provide a special case of CO2-rich porphyry Mo system in non-collision settings.

Non-linear growth kinematics of opening-mode fractures

Fracture aperture is commonly assumed to be a linear function of fracture length, stress, and elastic material properties. Under constant stress, fracture opening displacement may change without concurrent length or height growth if the material effectively weakens after initial linear elastic fracture growth by changes in elastic properties or by non-elastic deformation processes. To investigate the kinematics of fracture opening and its dependence on length and height growth, we reconstructed the opening history of three opening-mode fractures that are bridged by crack-seal quartz cement. Similar crack-seal cement bridges had been interpreted to form by repeated incremental fracture opening and subsequent precipitation of quartz cement. Using scanning electron microscope cathodoluminescence imaging, we determined the thickness and number of crack-seal cement layers as a function of position along fracture length and height. Observed trends in crack-seal cement layer thickness and number of fracture opening increments are consistent with non-linear fracture growth kinematics, consisting of an initial stage of fast fracture propagation relative to aperture growth, followed by a stage of slow propagation and pronounced aperture growth. Consistent with earlier fluid inclusion observations indicating fracture opening and propagation occurring over 40–50 m.y., we interpret the second stage of pronounced aperture growth and slow propagation to result from fracture opening strain accommodated by solution-precipitation creep and concurrent slow, possibly subcritical, fracture propagation.

A search for shocked quartz grains in the Allerød‐Younger Dryas boundary layer

AbstractThe Younger Dryas impact hypothesis suggests that multiple airbursts or extraterrestrial impacts occurring at the end of the Allerød interstadial resulted in the Younger Dryas cold period. So far, no reproducible, diagnostic evidence has, however, been reported. Quartz grains containing planar deformation features (known as shocked quartz grains), are considered a reliable indicator for the occurrence of an extraterrestrial impact when found in a geological setting. Although alleged shocked quartz grains have been reported at a possible Allerød‐Younger Dryas boundary layer in Venezuela, the identification of shocked quartz in this layer is ambiguous. To test whether shocked quartz is indeed present in the proposed impact layer, we investigated the quartz fraction of multiple Allerød‐Younger Dryas boundary layers from Europe and North America, where proposed impact markers have been reported. Grains were analyzed using a combination of light and electron microscopy techniques. All samples contained a variable amount of quartz grains with (sub)planar microstructures, often tectonic deformation lamellae. A total of one quartz grain containing planar deformation features was found in our samples. This shocked quartz grain comes from the Usselo palaeosol at Geldrop Aalsterhut, the Netherlands. Scanning electron microscopy cathodoluminescence imaging and transmission electron microscopy imaging, however, show that the planar deformation features in this grain are healed and thus likely to be older than the Allerød‐Younger Dryas boundary. We suggest that this grain was possibly eroded from an older crater or distal ejecta layer and later redeposited in the European sandbelt. The single shocked quartz grain at this moment thus cannot be used to support the Younger Dryas impact hypothesis.

Time Scales of Porphyry Cu Deposit Formation: Insights from Titanium Diffusion in Quartz

Porphyry dikes and hydrothermal veins from the porphyry Cu-Mo deposit at Butte, Montana, contain multiple generations of quartz that are distinct in scanning electron microscope-cathodoluminescence (SEM-CL) images and in Ti concentrations. A comparison of microprobe trace element profiles and maps to SEM-CL images shows that the concentration of Ti in quartz correlates positively with CL brightness but Al, K, and Fe do not. After calibrating CL brightness in relation to Ti concentration, we use the brightness gradient between different quartz generations as a proxy for Ti gradients that we model to determine time scales of quartz formation and cooling. Model results indicate that time scales of porphyry magma residence are ~1,000s of years and time scales from porphyry quartz phenocryst rim formation to porphyry dike injection and cooling are ~10s of years. Time scales for the formation and cooling of various generations of hydrothermal vein quartz range from 10s to 10,000s of years. These time scales are considerably shorter than the ~0.6 m.y. overall time frame for each porphyry-style mineralization pulse determined from isotopic studies at Butte, Montana. Simple heat conduction models provide a temporal reference point to compare chemical diffusion time scales, and we find that they support short dike and vein formation time scales. We interpret these relatively short time scales to indicate that the Butte porphyry deposit formed by short-lived episodes of hydrofracturing, dike injection, and vein formation, each with discrete thermal pulses, which repeated over the ~3 m.y. generation of the deposit.

Scanning electron microscopy cathodoluminescence of quartz: Principles, techniques and applications in ore geology

Scanning electron microscopy cathodoluminescence (SEM-CL) of quartz has been a prevalent research technique in porphyry and epithermal systems for the past two decades. Quartz from specific geological environments reveals unique textures in SEM-CL, which can be used to constrain the evolution of these ore-bearing systems when complemented by fluid inclusion, hyperspectral mapping, and trace element studies. We review SEM-CL principles and instrumentation, sample preparation and handling, and experimental conditions of quartz SEM-CL imaging that result in the high quality CL images. The effects of sample polishing, accelerating voltage, beam spot size, working distance, vacuum conditions, image acquisition, and post-processing were examined through experimental trial. For the XL 30 ESEM and the attached Gatan PanaCL detector used, the optimum experimental conditions to obtain high quality panochromatic SEM-CL images of quartz at high vacuum mode for carbon-coated conductive samples are as follows: 15kV accelerating voltage, relative beam spot size 6 (approximately 500nm in diameter), HT −570V to −580V photomultiplier tube (PMT) voltage. Low vacuum mode (with chamber H2O vapor pressure from 0.1 to 1.0Torr) working conditions are similar to the conditions at high vacuum mode except the PMT voltage should be reduced to −550V to −560V. Working distances vary based on the position of user's retractable CL detector. The sample surface should be as close as possible to the CL detector, but a 1mm clearance between the detector and the sample surface is recommended to prevent detector from possible damage by the sample. Several minutes of beam exposure prior to image acquisition at 320second scan speeds at 50×–1500× magnifications is recommended to generate the greatest CL emission. Monochromatic CL imaging requires three scans over the same area using red, green, and blue optical filters that can be merged to produce a “true color” image. The red and green filters require stronger PMT voltages to produce sufficient CL emissions by an increase of −200V to −300V and −150V to −200V, respectively, from the PMT voltage used for panochromatic imaging. Special attention is given to the challenges associated with imaging hydrothermal quartz veining in ore deposits and the value of CL data as a foundation for geochemical studies. SEM-CL imaging of vein quartz is explored through case studies of the Red Hills Porphyry Cu–Mo Deposit, Texas, USA, and the Ertsberg–Grasberg Cu–Au District, Papua, Indonesia to aid in vein paragenesis. The most common application of quartz SEM-CL in ore geology is to reveal the relative timing of mineral precipitation, mineral dissolution, and inherited structural features. Understanding of temporal relations among these events makes it possible to select specific generations of quartz within a vein for further studies such as the TitaniQ thermometry and fluid inclusion microthermometry in order to establish T–P–X fluctuations throughout the development of a hydrothermal system.

O-Hf isotope constraints on the origin of zircon in high-pressure mélange blocks and associated matrix rocks from Tinos and Syros, Greece

Most previously dated zircons in high-pressure melange blocks and associated matrix rocks from Tinos and Syros (Cyclades), Greece, yielded ion microprobe (SHRIMP II) U-Pb ages of ca. 80 Ma. In many cases it remains unclear whether the zircons are igneous or metamorphic/hydrothermal in origin. Oxygen and hafnium isotope ratios in the dated zircons have been determined to further constrain their mode of formation. Spot analysis of zircons from metagabbro, eclogite, glaucophanite, jadeitite and chlorite schists by ion microprobe (CAMECA IMS-1280) yields a large range in δ 18 O, varying from 2.0 ‰ to 7.6 ‰ VSMOW. The average δ 18 O values for most zircons in 11 samples however fall within a relatively small compositional range between 4.7 ‰ and 5.5 ‰, which is consistent with an igneous origin. These values suggest a relationship to magmas typical for modern oceanic crust or to precursors that had been in equilibrium with primitive magma compositions or the mantle (5.3 ± 0.6 ‰, 2 SD). The δ 18 O (mantle-like) and initial epsilon hafnium values [ɛ Hf ( t ) = +10 to +24] suggest that the 80 Ma old zircons are igneous in origin and crystallised from magmas that were derived from depleted mantle. Scanning Electron Microscope cathodoluminescence imaging indicates that lower-δ 18 O zircons (grains or domains; down to 2.0 ‰), mostly from one exceptional eclogite sample (average 4.3 ± 0.8 ‰, 2 SD, n = 23), either represent cauliflower-like internal structures or weakly zoned (or porous) features in outer rims. Previous age dating of zircon domains with cauliflower-like internal structures indicated apparent ages that are considerably younger ( ca. 54 Ma and ca. 57 Ma) than the Cretaceous age determined for the majority of the zircon population. Taken together, these observations suggest that the low-δ 18 O zircons (and three high-δ 18 O zircons, > 6.0 ‰) are genetically linked to a well-documented Eocene high-pressure metamorphic event ( ca . 53–40 Ma), but post-magmatic seafloor hydrothermal alteration cannot be completely ruled out.

Tracking meteoric infiltration into a magmatic-hydrothermal system: A cathodoluminescence, oxygen isotope and trace element study of quartz from Mt. Leyshon, Australia

The oxygen isotope and trace element composition of hydrothermal quartz has been integrated with scanning electron microscope-cathodoluminescence (SEM-CL) images and fluid inclusion properties to track fluid sources and hydrothermal processes in the Mt. Leyshon Au deposit, Australia. Oxygen isotope and trace element data were collected on parallel traverses across the same quartz sections, using secondary isotope mass spectrometry (SIMS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), respectively, with SEM-CL images obtained before analysis for petrographic context. Over the 280 to 650 °C quartz precipitation range suggested by fluid inclusion microthermometry, δ 18O quartz varies from 0.0 to 14.4‰, corresponding to a wide range of equilibrium δ 18O fluid values, from-6.1 to 10.2‰ (vs. V-SMOW). The δ 18O quartz signature varies systematically among distinct SEM-CL quartz generations (both within and between samples), and can be correlated with variations in temperature and fluid composition, but is independent of intra-generational oscillatory zoning. In contrast, Al and Li concentrations correlate broadly with CL intensity in oscillatory quartz, whereas their concentration is unpredictable in sealed fractures and overgrowths. Concentrations of B, Mg, Na, P, Cl, K, Ti, Mn, Fe, Ge, and Sn are independent of Al, Li, and oscillatory CL features, but Ti correlates with quartz precipitation temperature. Although no systematic correlation between δ 18O quartz and trace element concentrations was found, complementary patterns exist in narrow overgrowths of low δ 18O quartz (∼ 0‰) and high Al (> 10,000 ppma). These quartz zones likely formed during the incursion of 18O-depleted meteoric water into the magmatically-dominated Mt. Leyshon hydrothermal system. We interpret the highest Al concentrations as the result of high quartz precipitation rates, triggered by depressurisation of the hydrothermal cell. The decoupling of oxygen isotope and trace element patterns in quartz leads to the suggestion that (1) under most circumstances, temperature and fluid chemistry dominate δ 18O quartz, and (2) the trace element record, and in particular Al and Li, is influenced by the superimposed effects of quartz precipitation rate.

Copper deposition during quartz dissolution by cooling magmatic–hydrothermal fluids: The Bingham porphyry

Scanning electron microscope cathodoluminescence imaging is used to map successive generations of fluid inclusions in texturally complex quartz veinlets representing the main stage of ore metal introduction into the porphyry Cu–Au–Mo deposit at Bingham, Utah. Following conventional fluid inclusion microthermometry, laser ablation–inductively coupled plasma–mass spectrometry (LA-ICPMS) is applied to quantify copper and other major and trace-element concentrations in the evolving fluid, with the aim of identifying the ore-forming processes. Textures visible in cathodoluminescence consistently show that the bulk of vein quartz (Q1), characterized by bright luminescence, crystallized early in the vein history. Cu–Fe-sulfides are precipitated later in these veins, in a microfracture network finally filled with a second generation of dull-luminescing Q2 quartz. Mapping of brine and vapor inclusion assemblages in these successive quartz generations in combination with LA-ICPMS microanalysis shows that the fluids trapped before and after Cu–Fe-sulfide precipitation are very similar with respect to their major and minor-element composition, except for copper. Copper concentrations in inclusions associated with ore formation drop by two orders of magnitude, in a tight pressure–temperature interval between 21 and 14 MPa and 425–350 °C, several hundred degrees below the temperature of fluid exsolution from the magma. Copper deposition occurs within a limited P– T region, in which sulfide solubility shows strong normal temperature dependence while quartz solubility is retrograde. This permits copper sulfide deposition while secondary vein permeability is generated by quartz dissolution. The brittle-to-ductile transition of the quartz–feldspar-rich host rocks occurs in the same temperature range, which further enhances vein reactivation and promotes cooling and expansion of fluids ascending across the transition from lithostatic to hydrostatic conditions.

Hydrothermal Quartz Vein Formation, Revealed by Coupled SEM-CL Imaging and Fluid Inclusion Microthermometry: Shuteen Complex, South Gobi, Mongolia

Abstract. Scanning electron microscopy-cathodoluminescence (SEM-CL) imaging of vein quartz in the Cu-mineralised, Shuteen Complex (South Gobi, Mongolia) has revealed a complex history of crystal growth, dissolution and microfracture healing, associated with several hydrothermal events that could not be detected using other observational techniques (e.g. transmitted/reflected light microscopy, back-scattered electron imaging, or secondary electron imaging). The quartz initially grew as CL-bright/grey crystals in a 345±30d̀C liquid reservoir, as inferred by the analysis of primary liquid fluid inclusions (average Th of 343d̀C; 6.6∼7.7 wt% NaCleq). Quartz precipitation occurred at the edge of the crystals as reservoir fluids cooled to 260±25d̀C, as indicated by micron-scale CL-dark/CL-bright quartz growth bands containing abundant fluid inclusions (with an average Th values of 261d̀C). Pressure fluctuations were the likely cause of dissolution, as SEM-CL imaging reveals the quartz have corroded or rounded crystal edges, and precipitation of later quartz into open space. SEM-CL imaging shows the quartz contains healed microfractures that trapped low salinity fluids (3.9 wt% NaC1eq) with Th values of 173±15d̀C. SEM-CL imaging provides a means of deciphering the thermal and chemical evolution of the fossil Shuteen hydrothermal system, and the nature of hydrothermal quartz vein-forming processes, by facilitating the correlation of distinct fluid inclusion populations and their relative chronology, with specific hydrothermal events.

Fluid-rock interaction processes in the Te Kopia geothermal field (New Zealand) revealed by SEM-CL imaging

Scanning electron microscopy-cathodoluminescence (SEM-CL) imaging of hydrothermal quartz exposed by weathering in the Te Kopia geothermal field (New Zealand) has revealed a history of crystal growth, dissolution, overprinting and fracturing that cannot be detected using other observational techniques (e.g. transmitted or reflected light microscopy, back-scattered electron imaging or secondary electron imaging). The crystals initially grew as CL-dark quartz, at least 350 m below their present location on the Paeroa Fault scarp, in a neutral pH, 215±10 °C liquid reservoir (inferred from the analysis of primary liquid fluid inclusions: mean T h of 213 °C; 0.2–0.4 wt.% NaCl eq.). Relict quartz–adularia–illite alteration occurs at the surface, in the vicinity of the quartz crystals, and in drillcores from the nearby TK-1 exploration well. Repeated movement on the Paeroa Fault uplifted pyroclastic rocks hosting the quartz crystals, but also provided pathways for “pulses” of hot fluids to move through the system. Quartz precipitation occurred at the edge of the crystals as the reservoir fluids cooled, as indicated by micron-scale alternating CL-dark/CL-bright quartz growth bands, which contain fluid inclusions with T h values of 210±40 °C. Pressure fluctuations were the likely cause of dissolution, marked by corroded crystal edges, with subsequent precipitation of quartz into open space. SEM-CL imaging shows that the quartz crystals contain healed fractures, which trapped low salinity fluids with T h values of 201±6 °C. Low-pH fluids in the near-surface setting also rounded the quartz crystals, and coated them with kaolinite and CL-grey amorphous “silica residue”.

Fluid-rock interaction processes in the Te Kopia geothermal field (New Zealand) revealed by SEM-CL imaging

Scanning electron microscopy-cathodoluminescence (SEM-CL) imaging of hydrothermal quartz exposed by weathering in the Te Kopia geothermal field (New Zealand) has revealed a history of crystal growth, dissolution, overprinting and fracturing that cannot be detected using other observational techniques (e.g. transmitted or reflected light microscopy, back-scattered electron imaging or secondary electron imaging). The crystals initially grew as CL-dark quartz, at least 350 m below their present location on the Paeroa Fault scarp, in a neutral pH, 215±10 °C liquid reservoir (inferred from the analysis of primary liquid fluid inclusions: mean T h of 213 °C; 0.2–0.4 wt.% NaCl eq. ). Relict quartz–adularia–illite alteration occurs at the surface, in the vicinity of the quartz crystals, and in drillcores from the nearby TK-1 exploration well. Repeated movement on the Paeroa Fault uplifted pyroclastic rocks hosting the quartz crystals, but also provided pathways for “pulses” of hot fluids to move through the system. Quartz precipitation occurred at the edge of the crystals as the reservoir fluids cooled, as indicated by micron-scale alternating CL-dark/CL-bright quartz growth bands, which contain fluid inclusions with T h values of 210±40 °C. Pressure fluctuations were the likely cause of dissolution, marked by corroded crystal edges, with subsequent precipitation of quartz into open space. SEM-CL imaging shows that the quartz crystals contain healed fractures, which trapped low salinity fluids with T h values of 201±6 °C. Low-pH fluids in the near-surface setting also rounded the quartz crystals, and coated them with kaolinite and CL-grey amorphous “silica residue”.

Boron diffusion into 6H-SiC through graphite mask

Experimental results are reported of selective diffusion of boron in 6H−SiC. Photoluminescence spectroscopy scanning electron microscopy cathodoluminescence imaging, secondary-ion mass spectroscopy optical microscopy, and stain-groove technique were used to characterize the selectively doped regions fabricated by diffusion from the vapor phase through a graphite mask. Local p-doped regions of dimensions down to ∼20 μm in diameter were formed on an n-type substrate using the graphite mask. Maximum concentration of boron atoms at the surface, obtained by SIMS, varied from 3×1019 cm−3 to 6×1019 cm−3, depending upon the temperature of diffusion, while the p-n junction depth measured by the stain-groove technique varied from 0.5 μm to 1.2 μm. Planar p-n junction diodes fabricated on the diffused regions exhibited good rectification characteristics with a breakdown voltage of about 1000 V.

Cathodoluminescence and trace element zoning in quartz phenocrysts and xenocrysts

Quartz phenocrysts and xenocrysts are a common component of volcanic rocks but rarely contribute to our understanding of magmatic or eruptive processes due to an apparent internal chemical and structural homogeneity. Scanning electron microscopy cathodoluminescence imaging (SEM-CL) and secondary ion mass spectroscopy (ion microprobe) analysis (SIMS) are used here to demonstrate that quartz crystals in volcanic rocks show hitherto unrecognised internal structural and chemical zoning. Two contrasting crystal morphologies have been studied. Porphyritic euhedral bipyramidal quartz crystals in dacite from Bonne Nuit Bay, Jersey, UK show fine-scale growth zoning in SEM-CL. Zones are narrow (1–40 μm wide), planar, parallel to crystal faces, and overgrow more broadly zoned, rounded cores. These crystals are interpreted to have formed at small degrees of undercooling by diffusion-controlled growth. Quiescent magma conditions are indicated by the preservation of an Al-rich diffusive boundary layer which has not been removed by melt shearing during convection. In contrast, rounded and embayed porphyritic quartz grains in a quartz dacite from the Esperanza Mine, E1 Salvador, northern Chile show broad internal SEM-CL zones with irregular boundaries, some of which are truncated by grain margins indicating dissolution. Crystal rounding may occur by dissolution or melting due to thermal or thermal-chemical disequilibrium with surrounding melt, and embayments by gas bubble enhanced convective dissolution are consistent with the development of incipient ocelli textures (strings of amphibole inclusions adjacent to quartz rims). Several episodes of rounding can be recognised in some xenocrysts. Ion microprobe (SIMS) analysis of quartz from both samples shows that A1 contents are higher in areas of bright CL emission. Previous studies of CL in quartz have suggested that several factors control luminescence. Although A1 substitution for Si in SiO4 tetrahedra may be partly responsible for SEM-CL zoning recorded in volcanic quartz, additional mechanisms (including the effect of growth rate on defect density) probably also play a major role.

Identification of fluid inclusions in relation to their host microstructural domains in quartz by cathodoluminescence

The geometry of fluid trapping in relation to microfracture healing or filling was investigated using massive quartz vein samples characterized by multistage deformation and fluid trapping. The multidisciplinary approach used in this study includes • 1) mapping of oriented thin sections of quartz using the cathodoluminescence (CL) mode of a scanning electron microscope (SEM). SEM CL images display areas of different intensities from dark to white, which correspond to the intensity of the CL and are caused by trace elements present in the quartz. Microfracture healing by newly formed quartz crystallization may occur under physical-chemical conditions different from those which characterized the early quartz matrix formation. Thus, evidences of linear cathodoluminescent markers have been searched and identified by paralleling SEM CL images and transmitted light microphotographs. Data show that cathodoluminescence makes clearly visible the healed crack networks which are only detected in part by transmitted light microscopy. The technique is essential to establish the microcrack chronology and to estimate the width affected by healing along a fluid inclusion trail. Such data are useful for the estimation of the microfracture permeability changes throughout the history of fluid migration; • 2) systematic and statistical analysis of the microstructural markers in horizontal and oriented planes using an interactive videographic analyzer which gives the geometry of the paleofluid pathways (orientation, dip and width of the veinlets, fluid inclusion trails, and cathodoluminescent microdomains); • 3) microthermometric and Raman analyses of fluid inclusions in selected fluid inclusions, for the determination of the P-V-T-X conditions of fluid migration in a specific microcrack network. Studied quartz samples come from Au-mineralized quartz veins from the French Massif Central, and some quartz samples from the Cassiar Mountains in British Columbia, have been observed for comparison. In all samples, the quartz matrix exhibits intense microfracturing, suggesting strong brittle deformation which postdates the main stage of quartz lens formation. The quartz veins are characterized by multistage quartz crystallization. Healed microfractures, clear quartz bands, as well as quartz comb veinlets, were studied as functions of the physicochemical conditions in relation to stages of deformation and ore deposition. This study shows that systematic measurements of microstructural marker orientations together with detailed fluid inclusion and mineral petrography and microthermometry may lead to a precise determination of ore fluid pathways and chronology.