Brian Rusk

Scanning electron microscope–cathodoluminescence analysis of quartz reveals complex growth histories in veins from the Butte porphyry copper deposit, Montana

Scanning electron microscope–cathodoluminescence (SEM-CL) analysis of quartz reveals textures that cannot be observed using optical microscopy or backscattered electrons. These cryptic textures yield insight into timing and physical conditions of quartz growth, especially in environments with multiple quartz-precipitation events. Hydrothermal quartz from quartz-sulfide veins in the porphyry copper deposit in Butte, Montana, was analyzed by SEM-CL, revealing the following textures: euhedral growth zones, wide nonluminescing bands that cut across multiple quartz grains, rounded luminescent quartz grain cores with euhedral overgrowths, nonluminescing ‘‘splatters’’ of quartz connected by networks of cobweb-like nonluminescing quartz in otherwise luminescent quartz, concentric growth zones, and wide nonluminescent grain boundaries. These textures indicate that many veins have undergone fracturing, dilation, growth of quartz into fluid-filled space, quartz dissolution, and recrystallization of quartz. Precipitation and dissolution textures indicate that early quartz-molybdenite veins formed as a result of pressure fluctuations between lithostatic and hydrostatic at high temperatures, and later pyrite-quartz veins formed near hydrostatic pressure in response to temperature decrease through and beyond the field of retrograde quartz solubility.

Trace elements in hydrothermal quartz: Relationships to cathodoluminescent textures and insights into vein formation

High-resolution electron microprobe maps show the distribution of Ti, Al, Ca, K, and Fe among quartz growth zones revealed by scanning electron microscope-cathodoluminescence (SEM-CL) from 12 hydrothermal ore deposits formed between ~100 and ~750 °C. The maps clearly show the relationships between trace elements and CL intensity in quartz. Among all samples, no single trace element consistently correlates with variations in CL intensity. However in vein quartz from five porphyry-Cu (Mo-Au) deposits, CL intensity always correlates positively with Ti concentrations, suggesting that Ti is a CL activator in quartz formed at >400 °C. Ti concentrations in most rutile-bearing vein quartz from porphyry copper deposits indicate reasonable formation temperatures of <750 °C using the TitaniQ geothermometer. Titanium concentrations of <10 ppm in all veins that formed at temperatures <350 °C suggest a broad correlation between Ti concentrations and temperature of quartz precipitation. In quartz from most deposits formed at 2000 ppm, but in high-temperature quartz, Al concentrations are consistently in the range of several hundred ppm. Aluminum concentrations in quartz reflect the Al solubility in hydrothermal fluids, which is strongly dependent on pH. Aluminum concentrations in quartz therefore reflect fluctuations in pH that may drive metal-sulfide precipitation in hydrothermal systems.

Intensity of quartz cathodoluminescence and trace-element content in quartz from the porphyry copper deposit at Butte, Montana

Abstract Textures of hydrothermal quartz revealed by cathodoluminescence using a scanning electron microscope (SEM-CL) reflect the physical and chemical environment of quartz formation. Variations in intensity of SEM-CL can be used to distinguish among quartz from superimposed mineralization events in a single vein. In this study, we present a technique to quantify the cathodoluminescent intensity of quartz within individual and among multiple samples to relate luminescence intensity to specific mineralizing events. This technique has been applied to plutonic quartz and three generations of hydrothermal veins at the porphyry copper deposit in Butte, Montana. Analyzed veins include early quartz-molybdenite veins with potassic alteration, pyrite-quartz veins with sericitic alteration, and Main Stage veins with intense sericitic alteration. CL intensity of quartz is diagnostic of each mineralizing event and can be used to fingerprint quartz and its fluid inclusions, isotopes, trace elements, etc., from specific mineralizing episodes. Furthermore, CL intensity increases proportional to temperature of quartz formation, such that plutonic quartz from the Butte quartz monzonite (BQM) that crystallized at temperatures near 750 °C luminesces with the highest intensity, whereas quartz that precipitated at ~250 °C in Main Stage veins luminesces with the least intensity. Trace-element analyses via electron microprobe and laser ablation-ICP-MS indicate that plutonic quartz and each generation of hydrothermal quartz from Butte is dominated by characteristic trace amounts of Al, P, Ti, and Fe. Thus, in addition to CL intensity, each generation of quartz can be distinguished based on its unique trace-element content. Aluminum is generally the most abundant element in all generations of quartz, typically between 50 and 200 ppm, but low-temperature, Main Stage quartz containing 400 to 3600 ppm Al is enriched by an order of magnitude relative to all other quartz generations. Phosphorous is present in abundances between 25 and 75 ppm, and P concentrations in quartz show little variation among quartz generations. Iron is the least abundant of these elements in most quartz types and is slightly enriched in CL-dark quartz in pyrite-quartz veins with sericitic alteration. Titanium is directly correlated with both temperature of quartz precipitation, and intensity of quartz luminescence, such that BQM quartz contains hundreds of ppm Ti, whereas Main Stage quartz contains less than 10 ppm Ti. Our results suggest that Ti concentration in quartz is controlled by temperature of quartz precipitation and that increased Ti concentrations in quartz may be responsible for increased CL intensities.

Is Quartz Cathodoluminescence Color a Reliable Provenance Tool? A Quantitative Examination

ABSTRACT We examined cathodoluminescence (CL) colors of quartz by using red (590-780 nm), green (515-590 nm), and blue (380-515 nm) optical filters interfaced with a cathodoluminescence (CL) detector attached to a scanning electron microscope (SEM). SEM/CL images taken through these filters were captured digitally and transferred to a computer. Luminescence intensities (luminosities) of the images were measured by using available commercial software. Measured luminosities of these CL images are directly related to relative intensities of red, green, and blue CL emissions. Luminosity data were then used to construct plots that display relative luminosities of the CL images acquired through the red, green, and blue filters. An unfiltered CL image of each quartz grain, generated by photons with wavelengths ranging from 200-700 nm, was also acquired. By subtracting the numerical luminosity values of the images acquired through the red, green, and blue filters from the luminosity value of the unfiltered image, the contribution to total luminosity provided by CL emission in the near ultraviolet (UV) was calculated. The CL colors of quartz from a variety of volcanic, plutonic, and metamorphic rocks and hydrothermal deposits were examined. Volcanic quartz phenocrysts have the most restricted CL color range, with strongest emission intensity in the blue wavelength band. CL colors of plutonic quartz overlap those of volcanic phenocrysts but extend over a broader range to include quartz that displays higher intensity of red emission. CL emission in hydrothermal (vein) quartz is similar to that in plutonic quartz, although some hydrothermal quartz exhibits stronger green-CL emission than does plutonic quartz. The CL colors of metamorphic quartz exhibit the widest variation, overlapping the color fields of both volcanic and plutonic quartz and extending further into the red. CL emission in the near UV makes a significant contribution ( 5-85 percent) to the total luminosity of SEM/CL images, particularly images of plutonic quartz. Because of overlap in the CL color ranges of volcanic, plutonic, metamorphic, and hydrothermal quartz, unambiguous identification of quartz provenance on the basis of CL color alone is problematic. It is difficult to distinguish between volcanic and some plutonic quartz, and between some plutonic and hydrothermal quartz, or to distinguish magmatic quartz from metamorphic quartz that exhibits blue CL color. Only metamorphic quartz that exhibits moderately strong red emission appears distinguishable (on the basis of color) from quartz of other origins. Our work thus suggests that CL color is not a reliable indicator of quartz provenance.

Improved electron probe microanalysis of trace elements in quartz

Abstract Quartz occurs in a wide range of geologic environments throughout the Earth’s crust. The concentration and distribution of trace elements in quartz provide information such as temperature and other physical conditions of formation. Trace element analyses with modern electron-probe microanalysis (EPMA) instruments can achieve 99% confidence detection of ~100 ppm with fairly minimal effort for many elements in samples of low to moderate average atomic number such as many common oxides and silicates. However, trace element measurements below 100 ppm in many materials are limited, not only by the precision of the background measurement, but also by the accuracy with which background levels are determined. A new “blank” correction algorithm has been developed and tested on both Cameca and JEOL instruments, which applies a quantitative correction to the emitted X-ray intensities during the iteration of the sample matrix correction based on a zero level (or known trace) abundance calibration standard. This iterated blank correction, when combined with improved background fit models, and an “aggregate” intensity calculation utilizing multiple spectrometer intensities in software for greater geometric efficiency, yields a detection limit of 2 to 3 ppm for Ti and 6 to 7 ppm for Al in quartz at 99% t-test confidence with similar levels for absolute accuracy

Visualizing trace element distribution in quartz using cathodoluminescence, electron microprobe, and laser ablation-inductively coupled plasma-mass spectrometry

Abstract Cathodoluminescent (CL) textures in quartz reveal successive histories of the physical and chemical fluctuations that accompany crystal growth. Such CL textures reflect trace element concentration variations that can be mapped by electron microprobe or laser ablation-inductively coupled plasmamass spectrometry (LA-ICP-MS). Trace element maps in hydrothermal quartz from four different ore deposit types (Carlin-type Au, epithermal Ag, porphyry-Cu, and MVT Pb-Zn) reveal correlations among trace elements and between trace element concentrations and CL textures. The distributions of trace elements reflect variations in the physical and chemical conditions of quartz precipitation. These maps show that Al is the most abundant trace element in hydrothermal quartz. In crystals grown at temperatures below 300 °C, Al concentrations may vary by up to two orders of magnitude between adjacent growth zones, with no evidence for diffusion. The monovalent cations Li, Na, and K, where detectable, always correlate with Al, with Li being the most abundant of the three. In most samples, Al is more abundant than the combined total of the monovalent cations; however, in the MVT sample, molar Al/Li ratios are -0.8. Antimony is present in concentrations up to -120 ppm in epithermal quartz (-200-300 °C), but is not detectable in MVT, Carlin, or porphyry-Cu quartz. Concentrations of Sb do not correlate consistently with those of other trace elements or with CL textures. Titanium is only abundant enough to be mapped in quartz from porphyry-type ore deposits that precipitate at temperatures above -400 °C. In such quartz, Ti concentration correlates positively with CL intensity, suggesting a causative relationship. In contrast, in quartz from other deposit types, there is no consistent correlation between concentrations of any trace element and CL intensity fluctuations.

Cathodoluminescent textures and trace elements in hydrothermal quartz

When viewed with scanning electron microscope-cathodoluminescence (SEM-CL), hydrothermal vein quartz displays textures that are unobservable using other techniques. These textures provide unique insights into the sequence of quartz precipitation and dissolution events during hydrothermal vein formation. Such textures relate specific quartz generations to specific mineralization events or fluid inclusion populations and may also relate quartz isotopic or trace element data to specific hydrothermal events. The most commonly observed CL textures in hydrothermal quartz include: (1) euhedral growth zones of oscillating CL intensity; (2) chalcedonic, coliform, and spheroidal textures; (3) mosaic textures; (4) CL-dark bands; (5) spider and cobweb texture; (6) rounded cores with overgrowths; (7) microbrecciation; (8) rounded or wavy concentric zonation; and (9) homogeneous (or slightly mottled) texture. These textures are present to varying degrees in quartz from different types of hydrothermal ore deposits depending on the pressure, temperature, or composition of hydrothermal fluids, and the rates and magnitude of fluctuations in these variables. In samples, where the geologic setting of quartz is not clear, CL textures distinguish among quartz derived from epithermal, porphyry-type, and orogenic Au deposits. CL textures result from defects in the quartz lattice, including those caused by trace element concentration variations. Like CL textures, trace element abundance and distribution result from variations in the physical and chemical conditions of quartz precipitation, followed by any subsequent solid-state changes in quartz chemistry. Here we show that CL textures, CL spectra, and trace element concentration vary systematically between quartz from various types of hydrothermal ore deposits. The information derived from quartz analysis can therefore be used to fingerprint the origin of quartz and make some inferences about the pressure, temperature, and fluid compositional changes that accompany hydrothermal quartz precipitation.

Tectonic settings of porphyry Cu–Mo–Au deposits in the Himalayan–Tibetan orogen, East Tethys

Porphyry Cu (Mo–Au) deposits in the Himalayan–Tibetan orogen formed during the Late Triassic, Early Cretaceous, Eocene, Oligocene, and Miocene and can be classified into different metallogenic belts according to their petrologic features, mineralization ages, and tectonic settings. A close spatial relationship to regional strike–slip faults is evident in all five belts. Porphyry Cu (Mo–Au) deposits exist in a wide range of tectonic environments, including island arc, syn-collision, post-collisional convergence, and continental-transform plate boundaries. Porphyry Cu deposits cluster in the southernmost part of the Yidun–Zhongdian Belt, along the N–S-trending Gaze River dextral strike–slip fault. Porphyry Cu deposits in the Lijiang–Jinping Belt lie along the Ailaoshan–Red River continental–transform shear zone and the associated strike–slip faults. The Yulong–Malasongduo porphyry belt is controlled by the Cesuo Fault, a NNW-trending regional dextral transcurrent fault that is associated with Palaeogene westward continental oblique subduction along the Jinsha suture. In the Gangdis Belt, Miocene porphyry Cu deposits are localized along N–S-trending normal faults, which were produced by transpression within the regional NW–SE-trending Karakoram–Jiali fault zone (KJFZ). A close spatial relationship between porphyry Cu deposits and strike–slip faults also exists for the Bangong–Nujiang Belt.

Using mineral geochemistry to decipher slab, mantle, and crustal input in the generation of high-Mg andesites and basaltic andesites from the northern Cascade Arc

Abstract To better understand the role of slab melt in the petrogenesis of North Cascades magmas, this study focuses on petrogenesis of high-Mg lavas from the two northernmost active volcanoes in Washington. High-Mg andesites (HMA) and basaltic andesites (HMBA) in the Cascade Arc have high Mg# [molar Mg/(Mg+Fe2+)] relative to their SiO2 contents, elevated Nd/Yb, and are Ni- and Cr-enriched. The rock units examined here include the Tarn Plateau HMBA (51.8–54.0 wt% SiO2, Mg# 68–70) and Glacier Creek HMA (58.3–58.7 wt% SiO2, Mg# 63–64) from the Mount Baker Volcanic Field, and the Lightning Creek HMBA (54.8–54.6 SiO2, Mg# 69–73) from Glacier Peak. This study combines major and trace element compositions of minerals and whole rocks to test several petrogenetic hypotheses and to determine which, if any, are applicable to North Cascades HMA and HMBA. In the Tarn Plateau HMBA, rare earth element (REE) equilibrium liquids calculated from clinopyroxene compositions have high Nd/Yb that positively correlates with Mg#. This correlation suggests an origin similar to that proposed for Aleutian adakites, where intermediate, high Nd/Yb slab-derived melts interact with the overlying mantle to become Mg-rich, and subsequently mix with low Nd/Yb, mantle-derived mafic magmas with lower Mg#. In the Glacier Creek HMA, elevated whole-rock MgO and SiO2 contents resulted from accumulation of xenocrystic olivine and differentiation processes, respectively, but the cause of high Nd/Yb is less clear. However, high whole-rock Sr/P (fluid mobile/fluid immobile) values indicate a mantle source that was fluxed by an enriched, hydrous slab component, likely producing the observed high Nd/Yb REE signature. The Lightning Creek HMBA is a hybridized rock unit with at least three identifiable magmatic components, but only one of which has HMA characteristics. Cr and Mg contents in Cr-spinel and olivine pairs in this HMA component suggest that its source is a strongly depleted mantle, and high whole-rock Sr/P values indicate mantle melting that was induced through hydration, likely adding the component responsible for the observed high Nd/Yb REE pattern. The elevated SiO2 contents (54.6 wt%) of the HMA component resulted from differentiation or high degrees of partial melting of ultramafic material through the addition of H2O. Therefore the Lightning Creek HMBA is interpreted to have originated from a refractory mantle source that underwent melting through interaction with an enriched slab component. Our results indicate that in addition to slab-derived fluids, slab-derived melts also have an important role in the production of HMA and HMBA in the north Cascade Arc.

Rutile inclusions in quartz crystals record decreasing temperature and pressure during the exhumation of the Su-Lu UHP metamorphic belt in Donghai, East China

Abstract Donghai County in the Jiangsu Province of East China is known for its large-scale production of high-quality quartz crystals. The quartz crystals are hosted within the Su-Lu ultrahigh-pressure (UHP) metamorphic belt. They form in quartz veins hosted by eclogite and gneiss, or along contacts between eclogite and gneiss and are mined as placer deposits in the Quaternary sediments. Backscattered electron imaging of the rutile in rutile-bearing euhedral quartz crystals from this region reveal three compositionally and texturally distinct generations of rutile, encapsulated within the host quartz crystal. The earliest generation of rutile (R1) is brightest in BSE, reflecting enrichment in Fe2O3 (0.75-2.08 wt%), Nb2O5 (0.37-0.93 wt%), WO3 (0.48-2.99 wt%), and ZrO2 (0.005-0.105 wt%), relative to R2 and R3. R2 rutile overgrows R1 rutile and contains 0.66-1.11 wt% Fe2O3, 0.45-0.74 wt% Nb2O5, 0.09-0.20 wt% WO3, and <0.001-0.013 wt% ZrO2. R3 rutile, which is the only rutile generation in contact with the host quartz crystals is darkest in BSE and always overgrows R1 and rounded aggregates of R2. R3 rutile contains 0.40-0.68 wt% Fe2O3, 0.13-0.37 wt% Nb2O5, 0.03-0.12 wt% WO3, and <0.001 wt% ZrO2. Oxygen isotopes decrease progressively from 0.6 to -1.5‰ in R1 rutile to 0.1‰ in R2 rutile to -5.1 to -0.3‰ in R3 rutile, consistent with decreasing temperature of rutile crystallization. Combined application of the Zr in rutile and Ti in quartz thermobarometers indicate that these rutile generations precipitated at progressively lower pressures and temperatures, consistent with previously determined retrograde metamorphic conditions of the Su-Lu UHP belt. R1 grew after peak metamorphism during retrograde eclogite facies metamorphism at temperatures between 720 and 800 °C and pressures between 21.7 and 26.6 kbar. During subsequent retrograde metamorphism, R1 was fractured and partially brecciated and overgrown by R2 during epidote-amphibolite facies retrograde metamorphism in the range of 400 to 500 °C and 5 kbar. Zirconium concentrations of <14 ppm in R3 rutile and Ti concentrations of <1 ppm in the co-precipitated host quartz suggest that they both grew at low temperatures of <300° and pressures <2 kbars. Oscillatory growth zones in quartz revealed by cathodoluminescence suggest that no subsequent metamorphism or hydrothermal activity occurred after the formation of the host euhedral quartz crystals and is consistent with a low temperature of quartz crystallization. The random distribution of rutile crystals in the quartz and the lack of evidence for rutile transport by hydrothermal fluids indicate that the host quartz formed by a dissolution-replacement process, whereby the original host mineral, likely omphacite or garnet, was replaced by quartz, but the rutile was not significantly replaced or dissolved due to the low solubility of Ti in low-temperature hydrothermal fluids.