OH defects in quartz in the system quartz–albite–water and granite–water between 5 and 25?kbar

The incorporation of OH defects in quartz from the systems quartz–water, quartz–albite–water and granite–water at pressures between 5 and 25?kbar and temperatures between 800 and 1,000?°C was investigated by IR spectroscopy. The two most important OH absorption features can be assigned to hydrogarnet defects (absorption band at 3,585?cm^?1) and coupled substitutions involving Al³⁺ (Al–H defects, absorption bands at 3,310, 3,378 and 3,430?cm^?1). Al incorporation in quartz is controlled by mineral/melt partitioning (D _Al ^Qz/Melt ?=?0.01) and exhibits a negative pressure dependence. This trend is not clearly reflected by the concentration of Al–H defects, which shows positive deviations from the theoretical 1:1 correlation of Al/H for some samples. In contrast to the Al–H defects, formation of hydrogarnet defects appears to be positively correlated to pressure and water activity, and may be used a petrological indicator. The overall water concentration exhibits only minor changes with pressure and temperature, but a clear correlation of water activity (controlled by various amounts of dissolved salts) and hydrogarnet substitution could be established.     

Dissolution–precipitation reactions in hydrothermal experiments with quartz–feldspar aggregates

Batch and flow-through experiments were performed on quartz–feldspar granular aggregates at hydrothermal conditions (up to ≈150 °C, up to 5 MPa effective pressure, and near-neutral pH) for up to 141 days. The effect of dissolution–precipitation reactions on the surface morphology of the mineral grains was investigated. The starting materials as well as the solids and fluids resulting from the experiments were characterized using BET, energy dispersive X-ray spectroscopy, electron microprobe analysis, inductively coupled plasma-optical emission spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, and X-ray fluorescence spectroscopy. The electrical conductivity of fluid samples was used as a proxy for the evolution of the fluid composition in the experiments. The chemical analyses of the fluids in combination with hydrogeochemical simulations with PHREEQC suggested the precipitation of Al–Si-bearing solid phases. Electron microscopy confirmed the formation of secondary amorphous Al–Si-bearing solid phases. The microscopic observations are consistent with a process of stoichiometric dissolution of the mineral grains, transport of dissolved ions in the fluid phase, and spatially coupled precipitation of sub-μm sized amorphous particles on mineral surfaces. These findings shed light onto early stages of diagenesis of quartz–feldspar sands and indicate that amorphous phases may be precursors for the formation of crystalline phases, for example, clay minerals.     

Zircon–quartz–calcite segregations in carbonate–alkaline metasomatic rocks of the western Baikal region and their petrogenetic implications

Fine-grained segregations up to 5 mm in size composed of graphic intergrowths of zircon, quartz, calcite and containing up to 0.8 wt % SrO have been found in albite–riebeckite and dolomite–biotite metasomatic rocks formed after alaskite granite. They contain magnetite, titanomagnetite (25.4 wt % TiO₂), cerite-(Ce,Nd), rutile (up to 1.2 wt % Nb₂O₅), as well as rare micrograins of monazite-(Ce), bastnaesite-(Ce), and barite (up to 5.7 wt % SrO). The fine-grained structure of mineral aggregates suggests a metacolloidal nature. It is assumed that the zircon–quartz–calcite assemblage was formed due to exchange decomposition reaction between the salt phase of hydrothermal solution with predominant Na₂CO₃, elevated Zr and, to a lesser extent, Fe, Ti, LREE, Nb contents and dissolved calcium and silica compounds of a Na₂SiO₃ type.     

New data on ore geochemistry of the Rodionovskoe gold–quartz deposit,Northeast Russia

The enrichment of gold–quartz ores from the Rodionovskoe deposit in chalcophile elements (Au, Ag, As, Sb) is established. The ores are characterized by small negative Eu anomalies and low REE contents, which are typical of magmatic fluids. Slight enrichment of ores in Bi is evidence of the possible involvement of magmatic fluid in ore formation, which may have been superimposed on early metamorphic quartz veins and veinlets. The variously oriented REE patterns also indicate the presence of another magmatic fluid source, which could be related to the post-ore granitic intrusion. These results generally confirm the metamorphic–magmatic model of the formation of the gold–quartz deposits of the Yana–Kolyma belt. Our data are of practical interest for regional metallogenic forecasts, search, and evaluation of gold deposits.     

The crystallization temperature of vein quartz estimated from the concentration of the titanium paramagnetic center in quartz: A case study of the Peschanka porphyry copper–molybdenum–gold deposit,Western Chukchi Peninsula,Russia

The concentration of the Al and Ti paramagnetic impurity centers in pre-ore and ore-stage quartz at the Peschanka porphyry copper–molybdenum–gold deposit in the Western Chukchi Peninsula, Russia were determined using electron paramagnetic resonance spectroscopy (EPR). The [AlO ₄ ^- /h⁺]⁰ concentration in pre-ore and ore-stage quartz varies from 29 to 124 and from 13 to 101 at. ppm, respectively. The contents of the [TiO ₄ ^- /Li⁺]⁰ and [TiO ₄ ^- /H⁺]⁰ centers reach 20 and 6.3 at. ppm, respectively. Pre-ore quartz associated with the formation of biotite–potassium feldspar–quartz alteration and ore-stage quartz associated with the formation of quartz–sericite rocks followed by the ore deposition differ considerably in the Ti center content, especially the [TiO ₄ ^- /H⁺]⁰ center. The [TiO ₄ ^- /H⁺]⁰ concentration is much higher in the pre-ore quartz (>2 at. ppm) than that in the ore-stage quartz related to copper mineralization (<2 at. ppm). The [TiO ₄ ^- /Li⁺]⁰ concentration also decreases from pre-ore to ore-stage quartz. Taking the data we obtained into account, the formation temperature of pre-ore and ore-stage quartz estimated from a titaniumin-quartz geothermometer is 590–470°C (weighted average 520°C) and 510–310°C (weighted average 430°C), respectively. The obtained temperature range of 590 to 310°C is similar to that determined from homogenization of fluid inclusions in quartz.     

Geochemical features of gold–quartz veins in granitoid intrusives and terrigenous masses of the Yana–Kolyma folded belt in the northeast of Russia

The main task of this study was to reveal geochemical and distinctive features of gold–quartz vein ores of deposits in granitoid intrusive bodies and in terrigenous black-schist masses of the Yana-Kolyma folded belt. The results obtained point to the significant role of metamorphism of the enclosing terrigenous carbonaceous masses in ore formation of both types of deposits. The established facts are not contradictory to the metamorphic–magmagene model of the formation of gold deposits in the Yana–Kolyma belt. The geochemical similarity of both types of deposits shows that these are products of the same orogenic system, which confirms the validity of combining these deposits to form a unified gold–quartz formation.     

New data for geology and geochemistry of the Rodion gold–quartz deposit,northeastern Russia

This paper is focused on the new data for geology, mineralogy, and geochemistry of stockworks consisting of steep and gentle quartz veins and veinlets forming a complex multilevel structure at the Rodion deposit. These stockworks range from 25 to 150 m in thickness. Average gold grade is 1.8 g/t. Ore minerals pyrite, arsenopyrite, chalcopyrite, sphalerite, galena, and native gold are predominantly concentrated on the vein and veinlet walls. Thermal metamorphism caused by the intrusion of the Ulakhan granodiorite pluton is the important singularity of the deposit. The deposit ore is enriched in chalcophile microelements Au, Ag, As, Sb, Cu, Pb, Zn, and Bi as compared to the average composition of the upper crust and hosting Permian sequences. The enrichment factors range from a few to hundreds of times. Bi, W, Pb, Ag, and Na₂O are positively correlated between each other and with Au. The highest correlation coefficient 0.59 is between Au and Bi. Au is negatively correlated with Ba, Li, Co, Ni, Mn, Ti, and Be. The stockwork ores were formed involving homogeneous low-saline (9.4–4.3 wt % NaCl equiv) substantially aqueous bicarbonate-chloride fluid at 275–330°C and 300–1840 bar fluid pressure. Fluid has a high concentration of CO₂ (up to 349 g/kg of water) and is reductive (СО₂/СН₄ = 17–37.3). Na and Ca are the major cations in the fluid, whereas K and Mg are minor. In addition, many microelements were detected in the fluid: As, Li, Rb, Cs, Mo, Ag, Sb, Cu, Zn, Cd, Pb, U, Ga, Ge, Ti, Mn, Fe, Co, Ni, V, Cr, Y, Zr, Sn, Ba, W, Au, Hg, and REE. The results obtained are consistent with the metamorphic–magmatic formation model of orogenic gold–quartz deposits within the Yana–Kolyma belt.     

Gold–quartz deposits of the Zhdaninsky ore–placer cluster,eastern Yakutia: Structural control and formation conditions

Gold deposits and occurrences small in reserves and high in Au grade conventionally determine the line of prospecting in terrigenous sequences of the Verkhoyansk–Kolyma region. In this paper, the geological structure of such gold objects is considered with the example of the deposits and prospects making up the Zhdaninsky ore–placer cluster in the Republic of Sakha (Yakuia). From lithological, structural, and mineralogical–geochemical data, the formation conditions of ore-bearing complexes are specified, the geological evolution history of the northern Ol’chan Zone of the Kular–Nera Belt is reconstructed, and the zonal distribution of mineralization within the ore–placer cluster is revealed. The structural–compositional complexes were formed in the following succession: (1) sedimentation at the shelf of the passive margin accompanied by synsedimentation deformations; (2) metagenesis of sediments and the development of bedding-plane intraformational detachments of collision stage D₁ under conditions of tangential compression and accompanied by the formation of carbon dioxide–aqueous metamorphic fluid at a temperature of 300°C and under a pressure of 1.4 kbar; (3) folding and faulting of orogenic stage D₂ with the formation of synkinematic magmatic bodies, metasomatic alteration, and Au-bearig mineral assemblages. Small Au-bearing objects with veined mineralization and high Au grade are localized in structures of stage D₂ transverse to bedding-plane schistosity S₁. They form at the collision stage above intraformational detachment surfaces and are controlled by shear structures of the orogenic stage with misalignment of these deformations. The ore zoning is determined by the distribution of Co and Ni minerals and by variations in the anionic composition of ore (S, As, Sb).     

Deformation behavior of migmatites: insights from microstructural analysis of a garnet–sillimanite–mullite–quartz–feldspar-bearing anatectic migmatite at Rampura–Agucha,Aravalli–Delhi Fold Belt,NW India

International Journal of Earth Sciences - In the present study we investigate the microstructural development in mullite, quartz and garnet in an anatectic migmatite hosted within a Grenvillian-age...     

The effects of Dissolution–Precipitation Creep on quartz fabrics within the Purgatory Conglomerate,Rhode Island

Quartz Crystallographic Preferred Orientation (CPO) patterns are most commonly a result of deformation by dislocation creep. We investigated whether Dissolution–Precipitation Creep (DPC), a process that occurs at lower differential stresses and temperatures, may result in CPO in quartz. The Purgatory Conglomerate is part of the SE Narragansett basin where strain intensity increases from west to east and is associated with top-to-the-west transport and folding during the Alleghanian orogeny. Within the Purgatory Conglomerate, DPC led to quartz dissolution along cobble surfaces perpendicular to the shortening direction, and quartz precipitation in overgrowths at the ends of the cobbles (strain shadows), parallel to the maximum extension direction. Quartz c-axis orientations as revealed by Electron Backscatter Diffraction (EBSD) methods were random in all analyzed domains within the cobbles and strain shadows irrespective of the intensity of strain or metamorphic grade of the sample. Quartz dissolution probably occurred exclusively along the cobbles' margins, leaving the remaining grains unaffected by DPC. The fact that quartz precipitated in random orientations may indicate that the strain shadows were regions of little or no differential stress.     

Metamorphic alteration,mineral paragenesis and geochemical re-equilibration of early Archean quartz–amphibole–pyroxene gneiss from Akilia,Southwest Greenland

Quartz–amphibole–pyroxene gneiss from the island of Akilia, Southwest Greenland has been claimed to contain the earliest traces of life on Earth in the form of biogenic carbonaceous matter encapsulated as inclusions in apatite crystals. Various lines of evidence, including petrography, geochronology, field relations, and geochemistry, have, however, been presented that challenge this interpretation. Textural relationships and geochemical signatures in this controversial gneiss presented here manifest a complex, spatially variable metamorphic history that includes granulite- and amphibolite-facies overprints and metasomatism. A peak metamorphic, granulite-facies, quartz–orthopyroxene–clinopyroxene–amphibole–magnetite assemblage is preserved in only a few centimeter-scale layers within the 5-m-thick, quartz–amphibole–pyroxene gneiss unit. Calcite veinlets that appear to postdate the peak metamorphism occur in pyroxene. The quartz–amphibole–pyroxene gneiss unit has subsequently experienced isochemical (except hydration) amphibolite-facies alteration during which pyroxenes were retrogressed to amphiboles and magnetite, and calcite was consumed. Parts of the quartz–amphibole–pyroxene gneiss that contain texturally late hornblende have experienced metasomatic alteration by Al-carrying fluids. These fluids controlled the alteration of pyroxenes and amphiboles to hornblende, and modified the trace-element composition by remobilizing LREE and Eu. Apatite has variable REE composition and ⁸⁷Sr/⁸⁶Sr in the quartz–amphibole–pyroxene gneiss, but on the local scale (cm) is in equilibrium with co-existing silicates. Effective recrystallization of apatite crystals as well as co-existing silicates during several stages of the metamorphic history makes the intact preservation of diagenetic apatite with encapsulated primary carbonaceous matter implausible. Hence, it is highly unlikely that Akilia apatite could serve as repository of the earliest traces of life on Earth.     

Generation of hydrogen ions and hydrogen gas in quartz–water crushing experiments: an example of chemical processes in active faults

To understand the fundamental chemical processes of fluid–rock interaction during the pulverization of quartz grains in fault zones, quartz grains were crushed within pure water. The crushing experiments were performed batch style using a shaking apparatus. The crushing process induced a decrease in pH and an increase in hydrogen gas with increased shaking duration. The amount of hydrogen ions generated was five times larger than that of the hydrogen gas, which was consistent with the amount of Si radicals estimated from electron spin resonance measurements by Hochstrasser and Antonini (1972). This indicates that hydrogen gas was generated by consuming most of the Si radicals. The generation of hydrogen ions was most likely related to the presence of silanols on the newly formed mineral surface, implying a change of proton activities in the fluid after pulverization of quartz.     

Geological characteristics, tectonic setting and preliminary interpretations of the Jilau gold–quartz vein deposit, Tajikistan

The southern Tien Shan metallogenic province of Central Asia hosts a number of important gold resources including the Jilau gold–quartz vein system in western Tajikistan. These deposits were formed at the late stages of continent–continent collision in association with subduction-related magmatism, metamorphism and continental margin deformation attributed to the Central Asian Hercynian Orogeny. Jilau is hosted by a Hercynian syntectonic granitoid intrusive that was emplaced into bituminous dolomite country rocks. Economic mineralisation is associated with a dilational jog within a high-angle, oblique dextral-reverse slip shear zone that was undergoing brittle–ductile deformation. The orebody takes the form of shear-zone subparallel quartz veins and lenses that emanate from a steeply plunging ore shoot of veins and stringers within a silicified and sulphidised granodiorite core. It is thought to have formed by a dynamic process in which fluid flow was governed by a fault-valve mechanism. Numerous cycles of fluid pressure build-up, fault failure, jog dilation, fluid flow, phase separation of low salinity H₂O–CO₂–CH₄(–N₂) fluids, and sealing took place. Gold appears together with scheelite and bismuth minerals predominantly as inclusions in arsenopyrite in quartz veins and altered wall-rock, and is mainly associated with quartz containing fluid inclusions enriched in CH₄. The correlation between high gold grades and high CH₄ concentrations suggests that components of the mineralising fluids were derived from, or passed through, the reducing, carbonaceous rocks in the contact aureole of the intrusive. The occurrence of Au and W in an adjacent Hercynian skarn deposit and in the Jilau orebody, infers that the ore metals in both these systems were ultimately derived from a magmatic source. Received: 15 April 1999 / Accepted: 30 December 1999     

Kinetics of feldspar and quartz dissolution at 70–80°C and near-neutral pH: effects of organic acids and NaCl

Effects of the organic acid (OA) anions, oxalate and citrate, on the solubility and dissolution kinetics of feldspars (labradorite, orthoclase, and albite) at 80°C and of quartz at 70°C were investigated at pH 6 in separate batch experiments and in media with different ionic strength (0.02–2.2 M NaCl). Although it has been shown that OAs can increase rates of feldspar dissolution, prior experiments have focused primarily on dilute, highly undersaturated and acidic conditions where feldspar dissolution kinetics are dominated by H⁺ adsorption and exchange reactions. Many natural waters, however, are only weakly acidic and have variable ionic strength and composition which would be expected to influence mineral surface properties and mechanisms of organic ligand-promoted reactions.Oxalate and citrate (2–20 mM) increased the rate of quartz dissolution by up to a factor of 2.5. Quartz solubility, however, was not increased appreciably by these OAs, suggesting that Si–OA complexation is not significant under these conditions. The lack of significant OA–SiO₂ interaction is important to understanding the effects of OAs on the release of both Si and Al from feldspars. In contrast to quartz, both the rates of dissolution and amounts of Si and Al released from the three feldspars studied increased regularly with increasing OA concentration. Feldspar dissolution was congruent at all but the lowest OA concentrations. Total dissolved Al concentrations increased by 1–2 orders of magnitude in the presence of oxalate and citrate, and reached values as high as 43 mg/l (1.6 mM). Si concentrations reached values up to 65 mg/l (2.3 mM) in feldspar–OA experiments. Precipitation of authigenic clays was observed only in experiments without or at very low concentrations of OAs. The high concentrations of dissolved Si attained during dissolution of feldspars in OA solutions, relative to Si concentrations in quartz–OA experiments, is attributed to concomitant release of Si driven by strong Al–OA interactions.Modeling of the dependence of feldspar dissolution rates on OA concentration in natural diagenetic environments is complicated by the competing effects of overall solution chemistry and ionic strength on the dissolution mechanism. Results of experiments using labradorite (An₇₀) indicate that in OA-free solutions, dissolution is progressively slower at increasing NaCl concentrations (up to 2.2 M), in agreement with prior experiments on the effects of alkali metals on feldspar dissolution. The combined effects of oxalate and NaCl on labradorite dissolution rates are such that the rate increase due to oxalate is suppressed by the addition of NaCl. Thus, feldspar dissolution kinetics should be most significantly affected by a given concentration of OAs in low ionic strength solutions.     

Mechanisms of hydrothermal crystallization of quartz at 250°C and 15 kbar

Oxygen isotopic exchange between quartz and water, using a novel technique in which both ¹⁸O/¹⁶O and ¹⁷O/¹⁶O fractionations were measured, yielded an equilibrium fractionation Δ¹⁸ = 9.0 at 250°C and 15 kbar. The reaction proceeds predominantly by solution of fine grains and growth of larger grains. Exchange by solid-state diffusion is immeasurably slow at this temperature. Under the same experimental conditions, cristobalite behaves quite differently, becoming transformed to sub-micron quartz crystals in a few minutes. The phase transformation is accompanied by a kinetic isotope effect yielding quartz in isotopic disequilibrium with water. It is possible that such disequilibrium products are also formed in other experiments involving phase transitions or mineral syntheses.     

Formation of kyanite–quartz veins of the Alpe Sponda,Central Alps,Switzerland: implications for Al transport during regional metamorphism

In this study, we have investigated the formation of quartz–kyanite veins of the Alpe Sponda, Central Alps, Switzerland. We have integrated field observations, fluid inclusion and stable isotope data and combined this with numerical geochemical modeling to constrain the chemical processes of aluminum transport and deposition. The estimated P–T conditions of the quartz–kyanite veins, based on conventional geothermometry (garnet–biotite, white mica solvus and quartz–kyanite oxygen isotope thermometry) and fluid inclusion data, are 550 ± 30°C at 5.0 ± 0.5 kbar. Geochemical modeling involved construction of aqueous species predominance diagrams, calculation of kyanite and quartz solubility, and reaction–path simulations. The results of the modeling demonstrate that (1) for the given chemical composition of the vein-forming fluids mixed Al–Si aqueous species are dominant in transporting Al, and that (2) fluid cooling along a small temperature gradient coupled with a pH decrease is able to explain the precipitation of the quartz–kyanite assemblages in the proportions that are observed in the Alpe Sponda veins. We conclude that sufficient amounts of Al can be transported in typical medium- to high-grade regional metamorphic fluids and that immobile behavior of Al is not very likely in advection–dominanted fluid–rock systems in the upper and middle crust.     

The effect of Dauphiné twinning on plastic strain in quartz

We present an electron backscatter diffraction analysis of five quartz porphyroclasts in a greenschist facies (T = 300–400°C) granitoid protomylonite from the Arolla unit of the NW Alps. Mechanical Dauphiné twinning developed pervasively during the incipient stage of deformation within two porphyroclasts oriented with a negative rhomb plane {z} almost orthogonal to the compression direction (z-twin orientation). Twinning was driven by the anisotropy in the elastic compliance of quartz and resulted in the alignment of the poles of the planes of the more compliant positive rhomb {r} nearly parallel to the compression direction (r-twin orientation). In contrast, we report the lack of twinning in two porphyroclasts already oriented with one of the {r} planes orthogonal to the compression direction. One twinned porphyroclast has been investigated with more detail. It shows the localization of much of the plastic strain into discrete r-twins as a consequence of the higher amount of elastic strain energy stored by r-twins in comparison to z-twins. The presence of Dauphiné twins induced a switch in the dominant active slip systems during plastic deformation, from basal <a> (regions without twinning) to {π} and {π′} <a> (pervasively twinned regions). Dynamic recrystallization is localized along an r-twin and occurred dominantly by progressive subgrain rotation, with a local component of bulging recrystallization. Part of the recrystallized grains underwent rigid-body rotation, approximately about the bulk vorticity axis, which accounts for the development of large misorientation angles. The recrystallized grain size piezometer for quartz yields differential stress of 100 MPa. The comparison of this palaeostress estimate with literature data suggests that mechanical Dauphiné twinning could have a potential use as palaeopiezometer in quartz-bearing rocks.     

Alternative chronologies for Late Quaternary (Last Interglacial–Holocene) deep sea sediments via optical dating of silt-sized quartz

We summarize the results of a test on the potential of optical dating for the age assessment of Late Quaternary deep-sea sediments. Our approach combines a single aliquot regeneration (SAR) protocol for equivalent dose (D_e) estimation on fine silt-sized quartz with a time-dependent evaluation of supported and unsupported long-lived radioisotopes within the deposited sediment matrix. For this purpose nine samples from two independently dated deep-sea cores from the Indian Ocean were obtained. The SAR analysis resulted in mean D_e estimates with precisions ranging from 0.9% to 3.7%. Combination of these data with measured radioisotope concentrations resulted in stratigraphically sensible ages ranging from ca 7 to 120 ka. These ages exhibit good agreement with existing chronologies for the cores based on radiocarbon age determinations and correlation of key volcanic, palaeoecological and isotopic events. Optical dating may provide an important additional dating method for Late Quaternary deep-sea sediments.     

Molybdenite Re–Os and muscovite 40Ar/39Ar dating of quartz vein-type W–Sn polymetallic deposits in Northern Guangdong, South China

Northern Guangdong is an important part of Nanling tungsten–tin metallogenic belt, South China. The tungsten mineralization in this area consists of mainly quartz–wolframite vein-type mineralization, with W–Sn polymetallic deposits mostly distributed at the outer contact zone between concealed Late Jurassic granitic stocks and Cambrian–Ordovician low-metamorphosed sandstones and shales. Molybdenite Re–Os and muscovite ⁴⁰Ar/³⁹Ar isotopic dating of three typical tungsten vein-type deposits (Yaoling, Meiziwo, and Jubankeng) in northern Guangdong, show that two episodes of Late Jurassic W–Sn polymetallic mineralization occurred in this area: an early episode during the Late Jurassic (158–159?Ma) represented by the Yaoling, Hongling, and Meiziwo tungsten deposits, and a younger event during the Early Cretaceous (138?Ma) represented by the Jubankeng deposit. Analysis of available radiometric ages of several W–Sn deposits in the Nanling region indicate that these deposits formed at several intervals during the Mesozoic at 90–100, 134–140, 144–162, and 210–235?Ma, and that large-scale W–Sn mineralization in this region occurred mainly between 150 and 160?Ma.