The reflectance behaviour of gold at temperatures up to 500°C

Summary Reflectance spectra for gold have been determined at temperatures of 25°, 300°, and 500°C between 400 and 700 nm. The extremely steep dispersion curve is significantly flattened by the increase of temperature (higher reflectances in the short wavelengths range, lower reflectances in the middle part of the spectrum and in the long wavelengths range, no temperature dependent reflectances at c. 500 nm). This temperature dependent shifting of the spectral reflectance curve is a reversible process.

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Das Reflexionsverhalten von Gold bei Temperaturen bis zu 500°C

Zusammenfassung Die Reflexionsspektren von Gold wurden bei Temperaturen von 25°, 300° und 500°C zwischen 400 und 700 nm bestimmt. Die außerordentlich steile Dispersonskurve verflacht mit steigender Temperatur (höhere Reflexion im kurzwelligen Spektralbereich, niedrigere Reflexion im mittleren und langwelligen Spektralbereich, temperaturunabhängige Reflexion bei etwa 500 nm). Die temperaturabhängige Änderung des Kurvenverlaufs ist ein umkehrbarer Vorgang.

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With 2 Figures     

Confined fission track lengths in apatite: a diagnostic tool for thermal history analysis

Fission-track ages in apatite are generally accepted as giving a measure of the time over which a sample has been exposed to temperatures below approximately 100° C. A compilation of the lengths of confined fission tracks in a wide variety of apatites from different geological environments has shown that the distribution of confined track lengths can provide unique thermal history information in the temperature range below about 150° C over times of the order of 10⁶ to 10⁹ years. The distribution of confined lengths of freshly produced induced tracks is characterised by a narrow, symmetrical distribution with a mean length of around 16.3 m and a standard deviation of the distribution of approximately 0.9 m. In volcanic and related rocks which have cooled very rapidly, and never been reheated above about 50° C, the distribution is also narrow and symmetric, but with a shorter mean of 14.5 to 15 m, and a standard deviation of the distribution of approximately 1.0 m. In granitic basement terrains which are thought never to have been significantly disturbed thermally, since their original post-emplacement cooling, the distribution becomes negatively skewed, with a mean around 12 or 13 m and a standard deviation between 1.2 and 2 m.This distribution is thought to characterise slow continuous cooling from temperatures in excess of 120° C, to ambient surface temperatures. More complex thermal histories produce correspondingly complex distributions of confined tracks. The continuous production of tracks through time, coupled with the fact that the length of each track shrinks to a value characteristic of the maximum temperature it has experienced, gives a final length distribution which directly reflects the nature of the variation of temperature with time. Most distinctive of the myriad possible forms of the final distribution are the bimodal distributions, which give clear evidence of a two-stage history, including high and low temperature phases. The study of confined length distributions therefore offers invaluable evidence on the meaning of any fission-track age, and bears the potential of providing rigorous constraints on thermal history in the temperature regime below about 150° C. The results of this study strongly suggest that any apatite fission-track age determination should be supported by a confined track length distribution.     

Petrographic and geochemical evidence for hydrothermal evolution of the North Deposit,Mt Tom Price,Western Australia

High-grade iron mineralisation (>65%Fe) in the North Deposit occurs as an E-W trending synclinal sheet within banded iron formation (BIF) of the Early Proterozoic Dales Gorge Member and consists of martite-microplaty hematite ore. Three hypogene alteration zones between unmineralised BIF and high-grade iron ore are observed: (1) distal magnetite-siderite-iron silicate, (2) intermediate hematite-ankerite-magnetite, and (3) proximal martite-microplaty hematite-apatite alteration zones. Fluid inclusions trapped in ankerite within ankerite-hematite veins in the hematite-ankerite-magnetite alteration zone revealed mostly H₂O–CaCl₂ pseudosecondary and secondary inclusions with salinities of 23.9±1.5 (1, n=38) and 24.4±1.5 (1, n=66) eq.wt.% CaCl₂, respectively. Pseudosecondary inclusions homogenised at 253±59.9°C (1, n=34) and secondary inclusions at 117±10.0°C (1, n=66). The decrepitation of pseudosecondary inclusions above 350°C suggests that their trapping temperatures are likely to be higher (i.e. 400°C). Hypogene siderite and ankerite from magnetite-siderite-iron silicate and hematite-ankerite-magnetite alteration zones have similar oxygen isotope compositions, but increasingly enriched carbon isotopes from magnetite-siderite-iron silicate alteration (–8.8±0.7, 1, n=17) to hematite-ankerite-magnetite alteration zones (–4.9±2.2, 1, n=17) when compared to the dolomite in the Wittenoom Formation (0.9±0.7, 1, n=15) that underlies the deposit. A two-stage hydrothermal-supergene model is proposed for the formation of the North Deposit. Early 1a hypogene alteration involved the upward movement of hydrothermal, CaCl₂-rich brines (150–250°C), likely from the carbonate-rich Wittenoom Formation (¹³C signature of 0.9±0.7, 1, n=15), within large-scale folds of the Dales Gorge Member. Fluid rock reactions transformed unmineralised BIF to magnetite siderite-iron silicate BIF, with subsequent desilicification of the chert bands. Stage 1b hypogene alteration is characterised by an increase in temperature (possibly to 400°C), depleted ¹³C signature of –4.9±2.2 (1, n=17), and the formation of hematite-ankerite-magnetite alteration and finally the crystallisation of microplaty hematite. Late Stage 1c hypogene alteration involved the interaction of low temperature (~120°C) basinal brines with the hematite-ankerite-magnetite hydrothermal assemblage leaving a porous martite-microplaty hematite-apatite mineral assemblage. Stage 2 supergene enrichment in the Tertiary resulted in the removal of residual ankerite and apatite and the weathering of the shale bands to clay.Editorial handling: B. Lehmann     

Tridymite: Effect of hydrostatic pressure to 6 kbar on temperatures of two rapidly reversible transitions

Trajectories of two reversible phase transitions in a low-Na synthetic tridymite have been determined to 6 kbar by differential thermal analysis (DTA) in hydrostatic apparatus using Ar or CO₂. The temperature of the lower transition increases from 111 ° C at 1 bar linearly with pressure with slope 1₅ deg kbar^–1. Pressure raises the temperature of the upper transition from 157 ±2 ° or 159 ° C (independently determined) at 1 bar wit a slope of 53 deg kbar^–1, up to 0.7 kbar; for the data above that pressure, the initial slope is 64 deg kbar^–1. Above 2–1/2 kbar, the variation is linear with slope 70 deg kbar^–1. No evidence for other transitions was found at any of the apparent changes of slope. Hystereses for both transitions decreased at high pressures compared to 1-bar. Preferred values for the transition enthalpies, together with these slopes and the Clausius-Clapeyron equation, yield estimates for the volume changes at the transitions of 0.01 (lower) and 0.15 to 0.25 (upper) cm³ gfw^–1. These calculated volume changes are not consonant with many of the high temperature volumetric data on tridymites of varying origins.     

Dehydration-melting of amphibolite at 10 kbar: the effects of temperature and time

We have simulated the dehydration-melting of a natural, low-K, calcic amphibolite (67.4% hornblende, 32.5% anorthite) in piston-cylinder experiments at 10 kbar and 750–1000°C, for 1–9 days. The solidus temperature is lower than 750°C; garnet appears at 850°C. The overall reaction is: Hb+PL+Cpx+Al-Hb+Ca-Hb+Ga+Opx. Three stages of reaction are: (1) melting dominated by the growth of clinopyroxene and garnet, with little change in composition of liquid or garnet, (2) a reversal of this reaction between 875°C and 900°C, with decreases in the amounts of liquid and garnet, and (3) a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase. Garnet is enriched in pyrope and zoned from Fe-cores to Mg-edges (range 3 mol % pyrope); liquid composition is enriched first in An (to 950°C) and then in Ab. The liquids are more calcic and aluminous than natural tonalites, which is attributed to the plagioclase composition (An₉₀). The formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite — not H₂O-saturated conditions. The results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet-absent field (900–1000°C), but with a solidus backbend at 7–9 kbar, coincident with the garnet-in boundary. Hornblende breakdown due to garnet formation in a closed system must make H₂O available for H₂O-undersaturated melting right down to the H₂O-saturated solidus, below 700°C, which defines a large low-temperature PT area where hydrous granitoid melts can be generated with residual garnet and hornblende.     

Stability relations of chromium kyanite at high pressures and temperatures

The pseudobinary section Al₂SiO₅-Cr₂SiO₅ of the system Al₂O₃-Cr₂O₃-SiO₂ has been investigated at 20 and 30 kb pressure in the temperature range 1,000–1,600° C. The solubility of Cr₂SiO₅ in kyanite (Al₂SiO₅) extends up to 24 mole % Cr₂SiO₅ at 20 kb and 31 mole % at 30 kb. The extent of this solid solution is not notably dependent on temperature in the range studied. The stability of kyanite is increased by the substitution Cr Al by about 80° C at 20 kb as compared to Cr-free kyanite. Cr-poor sillimanite and Cr-rich kyanite can stably coexist in a temperature interval of up to 80° C at 20 kb and about 150° C at 25 kb.     

Conditions of formation of gold-bearing arsenopyrite: a comparison of synthetic crystals with samples from Le Chatelet gold deposit, Creuse, France

In order to understand the formation mechanisms of gold-bearing arsenopyrites, hydrothermal experiments have been performed. Needle-shaped gold-rich (up to 1.7 wt % as compared to 1.6 wt % in some natural occurrences) zoned arsenopyrites were obtained at 400 °C and 500 °C under 1 and 2 kbar PH₂O. Comparisons with natural needle-like gold-rich arsenopyrite from the Le Châtelet deposit (Creuse, France) revealed analogous zoning of As/S ratio, with similar gold content and zonations. Fluid inclusion studies carried out in the host quartz of the latter indicate high formation temperatures and variable redox conditions. Compared with the neighbouring Villeranges gold deposit, where arsenopyrite was formed at about 200 °C with gold being mainly trapped within its structure, it appears that temperature is not likely to be the major factor for such an incorporation. Alternatively, it is suggested that non-equilibrium rapid crystallization may induce gold trapping under an extended range of T-P-fO₂ conditions. The practical importance of the needle-like habit of arsenopyrite in areas where geochemical gold anomalies are known without visible gold being detected, is emphasized.     

Hydrothermal alteration of oceanic crust in the West Philippine Sea Basin (Ocean Drilling Program Leg 195, Site 1201): inferences from a mineral chemistry investigation

Summary Secondary minerals of a 91 meters-thick sequence of pillow basalts cored during ODP Leg 195 (Site 1201, West Philippine Basin) were investigated to reconstruct the hydrothermal alteration history and regime. The basement was first buried by red clays, and then by a thick turbidite sequence, thereby isolating it from seawater. The basalts are primitive to moderately fractionated, texturally variable from hypocrystalline and spherulitic to intersertal, sub-ophitic and intergranular. Relic primary minerals are plagioclase, clinopyroxene and opaques. Hydrothermal alteration pervasively affected the basalts, generating secondary clay minerals (mostly glauconite, minor Al-saponite and Fe-beidellite), iddingsite, Ca–Na-zeolites, minor alkali-feldspar and calcite. The secondary mineral paragenesis and mutual relationships suggest that the hydrothermal alteration occurred under zeolite-facies conditions, at temperatures <100–150°C. The main phase of alteration occurred under oxidizing conditions, with a high seawater/rock ratio, in an open-circulation regime, at temperatures of 30–60°C, with precipitation of abundant glauconite and iddingsite. A later stage of alteration occurred at ca. 70°C, with precipitation of abundant Ca–Na-zeolites and minor calcite, in a more restricted circulation regime as a consequence of basement burial under the sedimentary cover, which supplied an altered, Ca-rich and Mg–K-sulfate-poor water causing precipitation of almost pure calcite.     

Alteration and mass transfer in cataclasites and mylonites in 6.6 km of granitic crust at the Siljan impact structure,central Sweden

Granitic rocks deformed by cataclasis and mylonitization on macro- (a few meters) and micro- (thin section) scales are found at depths down to 6.6km in the Siljan impact structure in central Sweden. Granites near fault planes exhibit: (1) fracturing, kinking, fragmentation, and recrystallization of feldspars into pure K and Na endmember varieties, (2) fragmentation, polygonization and development of undulose extinction in quartz, and (3) kinking, appearance of wavy extinction and alteration of biotite, chlorite, amphibole, and alteration of ilmenite and magnetite. Whole-rock chemical analyses of deformed and undeformed rocks show that deformed rocks are enriched in SiO₂ (by about 5 wt.%) and depleted in other oxides by variable percentages. Apart from Rb and Co, the concentrations of other trace elements (including Ba, Sr, Zn, Zr, Pb, Cd, Cu, Cr, Ni, V, U, Th, La, and Li) are lower in deformed relative to undeformed rocks. Mass-balance calculations for a 1000 cm³ model granite which were based on modal mineralogy, whole-rock chemistry, and mineral analyses suggest that the break down of primary biotite, chlorite, and amphibole in deformed zones released elements to circulating fluids. These calculations also indicate liberation of water and a doubling of porosity (from 1 to 2%) during the deformation episodes. Later precipitation of minerals in shear and tension fractures reduced this porosity. Within the upper 2000 m of the Gravberg-1 well, the formation of fracture-filling minerals (smectite, calcite, hematite, chlorite, and albite) is impact-related, and was favored by active circulation of meteoric water. Fracture-filling minerals in the upper 2000 m of the borehole formed at temperatures of 70° to 200°C. Between depths of 2000 and 3500 m, fracture-filling mineral assemblages (dominated by Fe–Mg chlorite, sphene and epidote) suggest formation temperatures in the range of 150° to 300°C. Occurrence of pumpellyite and prehnite in some altered biotite and chlorite of the deformed zones between 3500 and 5500 m suggest preimpact metamorphism and formation temperature above 150°C. Below 5500 m, the mineral assemblages in the fractures are dominated by quartz, sphene, epidote, and some muscovite and chlorite, indicating a temperature range between 300° and 450°C. One of the possible origins for the CH₄ and H₂ gases detected in the Gravberg-1 well is a combination of hydrogen ions released by decomposition of hydrated silicates (biotite, chlorite, hornblende) with carbon. The presence of iron in the deformed granitic rocks prevented the resulting CH₄ from being oxidized.     

The pressure and temperature stability limits of lawsonite: implications for H2O recycling in subduction zones

The stability relations of lawsonite, CaAl₂Si₂O₇(OH)₂H₂O, have been investigated at pressures of 6 to 14 GPa and temperatures of 740 to 1150°C in a multi-anvil apparatus. Experiments used the bulk composition lawsonite+H₂O to determine the maximum stability of lawsonite. Lawsonite is stable on its own bulk composition to a pressure of 13.5 GPa at 800°C, and between 6.5 and 12 GPa at 1000°C. Its composition does not change with pressure or temperature. All lawsonite reactions have grossular, vapour and two other phases in the system Al₂O₃-SiO₂-H₂O (ASH) on their high-temperature side. A Schreinemakers analysis of the ASH phases was used to relate the reactions to each other. At the lowest pressures studied lawsonite breaks down to grossular+kyanite+coesite+vapour in a reaction passing through 980°C at 6 GPa and 1070°C at 9 GPa. Above 9 GPa the reactions coesite=stishovite and kyanite+vapour=topaz-OH are crossed. The maximum thermal stability of lawsonite is at 1080°C, at 9.4 GPa. At higher pressures the lawsonite breakdown reactions have negative slopes. The reaction lawsonite=grossular+topaz-OH+stishovite+vapour passes through 1070°C at 10 GPa and 1010°C at 12 GPa. At 14 GPa, 740–840°C, lawsonite is unstable relative to the assemblage grossular+diaspore+vapour+a hydrous phase with an Al:Si ratio of 1:1. Oxide totals in electron microprobe analyses suggest that the composition of this phase is AlSiO₃(OH). Two experiments on the bulk composition lawsonite+pyrope [Mg₃Al₂Si₃O₁₂] show that at 10 GPa the reaction lawsonite=Gr-Py_ss+topaz-OH+stishovite+vapour is displaced down temperature from the end-member reaction by 200°C for a garnet composition of Gr₂₀Py₈₀. Calculations suggest similar temperature displacements for reaction between lawsonite and Gr-Py-Alm garnets of compositions likely to occur in high-pressure eclogites. Temperatures in subduction zones remain relatively low to considerable depth, and therefore slab P-T paths can be within the stability field of lawsonite from the conditions of its crystallisation in blueschists and eclogites, up to pressures of at least 10 GPa. Lawsonite contains 11.5 wt% H₂O, which when released may trigger partial melting of the slab or mantle, or be incorporated in hydrous phases such as the aluminosilicates synthesised here. These phases may then transport H₂O to an even greater depth in the mantle.     

Sidewall crystallization in the Klokken intrusion: zoned ternary feldspars and coexisting minerals

The syenitic layered series in the Klokken intrusion is surrounded by a zone (500 m thick) of nearly structureless unlaminated syenite followed outwards by a zone of vertically banded gabbro (200 m thick) at the outer rim. The unlaminated syenite is intrusive into the gabbro and develops a thin (2 m) transition zone of syenodiorite at the contact. A traverse across the vertical transition zone and inwards towards the layered series was sampled with a portable drill. Mafic silicates (olivine, clinopyroxene, biotite) show inward evolution in Fe/(Fe+Mg) across the syenodiorite-unlaminated syenite zones. Feldspars change rapidly across the syenodiorite zone from rocks dominated by plagioclase, in some cases together with two alkali feldspars, one a mesoperthite or cryptomesoperthite, the other a cryptoperthite, to rocks in which plagioclase is seen only rarely as cores to cryptomesoperthitic alkali feldspar crystals. Plagioclase is absent from the layered series.Alkali feldspars occurring in pairs have bulk compositions on solvus isotherms in the Or-Ab-An ternary system, estimated at 950° C in a syenogabbro and 910° C in a syenodiorite, at 1 kbar. The more calcic liquids from which they crystallized fractionated on paths that intersected the two- feldspar surface, whereas the more syenitic members crystallized from liquids which terminated crystallization in the one- feldspar field at 900° C. Plagioclases evolve from calcic andesine in syenodiorites, to very rare sodic oligoclase in the most evolved unlaminated syenites. The boundaries between plagioclase cores and alkali feldspar rims, which are usually optically abrupt, involve complex mixed zones on the m -scale, consistent with arrested reaction between plagioclase primocrysts and crystallizing syenitic liquid. Ternary liquidus-solidus relationships are in qualitative agreement with this interpretation. The syenodiorites are cumulates produced during sidewall crystallization of a trachytic magma against a gabbroic chamberlining. This magma changed little in bulk composition as it evolved, giving rise to the unlaminated syenites by further sidewall crystallization. Water build- up in this liquid probably caused a change in style of chamber filling, giving rise to the layered series by bottom accumulation. Microtextures in the zoned feldspars are described in an accompanying paper.CRPG contribution 729     

Indicator kriging for unit vectors: Rock joint orientations

Indicator kriging (IK) is extended to analyze three-dimensional random unit vectors and evaluate the local probability distribution of rock joint orientations in geological formations. The pole vector representing joint orientations is regionalized and projected on a plane normal to the mean attitude of the joint family and centered at the mean. A two-dimensional cutoff system is developed to define the indicator variable, and corresponding indicator variograms and indicator kriging. The cutoff system defines probability regions similar to those of a bivariate distribution, concentric rings sliced into radial sectors. A case study made on an open pit mine proved positively the efficiency of IK and encourages its applications to localized probabilistic structural modeling for geotechnical or geohydrological analysis and oil and gas reservoir analysis.     

Stability of manganocordierite and related phase equilibria in part of the system MnO-Al2O3-SiO2-H2O

The pressure temperature stability of the phase Mn-cordierite hitherto not recorded as a mineral has been determined at temperatures ranging from 400° C up to the melting mainly using standard hydrothermal techniques at the oxygen fugacities provided by the buffering power of the bomb walls. Manganocordierite is a pronounced low-pressure phase with a maximum pressure stability of about 1 kb near 400° C and decreasing pressure limits at higher temperatures. Throughout the temperature range investigated the stable high-pressure breakdown assemblage of Mn-cordierite is spessartine, an Al-silicate, and a SiO₂-polymorph. Due to the variable water contents of Mn-cordierite and spessartine there is a pronounced curvature in the negative dP/dT-slope of the requisite upper pressure breakdown curve of Mn-cordierite. Only theoretical deductions were possible concerning the stable hydrous low-temperature breakdown assemblage of Mn-cordierite below about 400° C.The manganocordierites synthesized are orthorhombic low-cordierites with distortion indices increasing with temperature, water pressure, and duration of heating. Their mean refractive indices increase with rising contents of absorbed water in the structural channels. Based on experiments with natural material the upper temperature stability limit of the mineral carpholite must lie at temperatures below about 400° C for water pressures up to 2.5 kb.The absence of Mn-cordierite from natural rocks studied thus far cannot be explained on chemical grounds, but must be due to its narrow pressure temperature stability range. The phase may yet be discovered as a mineral in manganiferous metasediments formed by lowpressure contact metamorphism.     

Subcalcic diopsides from kimberlites: Chemistry,exsolution microstructures,and thermal history

Twenty-six subcalcic diopside megacrysts (Ca/(Ca+ Mg)) = 0.280–0.349, containing approximately 10 mol% jadeite, from 15 kimberlite bodies in South Africa, Botswana, Tanzania, and Lesotho, have been characterized by electron microprobe analysis, X-ray-precession photography, and transmission electron microscopy. Significant exsolution of pigeonite was observed only in those samples for which Ca/(Ca+Mg)0.320. The exsolution microstructure consists of coherent (001) lamellae with wavelengths ranging from 20 to 31 nm and compositional differences between the hosts and lamellae ranging from 10 to 30 mol% wollastonite. These observations suggest that the exsolution reaction mechanism was spinodal decomposition and that the megacrysts have been quenched at various stages of completion of the decomposition process.Annealing experiments in evacuated SiO₂ glass tubes at 1,150° C for 128 hours failed to homogenize microstructure, whereas, at 5 kbar and 1,150° C for only 7.25 hours, the two lattices were homogenized. This pressure effect suggests that spinodal decomposition in the kimberlitic subcalcic diopside megacrysts can only occur at depths less than 15 km; the cause of the effect may be the jadeite component in the pyroxene. Apparent quench temperatures for the exsolution process in the megacrysts range from 1,250° C to 990° C, suggesting that decomposition must have commenced at temperatures of more than 1,000° C.These P–T limits lead to the conclusion that, in those kimberlites where spinodal decomposition has occurred in subcalcic diopside megacrysts, such decomposition occurred at shallow levels (<15 km) and, at the present erosion level, temperatures must have been greater than 1,000° C.     

Caractéristiques de la phase fluide associée à la genèse des gisements d'étain d'Abbaretz et de La Villeder (Bretagne Meridionale)

The fluid inclusions occurring in quartz of cassiterite-bearing quartz veins from two localities of Southern Brittany have been studied (microthermometry and chemical analysis). In both localities, two sorts of fluids have been recognized: 1. Early fluids, related to the precipitation of cassiterite. Those fluids are closed to a chloride-bearing aqueous solution, with very little CO₂ and hydrocarbons. The salinity is rather low (6 to 9 wt % eq. NaCl). The inclusions homogenize between 150 à 300 °C. The K/Na atomic ratio is about 0,1. From these data and the mineralogical associations (muscovite + kaolinite), the physical and chemical properties of the solution at the time of cassiterite crystallization have been calculated: temperature 350°C, pressure 800 bars; molalities of NaCl, KCl and HCl are, respectively about 1, 0.1 and 0.01 (pH at 25 °C, 1 bar 2, lower than 3 in any case). 2. Late fluids, related to an important kaolinization. They are generally colder, and have a either lower or higher salinity, than the early fluids.     

Application of mineral thermometers and barometers to granitoid igneous rocks: the Etive Complex, W Scotland

Summary The Etive complex, one of the Caledonian Newer Granites of Scotland, is a ring complex of Devonian age, ranging in composition from pyroxene-diorite to leucogranite. Six samples, representing the major rock units in the southern parts of the Etive complex were chosen for mineral chemical studies and for estimation of the pressure and temperature conditions of magmatic crystallisation. Application of Al- in-hornblende barometry and crossite contents of amphiboles indicates a pressure <3kbar for the intrusion, in good agreement with published independent pressure estimates of 2kbar from mineral equilibria in metasedimentary hornfelses in the Etive thermal aureole. Thermometry, using ternary and binary feldspar systems, yields low temperatures, which probably reflect late-stage, post-magmatic re-equilibration of these minerals. Several geothermometers have been applied to the Quarry Diorite, the outermost intrusion of the complex. The highest temperature for the rocks comes from orthopyroxene–clinopyroxene solvus thermometry, and is 1000°C; this is interpreted to reflect the initial crystallisation of the diorite magma immediately after its emplacement. The maximum temperature from hornblende-plagioclase thermometer is 816°C, which probably reflects late-stage crystallisation of the magma.     

Inversions in sub-potassic nephelines

Sub-potassic nephelines in the system NaAlSiO₄(Ne)-KAlSiO₄(Ks) were synthesized under a variety of conditions and studied at room temperature and up to 1000 °C using an X-ray powder diffractometer. At low temperatures they do not have the hexagonal structure determined by Hahn and Buerger (1955) for natural nepheline. Samples with 0.7 to 2.5 mole % Ks have an orthorhombic supercell with parameters equivalent to a, 3a, 3c where a and c are Hahn and Buerger structure cell parameters. Nephelines with 0 to 0.7% Ks consist of two phases with different c axes; one of these phases has the orthorhombic supercell.Pure-Na nephelines (NaAlSiO₄) invert to a hexagonal phase with the Hahn and Buerger structure at 190 °±10 °C; this inversion temperature decreases with increasing Ks and a sample with 0.5% Ks inverts at 170 °±5 °C. The inversion is reversible and is displacive. Another reversible inversion begins at 875 °±10 °C in pure-Na nepheline; this inversion increases in temperature with increasing Ks and a sample with 1.8% Ks begins to invert at 960 °±10 °C.Superstructures with anomalous low-temperature cell parameters in sub-potassic nephelines are attributed to reversible collapse of the framework about the larger cation sites which must be occupied by small Na in subpotassic nephelines. Superstructures in natural nephelines are also related to framework collapse at a displacive inversion.     

Activity-composition relationships for pyrope-grossular garnet

Activity coefficients () for grossular in pyrope-grossular garnet have been determined experimentally using the divariant assemblage garnet-anorthite-sillimanite (kyanite)-quartz. Values of for garnets with 10–12 mole % grossular have been obtained at 1000 °, 1100 °, 1200 ° and 1300 ° C at pressures between 15 and 21 Kb. The data are consistent with a symmetrical regular solid model for grossular-pyrope solid solutions. The interaction parameter (W) increases linearly with decreasing temperature and is given by W = 7460-4.3 T cals (T in °K). A solvus in the pyrope-grossular solid solution is predicted with a temperature of critical mixing of 629°C±90 ° C.     

Das Cu-W-S-System und seine Mineralien sowie ein neues Tungstenitvorkommen in Kipushi/Katanga

Zusammenfassung Die Phasenbeziehungen im Cu-W-S-System wurden zwischen 900 °C und Zimmertemperatur durch DTA- und Abschreckungsexperimente in Quarzglas-und in unter Druck kollabierenden Goldampullen, sowie durch eine Reihe von Verwitterungsversuchen und Umsetzungen in wässrigen Lösungen untersucht. Die im System auftretenden Verbindungen sind Cu₂S (Kupferglanz), Cu_1.97S (Djurleit), Cu_1.75S (Anilit), Cu_1+xS (blaubleibender Covellin), CuS (Covellin) und oberhalb 70°C Cu_1.8S (Digenit), sowie WS₂ (Tungstenit). Es gibt keine ternären Verbindungen. Von allen genannten Phasen ist nur der Tungstenit über den gesamten untersuchten Temperaturbereich stabil. Das System zeigt bei 900 °C neben Schwefelschmelze (L₁) eine Sulfidschmelze (L₂). Es handelt sich um das oberhalb 813 °C auftretende Monotektikum im Randsystem Cu-S, welches im ternären System 0.5 Gew.-% WS₂löst. Die Phase WS₂ koexistiert bei 900 °C mit L₁, L₂ und mit der bei dieser Temperatur lückenlosen Kupferglanz-Digenit-Mischkristallreihe sowie mit W. Außerdem besteht eine Konode zwischen W und Cu₂S. Das gegenseitige Lösungsvermögen der Verbindungen ist selbst bei 900 °C gering. Während Digenit 0.5 Gew.-% WS₂ in fester Lösung aufzunehmen vermag, beträgt umgekehrt die Löslichkeit von Kupfersulfid in WS₂0.2%. Die Phasenbeziehungen unter 900 °C sind charakterisiert durch das Stabilwerden des Covellins bei 507 °C. Kurz unterhalb dieser Temperatur werden WS₂ und CuS nebeneinander stabil. Die Mischungsreihe zwischen Digenit und Kupferglanz ist unterhalb 430 °C nicht mehr lückenlos. Das System Cu-W-S zeigt daher bei 400 °C Konoden von WS₂ zu Covellin, Digenit und Kupferglanz. Unterhalb 70 °C zerfällt der mit Tungstenit koexistierende Digenit zu Anilit und Djurleit. Bei künstlicher Verwitterung von Kupferglanz oder Digenit mit WS₂-Einschlüssen konnten durch teilweise Oxidation mit verdünnter Fe-Sulfat- oder Cu-Sulfatlösung die Kupfersulfide in blaubleibenden Covellin überführt werden, während Tungstenit unter gleichen Bedingungen den Agenzien widerstand, wodurch sich die Koexistenz zwischen Cu_1+xS und WS₂ nachweisen ließ. Die bei niedrigen Temperaturen mit Tungstenit im Cu-W-S-System koexistierenden Phasen sind: Kupferglanz, Djurleit, Anilit, blaubleibender und normaler Covellin; bei Spuren von im Digenitgitter gelösten Fe tritt Anilit nicht auf, statt dessen ist Digenit mit Tungstenit stabil. Ein neues natürliches Tungstenitvorkommen (Kipushi/Katanga) wird beschrieben, das Mineral ist orientiert in massivem Kupferglanz eingewachsen (Abb. 4 und 5).

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The phase relations in the Cu-W-S-system were investigated at various temperatures ranging from almost room temperature up to 900 °C. The experiments were performed in evacuated silica glass tubes with a minimum vapor space. At low temperatures alteration experiments were carried out in water solutions containing copper(II)-sulfate or iron(III)-sulfate. No ternary phase exists in the system. At 900 °C Cu₂S and W are coexisting phases. Tie lines connect WS₂ with the digenite-chalcocite solid solution and with a sulfuric liquid containing 0.5 wt.-% WS₂. Below 813 °C the sulfuric liquid disappears in the Cu-S system (monotectic). On continuous cooling CuS will appear at 507 °C in the Cu-S system and shortly below this temperature covellite coexists with tungstenite. At temperatures below 70 °C tungstenite can coexist with covellite, blaubleibender covellite, anilite, djurleite, and with chalcocite in the pure system. If traces of iron are present anilite will not be formed and digenite remains stable with tungstenite. A new occurrence of tungstenite was observed from the Kipushi mine (Katanga), displaying excellent intergrowth with chalcocite (fig. 4 and 5).

     

Zn-Pb mineralisation in the Silvermines district,Ireland: a product of burial diagenesis

Carbonate-sulphide cement stratigraphic relationships in the host rock and ore have been used to constrain the age of mineralisation at the Silvermines zinc-lead-barium deposit. The base-metal sulphides post-date planar dolomite and replace stylolites. Furthermore, the pre-mineralisation planar dolomites also replace stylolites. These and other diagenetic observations indicate that the base-metal sulphides formed at burial depths greater than 800 m, but probably predate the Variscan deformation (since pressure shadows overgrow base metal sulphides). This indicates that the sulphides are of epigenetic origin, constraining the age of mineralisation to between the late Chadian (347 Ma) and the late Westphalian (307 Ma). However, the most likely age for mineralisation, (based on widespread macro-stylolite development) is Asbian (339 Ma) or younger. No evidence of synsedimentary sulphides (in the form of hydrothermal chimneys, vent faunas, or sulphides intergrown with marine cements) was observed at Silvermines. Mineralised breccias (black matrix breccias), late-stage internal sediments, and dissolution zones within the carbonate cements all appear to be produced by hydrothermal karsting that occurred during the mineralisation process. Fluid inclusion homogenisation temperatures for ore-stage calcites (up to 300 °C) approach the peak temperature estimates derived from regional maturation parameters (270 to 310 °C from conodont alteration indices and vitrinite reflectance). This suggests that homogenisation temperatures represent maximum heating temperatures (probably during Variscan time) rather than mineralisation temperatures.Editorial handling: J. Mengue