Showing papers in "Mineralogical Magazine in 1991"

Oscillatory zoning in metamorphic minerals: an indicator of infiltration metasomatism

Abstract: Examples of oscillatory zoning in metamorphic minerals, imaged using a Back-Scattered Electron Detector on the SEM, are described from a series of contrasting environments. These are a prehnite vein sampled by drilling in the Mirvalles geothermal field, Costa Rica, a pyroxene vein developed in a regional metamorphic shear zone in the Yilgarn block, Western Australia, and a bedded metasomatic diopside rock from regionally metamorphosed metasediments in Connemara, Ireland. In each case the formation of oscillatory zoning can be ascribed to mineral growth under supersaturated conditions due to fluid infiltration. Oscillations can be related in the first example to periodic episodes of pressure release and boiling in the geothermal field, but in the regional metamorphic examples actualistic models are harder to define. The development of oscillatory zoning is likely to be a characteristic feature of infiltration metasomatism and can be used as a criterion in the recognition of metasomatic mineral growth in metamorphic rocks outside the vein environment.

Empirical ferric iron corrections: necessity, assumptions, and effects on selected geothermobarometers

Abstract: The ferromagnesian silicate minerals, such as garnets, pyroxenes, micas, and amphiboles, appear in a variety of geothermometers and geobarometers. Where complete chemical analyses are available and regardless of bulk composition (metamorphosed pelitic or mafic), the aforementioned minerals commonly contain ferric iron. In mineral analyses using the electron microprobe, ferric and ferrous iron are not distinguished, and all the iron is treated as FeO. In ferric Fe-bearing minerals, this treatment results in (1) low analytical sums and (2) excess cations in the mineral formulae. Assuming ideal stoichiometry (ideal formula cations and oxygens) allows direct ferric estimates in garnets and pyroxenes; amphiboles require additional assumptions concerning site occupancies, and, for micas, no acceptable constraint exists for a ferric estimate. Based on ferric iron determinations for some metamorphic ferromagnesian silicates, the proportion of ferric to total iron increases at higher Xug values. The influence of ferric estimates on T and P calculations depends on the model used and on the extent the ferric estimate alters the relative proportions of end-members. Several examples suggest that, in general, if ferric estimates (or determinations) are made, they should be made for all the relevant minerals.

Geothermometry and geobarometry of high-grade rocks: a case study on garnet-pyroxene granulites in southern Sri Lanka

Abstract: In the central granulite belt of Sri Lanka, garnet-pyroxene granulites of granitic and gabbro-noritic composition are the most abundant rock types. The micro-structures and mineral chemistry data prove complete attainment of textural and large-scale chemical equilibrium during and following a phase of extreme penetrative deformation at conditions of the granulite facies (800–850 °C, 5 to 9 kbar). On a local scale, especially along the intergranular system, continued cation exchange decoupling from the early ceasing net-transfer reactions destroyed the near-peak metamorphic equilibrium. The extreme compositional variation of the coexisting ferro-magnesian phases (Fe/(Fe + Mg): gar 0.98−0.65, opx 0.92−0.40, cpx 0.88−0.28) and the near-isothermal conditions of equilibration throughout the studied area enabled examination of the effects of non-ideal mixing in garnet and pyroxenes on the equilibrium constants of reactions used in geothermobarometry, and tests on the quality of commonly applied thermometers/barometers and the validity of activity models adopted in the calibrations. The Sri Lankan data set reveals more or less pronounced compositional dependences for all of the tested gar-opx/gar-cpx Fe-Mg exchange thermometers and the opx + plg ⇌ gar + qtz barometers. Evidently the recommended solution models do not adequately describe the mixing properties of the involved ferro-magnesium mineral phases (garnet, orthopyroxene and clinopyroxene).

Competitive Diffusion-Controlled Growth of Porphyroblasts

Abstract: In a diffusion-controlled process of nucleation and growth, adjacent porphyroblasts compete with one another for nutrients. When the effects of this competition are evaluated quantitatively for garnet porphyroblasts in pelitic rocks from the Picuris Range of New Mexico (U.S.A.), significant correlations arise between crystal sizes and the volumes of the domains from which the crystals drew their nutrients. These correlations strengthen the conclusion drawn from earlier work on spatial dispositions, zoning patterns, and crystal size distributions that the kinetics of intergranular diffusion governed the crystallisation of these porphyroblasts. Computer simulations indicate that competition for nutrients during diffusion-controlled growth may have small but detectable effects on crystal size frequency distributions. Diffusional competition therefore introduces relatively minor inaccuracies into attempts to extract quantitative information on crystallisation processes from size distributions using models for the growth of isolated porphyroblasts. In contrast, the effects of diffusional competition on patterns of compositional zoning may be substantial, especially for porphyroblasts in rocks for which chemical inhomogeneity of the precursor leads to strongly clustered spatial dispositions. In such rocks, clustering may alter the patterns of compositional zoning in ways that obscure evidence for diffusion-controlled growth. KEVWORDS: metamorphic crystallisation; intergranular diffusion; reaction kinetics.

The Garnet-Orthopyroxene Al Barometer: Problematic Application to Natural Garnet Lherzolite Assemblages

Abstract: The garnet-orthopyroxene AI barometer specifically considers the A1 content of orthopyroxene in equilibrium with garnet resulting from Mg-Tschermaks substitution. It is demonstrated that P-T calibrations of this barometer derived solely from experimental data for the MAS system, such as that favoured by Finnerty and Boyd (1984, 1987) based on the data of MacGregor (1974), cannot be expected to yield meaningful pressure estimates for natural garnet lherzolite assemblages. The presence of additional CaO, FeO and Cr20 3 components in natural garnet lherzolites can be expected to influence substantially the A1 partitioning between orthopyroxene, garnet and/or spinel at any particular P and T. Thus a more comprehensive barometer formulation is required, such as the one provided by Nickel and Green (1985) that is based on experimental data for the CMAS and SMACCR systems with thermodynamic modelling and addition of an Fe correction term. It is further emphasised that for orthopyroxenes in natural garnet lherzolites the amount of A1 introduced as Mg-Tschermaks substitution cannot be assessed simply as the total AI cation content since such orthopyroxenes frequently contain AI cations linked to Na substitution in M2 sites or to Cr, M1 Ti and possibly Fe 3+ in M1 sites. Revised algorithms for the calculation of specific XAl(Ts) orthopyroxene contents are presented. Application to analytical data sets for garnet lherzolite zenolith suites in the Thaba Putsoa and Mothae kimberlites generates revised upper mantle P-T arrays which refute the widely accepted advocacy by Finnerty and Boyd (1984, 1987) and Finnerty (1989) of an upper-mantle palaeogeotherm beneath northern Lesotho that is markedly inflected to a higher thermal gradient at the depths of derivation of the more chemically fertile, porphyroclastic textured, xenoliths. KEVWORDS: garnet, orthopyroxene, AI barometer, Iherzolite.

Porphyroblast textural sector-zoning and matrix displacement

Abstract: There is an association between the development of cleavage domes, a texture reflecting the displacement of insoluble matrix grains by porphyroblasts growing under a bulk hydrostatic stress, and textural sector-zoning. This has been found in garnet, staurolite, chiastolite, pyrite and possibly emerald porphyroblasts. Sector-zoned porphyroblasts form by lineage growth normal to the crystal faces. This causes several distinctive textures (type 1 inclusions and type 2 intergrowths, inclusion bands, growth prongs), all of which are directly or indirectly related to displacement growth. Graphite or other carbonaceous material is ubiquitous in samples showing textural sector-zoning.

The Crystal Structure of Fedotovite, K2Cu3O(SO4)3

Abstract: The crystal structure of fcdotovite, K1CU30(S04h has been determined, space group C2/e, a 19.037(6), h 9.479(2), e 14.231(5) A, ~ 111.04(3t, Z = 8, Dx = 3.09 g/cm3. The main units of the fedotovite structure are formed around two additional oxygen atoms and consist of edge-sharing [OCU4Jtetrahedra and four [S04J tetrahedra attached to them. The units are further connected by two [S04J tetrahedra, building distinct layers parallel to the yz plane. These layers are interconnected by potassium atoms. In the fedotovite structure, the three kinds of copper atoms are fivefold (4 + 1) coordinated to oxygen atoms with a strong Jahn-Teller effect. The coordination polyhedra of Cui and Cu2 atoms are distorted and flattened orthorhombic pyramids with Cu-O distances varying from 1.912 to 2.333 A,. the sixth neighbour of the both atoms is the copper atom lying at 2.975 and 2.981 Afor Cu2 and Cui respectively. The coordination environment of the Cu3 atom is a distorted elongated orthorhombic pyramid with four Cu-O distances from 1.943 to 1.961 A, a fifth at 2.558 A, and further sixth and seventh neighbours (oxygen and copper atoms) at 2.809 and 2.806 A, respectively.

Cathodoluminescence and growth of cassiterite in the composite lodes at South Crofty Mine Cornwall, England

Abstract: The cathodoluminescence (CL) of cassiterite (SnO2) from the paragenetic sequence in the lodes at South Crofty Mines has been used in the interpretation of the processes which govern mineral deposition. Luminescence is controlled by the substitution of Ti, Fe and W for Sn in the cassiterite lattice. Ti and W behave as luminescence activators whereas Fe quenches luminescence. The observed colours and intensity of the luminescence are believed to result from the interplay between these effects. Successive generations of cassiterite formed during the separate stages of paragenetic evolution show different luminescence colours. These are believed to reflect changes in bulk composition of the fluid, which is indicated by the systematic variation in the composition of tourmaline in the associated gangue. CL also reveals details of growth and deformation in individual crystals. Growth textures observed in CL are broad concentric growth banding, oscillatory zoning and sector zoning. Sector boundaries often show a zig-zag form which is indicative of competitive growth of adjacent crystal faces. Interpretation of CL textures is used to support the hypothesis that cassiterite growth took place under two main regimes. These are, firstly, conditions in which fluid pressure decreased abruptly when tectonic reactivation caused dilation of parts of the lode fracture system and secondly, conditions of sustained hydraulic pressure within fractured vein fill. In tectonically induced fractures the crystals are stubby, have non-equivalent growth faces and are commonly tetragonal bipyramidal forms in which sector zoning is invariably present. In hydraulic fractures crystals take a predominantly tetragonal prismatic form. Thus, it is believed that growth under these contrasting conditions leads to distinctive morphology and zoning in cassiterite, providing a basis for typomorphic characterisation.

The reaction antigorite → olivine + talc + H2O in the Bergell aureole, N. Italy

Abstract: The low-grade contact metamorphic decomposition of antigorite serpentine to olivine and talc has been studied using TEM. The reaction was more complex than has been assumed by previous workers in that (i) FeO played a significant role in controlling the temperature of reaction, and (ii) minor amounts of an aluminous phase (chlorite) were probably produced. The reaction occurred over a range of temperatures within which the assemblage antigorite + talc + olivine was stable and the antigorite became progressively more Mg-rich as the reaction proceeded. Oriented nucleation of talc occurred within the antigorite. However, the olivine breakdown product did not require nucleation; rather it appears to have grown on pre-existing olivine grains. Communication between the sites of reaction was probably facilitated by an ubiquitous grain-boundary fluid.

Corroded plagioclase feldspar inclusions in orthopyroxene and olivine of the Lower and Critical Zones, Western Bushveld Complex

Abstract: The occurrence of spheroidal, ovoid or deeply embayed plagioclase inclusions within cumulus bronzite and olivine grains of both pyroxenites and norites here implies the existence of plagioclase feldspar in melts prior to the separation of ultramafic cumulates from these melts. Variations of Sr-isotope initial ratio are shown to exist within the feldspar populations of such rocks, even on a grain-to-grain scale. Five separate stratigraphic intervals within the Lower and Critical Zones exhibit this texture, and in each case these are intervals wherein normal fractionation trends (decline in both whole-rock and orthopyroxene Mg/(Mg+Fe2+); decline in Cr contents of orthopyroxene) became reversed. The data support a model for evolution of the texture during major episodes of influx of hot, primitive magmatic liquid into the chamber. Progressive mixing of the new liquid with resident liquids (bearing feldspar on the liquidus) effected progressive changes in the composition of the hybrid liquids produced, and resorption and imposition of reversed zoning upon the inclusions.

Vanadian allanite-(La) and vanadian allanite-(Ce) from the Hemlo gold deposit, Ontario Canada

Abstract: Allanite-(La) (containing up to 7.80 wt.% V2O3 and with La/ΣREE and La/Ce atomic ratios up to 0.54 and 1.45, respectively) and allanite-(Ce) (up to 8.46 wt.% V2O3) occur in close association with vanadian muscovite, barian tomichite and vanadian titanite in the main ore zone of the Hemlo gold deposit, Ontario, Canada. Allanite-(Ce) generally occurs as a minor constituent in cross-cutting veins or along foliation planes, whereas allanite-(La) invariably occurs in direct contact with titanite. The high V concentrations in the allanites from Hemlo are readily attributable to an adequate local source of this element, and are most likely controlled mainly by a simple substitution of V for Al in octahedral coordination. Vanadian allanite-(La) and vanadian allanite-(Ce), without any systematic differences in other constituents, are clearly distinct in REE composition, in respect to both the relative concentrations of La and Ce and abundances of other REE. The formation of both allanites (Ce- and La-rich) indicates very localised remobilisation and concentration of REE during a late hydrothermal alteration. The unusual REE composition of vanadian allanite-(La) directly reflects partitioning of REE with coexisting titanite, and the formation of this unusual phase may be attributable to replacement of earlier titanite with redistribution of REE in the solid state.

Fluid inclusion and stable isotope evidence for the origin of mineralizing fluids in south-west England

Abstract: The oxygen and hydrogen isotope composition of hydrothermal fluids associated with the Variscan granites of southwest England has been inferred from analysis of various silicate minerals (predominantly quartz) and by direct analysis of fluid inclusions within quartz and fluorite. These data have been combined with the results of a fluid inclusion study to develop a model for the origin and evolution of hydrothermal fluids in the region. Magmatic fluids expelled from the granites had compositions in the range 6D = -65 to -15%o, and 6180 = 9 to 13%o. Respective temperature, salinity, fluid 6D, and fluid 8180 values for the (i) early Sn-W mineralization, (ii) later Cu-Pb-Zn sulphide mineralization, and (iii) latest 'crosscourse' Pb-Zn-F mineralization are: (i) 230-400 ~ 5-15 wt.% NaC1 equiv,, -39 to -16%o, and 7.0 to 11.2%o, (ii) 220-300 ~ mostly 2-8 wt.% NaC1 equiv., -41 to -9%0, and 2.3 to 8.1%o, and (iii) 110-150~ 22-26 wt.% NaC1 equiv., -45 to +2%0, and -1.8 to +5.5%o. These data highlight the important role of both magmatic fluids exsolved from the crystallizing granite, and basinal brines circulating within restricted fracture systems.

Cathodoluminescence and Microporosity in Alkali Feldspars from the Blå Måne Sø Perthosite, South Greenland

Abstract: Samples from a traverse across the BI~ M~ne Sr perthosite unit in the Tugtut6q Central Complex of the Gardar province, South Greenland have been examined for cathodoluminescence characteristics, microporosity and 6180 isotopic values. Reddening of cathodoluminescence colours in alkali feldspars (normally blue) from the unit may be correlated with increased microporosity of the feldspars as determined using scanning electron microscopy. 6180 values of all samples lie within the range of values expected of juvenile fluids, independent of the level of alteration indicated by cathodoluminescence studies. Observations are consistent with previous suggestions that levels of alkali feldspar microporosity and levels of fluid alteration (as determined by cathodoluminescence of alkali feldspars) are related phenomena. Oxygen isotope ratios suggest that the fluid is largely juvenile in origin, with, perhaps, some meteoric (low 6180) component.

Fluid inclusion and geochemical evidence for fluid mixing in the genesis of Ba-F (Pb-Zn) lodes of the Spanish Central System

Abstract: Fluid inclusion data and geochemical evidence lead to a genesis of Ba-F (Pb-Zn) lodes of the Spanish Central System as related to fluid mixing of hot (>300 °C), low saline ( 2.8 molal), Na-K-Ca-Mg brines of phraeatic origin. Ore formation took place at relative low depth and temperatures (from 270 to 120 °C) in a regime of increasing f O2 , (Ca + Mg)/Na ratio and pH of the fluids towards the surface. Such evolution destabilizes the chloride metal complexes, allowing for the precipitation of Zn and Pb carried by the deep solution. Vertical fluorite-baryte zonation is explained in terms of mineral solubilities. Fluorite deposition in the deeper mineralized zone is related to a slight increase of mCa2+ of the fluid in the lower part of the fluid mixing zone; further increase of mCa2+ and mMg2+ towards the surface promotes fluorite dissolution. Increase of f O2 in the shallow part of the hydrothermal system promotes the oxidation of , resulting in baryte formation. We propose an ore genesis related to fluid mixing in shallow hydrothermal systems associated with an extension of Permo-Triassic age. Such interpretation is in agreement with the present day ideas for the genesis of many of the Ba-F deposits in the Hercynian Belt of Europe.

The Paragenesis of Upper-Mantle Xenoliths from the Quaternary Volcanics South-East of Gees, West Eifel, Germany

Abstract: Group I xenoliths, orthopyroxene-rich and orthopyroxene-free, contain Cr-spinel and clinopyroxene + phlogopite, and occur together with Group II clinopyroxenites + Ti-spinel + phlogopite in K-mafic pyroclastics southeast of Gees. The petrography and clinopyroxene chemistry of orthopyroxene-rich (opx-rich sub-group) Group I xenoliths is consistent with an 'original' harzburgitic mantle that has been transformed to lherzolite by the addition of endiopside. In harzburgites, orthopyroxenes are reacting to diopside + olivine + alkali-silicate melt, and, by inference, the orthopyroxene-free (opx-free subgroup) Group I, dunite-wehrlite series can be linked to the opx-rich sub-group via this reaction. Progressive enrichment of dunitic material in endiopside-diopside has resulted in the formation of wehrlite. Phlogopite is titaniferous and occurs as a trace mineral in opx-rich, Group I xenoliths, whereas substantial phlogopite vein-networks are confined to the opx-free sub-group (dunite-wehrlite series). Interstitial, alkali-felsic glass occurs are veins within, and as extensions of, the phlogopite networks. Clinopyroxenes in phlogopite-veined xenoliths are decreased in Mg/(Mg + FeTotal) (mg) and Cr and increased in Ti, AI and Ca, compared with clinopyroxenes in xenoliths which have trace phlogopite. It is proposed that harzburgitic and dunitic mantle has been infiltrated by a Ca- and alkalirich, hydrous silicate melt rather than an ephemeral carbonatite melt. Dunite has been transformed to phlogopite wehrlite by the invasion of a Ca-, AI-, Ti- and K-rich, hydrous silicate melt. Ca-activity was high initially in the melt and was reduced by clinopyroxene precipitation. This resulted in enhanced K-activity which led to phlogopite veining of clinopyroxene-rich mantle. Group II phlogopite clinopyroxenites contain Ti-spinel and salites that are distinct in their Ti, A1 and Cr contents from endiopsides and diopsides in Group I xenoliths. It is unlikely that these Group II xenoliths represent the culmination of the infiltration processes that have transformed dunite to wehrlite, nor can they be related to the host melt. These xenoliths may have crystailised from Ca- and K-bearing, hydrous silicate melts in mantle channelways buffered by previously precipitated clinopyroxene and phlogopite. Gees lherzolites contain pyroxenes and spinel with distinctly lower A1 contents than these same minerals in lherzolites described previously from other West Eifel localities, which may reflect a distinctive lithology and/or processes of modification for the Gees mantle.

Conversion of nepheline to sodalite during subsolidus processes in alkaline rocks

Abstract: Cathodoluminescence (CL) petrography of nepheline syenites of the Igaliko complex, Gardar province, South Greenland shows that sodalites possess embayed contacts against nepheline, and have formed by a process of metasomatic replacement. This texture is demonstrated clearly by CL, since sodalite luminesces bright orange and nepheline is poorly luminescent. The transformation from nepheline to sodalite results in a volume change which leads to a network of fractures in which deep-blue luminescent fluorite is precipitated. Fluorite is formed since the chlorination process involved in the transformation causes localised reductions of the salinity of the fluid and therefore a decrease in the solubility of fluorite. Sodalite-fluorite textures observed using CL allow sodalites of secondary origin in alkaline igneous rocks to be identified. Nephelines and sodalites, when observed using scanning electron microscopy, possess small micropores. By analogy with recent work on alkali feldspars, pervasive alteration of nephelines may occur by fluid flow assisted by a permeable micropore network.

Modification of antiphase domain sizes in omphacite by dislocation glide and creep mechanisms and its petrological consequences

Abstract: A T.E.M. study of omphacites from the western Italian Alps (Sesia Lanzo Zone and Monviso eclogites) has revealed a bimodal size distribution of antiphase domains: (a) 250-350 ]k, (b) >- 2500 ~. In addition observed dislocation substructures and 'large-scale' antiphase domains are intimately interconnected. A model is presented that can explain modification of the antiphase domain sizes by the interplay of cooling/growth and dislocation glide and/or creep mechanisms. Subsequent coarsening of the modified antiphase domains is inferred to be the result of surface free-energy processes. The model clearly illustrates that only the 'relatively undeformed' areas containing the small-scale antiphase domains can be used for thermometric methods.

Ore textures: problems and opportunities

Abstract: Over the past several decades, thinking about chemical processes in rocks had been dominated by experimental and theoretical treatments of mineral equilibrium, which is the state from which the time variable has been excluded. But, to an extent exceeding that of any of our sister sciences, we in geology are concerned with the behaviour of things as a function of time; thus equilibrium is but one of several interesting boundary conditions. Textures, (defined as the spatial relations within and among minerals and fluids, regardless of scale or origin) provide a means to sort out and identify successive states. In fact, it is the pattern of evolution of those states that enables us to deduce the processes. We may well draw the analogy with thermodynamics and kinetics, respectively: equilibrium textures and phase assemblages, via thermodynamic analysis → definition of conditions of equilibration, whereas kinetics, as displayed in disequilibrium textures → sequence of events and processes of mineralization. The interpretation of textures is one of the most difficult yet important aspects of the study of rocks and ores, and there are few areas of scientific endeavour that are more subject to misinterpretation. Although the difficulties are many, the opportunites for new understanding are also abundant. Textural interpretations have many facets: some are well established and accepted; some that are accepted may be wrong; others are recognised to be speculative and controversial; and we trust that still other textural features remain to be described and interpreted. This paper will deal principally with low-temperature, epigenetic ore deposits, and will emphasise silica and sphalerite; but extension to other materials is not unreasonable. Ore and gangue minerals react internally, or with their environment, at widely ranging rates, ranging from the almost inert pyrite, arsenopyrite, well-crystallised quartz, and tourmaline to the notoriously fickle copper/iron and copper/silver sulfides. Arrested or incomplete reactions may be identifed by textural criteria and, when appropriately quantified, can provide guides to the duration of geological processes. In recent years so much emphasis has been placed on isotopes, fluids, chemistry, and deposit and process models that the textural features have been ignored. In part this oversight occurs because we have grown accustomed to using superposition, cross-cutting, pseudomorphism, mutual intergrowths, exsolution and so on as off-the-shelf tools, to be grasped and applied without evaluation or even description. Surely science must build on previous work without constant and exhaustive reassessment, but for mineral textures a little reassessment may yield substantial benefit.

An exploratory study of acetate decomposition and dissolution of quartz and Pb-rich potassium feldspar at 150°C, 50 MPa (500 bars)

Abstract: Experiments to explore the dissolution behaviour of Pb-rich orthoclase (1% PbO) and quartz have been carried out in the presence of pH buffered and unbuffered potassium acetate and lithium acetate solutions at 150°C and 50 MPa (500 bars). In pH-unbuffered potassium acetate solutions Pb and Na solubilities (and pH) increase with increasing fluid acetate content, reflecting increased bulk dissolution of the feldspar; silica solubility decreases despite an increase in measured pH from 7.5 to 8.9. Similarly, in experiments at pH 6 using a potassium acetate pH buffer, quartz solubility decreases with increasing acetate content. The use of lithium acetate pH buffers (pH 6 at 25°C) in experiments with orthoclase plus quartz results in the precipitation of the lithium chlorite cookeite, complicating interpretation of the fluid chemistry. It is also apparent that in the presence of orthoclase plus quartz (but not albite alone) acetate decarboxylation takes place at much higher rates than expected for the experimental configuration used. The observed effects are unlikely to be due to the presence of acetate alone; the influence of species produced by acetate decay (especially carbonate) must also be considered. This study provides little support for models which call upon acetate to enhance the solubility of aluminosilicate minerals, and suggests that acetate decarboxylation in nature may limit its involvement in dissolution processes. It emphasises the potential of feldspars as sources of elements for mineralisation, such as Pb.

Arsenic-silver incompatibility in fahlore

Abstract: Silver-bearing zinc-iron tetrahedrite-tennantite and freibergit fahlores approximating the simplified formula (Ag,Cu)m(Fe,Zn)2(As,Sb)4S~3 have been equilibrated with excess electrum (AuxAgl_~) and chalcopyrite + pyrite + iron-bearing sphalerite (CuFeS2 + FeS2 + Fe0.05Zn0.95S) in evacuated silica tubes at 300 ~ in reversed silver-copper exchange experiments with less than 0.1 mg NH4CI added as a transport medium. A thermodynamic formulation and parameters describing As-Ag incompatibility at 400 ~ (Ebel and Sack, 1989), which incorporate large temperature dependencies of standard-state properties and composition-ordering systematics, are shown to apply equally well to these 300~ results. A generalised graphical model for this mineral assemblage is presented, describing fahlore composition as a function of temperature and the compositions of coexisting electrum and (Fe,Zn)S, which define the Ag(Cu)_l and Fe(Zn)_l exchange properties controlling fahlore compositions.

A new method for measuring the crystallinity index of quartz by infrared spectroscopy

Abstract: A new method for measuring the crystallinity index of quartz is presented. It is based on the measurement of the intensity of the first derivative of the infrared absorption spectrum at the shoulder at c. 1145 cm -1. The results correlate well with X-ray and D.T.A. crystallinity measurements. KEYWOROS: quartz, chert, infrared spectroscopy, crystallinity index, 1R first derivative.

C-O-H-N fluid inclusions associated with gold- stibnite mineralization in low-grade metamorphic rocks, Mari Rosa mine, Caceres, Spain

Abstract: The Marl Rosa mine lies within a low-grade Precambrian alternating series of black shales and metagreywackes in the Spanish Hercynian massif. There are two generations of mineralized veins: V2, gold-(stibnite)-bearing quartz veins, parallel to the main cleavage, and V3, stibnite-bearing quartz veins which postdate the main deformation event. Four main types of inclusions have been identified. Type I, II and IV are aqueous-carbonaceous inclusions, with variable degrees of filling, while type III are non-aqueous and typically single-phase at room temperature. Except for type I (absent in V3), similar inclusions have been observed in both V2 and V3 veins. Gas compositions are always characterised by CH4-Nz-CO2 assemblages, ranging from CO2-rich mixtures in the earliest inclusions (type I), to N2-rich mixtures in the latest inclusions (type IV). Gold precipitation in V2 veins can be related to type I inclusions at T> 380 ~ (TH = 300-380 ~ A subsequent drop in Xco2 and cooling are recorded in type II and III inclusions, interpreted to be the result of unmixing of a previously homogeneous fluid derived from type I. This boiling would provoke the precipitation of stibnite at 300 ~ and 1 kbar. The type IV inclusions, which are the richest in H20, represent a late fluid circulation at lower temperatures (TH = 190-280 ~ KEYWOROS: fluid inclusions, gold, stibnite, Caceres, Spain.

Textural Evolution of W-Cu-Sn-bearing Hydrothermal Veins at Minas da Panasqueira, Portugal

Abstract: Combined structural and textural studies of the quartz veins of the Panasqueira W-Cu-Sn deposit of central Portugal have revealed the presence of coeval forms of crack-seal and cavity-fill mineral textures. The textures result from the dilational and infilling histories of brittle fractures produced within the host rock by the intrusion of pressurised fluids. Crack-seal textures were produced wherever or whenever crystal growth rates matched or were greater than fracture dilation rates, whereas cavity-fill textures developed wherever or whenever dilation rates were quicker than those of crystal growth. Rates of dilation within the blade-like fissures were naturally lowest at the edges, where types of crack-seal textures were produced, and highest in the cores, where cavity-fill texture formed. During propagation the linkage of adjacent echelon blades, by the rupture of the host-rock straps (bridges) between them, enhanced dilation rates and led to the widespread development of cavity-fill textures. Within the composite fissures so produced, environments in which types of crack-seal textures could form were restricted to the fissure margins and to areas where the tensile strain was sympathetically shared between closely spaced fissures. Textures and minerals characteristic of each fracture environment were able to form simultaneously within the fracture system.

The synthesis and composition of georgeite and its reactions to form other secondary copper(II) carbonates

Abstract: Comparison of the infrared spectra of georgette and a phase which can be reproducibily synthesised in the laboratory shows that the mineral is an amorphous analogue of malachite, Cu2CO3(OH)2. Synthetic studies also explain the chemical conditions under which georgette may form, as well as those which can cause it to react to either malachite or chalconatronite. Parallels may be drawn between the laboratory observations and known mineral associations of georgette.

Petrogenetic Implication of the Mössbauer Hyperfine Parameters of Fe3+-Chromites from Sukinda (India) Ultramafites

Abstract: Chromites from two horizons of the Sukinda area (India) marked as 'grey ore' and 'brown ore' zones have been studied by 57Fe M6ssbauer spectroscopy, which revealed that both chromite types are oxidised and have a type of disordered spinel structure in which octahedral sites are occupied by Fe e+ ions. The spectra of the grey ore sample can be fitted to three doublets corresponding to Fe 2+ (A), Fe 3+ different next-nearest neighbour configurations and a third doublet for Fe z+ at the B site. The brown ores have higher chromium and Fe 3+ content and have lesser amounts of Ni and A1 in comparison to the grey ores. Megascopically, the former shows larger crystal sizes. The high Fe 3§ content in the brown ore suggests that this type of chromite was formed in a region of high fo2 in the magmatic environment. This perhaps occurred at the part of the mantle where the temperature was higher and the rate of cooling was slower than that of the grey ores which crystallised in the magmatic melt. KEYWOROS: spinel, chromite, electron hopping, electron localisation, M6ssbauer spectra, Sukinda, ultramafites, India.

Kornerupine breakdown reactions in paragneisses from southern Madagascar

Abstract: Kornerupine-rich layers up to several centimetres thick with minor sillimanite, spinel, Fe oxide and ilmenite occur in a diopsidite in sillimanite-cordierite gneiss south of Beraketa (24~ 46~ southern Madagascar. Kornerupine, sillimanite, spinel and hematite grains up to 1 mm across have mutual polygonal boundaries indicating textural equilibrium at their crystallisation. Kornerupine has XMg 0.67--0.80 and 0.9 to 2.6 wt.% B203. Sillimanite contains up to 2.0 wt.% Fe203. Spinel is essentially (Mg,Fe 2+) A1204 with an XMg range of 0.29-0.40 and exsolution lamellae of Fe oxide. Textural relations demonstrate two limited reactions, each confined to areas less than 500 ~tm across: (1) Kornerupine and spinel reacted along grain contacts to form very fine-grained tourmaline, corundum and chlorite. The replacing phases are symmetrically zoned with a central tourmaline and hematite, bordered by an aggregate of chlorite, tourmaline and corundum, followed outwards by a rim of chlorite against the kornerupine and spinel. (2) Within kornerupine grains, zoned, round aggregates consist of very fine-grained chlorite, tourmaline and corundum of different composition than in (1). They define the terminal reaction of kornerupine breakdown. Geothermobarometry indicates that the early kornerupine-bearing assemblage was stable at 7.0 kbar and 700 ~ This P-T point lies close to the retrograde, nearly isothermal trajectory defined independently by nearby sapphirine-bearing assemblages. The fine-grained aggregates formed most likely during further cooling, or by increasing water fugacity.