Showing papers in "Mineralium Deposita in 1969"

The Chemistry of Pyrite Formation in Aqueous Solution and its Relation to the Depositional Environment

Abstract: Experimental investigations on pyrite synthesis indicate that before pyrite can be produced by a reaction involving ferrous iron, the disulphide ion must be formed; in experiments described the ion was obtained by the action of H2S in aqueous solution on elemental sulphur. Conditions under which the experiments were conducted indicate that pyrite will not form above pH 6.0. The reaction to produce pyrite is fastest when oxygen is excluded and elemental sulphur is produced from the oxidation of H2S by ferric iron. A reaction between FeS and elemental sulphur will yield pyrite at a much slower rate, although the same basic reaction is involved. An attempt has been made to relate the occurrence of pyrite in different sedimentary environments to this basic chemistry.

Volcanic activity related to the formation of the Kuroko-type deposits in the Kosaka district, Japan

Abstract: In the Miocene Kosaka formation of NE-Japan, submarine volcanic sedimentary deposits of the Kuroko-type occur. This formation consists mainly of volcanics which erupted in a submarine environment. The “Kosaka Volcano” was built up by nine or more volcanic events of a single eruptive cycle. The mode of eruption during a representative single eruptive cycle changed as follows: The Uwamuki tuff breccia is a contact product between dacitic magma and the sea water. Dacitic magma pushed the Motoyama dacite dome upward, as a result of the decrease in vesicularity and perhaps also in temperature. Next, a steam explosion occurred at a flank of this lava dome. The hydrothermal activity which began in this steam explosion center is responsible for the formation of the Kuroko-type Cu-Zn-Pb-mineral deposits. Similar examples of a single eruptive cycle as at Kosaka are also found in Quaternary terrestrial volcanoes of Japan.

The mineralogy and geochemistry of a nickeliferous laterite profile (Greenvale, Queensland, Australia)

Abstract: A laterite profile on serpentinite at Greenvale, Australia, has been investigated in order to elucidate the formation of the secondary minerals and the trace element behaviour during tropical weathering. Mineralogical and chemical studies indicate that the serpentine alters to montmorillonite, aluminous goethite, and quartz. Chromiferous chlorite, a stable component, becomes concentrated in the weathering profile. No members of the kaolin group or bauxite group were identified, and the alumina occurs chiefly in solid solution with the goethite. In the upper levels of the profile serpentine and mont-morillonite disappear completely, and the amount of alumina substituting in the goethite increases. With increasing depth the Fe2O3 and Al2O3 content falls, whereas SiO2 and MgO increase. This is in accordance with the usual trends of lateritic weathering. The trace elements Ni, Co, Mn, Cu and Cr are all concentrated during the weathering process. Evidence suggests that Ni is associated with goethite, and possibly is incorporated in the lattice of this mineral. No appreciable nickel is associated with the manganese minerals. The concentration of Cr takes place mainly through the increase of the stable chromiferous chlorite.

The system Ag-Sb-S from 600°C to 200°C

Abstract: The system Ag-Sb-S was studied between 600°C and 200°C in evacuated silica glass tubes. Results from lower temperature runs require shifts in the stable tie-line configuration found by Barstad at 400°C. It is proposed that the configuration changes near 300°C, and that at 200°C the equilibrium assemblages correspond to those usually reported for minerals in ores. Most of the minerals of the system were synthesized. In addition, the synthetic phase Ag7SbS6 (antimony analogue of the arsenic mineral billingsleyite) is characterized, and the ease of its synthesis in the composition area bounded by argentite-pyrargyrite-sulfur suggests the probable existence of a mineral of this composition. The relatively common mineral stephanite (Ag5SbS4) was not formed as a synthetic product in the temperature range of this study. Combined DTA and X-ray data show that at 197±5°C stephanite decomposes in the absence of sulfur to form pyrargyrite plus argentite, whereas with excess sulfur the products are Sb-billingsleyite plus pyrargyrite. Pyrostilpnite (Ag3SbS3), the low temperature dimorph of pyrargyrite, is unstable above 192±5°C.

La formation actuelle d'un gisement sous-marin de sulfures fumerolliens a Vulcano (mer tyrrhénienne)

Abstract: Les solfatares de l'ľe de Vulcano sont bien connues et l'auteur rappelle leur extreme variabilite depuis la derniere eruption du volcan trachytique de la Fossa (1890). Parce qu'elles provoquent la formation actuelle d'un gisement de sulfures, les fumerolles de la rive W et du fond du Porto di Levante ont ete tout specialement etudiees. Il s'agit essentiellement d'emissions sous-marines de CO2 et de vapeur d'eau a la temperature d'ebullition qui traversent des tufs et des sables imbibes d'eau de mer entre le niveau de la plage et 15–20 m de profondeur. Les tufs, au depart tres poreux, contiennent actuellement jusqu'a 29% de pyrite et de marcassite; ces sulfures de fer se presentent: 1. en couches colloformes sur les diaclases, associes a l'alunite, au soufre et a l'opale; la melanterite, la chalcantite, le gypse, etc. ... apparaissent apres l'echantillonnage. 2. en remplacement de la titanomagnetite, des pyroxenes et de la mesostase des lapilli. 3. en cristaux subautomorphes isoles dans les espaces intergranulaires. 4. en mouches incrustees sur les lapilli, remplissant progressivement les espaces intra- et intergranulaires. 5. en spherules de 5 types: fibroradiees, framboidales, a enveloppes ... A cote de la marcassite toujours bien cristallisee, la pyrite procede d'au moins deux generations: des cristaux bien definis de pyrite claire ou des enduits homogenes de pyrite brunâtre; cette derniere semble provenir de la recristallisation d'une phase sulfuree instable.

Isotope studies on the Rosebery, Mount Farrell and Mount Lyell ores, Tasmania

Abstract: Preliminary studies have been made on the distributions of oxygen and sulphur isotopes in the Rosebery, Mount Farrell, and Mount Lyell ores. These ores lie in Cambrian geosynclinal volcanic rocks in West Tasmania. At each locality the sulphur of the sulphide minerals has a distinctive degree of enrichment in δ34S in relation to sulphur in meteorites and a narrow range of δ34S values. The dominant ore at Mount Lyell (mainly pyrite-chalcopyrite) has an average δ34S value of +7.0‰, the main lode at Rosebery (pyrite-sphalerite-galenachalcopyrite) averages +10.9‰, and the Mount Farrell ore (galena-sphalerite) averages +14.1‰. The degree of enrichment does not appear to be related to local, near-surface geological factors. Other ores of geosynclinal volcanic type with similar mineralogy also show narrow ranges in δ34S and varying enrichments in 34S. Barite from a concordant sulphide-barite-carbonate lode at Rosebery has an average δ34S of +38.1‰ and an average δ18O of +10.7‰. Barite from veins at Mount Lyell has an average δ34S of +25.3‰ and an average δ18O of +10.6‰.

Comportement géochimique des éléments métalliques en milieu réducteur et diagrammes (log S, pH)

Abstract: L'existence de relations entre les teneurs en carbonates et en sulfures dans les eaux des bassins reducteurs, ainsi que l'irreversibilite de la reaction sulfate-sulfure, conduisent a proposer le remplacement des diagrammes Eh. pH, par les diagrammes log S, pH (S designant la concentration totale de S (-II)). Nous donnons ici le principe de construction, des exemples de diagrammes et quelques applications.

High strontium contents and Sr/Ca-ratios in Lake Constance waters and carbonates and their sources in the drainage area of the Rhine River (Alpenrhein)

Abstract: Algal and mollusk carbonates in Lake Constance sediments proved Sr/Ca-ratios unusually high for fresh water environments. In most cases the observed Sr/Ca-ratios for mollusk shells fall into the range of marine mollusks. These high ratios can be attributed to a high Sr/Ca-ratio (6 ± 0.5) in Lake Constance water. In the Alpenrhein River, the main influent of Lake Constance, Sr/Ca-ratios were found to vary between 4.7 (at high water discharge) and 10.7 (at low water level). High Sr/Ca-ratios correspond to high sulfate contents. From all tributaries of the Alpenrhein, the Ill River revealed to have the highest Sr-contents and Sr/Ca-ratios. A detailed study of its drainage area led to the detection of exceptionally high Sr-contents (up to 11.6 mg/l) and Sr/Ca-ratios (up to 116.5) in springs of the Triassic Ladinian stage of the Austroalpine nappe zone (“Ostalpine Deckenzone”). The fact that high Sr-concentrations occur together with high sulfate contents leads to the assumption that celestite is leached within Ladinian rocks. This assumption is confirmed by the evidence that celestite deposits have been reported from the same stratigraphic section (“Wettersteinkalk”) from other localities in the Northern Calcareous Alps.

Erzmikroskopische und mikroanalytische Untersuchungen an Manganerzkonkretionen aus dem pazifischen Ozean

Abstract: Die Verteilung von Eisen, Mangan, Kupfer und Nickel in Manganerzkonkretionen wurde mit Hilfe der Elektronenstrahl-Mikroanalyse untersucht. Die einzelnen Manganknollen zeigen einen inhomogenen Aufbau aus Mn- und Fe-reichen Zonen, die sich erzmikroskopisch unterscheiden lassen. Auffallend sind die Korrelationen zwischen Mangan, Kupfer und Nickel auch im Mikronbereich. Die Zusammenhange zwischen den spektralen Reflexionsfunktionen und der chemischen Zusammensetzung der Manganerzkonkretionen werden diskutiert.

Compositional variations in enargite and luzonite

Abstract: Electronprobe microanalyses were made of enargite and luzonite from various localities. It was shown that enargite can contain as much as 6 wt.% antimony. Complete solid solution was observed between the tetragonal phases luzonite and famatinite. The compositions of enargite and of luzonite-famatinite solid solution were found to be very variable even within one polished section.

The relationship of silver and gold in the basal reef of the Witwatersrand System, South Africa

Abstract: Ag/Au ratios of individual gold particles and of ore-samples obtained from the Basal Reef conglomerate horizon of the Upper Witwatersrand System in the Orange Free State Goldfield (South Africa) are discussed. The Ag/Au values of individual gold grains were found to be exceptionally constant at about 0.0812, whereas the Ag/Au ratios of the ore-samples have markedly higher values and show a large variation. These values indicate that the detrital gold has been homogenized during transportation and sedimentation and that part of the original silver content has been redistributed. It is likely that the first refining of detrital gold particles occurred in streams which eventually drained into the Witwatersrand basin. Further redistribution of the silver relative to the gold took place in the littoral zone of the basin, where the conglomerates were reworked by wave action and erosion. It is contended that supergenic enrichment of gold might also have taken place close to the original shore-line. These processes are believed to account for an increase in the Ag/Au values of ore-samples towards the northwest in the area investigated. Finally, the gold has been homogenized during the metamorphic period of the Witwatersrand System, when pseudohydrothermal transport and redeposition of certain constituents took place. It is suggested that the main portion of the silver content of the original alloy, which was redistributed during the various processes outlined above, is now present as finely dispersed silver minerals. The hypothesis of an elevation control of the Ag/Au values has been found to be improbable.

Recherches préliminaires sur les fluides associés à la genèse des minéralisations en uranium du Limousin (France)

Abstract: L'etude thermometrique des inclusions montre que les phases fluides etaient riches en CO2, a temperature elevee et sous une pression relativement basse lors de la formation des minerais d'uranium du Limousin.

Stratabound barite deposits in Nevada

Abstract: The barite deposits in the Shoshone Range contain rhythmic layering of several types One type includes interlayered barite and clay seams with iron oxide grains and stains, where the barite beds gradually change in thickness upwards Top-bottom (geopetal) features are present but differ from those observed in the Arkansas Stanley Shale deposits Chert and very fine-grained quartz are interlayered with the barite The thickest barite beds lie where the chert and shaly layers are thinnest These are features one would normally find in a sediment These and other observations suggest a formation by syn-diagenetic processes

Les minéralisations plombo-zincifères de Carthagène (Cordillères bétiques, Espagne)

Abstract: A Carthagene, trois episodes volcaniques d'âge tres different remettent en mouvement les mineralisations preexistantes. Les deplacements sont verticaux et ascendants; ils conduisent a des gisements superposes sur une aire tres restreinte. Le «manto pyriteux» (quartz, pyrite, Zn, Pb) correspondrait a une activite keratophyrique. Le «manto des silicates» (greenalite, magnetite, pyrite, Pb, Zn) ainsi que des alterations locales et le remplissage de certains filons sont lies, a une activite doleritique. Les filons (pyrite, Zn, Pb) encaisses dans des trachys andesites d'âge pliocene, sont lies a ce sub-volcanisme tardif. A ces tross deplacements contemporains des volcanismes s'ajoutent des deplacements superficiels. Une concentration paleopedologique discordante comprend un niveau plombifere inclus dans des schistes sombres blanchis et des poches a calamine (Zn) dans les calcaires. Des conglomerats interstratifies d'âge miocene moyen sont mineralises (galets de mineral). La filiation des mineralisations a partir d'un stock unique repose — pour les gisements en relation avec des volcanismes, sur le fait que, en dehors de l'aire mineralisee primitive, les volcanismes sont depourvus de gisements — pour la concentration pedologique, sur l'existence du paleosol immediatement sous les depots sedimentaires miocene moyen et sur la dissociation du Pb et du Zn — pour les depots mineralises sedimentaires, sur la presence, entre autres, de magnetite detritique et de galets de minerai provenant du demantelement des corps mineralises existants.

Sulfur isotopes of the Nairne Pyrite deposit, South Australia

Abstract: The Nairne Pyrite deposit occurs in at least five horizons in a succession of graywackes, quartzites, and siltstones of Cambrian age. Pyrite and pyrrhotite have been traced along bedding planes for almost 100 km suggesting, of course, a syngenetic origin for the sulfides. Sulfur isotopic analyses of samples from the Nairne Pyrite open-cut project provide δS34 values ranging from −12.8 to −20.6 permil. Such enrichment in S32, even though the spread in δS34 values is comparatively narrow, is suggestive of bacteriogenic sulfur which corroborates the geological evidence that the deposit is bacteriogenic — syngenetic in origin.

La région cuprifère à gisements porphyriques de Kerman (Iran)

Abstract: The recent discovery (1967) of a «porphyry copper» deposit in the Tertiary volcano-sedimentary complex of the Kerman region has encouraged detailed prospecting in this relatively unexplored region. This complex extends intermittently from Turkey to Baluchistan with a general south-easterly trend and with relatively uniform character. South of Kerman it is bordered by the metamorphic basement of Sirjan. The complex is folded in an east-west direction and is intersected by a system of north-south echelon faults along which dykes have been intruded. This network of fractures conforms with the «Oman high» which limits the basement and the copper mineralizations to the east and particularly seems to represent a paleogeographic limit. The Paleogene lavas, pyroclastics and sediments are cut by Neogene subvolcanic intrusions. Dykes, sills, and stocks of porphyritic granodiorite invade the cores and flanks of anticlines. Copper mineralization occurs mainly as disseminations around and in the altered stocks, and in the form of veins and stockworks and as impregnations in the surrounding lavas and pyroclastic rocks. Other regions in Iran showing copper mineralizations associated with the Tertiary volcanosedimentary complex have been studied in less detail than the Kerman region. The distribution of the deposits seems to depend upon paleogeographic and structural conditions.

Entwurf zu einer salinar-sedimentären Entstehung der Magnesite vom Typ Entachen (Salzburg)

Abstract: McKalvey veroffentlichte 1967 in einer Studie uber die Genese der Phosphatlagerstatten eine fur die sedimentaren Phosphate charakteristische Abfolge (dunkle kohlenstoffreiche Schiefer, phosphatische Schiefer, Phosphorite, Dolomite, Hornsteine oder Diatomite, verschiedene Carbonate, salinare Sedimente, rote Sande) Diese Serie legt einen Vergleich mit der Gesteinsabfolge im Rahmen der Magnesitlagerstatte Entachen (Salzburg) nahe Der Nachweis von 3% C, 2% P2O5 und die Anwesenheit von Lydit im liegenden schwarzen Magnesit, der Ubergang zu stark rotpigmentierten „tonigen“ Magnesiten im Hangenden unterstreicht die Ahnlichkeit In einem Schema wird der Ablauf der Sedimentation mariner Phosphate mit einer subsalinaren Dolomit- bzw Magnesitbildung in Verbindung gebracht Neben zurucktretender direkter Magnesitabscheidung wird dolomitisches Sediment wahrend der Diagenese metasomatisch zu Magnesit umgesetzt Das Vorhandensein von Chlor in verschiedenen Magnesiten kann als weiterer Hinweis auf die im w S salinare Entstehung derselben angesehen werden In dieser Richtung werden die Untersuchungen fortgesetzt

Die Eisenmanganerze der Grube Dr. Geier bei Bingen/Rhein als Verwitterungsbildungen des Mangans vom Typ Lindener Mark

Abstract: Die Eisenmanganerze der Grube Dr. Geier bei Bingen am Rhein/Westdeutschland gehoren zu den Verwitterungslagerstatten des Mangans. Sie sind an mitteldevonische Dolomite einer isoklinalen Mulde gebunden, die von marinen Sanden des Oligozans uberlagert werden. Ihre Bildungsraume sind Dolinen des Dolomitkarstes. Die Anlage der Dolinen ist gebunden an die Verbreitungsbereiche massiger Dolomite sowie an Zonen starker tektonischer Beanspruchung, in denen sie sich durch Mischungskorrosion gebildet haben. Ihre Fullungen bestehen bis zu 50% aus unvererzten tertiaren Feinsanden und Tonen. Die Eisenmanganerze mit einem durchschnittlichen Fe/Mn-Verhaltnis von 2:1 entstanden aus deszendenten Verwitterungslosungen durch eine gegenseitige Ausfallung von Hydroxidsolen des Fe und Mn. Mineralogisch bestehen sie aus Muskovit (10–50%), Goethit (30–50%), rontgenamorphen Mn-Oxiden (20–30%) sowie Hartmanganerzkonkretionen (0–22%) aus Manganit und Polianit/Pyrolusit. Die Herkunft der Mn-fuhrenden Verwitterungslosungen wird sowohl aus den benachbarten Tonschiefern als auch aus den Dolomiten angenommen.

Schwefel-Isotope aus Blei-Zink-Erzen von Oberschlesien

Abstract: Extrem schwerer Schwefel (bis zu δ34S=+67‰) in den Sulfiden aus den Pb-Zn-Erzen in Carbonatgesteinen Oberschlesiens (Gorny Śląsk) wird als Beweis fur eine Beteiligung sulfat-reduzierender Bakterien (und wahrscheinlich hochsalinarer Losungen) bei der Erzbildung angesehen.

Physical properties and the chemical composition of sphalerites from Yugoslavia

Abstract: Chemical and spectral analyses are given for 16 sphalerites from Yugoslavia. Since they were formed under various conditions the content of Fe and other metals (Mn and Cd especially) varies widely. The relation between the dimensions of elementary cell and the molar percent of FeS in the sphalerites of Yugoslavia shows only slight differences from Vegard's rule. The indices of refraction and reflectivity as well as dispersion for the wavelengths 440, 460, 480, 500, 540, 580, 620, and 660 nm were measured. For the wavelengths mentioned, diagrams are given showing the relation between reflectivity and microhardness of sphalerite and its chemical composition.

Sur trois principes généraux de la Métallogénie et de la recherche minérale

Abstract: Des publications recentes conduisent a insister sur trois demarches, ou trois principes fondamentaux sans lesquels la Metallogenie (comme la Petrologie) et la recherche minerale ne peuvent progresser. 1. Sortons des «types» connus et posons en principe que tout element chimique peut se concentrer a diverses etapes du cycle geologique et geochimique; recherchons donc ces concentrations et traquons les par tous les moyens mineralogiques et geochimiques a notre disposition. 2. Recusons la confusion des concepts chronologiques et des concepts spatiaux, et en particulier la superposition totalement erronee d'«epigenetique» et d'«etranger». 3. Tout est herite d'un passe plus ou moins lointain; principe qui rejoint le premier et nous conduira, dans certains cas favorables, vers des concentrations anterieures.

Sediment-ore-structure relations in barite and associated ores and sediments in the Upper Mississippi valley zinc-lead district near Shullsburg, Wisconsin

Abstract: The deposits which have been studied include the Blackstone, Temperley, and Teasdale mines. Most of the observations were made on sediment-ore-structure relationships. The ores lie in layers and breccia near, and in sag or basin structures at the base of the Quimbys Mill Member of the Platteville formation. The structure and bedding relationships of the contorted calcite and sphalerite bearing limestone layers show that the ores were present in the sediment during deformation. Geopetal and compaction features such as sphalerite nodules in carbonaceous limestones and the bulged ore layers further indicate a sedimentary and diagenetic formation for the ores.

Magnetites from a sulphide bearing iron ore formation in Sweden

Abstract: Magnetite coexisting with sulphides from an iron ore formation in northern Sweden has been investigated with micro-probe and X-ray methods. The Mg/Mg + Fe ratio in magnetite can be correlated with the presence or absence of coexisting pyrite, indicating varying sulphur pressure in the rocks. Sulphurization of magnetite is assumed to be responsible for the introduction of Mg into the magnetite structure.

Paragenetic evolution in Peejay Field, British Columbia, Canada

Abstract: Triassic sediments from the Halfway Formation of Peejay Field in British Columbia, Canada, are described here with special reference to the mineral alterations during diagenesis. These sediments were deposited along the irregular coastline during the middle Triassic transgression. The vertical lithologic succession is complex and considerably altered. The major lithologies concerned are silty sandstones, dolarenite, dololutite, sporadic coquinoid, and accumulated shell debris largely composed of pelecypods, which have been diagenetically altered to dolomite. Evaporites were either introduced through solution and precipitation or by replacement. In certain sandstone units, intense corrosion to complete replacement of clastic quartz as well as matrix by dolomite is observed. The introduction or removal of minerals in the sediments during diagenesis may result from physico-chemical changes in the equilibrium between sediments and the interstitial fluid caused by burial. Hence, the study of diagenetic minerals and paragenesis may be an important tool for interpreting the environment of deposition and the postdepositional history.

High temperature telescoped tungsten-antimony mineralization, Guatemala

Abstract: In the course of a geologic mapping project in northwestern Guatemala an area of unusual stibnite and scheelite mineralization was encountered near Ixtahuacan. The ore is localized along two intersecting faults in the upper part of the Pennsylvanian-Permian Tactic shale and is apparently a telescoped xenothermal deposit. The source of the hydrothermal tungsten-antimony-bearing solutions is uncertain, but field evidence suggests that the fluids could have been associated with a volcanic center less than 4 km to the south. On the basis of the estimated temperature of deposition and experimentally-determined factors controlling scheelite and stibnite solubility, it is suggested that this deposit formed at 400 to 500°C by an extremely rapid reaction between an alkaline hydrothermal solution and Chochal limestone or calcareous layers in the upper Tactic shale. This result contrasts with those for similar deposits which contain minerals thought to form at lower temperatures by a rapid decrease in temperature. Strontium isotopic evidence on Guatemalan volcanic rocks related to those thought to have been the source of the Ixtahuacan metallization suggests that the metals had their source in the upper mantle or basaltic lower crust.

Geology of the San Isidro iron ore deposit, Venezuela

Abstract: The iron ore deposits of Cuadrilatero Ferrifero de San Isidro represent the largest iron ore reserves in Venezuela. The district is a part of the iron metallogenic province of northern Guayana, one of the richest iron-bearing regions of the world. All presently known iron ore deposits of Venezuela are situated within this province: Cerro Bolivar, Altamira, Rondon, San Isidro, Maria Luisa, El Pao and others. Their total ore reserves amount to 2,000 million tons (disregarding the unenriched or slightly enriched iron-formation). The Imataca belt to which the iron ore deposits are confined consists of metamorphosed sedimentary and igneous rocks of Early Precambrian age, the oldest rocks presently known in South America. This belt extends some 450 km from the Orinoco delta southwesterly to the Cauro River. Iron ore is formed from banded iron-formation, a member of the Imataca complex, by removal of silica. The process of supergene enrichment is controlled to a certain degree by structural elements. There are five ore bodies in the San Isidro district, extremely varied in shape and size. Single bodies extend up to 3–4 km in length, approximately parallel to the regional structure pattern, and a few hundred meters in width. The morphology of the bottom of the ore bodies is rather irregular, particularly in transversal sections. Contacts between ore and the unaltered iron-formation beneath are gradational. Maximum vertical section through ore is 260 m; the average is 60 m approximately. The stratigraphic thickness of iron formation has been magnified by structural deformations. The primary stratigraphic thickness is estimated to be some 50–150 m. The iron ore is classified into two main types: a) hard, crustal ore, b) soft, friable ore. Hematite grains which remained after the leaching of silica, and goethite (as cement) are the two main constituents of crustal ore. Hematite and magnetite and a minor amount of quartz are almost the only constituents of friable ore. The crustal ore forms a 15–60 m thick mantle covering friable ore. The overall volume ratio between the friable and the crustal ore is about 2:1. However, it varies in different zones. The mean composition of iron ore on the basis of 10,800 chemical analyses is 64.41% Fe, 2.62% SiO2, 0.6% Al2O3. The ore contains a minor amount of Mn, P, Ti (no S, As, Ba). The ore reserves amount to 750 million tons; in addition, 180–300 million tons of possible ore reserves are estimated.

Neue Beobachtungen zur Deutung der knolligen Magnesite im Tonschieferkomplex des alpinen Buntsandsteins der Nördlichen Kalkalpen

Abstract: Magnesitknollen in den roten Tonschiefern des alpinen Buntsandsteins („Werfener Schiefer“) wurden in fast allen der bearbeiteten Profile zwischen Innsbruck und Saalfelden angetroffen. Es handelt sich um drei Typen von Magnesitknollen, die in ihrer Genese voneinander abweichen: 1. Kryptokristalline Konkretionen fruhdiagenetischer Ausfallung; 2. Syntektonische Rekristallisationsgefuge von knolligem Habitus, die in ihrer Entstehung zeitlich mit der Durchbewegung der sandig-tonigen Nebengesteine zusammenfallen; 3. Postdeformative Spatmagnesite, welche innerhalb des schiefrigen Starrgefuges als Konkretionen auskristallisierten und die Wegsamkeit der Schieferung als bevorzugte Wachstumsrichtung benutzten.

Barite deposits of the ``Montagne Noire'', Southern France

Abstract: Several small barite deposits of Devonian age are known in the Monts de Cabrieres region, Montagne Noire (southern France). A field and laboratory investigation of these stratabound deposits showed their possible diagenetic origin and a limited economic value.

Phase changes in aged sesquioxide gels and some analogies with katamorphic processes

Abstract: The phase changes detected in gel mixtures aged under different conditions offer certain analogies with natural katamorphic phenomena, particularly bauxitisation. pH is by far the most important single factor while composition and order of mixing come a close second. Certain factors also influence the distribution of aluminium in 4- and 6-fold co-ordination, while early stabilization of the latter in a boehmitic phase is an essential precursor to significant yields of kaolinite (including halloysite and hydrated and curled kaolinitic phases) but not for montmorillonite. Boehmite is the earliest phase in most gels and can be retained and crystallised under increased temperatures, pCO2, or reduced pH conditions. Generally however, it is replaced by bayerite, gibbsite, and/or nordstrandite. In the light of these experimental and relevant geological data three main paragenetic sequences are examined: a) Montmorillonite-halloysite-(boehmite) associated in stagnant or waterlogged conditions; b) Kaolinite with subordinate boehmite (and gibbsite) under moderately drained conditions; c) Gibbsite from alkali-rich silica rocks with optimum drainage. As phase changes are in the large part determined by pH conditions attention is drawn to the importance of parent rock type in determining such levels. However, CO2 is also shown to play a significant role. Finally a brief correlation is made between the experimental observations made here and various other paragenetic phenomena- including bauxitic deposits associated with marine activity.

The bases of a scheme for identification of ore minerals by optical methods

Abstract: When the ore microscope is used under conoscopic conditions, it is possible to analyse light which is reflected at oblique incidence from a polished surface. This light is in general elliptically polarised. If the angle and plane of incidence of the incident plane-polarised light are fixed, the azimuth and degree of ellipticity of the reflected light are constants which are characteristic of the reflecting surface for each particular wavelength. These constants are measured with ease and rapidity and may be used for purposes of identification; they may also be used in a quick method for estimating reflectivity or for computation of refractive and absorption indices. The accuracy and significance of the results are discussed.