Copper deposits in the basal breccias and volcano-sedimentary sequences of the eastern ligurian ophiolites (Italy)

A large number of the copper deposits, associated with the eastern ligurian ophiolites, are linked to the volcano-sedimentary sequences and, in minor amounts, to the ophiolitic basal breccias partially covering the intrusive rocks (ultramafites and gabbros). Some of these Fe-Cu-Zn mineralizations were selected because of their well defined stratigraphic and structural features, which, together with the textural and mineralogical characters and the trace elements in sulphides support the following considerations: - the stratiform mineralizations in the basal breccias show evidence of a sedimentary deposition preceding any consistent emplacement of a volcanic cover; - the stratabound deposits between the volcanic series and the sedimentary cover originate from hydrothermal or volcanic (exhalative) processes; - the stratabound deposits inside the volcanic pile show textural and mineralogical evidence of a volcanic origin with a relatively high temperature of formation; - the stockwork mineralizations, with their epigenetic characters, are of hydrothermal origin, probably related to subsea-floor convection cells.     

The limits of self-similarity

Numerical experiments on simulating ruptures of a one-dimensional plate boundary suggest that certain basic features of seismicity (including multiple ruptures and aftershock sequences) can be adequately reproduced if one assumes the existence of an elementary particle (or geon) of fault dynamics. Some general consequences of corpuscular assumptions in the earth sciences are discussed, with particular reference to the estimation of hazard.     

Thrust-controlled gold mineralisation at the Elandshoogte Mine,Sabie-Pilgrim's Rest goldfield,South Africa

Stratiform quartz-sulphide-gold veins, locally termed reefs are hosted within the Proterozoic Transvaal Sequence sedimentary succession, in the Sabie-Pilgrim's Rest goldfield, eastern Transvaal. These deposits have produced about 180 tonnes of gold and share many characteristics with those of Telfer, Western Australia. Detailed examination of the Elandshoogte Mine shows that gold deposition occurred in two stages, both linked to bedding-parallel thrust faulting within the sedimentary pile. Deformation being concentrated within incompetent shale beds, interlayered within more competent units. The majority of gold was introduced in the second stage of mineralisation and occurs within fractures in early-formed sulphide minerals. Deposition of competent quartz veins accompanying early sulphide and gold mineralisation resulted in a change in deformation style within the reef zone, from early shearing in shales to later duplex faulting of the quartz-reef. Fluids accompanying faulting are implied to have transported gold, and a magmatic source of mineralisation is suggested.     

Fracture mechanisms of offset rock joints-A laboratory investigation

To study the failure mechanisms of joints and rock bridges in jointed rock masses a series of uniaxial compression tests were performed on specimens made of rock-like material. Specimens of size 63.5cm × 27.9cm × 20.3cm, made of 72% silica sand, 16% cement (Type I) and 12% water by weight were tested. The joint inclination angle was maintained at 45°, while the offset angle i.e. angle between the plane of the joint and the line that connects the two inner tips of the joints, was changed from 0°–120° with an increment of 15°. The tests were performed using a 2000kN universal compression machine and a HP data acquisition system. In each sample, five LVDTs were fixed to measure the displacements along and across both the bridge and the joint segment, and the total displacement along the total length of the sample. The failure mechanism was monitored by visual inspection and a magnifier to detect cracks initiation. For each test the failure surfaces were investigated to determine the characteristics of each surface. In all of the tested samples curvilinear cracks called wing cracks initiated at the joints tips due to high tensile stresses concentration. These wing cracks were directed along the direction of the uniaxial load. The coalescence mechanism of two cracks was investigated. Results showed that open cracks could coalesce by shear failure or tensile failure. The coalescence path was found to be mainly dependent on the inclination of the rock bridge between the cracks.     

A New Method for Estimating Lengths for Partially Exposed Features

Length distributions from sedimentary bodies can be measured in outcrop. A common feature in many outcrops is the occurrence of partially exposed bodies, i.e., sedimentary bodies that are cut by either one or both edges of the outcrop. Ignoring these partials while establishing a length distribution leads to a bias toward the smaller length classes. This is because larger bodies have a larger probability to occur as partials than smaller bodies. We present a new method for estimating the expected complete length for a partially exposed feature, based on the theory of conditional probability. The method is demonstrated and tested on a sandstone outcrop exhibiting over 200 interbedded thin silt/shale streaks. It is shown that the method can significantly enlarge both the size and the representativeness of length distributions.     

Floor failure analysis at a longwall mining face based on the multiple sliding block model

This paper presents an analytical model of a floor failure at a longwall coal mining face based on the multiple sliding block model. During longwall mining, stresses and displacements of strata are constantly changing. High stress concentrations at the coal face can exceed the rock strength and initiate fractures in the strata that can, under unfavourable conditions, lead to large floor displacements and disruption of mining operations. Underground observations of the rock floor and the computational modelling of the longwall face indicate that two types of fracture dominate floor failure. Extensive lateral fracturing often develops along the numerous weak bedding planes that are typically present in the sedimentary strata while sub-vertical fractures form in response to changing stress abutments ahead of the longwall coal face. The fractures that initially develop ahead of the longwall face are subject to a secondary movement when exposed ahead of the longwall supports. In response to the stress relief as the coal is mined from above, strata move towards the opening and bending of the floor occurs. If fractures in the floor exist, the floor blocks will displace in response to the floor movement and interact at the fractured surfaces. This analysis attempts to explain how the stress distribution develops within the broken floor during an active movement of floor strata leading to high stress concentrations at the floor level. The analytical formulation in this paper is supplemented by numerical modelling, and results are presented to verify that the analytical solutions are in accordance with the numerical predictions.     

Gold mineralization in As-rich mesothermal gold ores of the Bogosu-Prestea mining district of the Ashanti Gold Belt,Ghana: remobilization of “invisible” gold

The Bogosu-Prestea mining district of southwestern Ghana is a 33 km section of the Early Proterozoic Ashanti Gold Belt. Greenschist facies carbonaceous and carbonate-bearing turbidites and greywackes, and mafic dikes host numerous economic mesothermal gold deposits. Structurally higher ores in the Bogosu concession have brittle deformation and consist of disseminated-sulphide lodes in tectonically-disrupted sedimentary rocks and carbonate-altered mafic dikes. Most gold occurs as micrometre-size particles in arsenian pyrite, and as invisible gold in arsenian pyrite and arsenopyrite. The structurally deeper ores of the adjoining Prestea concession are associated with brittle-ductile deformation and consist of extensive crack-seal quartz-veins and graphitic shear zones. Only minor amounts of invisible gold were detected; in these deeper lodes, gold occurs dominantly as abundant microscopic and larger particles in sulphide/arsenide minerals and in gangue. The gold distribution patterns revealed by SIMS microprobe analysis and ion maps, EMP and colour staining suggest that most of the primary gold in the Bogosu-Prestea system precipitated in solid-solution with sulphide/arsenide minerals. However, post-depositional concentration and redistribution occurred, in increasing degree with: 1) increase in metamorphic/hydrothermal gradients in the gold system (depth), 2) decrease in the refractory properties of the host mineral, and 3) increase in the amount of post-depositional, host-mineral recrystallization and deformation. Gold evolved from primary solid-solution within sulphide/arsenide minerals, to colloidal and micrometre-size particles concentrated in voids, fractures and internal grain boundaries, and finally to microscopic and larger particles at sulphide/arsenide grain margins and in the gangue assemblage. The general conclusions presented here are applicable to As-rich gold deposits of all ages, worldwide. The presence of gold in late fractures is insufficient evidence for late-stage introduction of gold in mesothermal gold systems.     

Dependency of unit shaft resistance on in-situ stress: Observations derived from collected field data

In this study, a database comprised of 30 pullout pile load tests was collected from geotechnical literature and analyzed to investigate the dependency of unit shaft resistance on effective vertical stress. The collected database consists of steel pipe, timber, and concrete piles, with varying normalized penetration depth with respect to pile diameter, driven into loose to very dense sand. Different correlations for the uplift lateral earth pressure coefficient K, Bjerrum-Burland ratio , and the average unit shaft resistance f _ave were derived using different assumptions. A comparison between measured and predicted capacities of the collected piles using the developed correlations indicated that the assumption of values of K and that were constant with depth did not provide a reasonable fit for the measured capacities of the collected piles and thus this assumption is inappropriate. The best correlations for K and that yield a reasonable fit to the measured capacities of the collected piles were found to be functions of sand relative density, pile diameter, and level of effective vertical stress. This indicates that average unit shaft resistance does not reach a limiting value, but rather continues to increase with depth. Moreover, the correlations for K and in terms of effective stress revealed that average unit shaft resistance increases as pile diameter decreases and this increase depends on initial sand relative density. Comparisons of measured and predicted pullout capacities of the collected piles using the best-obtained correlations for K and were made and compared to predictionsobtained from other methods. On the basis of these comparisons, it is concluded that the correlations for K and in terms of effective stress give results comparable to those obtained from other methods, without stipulating limiting values for the average unit shaft resistance.     

Analysis of spatial order in sandstones. I. Basic principles

The spatial arrangement of sedimentary rock components is a fundamental property of sedimentary rocks. If we assume that the size, shape, and composition of sedimentary rock components (mineral grains, pores) carry useful petrologic information, there is no reason to assume that their spatial arrangement does not. Spatial arrangement has been discussed in terms of texture and fabric, but it has had little objective measurement or classification. This deficiency is primarily due to the difficulties associated with the quantification of spatial phenomena. However, using digital-imaging techniques it is possible to generate petrographic images from thin-sections and quantify the spatial arrangement of selected rock components using a two-dimensional (2D) Fourier transform. A Fourier transform creates a spectral representation of the image similar to an x-ray diffraction pattern. This paper presents the fundamental framework of 2D Fourier analysis in petrology. This type of analysis provides a means to quantify and analyze the spatial arrangement of rock components in an objective, mathematical framework. 2D Fourier power spectra can be used to characterize the type and degree of spatial order in an image, both in terms of the classical concepts of long-range and short-range order and in terms of spatial patterns characteristic of sedimentary rock. The clearly defined mathematical relationship between an image and its Fourier power spectrum provide the opportunity to define the 2D structure of an image in the same manner that x-ray diffraction patterns are used to map 3D structure in minerals. In addition, a 2D Fourier power spectrum is easily transformed into a radial power spectrum. Radial power spectra can be used to characterize the density of objects in an image. They also provide a valid means to compare and contrast images in a multivariate framework, regardless of the type of order. One of the most desirable properties of a Fourier transform is its reversibility. Using selected components of the power spectrum, the inverse transform can be used to build synthetic images, which highlight those petrologic components that most affect the power spectrum. The inverse transform provides the means to translate the results of analysis into meaningful petrologic characteristics.     

The Kirkland-Larder Lake stratiform carbonatite

The auriferous zone long known as the Larder Lake Break and more recently as the Larder Lake Exhalite is here interpreted as an exhalative kimberlitic carbonatite on the basis of its K, Ba, Sr, Mg, Fe, Cr, Ni and Co contents, and by comparison with similar younger rocks. Gold is believed to have been deposited syngenetically with this stratiform carbonatite and to be genetically related to late Archean tectonic, volcanic, petrochemical, sedimentary and exhalative processes. Rocks of the Timiskaming Series which host the carbonatite appear to represent a post-uplift graben assemblage of sedimentary and alkalic igneous rocks analagous to modern alkalic-carbonatitic complexes located in rifted upwarps. Preliminary data seems to indicate that carbonatite in general has a higher gold abundance than any other rock type, including iron formations and ultramafic rocks. Ferromagnesian carbonatites, the least common variety, may be worthy untested targets for gold exploration programs, particularly if some pyrite or other sulphide mineral is present.     

Volatile production and transport in regional metamorphism

Calculations show that H₂O and CO₂ produced during devolatilization of an average pelite will occupy 12 vol. % of the rock at 500°C and 5 kb. Because the tensional strength of well foliated rock at metamorphic conditions is vanishingly small, such a volume of fluid having any vertical extent will fracture the rock and escape upward owing to its lower density.In a simplified model of a sudden increase of heat flow from 0.8 to 2.5 H.F.U., the average pelitic rock will have a rate of fluid production averaging 9.4×10^–10 g cm² s^–1 between 400°C and 600°C. The escape of this fluid can be accomodated by a single fracture 1 cm long and 0.2 wide per cm² of rock. If the fracture is reduced to 0.02 then 1,000 cm of fracture per cm² would be required. This width is the minimum original width as calculated from the volume of fluid observed in fluid inclusions trapped along annealed fractures within quartz in metamorphic terrains. Fluid flow will be laminar if the fracture is <0.025 cm wide. Additional calculations show that grain boundary diffusion is not an effective means of fluid transport in regional metamorphism.The commonly observed quartz segregations in pelitic terrains appear to mark the site of major channelways for fluid escape. In this case the bulk of escaping fluid is not able to react pervasively with rocks higher in the metamorphisc pile. Regionally metamorphosed rocks will have a discrete fluid phase only when devolatization reactions are actually taking place. At other times only an absorbed surface monolayer of volatiles on the minerals will be present.Died April 2, 1980     

The gold fraction of the Witwatersrand conglomerates from the Orange Free State goldfield (South Africa)

An investigation of the gold fraction of the Witwatersrand conglomerates from the Orange Free State goldfield (South Africa) has confirmed that the bulk of this metal is concentrated within the bottom parts of the reefs, mostly near the footwall contacts. This, together with other criteria, suggests that the distribution of the metal was governed by sedimentary processes. On the other hand, the shape of the gold grains, as well as the general fabric of the gold-enriched layers, often lack conspicuous sedimentary characteristics. Subsequent metamorphism has obviously produced marked structural modifications and a limited textural rearrangement of the gold and its host-rock, thus obliterating to a certain degree the primary alluvial character of the metallization. The large-scale reconstitution of the gold fraction was effected either by a mechanical reshaping of the malleable and ductile gold particles, or by an intermediate solution process followed by reprecipitation. These processes took place more or less in situ. Accordingly, the original sedimentary distribution pattern was not profoundly affected, whereas the individual allogenic gold grains were transformed into authigenic ones or sometimes even into major gold aggregates. Electron-probe microanalysis established that the gold contains on the average between 9.9 and 12.4% silver, which is also suggestive of an alluvial origin. In spite of some uncertainties arising from the camouflaging effect of pseudohydrothermal reconstitution processes and from the absence of hydraulic equilibrium between gold and several demonstrably detrital heavy minerals, an integration of all available data strongly supports a modified placer theory for the origin of the Witwatersrand gold.

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Zusammenfassung Eine Untersuchung der Goldführung von Konglomeraten des Oberen Witwatersrand-Systems (Oranje-Freistaat-Goldfeld, Südafrika) zeigt, daß die Verteilung des Metalls eindeutig durch sedimentäre Faktoren gesteuert worden ist, obwohl die Form der einzelnen Goldkörnchen sowie ihre Lagebeziehungen zueinander und zu den übrigen Mineralien nicht in jedem Falle mit gleicher Eindeutigkeit für ein sedimentäres Gefüge sprechen. Für diesen scheinbaren Widerspruch ist die Metamorphose verantwortlich, der die Lagerstätte im Laufe ihrer späteren geologischen Geschichte ausgesetzt war und bei der es zu einer teilweisen Umlagerung der Lagerstättensubstanz kam. Davon wurde auch die Goldfraktion betroffen, die dementsprechend heute kaum noch in ihrer ursprünglichen detritischen Form vorliegt, sondern meist pseudohydrothermale Aggregate bildet. Die Umkristallisation des Goldes erfolgte dabei meist in situ, so daß das ursprüngliche sedimentäre Verteilungsschema ausgezeichnet erhalten geblieben ist. — Der Silbergehalt des Goldes konnte mit 9.8–12.4% bestimmt werden, was gleichfalls für eine detritische Herkunft des Metalls spricht. Berücksichtigt man die späteren Umlagerungsvorgänge, dann bereitet der erzmikroskopische Befund einer Deutung der Konglomerate als Seifen keine Schwierigkeiten, sondern spricht mit überwältigender Wahrscheinlichkeit für eine Seifenumlagerungshypothese.

     

Mountain restoration: Soil and surface wildlife habitat

Much wildlife habitat is being destroyed by extractive resource industries in mountain environments. This article illustrates how mountain wildlife habitat was restored in a devastated area. A strip mine for coal on the east slopes of the Alberta Rockies, occupied during its operations by Rocky Mountain bighorn sheep (Ovis canadensis canadensis, Shaw 1803), was reclaimed as bighorn habitat. By considering the behaviour and habitat requirements of bighorns in conjunction with an operating coal mine and reclamation process, a large coal mining company has been able to reconstruct the mine site restoration to benefit mountain sheep. Although bighorn habitat requirements are well known, it was not until the animal and its requirements were studied against the backdrop of an operating coal mine that obvious recommendations regarding the maintenance and placement of high steep excavated rock faces were made and accepted by the government regulatory agencies. An open pit mine is a devastated landscape; trees, soil and overburden are removed to retrieve coal that may be up to 215 m underground. Overburden and soil are returned, but the disturbance compares to the barren landscapes left behind by glaciers. Contouring of the land, grading of overburden and soil, seeding with grass and legume mixtures and extensive fertilization are designed to speed plant colonization and soil development. By examining the bighorn's biological needs with respect to specific mining operations, a unique opportunity arose to employ McHarg's design with nature concept. This approach incorporates environmental and societal values into every aspect of development, and promotes the evaluation of the constraints and opportunities arising. The sheep numbered about 200. Their seasonal movements were similar to those found on native ranges. They used the reclaimed areas as winter range and for the mineral licks exposed during mining; in summer, the sheep moved to nearby alpine areas. Two thirds of all sightings were confined to 1.3 km² of reclaimed grassland; its average productivity (4190 kg/ha) exceeded native ranges (1700 kg/ha). The body mass of female sheep in autumn equalled that of the largest in Alberta, but the skull dimensions were not larger than those of adjacent ranges. Infestation with lungworms was moderate. Lamb production and survival were high. Design criteria should be: feeding areas should be dry and lie within 300 m of escape terrain, which should have a slope of 40% and contain at least three benches. Rock piles should be placed on grazing areas. Mineral licks, a vital welfare factor, already existed within the high walls created by strip mining.     

Prediction of hydrocarbons in sedimentary basins

To estimate the undiscovered hydrocarbon potential of sedimentary basins, quantitative play assessments specific for each location in a region may be obtained using geostatistical methods combined with the theory of classification of geological objects, a methodology referred to as regionalization. The technique relies on process modeling and measured borehole data as well as probabilistic methods to exploit the relationship between geology (the predictor) and known hydrocarbon productivity (the target) to define prospective stratigraphic intervals within a basin. It is demonstrated in case studies from the oil-producing region of the western Kansas Pennsylvanian Shelf and the gas-bearing Rotliegend sediments of the Northeast German Basin.     

Environmental significance of bedrock jointing in Southern Ontario,Canada

The bedrock surface of many glaciated areas is obscured by thick drift deposits. In southern Ontario, Canada, the buried bedrock surface is dissected by channels, infilled with glacial deposits as much as 150 m thick, that are part of a wider mid-continent preglacial fluvial system that predates formation of the modern Great Lake basins. The infills of bedrock channels form major groundwater aquifers, influence regional groundwater flows and contaminant migration to Lake Ontario, and may localize the release of thermogenic methane and radon within heavily urbanized surface environments. A quantitative comparison of the regional pattern of bedrock joints and the orientation pattern of buried bedrock channels and modern river valleys shows that all these orientation patterns are virtually coincident. Buried bedrock channels in south-central Ontario are not part of a simple antecedent drainage system but were likely predesigned by bedrock joint patterns that have subsequently been propagated upward into overlying Pleistocene sediments. Joints in sediments are of considerable environmental significance (for example, subsurface contaminant and gas migration in fine-grained clayey sediments) and of many origins (stress release, desiccation, etc.) but are widely assumed to be a predominantly surface-related phenomena; the existence of deeper joints has been noted by some authors but their origin is obscure. Data presented herein from south-central Ontario confirm that, in addition to surface-related joints, a second population of bedrock-related joints, reflecting the upward propagation of bedrock fractures, is present in Pleistocene sediments of south-central Ontario.     

Structural controls on Witwatersrand gold mineralisation

Structural controls on the distribution of gold in the Witwatersrand Basin can be seen at scales ranging from that of thin sections up to regional seismic data sets. At the thin section scale, gold occurs largely in fractures with hydrocarbons. These fractures are associated with pulses of thrusting deformation that occurred in the latter stages of the basin's development. The distribution of thrust–fracture networks (and therefore gold) is controlled by the mechanical attributes of the host stratigraphy. Thrust displacements are generally very low, with structures exploiting depositional contacts and mesoscale sedimentary bedforms. Many of the small faults are isolated from each other with bimodal vergence between larger-scale thrusts of the same age. Early-formed faults and fractures become folded and faulted by subsequent propagation of larger-scale thrusts. These characteristics are consistent with formation of the mineralised thrust–fracture networks in the frontal, low-displacement parts of a thrust system. In the Carletonville goldfield, the prospectivity of the Ventersdorp Contact Reef (VCR) has been evaluated by using underground observations to pinpoint the structural habitat of gold, and therefore characterise the size of prospective zones around structures intersecting the VCR, mapped within 3D seismic data. In the Welkom goldfield, the controls on gold distribution at the Tshepong mine have been characterised from 3D seismic data, drilling and underground observations, and used to develop an ore-body model that aids reserve estimation and production planning.     

Fe-Ti oxide minerals from regionally metamorphosed quartzites of western New Hampshire

The assemblages rutile-hematite, hematite, hematite-magnetite, hematite-ilmenite-magnetite, and ilmenite-magnetite occur in sillimanite- and kyanite-grade quartzites exposed in western New Hampshire. Different assemblages are found in interlayered sedimentary beds of single outcrops. Magnetites are nearly pure Fe₃O₄ and contain trace amounts of Al, Si, Ti, V, Cr, Mn, and Ni. Magnetites in contact with hematites contain up to 0.4 weight % MnO, but magnetites in contact with ilmenites containing up to 2.3 weight % MnO have no detectable Mn. Ilmenite is enriched in Mn relative to coexisting hematite, and hematite is so enriched with respect to magnetite. Systematic partitioning of elements between oxide minerals and absence of crossing tie lines suggest that the minerals attained chemical equilibrium during regional metamorphism. None of the assemblages are divariant because of the presence of components in addition to FeO, Fe₂O₃, and TiO₂; therefore, none of them constitute oxygen buffers. Nevertheless, gradients in between adjacent sedimentary beds can be measured using variations of oxide mineral composition in trivariant and quadrivariant phase assemblages. Oxygen behaved as an initial value component or inert component during regional metamorphism. It is likely that the gradients are due to differences in bulk composition of sedimentary beds at the time of deposition.     

Interpolation by regularized spline with tension: I. Theory and implementation

Bivariate and trivariate functions for interpolation from scattered data are derived. They are constructed by explicit minimization of a general smoothness functional, and they include a tension parameter that controls the character of the interpolation function (e.g., for bivariate case the surface can be tuned from a membrane to a thin steel plate), Tension can be applied also in a chosen direction, for modeling of phenomena with a simple type of anisotropy. The functions have regular derivatives of all orders everywhere. This makes them suitable for analysis of surface geometry and for direct application in models where derivatives are necessary. For processing of large datasets (thousands of data points), which are now common in geosciences, a segmentation algorithm with a flexible size of overlapping neighborhood is presented. Simple examples demonstrating flexibility and accuracy of the functions are presented.On leave from the Department of Physical Geography and Cartography, Comenius University, Mlynská dolina, Bratislava, Czechoslovakia.On leave from the Institute of Physics, Dúbravská cesta 9, Bratislava, Czechoslovakia.     

The Malines Cambrian carbonate-shale-hosted Pb-Zn deposit,France: Thermometric and isotopic (H,O) evidence for pulsating hydrothermal mineralization

Microthermometric, stable isotope (D/H, ¹⁸O/¹⁶O), and Raman spectroscopic data are given for four different mineralization events located beneath a Triassic unconformity. These events include the Pb-Zn-bearing K_II mineralization which fills geodes and fractures in the main karstic mineralization K_I and the F mineralization which fills fractures in the Cambrian carbonates and shales. Post-K_I, pre-K_II, and F mineralization fluids were hot (150°C), moderately saline (10 equiv. wt% NaCl), and precipitated red dolomite. Subsequently, equally hot but more saline (20 equiv. wt% NaCl) fluids deposited white dolomite and then sphalerite of the F mineralization; they were followed by another generation of dolomite at about 70°C from fluids with about 15 equiv. wt% NaCl and, even later, barites (165°C, 10 equiv. wt% NaCl). Variation in the homogenization temperature with salinity for the different dolomite generations suggests that the hot saline fluids were repeatedly diluted by a cooler, less-saline fluid. The fluids are interpreted to be of formation-water origin having possibly developed in the deep levels of the Rhone basins to the southeast, which was at least 3 km thick at the time of mineralization (post-Hetangian). The inferred metal-bearing hot brines were probably episodically expulsed during the dewatering of the basin, depositing their mineralization in the more permeable fault and karstic zones associated with the uplift of the St. Bresson horst. Cold surface waters probably invaded the mineralized zones between the pulses of hydrothermal solutions. This hydrothermal model with the introduction of sulfides can account for the precipitation and dissolution textures associated with the hydrothermal mineralizations.     

Implications of K-Ar ages of whole-rock and grain-size fractions of metapelites and intercalated metatuffs within an anchizonal terrane

White mica bearing fractions ranging in grain size from 0.4 m to 6.3–20 m were separated from metapelites and intercalated metatuffs of the eastern Rheinisches Schiefergebirge (FRG). The stratigraphic age of these rocks is Middle Devonian (Eifelian), and they contain detrital material of northwestern provenance (Old Red Continent, probably mainly derived from the Caledonian Orogen). Folding in the Carboniferous was associated with cleavage formation and an apparently synkinematic anchizonal metamorphism. Apparent K-Ar ages of metapelite fractions display a marked positive correlation with grain size illustrating the detrital influence which is diminished with decreasing grain size and increasing metamorphism (determined by illite crystallinity). Contrasting grain morphologies observed by SEM enable the interpretation of apparent age/ grain size relationship for coarse fractions. The anticipated lack of detrital mica in metatuffs is confirmed by the fairly consistent apparent K-Ar ages determined for the coarser than 0.63 m size fractions which date the anchizonal metamorphism at ca. 330 Ma. Comparison of metatuff and metapelite apparent ages suggests that the extent of rejuvenation in the latter was largely dependent on grain size. Rejuvenation was also somewhat controlled by the degree of anchizonal metamorphism as suggested by differences in K-Ar results of metapelites which were metamorphosed at variable anchizonal conditions. Fractions <0.63 m from upper anchizonal metapelites record ca. 330 Ma ages similar to those of the 0.63–20 m sizes in metatuffs. Together those results confirm the limited applicability of conventional K-Ar dating on bulk clay fractions (<2 m) of very-low grade (anchizonal) metamorphic rocks in dating metamorphic events and concomitant cleavage formation.