Michel Jébrak

Hydrothermal breccias in vein-type ore deposits: A review of mechanisms, morphology and size distribution

Abstract Breccias are among the most widely distributed rock textures found in hydrothermal vein-type deposits. Previous studies have mainly been interested in developing qualitative descriptive approaches, leading to a confusing profusion of terms. Brecciation originates in numerous ways, resulting in highly complex classification systems and frequent misinterpretations of facies. Field observations are difficult to reconcile with physical theories of fragmentation, partly due to the fact that few satisfactory quantitative tools have been developed. A review of the main brecciation processes occurring in hydrothermal vein-type deposits allows for the discrimination between chemical and physical mechanisms, including tectonic comminution, wear abrasion, two types of fluid-assisted brecciation (hydraulic and critical), volume expansion or reduction, impact and collapse. Each of these mechanisms can be distinguished using nonscalar parameters that describe breccia geometry, including fragment morphology, size distribution of the fragments, fabric, and dilation ratio. The first two parameters are especially important because: (1) the morphology of the fragments allows chemical and physical (mechanical) breccias to be distinguished, and (2) the particle size distribution (PSD) is a function of the energy input during breccia formation. The slope of the cumulative PSD (fractal dimension) ranges from high values for high-energy brecciation processes, to low values for low energy processes indicated by an isometric distribution. The evolution of a vein system can be divided into three stages: propagation, wear and dilation. These stages are separated by one threshold of mechanical discontinuity and one of hydraulic continuity. These two thresholds also mark the transition between different types of brecciation. Mineralization occurs during all three stages and may display different textures due to pressure variations. The use of quantitative parameters in fault-related hydrothermal breccias allows a better understanding of the physical parameters related to a vein environment, including structural setting and crustal level, as well as fluid–rock interactions. Recognition of the different breccia types could also be important during the early stages of mineral exploration.

High precision boundary fractal analysis for shape characterization

Abstract Numerous kinds of particles in geological and environmental sciences may be characterized by their boundary fractal dimension. Several methods are available: structured walk, box-counting, dilation and euclidean distance mapping (EDM). The precision and stability of these techniques is variable and usually low precision fractal dimensions are obtained (±0.1). Validation on mathematical fractals and tests of the effects of pixelization, size, resolution and topology were performed with three computer-derived methods (box-counting, dilation and EDM), using mathematical objects and fragments coming from impact and ore deposits breccias. Tests demonstrate that high precision results can be yielded with the right technique and caution. EDM showed the highest precision (±0.01) and strongest reliability with less sensitivity to size and resolution, with reproducible results for fragments as small as 10,000 pixels of area. It was also the most accurate for mathematical fractals.

Chemical brecciation processes in the Sue unconformity-type uranium deposits, Eastern Athabasca Basin (Canada)

Abstract Chemical brecciation in sandstone is common in many unconformity-type uranium deposits of the Athabasca Basin, and is expressed in some of them as ball zone breccia. Ball zones are composed of rounded argillized sandstone fragments, varying in size from several centimeters to 1 m, wrapped in a clay matrix. The Sue C open pit provided a unique opportunity to map and to study such ball zones. Here, they were up to 5 m wide with a 20–30 m vertical extension. They were mainly observed along a reverse fault controlling the Sue A and B uranium deposits, and were well developed at intersections with dextral NE-trending structures. Their maturity, characterized by the matrix percentage, increased toward the unconformity and at fault intersections. They are characterized by massive quartz dissolution, hematite leaching, (Ca,Sr,LREE)Al-phosphates crystallization and replacement of dickite by illite. Illite composition indicates formation temperatures of 240–280 °C, close to peak diagenesis conditions in the basin. Mass balance calculations show that V, K, Rb, B, LREE, Mg, Cr, Sr, U and Y were added and Si and Fe leached out with up to 85% volume loss. Ball zones were initiated by tectonic fracturing in sandstone during reverse faulting. Consecutive permeability increase induced basement fluid circulation in the sandstone with quartz dissolution along fractures. With a silica saturation of the fluid of 90%, a minimum fluid/rock ratio of 38,000 is obtained. The rounded morphologies of the breccia fragments are attributed to a diffusion-limited regime of dissolution. The resulting increase of clay content led to self-sealing of the hydrothermal system. Seismic reactivation may have been periodically rejuvenated the permeability. These processes seem to be coeval with the formation of structurally controlled high-grade unconformity-type uranium mineralization. Formation of the ball zones required probably more than 1 million years.

Multi-element relationships and spatial structures of regional geochemical data from stream sediments, southwestern Quebec, Canada

A regional geochemical survey, covering an area of 10000 km2 in the Grenville Province of southwestern Quebec, was carried out by the Quebec Department of Energy and Resources. The fine (<177 μm) and heavy fraction of stream sediments were analyzed for more than 25 chemical elements by ICPES and by INAA. This study aims to identify the main geological processes affecting the geochemical data of both media, by considering the multi-element relationships and spatial structures of the geochemical data. Fine fraction and heavy mineral concentrates were subjected to: (1) principal component analysis (PCA) to determine the inter-element relationships controlling the background variations; and (2) variogram analysis to establish the spatial continuity of geological phenomena associated with the factors of the PCA. The stability of PCA results was tested by using a bootstrap replication technique. Detailed geochemical and mineralogical studies were conducted on different densimetric and granulometric fractions in order to characterize the geochemical behavior of each fraction, and to relate the inter-element associations to a specific mineralogy. Our study shows that the fine and heavy fractions indicate different geochemical information. The background levels and the multi-element associations differ between both media. A principal component model shows that the fine fraction is strongly influenced by surficial processes, explaining approximately 55% of the data variability. Two factors accounting for only 23% of the data variability in the fine fraction are related to lithologies. The heavy fraction reflects lithological characteristics. Three factors, accounting for 51% of the variation in the data, represent mineralogical features of the principal lithologic units. Two other factors reflect geochemical components that could be related to mineralization or lithologies of interest for prospecting. The variograms reveal the different spatial variation components of the geochemical signals. They show high nugget effect (more than 40%), indicating the erratic occurrence of phases hosting geochemical signals in the sampled media. They allow the spatial scales of geochemical components to be recognized, and the geological phenomena associated with regional trends to be differentiated from those acting at local scales. Knowledge of spatial variation components provides useful information for estimating suitable sampling intervals with regards to geochemical surveys, and aids in proposing solutions for reducing the nugget effect.

Lateral transport of suevite inferred from 3D shape-fabric analysis: Evidence from the Ries impact crater, Germany

The transport mechanism of suevite parti- cles during impact cratering is poorly under- stood and was studied at the 15 Ma Ries crater in southern Germany. Two emplace- ment modes of suevite deposits are generally discussed: (1) fallback of plume material into the crater and its periphery upon collapse of an ejecta plume; and (2) horizontal transport of ejected material, akin to emplacement of pyroclastic deposits erupting from volcanic centers. In order to differentiate between the two emplacement modes of suevite deposition, we analyzed the shape fabrics of suevite components from two localities out- side the Ries crater by fishape-fabric ellipsoids to measured shape-fabric ellipses and by applying high-resolution, X-ray- computed tomography to analyze the three- dimensional shape and orientation of the suevite particles. We show that the preferred orientation of long axes of elongate particles is disposed either radially or concentrically with respect to the crater center. Our obser- vations indicate that suevite material was not only derived from an ejecta plume, but was transported by lateral fl ow under viscous conditions upon fallback. This fl ow regime resembles that known from pyroclastic fl ows.

3D modeling of uranium-bearing solution-collapse breccias in Proterozoic sandstones (Athabasca Basin, Canada)-Metallogenic interpretations

Unconformity-related uranium deposits are the highest grade, large tonnage uranium resources in the world. In the Athabasca Basin (northern Saskatchewan, Canada), which is the premier host for unconformity-type deposits, the ore deposits are frequently hosted and surrounded by breccias in sandstone. The significance of these breccias and their relation to mineralization are of major importance for the genesis of these high-grade deposits. Therefore, a modeling study, integrating results from structural geology and petrology, was performed with the gOcad 3D modeling software, in order to decipher geometrical and geological relationships between breccias, faults and mineralization zones. Mineralized bodies and the sudoite-dravite breccia bodies display strong spatial correlations. They appear to be controlled by reverse shear zones cross-cutting the unconformity and containing graphite in the basement. Geochemical computations evidenced that volumetric water-rock ratios up to 10,000 could be obtained in these breccia bodies for volume losses of up to 90%. Assuming reasonable values for quartz saturation, hydraulic conductivity and connected porosity, the minimal fluid volume and the time duration necessary to generate the sudoite-dravite breccia bodies were estimated at ca. 2km^3 and ca. 1Myr, respectively. The comparison of these results with literature data suggests that the formation of sudoite-dravite breccia and mineralization could have been coeval. It may be proposed that within the space created by the quartz dissolution in the breccia body, a mixing between basement and basinal fluids could have induced U deposition and allowed the development of high-grade mineralization. The first-order uranium solubility that this coeval formation would imply is consistent with literature data, which suggests that this conceptual model is reasonable.

Evolution of the subaqueous to near-emergent Joutel volcanic complex, Northern Volcanic Zone, Abitibi Subprovince, Quebec, Canada

Abstract The 5–7 km thick Archean Joutel volcanic complex (JVC), composed of basalt to rhyolite, is represented by five volcanic phases. Initial volcanic construction features effusive mafic volcanism characterized by massive, pillowed and pillow breccia flows with an interdigitated lapilli–tuff–breccia and lapilli–tuff (phases 1 and 1′). The principal construction phase of the JVC (phase 2) is host to volcanogenic massive sulphide deposits and is associated with massive, flow banded and lobate rhyolite to dacite flows with associated lapilli–tuff–breccias derived either from autoclastic or hydroclastic fragmentation processes. Felsic volcanism continued with the emplacement of massive to brecciated flows (phase 3b), whereas deposition of turbiditic tuffs, lapilli–tuffs, lapilli–tuff–breccias is associated with resedimented mafic volcanics (phase 3a). Phases 4 and 5 are associated with the construction of a second volcanic center. Phase 4 is interpreted as mostly pyroclastic deposits, whereas tuffs, lapilli–tuffs and lapilli–tuff–breccias of phase 5 are host to synvolcanic Au–Ag mineralisation and are interpreted as talus deposits. The Zr/Y and La/Yb ratios of phase 1 show a tholeiitic affinity, whereas phases 2 and 3 are transitional and phases 1′, 4 and 5 are calc-alkaline. Uranium–lead geochronology indicates 7 Ma between construction of the transitional and the calc-alkaline centers (2728–2721 Ma). The geochemistry suggests that all phases (except phase 1) are consistent with variable input of subduction-related processes. The mafic-felsic JVC, a composite volcano that formed on a paleo-sea floor, displays deep-water effusive dominant to shallow-water pyroclastic and volcaniclastic dominant volcanic construction, as indicated by pillowed flows, turbiditic tuffs, stromatolite fragments and synvolcanic alteration types. Geochemistry and physical volcanology of the JVC are consistent with construction in a rifted island arc or back arc setting.

A Cellular Automata Breccia Simulator (CABS) and its application to rounding in hydrothermal breccias

Dissolution processes are ubiquitous in surficial and hydrothermal environments. Solution breccias are formed when dissolution processes dominate and are widely observed in hydrothermal systems. Distinct fragment shapes develop during dissolution and can be used as diagnostic tools for characterizing the nature of the process. This paper presents a new simulator for hydrothermal breccias based on a 3D cellular automata model-thus called CABS (Cellular Automata Breccia Simulator)-that simulates various processes in virtual rocks, including dissolution, metosomatic exchange and precipitation. The code for this simulator is publicly available. The evolution of fragment shape complexity (boundary fractal dimension) during dissolution is evaluated using CABS. Results show that shape complexity increases for both surface-independent (kinetic regime) and dependant processes (diffusion-limited regime). In the case of a lognormal relationship between the probability of dissolution and the surface available for dissolution within the diffusion-limited regime, complexity remains constant and is accompanied by progressive rounding of the fragments. Increased porosity accelerates the rate of the process. The dissolution regime thus plays a crucial role in determining the relative importance of hydromorphic or clastic transport mechanism.

Gold mineralization in the Proterozoic Bleida ophiolite, Anti-Atlas, Morocco

Abstract The newly discovered (1998) West Bleida gold mineralization (3 tonnes metal Au) lies west of the main Moroccan Bleida copper deposit (1981–1991) in the central Anti-Atlas (southern Morocco). It is hosted by metamorphosed and deformed mafic to intermediate volcanic rocks that are part of the Neoproterozoic tholeiitic volcanosedimentary series forming the stratigraphically upper part of the Bou Azzer ophiolite sequence. Strong sericitization and local silicification are associated with mineralization. These altered rocks represent a proximal hydrothermal alteration halo around the West Bleida ore zones. Normative chlorite characterizes the metamorphic assemblage away from the ore zones. Gold mineralization primarily occurs as deformed gold-bearing quartz veins and disseminations in Cu-rich chert zones (chalcopyrite–malachite), Fe-rich lithofacies and breccia zones. Gold is accompanied by small amounts of copper sulphides (<1% modal chalcopyrite). Scanning electron microscope–energy dispersive spectrometry analyses of gold grains from veins and disseminations reveal the presence of palladium as inclusions of Pd–As–Sb, Pd–Bi–Se and Pd–Te mineral phases. An electron microprobe study confirms the presence of two types of gold. The first is an alloy of Au–Ag–Pd, typically bordered by small grains of Pd and Bi (Te,Sb) phases and associated with a metamorphic assemblage. Isomertieite, Pd11(Sb2,As2), was identified as one of the phases. The second type of gold is electrum (10% Ag, 90% Au), which is always associated with fractures and occurs with hematite and white mica. Based on its form and habits, West Bleida gold reflects two distinct generations of fluid activity. The primary event precipitated Au–Ag–Pd alloys from Au–Pd-bearing hydrothermal fluids and produced auriferous quartz veins and disseminations within mafic rocks of the Bleida ophiolitic accretionary complex. It was structurally and lithologically controlled. This early event is preserved in the deeper (and thus fresher) zones more than 80 m below the surface. Intense tectonic overprinting obscures the genetic relationship between vein and disseminated styles of mineralization, both of which contain Pd-rich gold, but some of the auriferous quartz veins are observed to crosscut disseminated mineralization. Two possible hypotheses are considered: the pre-tectonic root of a volcanogenic massive sulphide system, or a late tectonic orogenic (mesothermal) deposit. The presence of Pd minerals and anomalous cobalt concentrations suggest a source in ultramafic rocks. The second event, characterized by inclusion-free electrum, occurred much later and represents the alteration and weathering of the primary Pd-rich gold assemblage by oxidizing surface fluids. It affected all mineralized units and structures to a depth of 80 m. This post-tectonic surficial alteration also caused leaching of Cu-sulphides, which may explain their low abundances in the upper parts of the ore zones.

Geology and gold–molybdenum porphyry mineralisation of the Archean Taschereau–Launay plutons, Abitibi, Quebec

Abstract Porphyry gold deposits constitute a new challenge for mineral exploration in Archean greenstone belts, yet the relationships between intrusive rocks and mineralisation remain poorly understood. The Messegay gold occurrence is situated in the Taschereau–Launay plutonic complex of the Northern Volcanic Zone of the Abitibi greenstone belt. The Taschereau intrusion (2718±2 Ma) represents a complex network of sills composed of tonalite with minor amounts of diorite, monzonite and gabbro. The Launay granite, which intrudes the Taschereau pluton, is late-orogenic and its emplacement was controlled by an early reverse ductile N–S shear zone. Intrusive rocks have been subjected to four episodes of alteration: epidote–chlorite–carbonate, magnetite–hematite–microcline, albite–pyrite, and sericite–quartz. Shear zone hosted and metasomatic Au–Mo mineralisation is mainly associated with albite–pyrite assemblages, but also occurs within zones of intense magnetite–hematite–microcline and sericite–quartz alteration. The Messegay gold occurrence is interpreted as a new type of disseminated gold–molybdenum porphyry deposit associated with a zone of magmatic differentiation in a magnetite-series pluton.