Diagenesis of the Aphebian Mistassini regolith,Quebec, Canada

A well-developed regolith is preserved beneath early Proterozoic (Aphebian) rocks of the Otish and Mistassini Groups in Central Quebec, Canada. The regolith is covered by fluviatile clastic rocks (Otish Group) in the north, and by a thick sequence of stromatolitic and sandy dolomite (Mistassini Group) in the south.Where preserved beneath clastic rocks, the regolith exhibits the structures and textures of its crystalline parent rocks (tonalite, gneiss and amphibolite), despite the alteration of feldspars to clay minerals and the partial oxidation of biotite. A later event recrystallized the clay minerals to muscovite, while conserving the original outline of the feldspars. Beneath the dolomite, the regolith was largely replaced by dolomite, but retains many original textures. Dolomite replaced first the clay minerals, then quartz and unaltered feldspars and finally biotite. Repeated crustifications of dolomite with intervals of chert and minor anthraxolite surround unaltered blocks of crystalline rock within the regolith profile and similar complex veins fill many master joints. These veins are identical in composition to vug fillings throughout the overlying carbonate formations. Clasts of partly dolomitized regolith included in non-dolomitic sands filling channels and scours dug deep into the profile, suggest that dolomitization commenced very early, possibly related to a sabkha environment developed during the transgression.     

Gold-bearing iron-formation in a granulite terrane of the Canadian Shield: a possible deep-level expression of an Archean gold-mineralizing system

Gold occurs in metamorphosed iron-formation within a migmatized paragneiss terrane in the Eastern Superior Province of the Canadian Shield. The main gold prospect, which is located 100 m from a tonalite pluton, has undergone skarn-like metamorphism that occurred late during the granulite grade metamorphism. Despite its occurrence in a high-grade metamorphic host-rock, the mineralization shares many of the characteristics of lowergrade Archean greenstone belt gold mineralization, such as: (1) enrichment in Au, As, S, Sb, W, U and FeO; (2) a low base-metal/gold ratio; (3) the inclusion of gold in arsenopyrite; (4) the proximity to a felsic pluton; (5) an iron-rich, fractured host-rock; and (6) a similar mineralization time (around 2650 Ma). This gold occurrence occurs near the metamorphic peak and represents a deep level expression of the widespread crustal-scale mineralizing system documented for the late Archean. The main conclusion of this work is that the principal characteristics of Archean gold mineralization are present in the deep crust, and do not come from the leaching of upper-level rock piles, such as greenstone belts.     

The Dolodau dykes,Canada: An example of an archean carbonatite

Summary The Archean Dolodau carbonatite dykes occur near a late tectonic syenite stock located in the Northern Volcanic Zone of the Abitibi greenstone belt. The biotite sovite and amphibole-biotite silicocarbonatite dykes produced fenitization of the host rocks. The Dolodau carbonatites compare favourably with Phanerozoic carbonatites in petrography, mineralogy and geochemistry. An Archean age is suggested by field geology and the isotope data available. The similarities suggest that Archean carbonatite petrogenetic processes were similar to modern day processes.

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Die Dolodaugänge, Kanada: ein Beispiel eines archaischen Karbonatites

Zusammenfassung Die archaischen Karbonatitgänge von Dolodau liegen in der Nähe eines spättektonischen Syenit-Stockes in der nördlichen vulkanischen Zone des Abitibi Greenstone Belts. Die Biotit-Sovit- and Amphibol-Biotit Silicokarbonatit-Gänge führten zu Fenitisierung der umgebenden Gesteine. Die Dolodau-Karbonatite lassen sich gut mit phanerozoischen Karbonatiten vergleichen, was Petrographie, Mineralogie and Geochemie betrifft. Ein archaisches Alter wird durch Geländebeziehungen and Isotopendaten belegt. Diese Ähnlichkeiten weisen darauf hin, daß archaische Karbonatit-bildende Vorgänge ihren modernen äquivalenten ähnlich sind.

     

Gold mineralization patterns in relation to the lithologic and tectonic evolution of the Chibougamau mining district,Quebec, Canada

The Chibougamau mining camp, a major gold producer, has so far produced a total of 1,050 mt of gold at an average grade of 1.85 g/tonne. In contrast to a general tendency to group the deposits as vein type with varying compositions, this paper demonstrates the strong influence of regional lithologic and tectonic evolution of the area on the gold mineralization patterns. A syn-volcanic period of mineralization comprises both the volcanogenic massive sulphide and disseminated mineralization, and the subsequent epithermal mineralization. The latter is also associated with evolving volcanic landforms and syn-volcanic intrusions. The emplacement of the typical Archean lode-gold deposits and later Cu-Au deposits of uncertain origin coincides with the syn-deformational (Kenoran) period. The spatial relationship between the east-west-trending shear zones and a north-east-trending fault system is shown to be a possible mechanism for generating added dilatancy during the gold mineralizing phase. A late-to-post Kenoran shear system controlled the emplacement of the last major phase of gold mineralization within a stratiform intrusion.     

Petrologic and Geochemical Characteristics and Origin of Gusui Cherts,Guangdong Province,China

The characteristic structures of the Precambrian cherts from the Gusui section, Guangdong ,Chi-na, include bedded structure ,laminated structure ,massive structure and pseudobrecciated structure.The chert is characterized by consistently low abundance of TiO2,Al2O3 and most trace elements.Howevver ,it is enriched in Ba,As,Sb,Hg and Se.In Al-Fe-Mn ternary diagrams,it falls into the “hydrothermal field“ .Correspondence analysis and factor analysis show that many elements show up in the factor that represents the leaching of country rocks by hydrothermal solutions,and are the very characteristic element association fo the geochemically anomalous South China basement.Petrologic and geochemical evidence suggests a hydrothermal origin for the chert.The chert may have been formed in a Precambrian fift or an extension zone developed within the Yunkai marginal geosyncline, with a fault system linking it to an unknown heat source at depth.     

Geochemical migration of impurity trace elements and resultant fractal distribution patterns in source rocks

The migration of trace elements from the inner part of solid cells to the weaknesses is the bottleneck in the migration of these elements from their initial positions in the source rock to the final deposition site in ore bodies. Diffusion may play a key role in the reactivation of trace elements. The overall migration pattern of trace elements in source rocks is a fractal structure. There are two general tendencies for trace elements to migrate. One is from within solid cells toward sinks, and the other is from high temperature fields toward low temperature ones. High temperature enhances these two tendencies. Conjugate geochemical anomalies are the inevitable result of a closed geochemical system.     

Climatic and tectonic influences on fan deltas and wave- to tide-controlled shoreface deposits: evidence from the Archaean Keskarrah Formation, Slave Province, Canada

The 2.6 Ga Keskarrah Formation, located in the central Slave Province, Northwest Territories, Canada, is a late-orogenic, tectonically controlled sedimentary sequence that developed under unusual climatic and depositional conditions. The formation is adjacent to the crustal-scale, north-trending Beniah Lake Fault and overlies the 3.15 Ga Augustus Granite, the 2.69–2.7 Ga mafic volcanic Peltier Formation and the turbiditic Contwoyto Formation unconformably. Principal lithofacies in the Keskarrah Formation include conglomerate, sandstone and siltstone–sandstone. The conglomerate lithofacies represents coalescing gravelly streamflow-dominated fan deltas adjacent to topographic highs. Up-section quartz-rich arenites and quartz arenites of the sandstone lithofacies are interpreted to be shallow-water shoreface deposits influenced by wave action and tides. The overlying feldspathic litharenites of the siltstone–sandstone lithofacies are consistent with a lower shoreface to proximal offshore environment dominated by wave and tide interaction. Tidal influence in both sandstone-dominated lithofacies is inferred from the presence of mudstone laminae between bedforms and on foresets of cross-beds, as well as from abundant reactivation surfaces with local mudstone drapes. Intense chemical weathering during the Archaean, resulting from elevated atmospheric levels, higher temperatures and moist climatic conditions, played an important role in the development of quartz-rich arenites that appear to be first-cycle deposits. Few lithic fragments and feldspar grains are preserved due to in-situ host rock weathering, chemical weathering during transport and wave and tide action. Hydraulic sorting and abrasion in the shoreface environment contributed to the continued breakdown and transport of labile minerals. Increased proportions of lithic fragments in sandstone beds of the conglomerate lithofacies are the result of shorter transport distances from source areas to the depositional environment. Abundant conglomerate with up to 4-m large granitic boulders derived from the adjacent Augustus Granite and mafic clasts from the Peltier Formation indicate high relief and fault-related uplift and subsidence. The intimate association of fan deltas and wave- and tide-influenced shallow-marine deposits in association with quartz-rich sandstones forming in a high-relief area make the Keskarrah Formation remarkable in the rock record.     

Evolution du Complexe Alcalin de St.-Honoré (Québec) apres sa Mise en Place

Since its formation in earliest Cambrian time, the St-Honoré alkaline complex has fractured into large subhorizontal slabs under the effect of magmatic pulsations and thermal contraction. These slabs were rapidly segmented and cracked by the same stresses. The secondary permeability resulting from these fractures first aided the circulation of the rare earth mineralizing hydrothermal fluids in the center of the complex, and then, after the unroofing of the complex, instigated the development of a karstic drainage system within the carbonatites of the complex. The accumulated sediments in the karstic caverns derive from the periodic inwashing of the insoluble residues built up on the surface of erosion, were indurated after the sealing of the karstic openings with calcareous muds during the Ordovician transgression, and were faulted with their cover when the Saguenay graben formed. The last event, marked by low temperature regional mineralizating circulation (barite and calcite crystals coating geodes at NIOBEC), permitted the establishment of a new subterrean drainage system.