Benoît Dubé

New insights into the Appalachian Orogen from geology and geochronology along the Cape Ray fault zone, southwest Newfoundland

Reverse-oblique overthrusting of high-grade Port aux Basques Gneiss over the lower-grade, supracrustal Windsor Point Group along the Cape Ray fault zone occurred during late Silurian to early Devonian time and is attributed to major oblique collision between Gondwanan and Laurentian continental blocks. New U-Pb zircon, 40 Ar/ 39 Ar, and K/Ar data limit the timing of this orogenic event. The megacrystic granitic phase of the Cape Ray Igneous Complex and the Cape Ray tonalite have been dated at 488 ± 3 Ma and 469 ± 2 Ma, respectively. A black rhyolite from the Windsor Point Group yielded an Ordovician age (453 +5/−4 Ma). A pretectonic gabbro sill within the Windsor Point Group, dated at 424 +4/−3 Ma, is cut by the Windowglass Hill granite dated at 424 ± 2 Ma. Synkinematic hornblende and biotite from the Grand Bay Complex mylonites of the Port aux Basques Gneiss have 40 Ar/ 39 Ar ages of 407 ± 4 Ma and 403 ± 4 Ma, respectively, which is indicative of a late Silurian–early Devonian age for the amphibolite grade deformation. These ages are compatible with a previously determined 415 Ma monazite age, interpreted as the time of peak metamorphism in the Port aux Basques Gneiss, and suggests rapid exhumation. The east-west flexure of the Cape Ray fault zone represents a tear fault accommodating differential shortening between the Cape Ray fault zone and the correlative fault system in central Newfoundland. Mid-Devonian granites (386 ± 3, 384 ± 2 Ma) intervene between the thrusting events and a younger sinistral transcurrent movement. This deformation and magmatic history can be correlated throughout the northern Appalachians along other major fault boundaries.

Sulfur isotope and trace element data from ore sulfides in the Noranda district (Abitibi, Canada): implications for volcanogenic massive sulfide deposit genesis

We examine models for volcanogenic massive sulfide (VMS) mineralization in the ~2.7-Ga Noranda camp, Abitibi subprovince, Superior Province, Canada, using a combination of multiple sulfur isotope and trace element data from ore sulfide minerals. The Noranda camp is a well-preserved, VMS deposit-rich area that is thought to represent a collapsed volcanic caldera. Due to its economic value, the camp has been studied extensively, providing a robust geological framework within which to assess the new data presented in this study. We explore previously proposed controls on mineralization within the Noranda camp and, in particular, the exceptional Au-rich Horne and Quemont deposits. We present multiple sulfur isotope and trace element compositional data for sulfide separates representing 25 different VMS deposits and “showings” within the Noranda camp. Multiple sulfur isotope data for this study have δ34SV-CDT values of between −1.9 and +2.5 ‰, and Δ33SV-CDT values of between −0.59 and −0.03 ‰. We interpret the negative Δ33S values to be due to a contribution of sulfur that originated as seawater sulfate to form the ore sulfides of the Noranda camp VMS deposits. The contribution of seawater sulfate increased with the collapse and subsequent evolution of the Noranda caldera, an inference supported by select trace and major element analyses. In particular, higher concentrations of Se occur in samples with Δ33S values closer to 0 ‰, as well as lower Fe/Zn ratios in sphalerite, suggesting lower pressures and temperatures of formation. We also report a relationship between average Au grade and Δ33S values within Au-rich VMS deposits of the Noranda camp, whereby higher gold grades are associated with near-zero Δ33S values. From this, we infer a dominance of igneous sulfur in the gold-rich deposits, either leached from the volcanic pile and/or directly degassed from an associated intrusion.

Basaltic to andesitic volcaniclastic rocks in the Blake River Group, Abitibi Greenstone Belt: 2. Origin, geochemistry, and geochronology

In the Archean Blake River Group, mafic to intermediate fragmental units have controversially been proposed to have formed during the collapse of a giant submarine caldera. This paper describes and interprets these rocks, summarizing their physical characteristics, inferred origins, age relationships, and geochemical signatures. The widespread Stadacona member, south of Rouyn-Noranda, consists of several hundred meters of bedded volcaniclastic rocks interpreted to have been mostly deposited from aqueous density currents fed directly by explosive eruptions. The magmas involved in these eruptions were plagioclase-phyric tholeiitic to transitional basalts. The similarly widespread D’Alembert tuff, in the northern part of the Blake River Group, shares many physical characteristics with the Stadacona member and is thought to have a similar origin. However, the D’Alembert tuff is approximately six million years younger than the Stadacona member. It is composed mostly of transitional to calc-alkaline andesites and basaltic andesites with very distinct trace element profiles. Volcaniclastic rocks from other areas, such as Tannahill Township in Ontario and the Monsabrais area in Quebec, are interpreted to represent mostly in situ to remobilized hyaloclastite, with no explosive eruptions involved in their genesis. Our observations and interpretations are not compatible with models in which the volcaniclastic units are emplaced during a catastrophic event in relation with the collapse of a giant caldera. Instead, the fragmental rocks were produced by various mechanisms at many distinct times during the evolution of the Blake River Group.

Carlin-like gold mineralization in the Gaspé Peninsula, Canadian Appalachians

In the Gaspe Peninsula of the Canadian Appalachians, the Saint-Andre-de-Restigouche gold prospect is spatially associated with the Acadian dextral strike-slip Grand Pabos-Restigouche fault system. Mineralized veins hosted by Lower Silurian argillaceous and silty limestones are enclosed in a large halo of propylitic alteration. The veins are composed of quartz, stibnite, arsenopyrite and altered angular fragments of the host-rocks with arsenopyrite and framboidal pyrite. Grades average 6 g/t Au with the best values reaching 39 g/t. The host-limestone is silicified with local massive silica replacement. At a greater distance from the vein, the host-rock is carbonatized. Idiomorphic grains of pyrite are rimmed by arsenian pyrite. The metallic signature of the mineralization is characterized by Au-As-Sb-Hg without Ag and base metals. The geological setting, style of alteration, metallic signature and presence of auriferous arcenic-rich pyrite rims indicate that the Saint-Andre-de-Restigouche prospect has many attributes of Carlin-type gold de-posits in Nevada. It also shares analogies with distal disseminated gold deposits.