ALBERT

Description: The 2704 to 2695 Ma Blake River Group in the southern Abitibi greenstone belt comprises a well preserved submarine volcanic sequence that hosts a large number of VMS and important Au-rich VMS deposits, including the world-class Horne and LaRonde-Penna deposits. Establishing precise chronostratigraphic control on the VMS deposits within the Blake River Group is critical because numerous distinct events took place within a period of 9 m.y. Nineteen new high-precision U-Pb ages temporally constrain the host rocks of many poly-metallic VMS deposits and associated synvolcanic intrusions, demonstrating that these VMS deposits formed throughout the protracted volcanic evolution of the entire group. Ages on host rocks of the Horne (2702.2 ± 0.9 Ma), Quemont (2702.0 ± 0.8 Ma), and Fabie (2701.9 ± 0.9 Ma) deposits reveal that they are among the oldest VMS deposits in the Blake River Group. The giant Horne Au-rich VMS deposit had already formed when the Cu-Zn deposits of the Noranda mine sequence, including Millenbach and Amulet, were generated at ~2698 Ma and is thus unrelated, consistent with its different volcanological setting and metal content. Large Au-rich VMS deposits of the Bousquet Formation, including LaRonde Penna and Bousquet 2-Dumagami, were formed at 2698 to 2697 Ma and are distinctly younger than the Horne and Quemont deposits. There were, therefore, two major time-stratigraphic intervals within the Blake River Group that were favorable for the formation of Au-rich VMS deposits. Rhyolite hosting the large Bouchard-Hébert VMS deposits yielded an age of 2695.8 ± 0.8 Ma. Important mineralizing events in the Blake River Group occur at ca. 2-m.y. intervals apart and are associated with major magmatic episodes. Recognition of specific time-stratigraphic intervals for different styles of mineralization and geologic settings is essential to improve exploration models within the Blake River Group and for similar volcanic assemblages elsewhere in the Archean.

Description: An innovative approach to enhance volcanogenic massive sulfide (VMS) exploration in regions outside of mining camps is to first use physical volcanology with litho- and chemostratigraphy to establish the location of effusive centers in the volcanic units and use pyrite geochemistry in sulfide-bearing stratified intervals with whole-rock geochemistry in the underlying volcanic units to identify hydrothermal upflow zones. This methodology is illustrated by this study in the Archean Blake River Group within the Abitibi greenstone belt of Quebec and Ontario. The Blake River Group contains numerous VMS deposits, yet large segments remain underexplored, including the Hébécourt Formation which contains four tholeiitic units that range from basalt to rhyolite in composition. Effusive centers are located for three felsic units and subunits: (1) low Ti (porphyritic) subunit of the main rhyolite, (2) high Ti (aphyric) subunit of the main rhyolite, and (3) the upper rhyolite, and for a basaltic andesite unit. Stringer and disseminated Zn-Cu mineralization occurs within the flank breccia of the low Ti rhyolite dome. An inferred vent area for an overlying basaltic andesitic unit has also been identified in this area, illustrating the coincidence of hydrothermal upflow zones with volcanic vents. LA-ICP-MS analysis of pyrite grains from several sulfide-bearing stratified intervals indicates two broad areas of higher Cu, Zn, Au, and Ag contents. The eastern region corresponds to known volcanic vents and mineralization. The western region also indicates upflow of Cu-bearing hydrothermal fluids and corresponds to a possible effusive center for the high Ti subunit. The western region does not contain known mineralization at lower stratigraphic positions, but it has not been thoroughly explored.

Description: The hydrothermal system architecture related to the formation of the contemporaneous Au-bearing Horne and Quemont volcanogenic massive sulfide (VMS) deposits was visualized by employing kriging methods to map whole-rock oxygen isotope compositions, zones of silica addition and loss, and water contents in two- and three-dimension. Zones of alteration were mapped in three-dimensions in the vicinity of the steeply dipping Horne deposit, to depths of as much as 2 km. In all, nearly 300 samples were analyzed for oxygen isotopes and supplemented by previously published whole-rock analyses. Contents of SiO 2 , H 2 O, MgO, Al 2 O 3 , and S from chemical analyses of nearly 5,000 samples within the two- and three-dimensional study regions were used separately, and in combination with the oxygen isotope data, for modeling and hydrothermal mapping purposes. The Horne and Quemont deposits formed within a similar time frame, but in different magmatic-hydrothermal systems, distinguished by their mapped hydrothermal architecture. The Quemont deposit appears to be centered on the Powell pluton, which intruded late into an apparent volcanic-filled, rift-graben structure. Although structural complexities are apparent, we infer mineralizing high-temperature upflow in the footwall of the Quemont deposit to have emanated from a reaction zone above the Powell pluton (and its precursors), beneath a zone of extensive silicification. Faulting on the Andesite fault and Horne Creek faults, plus erosion, has removed evidence of the upflow zone in the hydrothermal system of the Horne deposit. Areas of silicification correspond, in general, with isotopic evidence of lower temperature alteration. Such alteration east of the Quemont deposit signaled the waning of hydrothermal activity. The suggested cooling, for the most part, promoted the precipitation of silica. In the case of the Horne deposit, mixing of metalliferous hydrothermal fluid with cold seawater in the permeable footwall rocks, in an apparently relatively stratigraphically stable and long-lived hydrothermal system, evidently led to marked footwall silicification. The silicified footwall may have contributed to an increased efficiency of sulfide precipitation in the Horne deposit. Continued intrusion and some post-VMS hydrothermal activity is recorded in the hanging-wall section to the Horne deposit. Our data suggest that deposition of the 10 million ounces (Moz) of Au within the Horne deposit was syngenetic, and not the product of subsequent hydrothermal activity.

Description: Two seismic reflection profiles acquired by Xstrata Canada in the Noranda mining camp were reprocessed and interpreted with the objective of providing key information on the geologic contacts and structures associated with volcanogenic massive sulfide (VMS) deposits at depth. The Amulet and Ribago seismic profiles run approximately from east to west and cross the volcanic rocks of the Noranda formation, which host most of the ore deposits in this camp. The seismic data interpretation relies strongly on a detailed three-dimensional geologic model built from an extensive number of exploration boreholes available in this area and is further supported by physical rock property measurements from in situ borehole logging data. Some reflections observed on the Amulet and Ribago seismic profiles correlate with rhyolite/andesite or silicified-andesite/andesite contacts that host the prospective exhalites of the Noranda formation. In particular, the silicified-andesite/andesite contact hosting the C-contact exhalite is imaged clearly down to a vertical depth of 1,100 m along the Ribago profile. The processing sequence included dip moveout (DMO) corrections and poststack migration. The seismic data reprocessing allowed the identification of two diffractions that correlate with known sulfide bodies intersected in boreholes located close to the Ribago profile. One of these diffractions, at approximately 1,200-m depth, coincides with the main massive sulfide intersection of the subeconomical Ribago orebody. Diorites cause several reflections observed within the Flavrian pluton. Some diorite units also have sufficient acoustic impedance contrast to produce strong reflections when juxtaposed against volcanic rocks. However, such reflections do not predominate in the Amulet and Ribago profiles, possibly because reflective diorite units are not so significant or have limited lateral continuity in this part of the Noranda formation. The reconciliation of the detailed three-dimensional geologic model and two-dimensional seismic data was not necessarily a straightforward task. A significant complication results from the inherent limitations of two-dimensional seismic imaging techniques in a complex three-dimensional geologic environment. Nevertheless, results presented in this paper indicate that seismic methods can image prospective contacts and deep-seated massive sulfide mineralization and can be a valuable exploration tool in the Noranda mining camp.

Description: The Misery syenitic intrusion in northern Quebec is host to a potentially important, recently discovered rare earth element (REE)-Zr-Nb prospect, containing significant concentrations of both light and heavy REEs, and is conspicuous on aeromagnetic maps as a well-defined, ring-shaped anomaly. Felsic syenite composed of idiomorphic perthite with interstitial mafic minerals, comprising fayalite, hedenbergite, ferropargasite and annite dominates the exposed outer part of the intrusion (the core is covered by Misery Lake). This unit is accompanied by ferrosyenite containing up to 50 vol % mafic minerals, including cumulate fayalite. The ferrosyenite, which occurs as amoeboid-like inclusions in the felsic syenite, is interpreted to have formed by fractional crystallization of ferromagnesian minerals, leaving behind a residual magma which produced the felsic syenites. This latter magma was progressively enriched in alkalis and silica, and only became saturated in ferromagnesian minerals at a very late stage of crystallization. The bulk of the REE mineralization was initially concentrated in pods and layers of fluorapatite that accumulated as a result of gravitational settling. The fluorapatite crystals were partly or completely replaced by britholite-(Ce), which further enriched the rocks in LREEs. Locally, thin quartz-fayalite dikes cut the felsic syenite. These dikes contain up to 10 vol % fergusonite-(Y) and 8 vol % zircon, and are interpreted to be products of an immiscible FeO-SiO 2 -rich magma into which Zr, Nb, and HREEs were preferentially fractionated from the conjugate felsic syenite magma.

Description: The Archean La Grande and Eastmain domains of the La Grande subprovince in the James Bay region of northwestern Québec are the focus of renewed and extensive exploration as a result of recent major discoveries made in this region (e.g., ~8 Moz Au Roberto deposit). A number of significant Neoarchean Au ± base metal occurrences are present in the La Grande domain, including the La-Grande-Sud Au-Cu prospect (Zone 32, inferred resource of 4.2 million metric tons (Mt) at 2.1 g/t Au and 0.2 wt % Cu). The La-Grande-Sud Au-Cu prospect is hosted in the 2734 ± 2 Ma synvolcanic La-Grande-Sud Tonalite in the 2751 to 2732 Ma Yasinski Group volcanic rocks. The La-Grande-Sud Tonalite is a synvolcanic calc-alkaline intrusion emplaced in an arc-like tectonomagmatic setting. Timing relationships in mineralized zones as well as associated hydrothermal alteration zones support a pre- to early-D 1 origin for at least a part of the mineralization in the La-Grande-Sud Tonalite. Deformation events were responsible for the overprinting of the early mineralization by auriferous shear zone-related systems associated with D 2 . Evidence for a pre-D 2 and pre- to early-D 1 alteration and mineralization system includes: (1) a broadly concentric pattern defined by the hydrothermal alteration assemblages within the La-Grande-Sud Tonalite, which suggests that the hydrothermal alteration was, at least in part, controlled by the geometry of the intrusion rather than by the D 2 deformation; (2) the development of a weak biotite-bearing potassic assemblage (biotite-epidote-plagioclase-muscovite-calcite ± pyrite) that is gradually replaced outward from the center of the intrusion by a chlorite-bearing propylitic assemblage (chlorite-epidote-muscovite-plagioclase-calcite-pyrite), and locally overprinted by a sericitic alteration assemblage (muscovite-quartz-albite-chlorite-pyrite-carbonate), which represents the most intense alteration in the La-Grande-Sud Tonalite; (3) the presence of mineralized hydrothermal breccias along the margins of the intrusion; (4) deformed disseminated and stockwork-style Au-Cu sulfide mineralization consisting of pyrite, chalcopyrite, tennantite, arsenopyrite, pyrrhotite, Bi sulfosalts, and native Au ± Bi in the sericite, chlorite, and biotite facies alteration zones; (5) S 1 -parallel elongated tonalite clasts and sulfide veinlets in D 2 -folded and transposed hydrothermal biotite breccias; and (6) pyrite porphyroblasts that are elongated parallel to the S 1 fabric (syn-D 1 metamorphic recrystallization) and that were folded during D 2 . D 2 was associated with the development of conjugate auriferous extensional quartz-tourmaline veins (orogenic-style veins) that were superimposed on the pre- to early-D 1 intrusion-hosted mineralized system. Gold in these syn- to late-D 2 veins could have been remobilized from preexisting mineralization (disseminated sulfides) or was related to a late (syn-D 2 ) auriferous hydrothermal event. The intrusion-hosted Au mineralization of the La-Grande-Sud Au-Cu prospect illustrates that Archean synvolcanic intrusions, regardless of their size, can host a range of styles of alteration and mineralization, including both "early" and "late" mineralizing systems.