Archean Blake River Group Research Articles

Using Physical Volcanology, Chemical Stratigraphy, and Pyrite Geochemistry for Volcanogenic Massive Sulfide Exploration: An Example from the Blake River Group, Abitibi Greenstone Belt,

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 Hebecourt 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.

Timing and characteristics of the Archean subaqueous Blake River Megacaldera Complex, Abitibi greenstone belt, Canada

The Archean Blake River Group of the Abitibi greenstone belt represents a megacaldera complex which evolved over 8–11M.y. from approximately 2704 to 2696Ma. The early Misema Caldera developed from a series of amalgamated shield volcano complexes identified by the remnants of mafic dyke and sill systems. These remnants are found in the Jevis-Clericy, Montsabrais-Renault, Clifford-Tannahill and Colnet regions, where summit calderas are delineated by circular ring dyke structures. The secondary 330°-trending New Senator Caldera formed within the envelope of the Misema Caldera and exhibits a box-work graben-type structure. Finally, the felsic-dominated, 070°-striking Noranda Caldera, well known for its VMS endowment, represents the final collapse of the megacaldera complex.Deformation has overprinted the calderas, but structural patterns can be used in order to reconstruct the pre-existing volcanic architecture. Structures such as mafic ring dyke complexes and rhyolitic dome-flow complexes have nucleated fold geometries and synvolcanic fractures were reactivated within zones of ductile deformation. Precise U-Pb geochronological analyses were conducted in selected areas with specific emphasis on the Misema and New Senator calderas. Felsic volcanism progressed throughout the evolution of the megacaldera complex and intermediate to felsic units have been dated to limit this evolution. The Misema Caldera formed between 2704 and 2702Ma via the amalgamation of shield volcanoes that were probably active prior to 2704Ma. The New Senator Caldera was generated between 2702 and 2700Ma during paroxysmal felsic volcanism, followed by the collapse of the Noranda Caldera culminating between 2700–2696Ma. The Misema Caldera was generated by a gravitational stress field consistent with the formation of ring and radial dyke architecture, whereas the SE-trending New Senator Caldera is more compatible with a SW-trending principal compression direction related to oblique convergence in the Abitibi belt. The Noranda Caldera is interpreted as a NE rift structure that formed in the final stages of megacaldera evolution.

Basaltic to andesitic volcaniclastic rocks in the Blake River Group, Abitibi Greenstone Belt: 1. Mode of emplacement in three areas1This article is a companion paper to Ross et al. 2011. Basaltic to andesitic volcaniclastic rocks in the Blake River Group, Abitibi Greenstone Belt: 2. Origin, geochemistry, and geochronology. Canadian Journal of Earth Sciences, 48: this issue.2MRNF Contribution BEGQ 8439-2010/2011-1. Natural

The Archean Blake River Group (BRG) of Ontario and Quebec is dominated by submarine mafic to intermediate lavas, with more restricted felsic volcanic rocks. Given the good quality of outcrop, and high level of preservation of some BRG rocks, the mafic to intermediate lavas were used in the 1970s and 1980s to better understand the evolution of massive and pillowed submarine flows, and their associated fragmental facies (pillow breccias, hyaloclastite). Potentially, the BRG could also represent a useful volcanic succession for the study of explosive submarine eruption products in the ancient record. Before this is possible, however, a regional inventory of the mafic to intermediate volcaniclastic units is needed to clarify their characteristics and origins. In this paper, we compare and contrast volcaniclastic rocks from three areas within the same formation of the northern BRG in Quebec: the Monsabrais area, the Lac Duparquet area, and the D’Alembert tuff area. Close examination reveals pronounced differences in terms of lateral continuity, thickness, grading, bedding, clast shapes, textures, etc. in the volcaniclastic rocks. These differences are interpreted to reflect vastly different emplacement processes, ranging from hyaloclastite generation as a result of self-fragmentation and lava contact with water (dominant in the Monsabrais and Lac Duparquet areas) to aqueous density currents likely fed directly by explosive submarine eruptions (dominant in the D’Alembert tuff).

Basaltic to andesitic volcaniclastic rocks in the Blake River Group, Abitibi Greenstone Belt: 2. Origin, geochemistry, and geochronology 1This article is a companion paper to Ross et al. 2011. Basaltic to andesitic volcaniclastic rocks in the Blake River Group, Abitibi Greenstone Belt: 1. Mode of emplacement in three areas. Canadian Journal of Earth Sciences, 48: this issue.2Ministère des Ressources naturelles

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 metres 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.

An Archean megacaldera complex: The Blake River Group, Abitibi greenstone belt

The Archean Blake River Group of the southern Abitibi greenstone belt is defined as a subaqueous megacaldera. Compelling evidence include: (1) a radial and concentric organization of synvolcanic mafic to intermediate dykes, (2) an overall dome geometry defined by the volcanic strata, (3) a peripheral distribution of subaqueous volcaniclastic units, (4) a zonal distribution of carbonate alteration, and (5) a distinct annular synvolcanic inner and outer ring fault pattern. Three caldera-forming events have been identified: (1) the early Misema caldera, (2) the New Senator caldera, and (3) the well-known Noranda caldera. The multi-vent BRG mafic volcanic complex, developed on a monotonous sequence of tholeiitic basalts, forming a submarine plain, and experienced a first major collapse that created the giant Misema Caldera (80 km in diameter). An endogenic dyke swarm intruded the synvolcanic fractures and an underlying magma chamber developed. Major volcaniclastic units were generated by local volcanic centers and summit calderas formed along the outer and inner ring faults. This fault system was used as a conduit for CO 2-rich hydrothermal activity. Renewed volcanic activity was associated with a resurgent central dome inside the Misema Caldera. This second collapse event created the 35 km by 14 km NW-trending New Senator Caldera. This caldera was produced by multi-step sagging after the underlying magmatic chamber migrated to the SE and formed the Flavrian-Powel Plutons. The final collapse resulted in the formation of the Noranda Caldera that generated a well-developed 070°-trending fracture pattern associated with several VMS deposits. The multiple-caldera setting provides an effective model to explain the presence of VMS mineralization along the synvolcanic fractures associated with the three collapse episodes. However, the importance of the New Senator Caldera fracture pattern is emphasized because of its role in the positioning of the giant Horne Mine deposit.

Regional-scale hydrothermal alteration in the Central Blake River Group, western Abitibi subprovince, Canada: implications for VMS prospectivity

The Late Archean Blake River Group is a thick succession of predominantly mafic volcanic rocks within the southern zone of the Abitibi greenstone belt. It contains a number of silicic volcanic centers of different size, including the large Noranda volcanic complex, which is host to 17 past-producing volcanogenic massive sulfide deposits. The Noranda complex consists of a 7- to 9-km-thick succession of bimodal mafic and felsic volcanic rocks erupted during five major cycles of volcanism. Massive sulfide formation coincided with a period of intense magmatic activity (cycle III) and the formation of the Noranda cauldron. Hydrothermal alteration in these rocks is interpreted to reflect large-scale hydrothermal fluid flow associated with rapid crustal extension and rifting of the volcanic complex. The alteration includes abundant albite, chlorite, epidote and quartz (silicification), which exhibit broad stratigraphic and structural control and correlate with previously mapped whole-rock oxygen isotope zonation. The Mine Sequence volcanic rocks are characterized by abundant iron-rich chlorite (Fe/Fe+Mg >0.5), hydrothermal amphibole (ferroactinolite) and coarse-grained epidote of clinozoisite composition ( 10 wt% Fe2O3) lacking the clinozoisite solid solution. Alteration in the Mine Sequence volcanic rocks persists along strike well beyond the limits of the main ore deposits (as far as several tens of kilometers) and can be readily distinguished from greenschist facies metamorphic assemblages at a regional scale. The lack of similar alteration in the pre-cauldron sequences is consistent with limited 18O-depletion and suggests that the early history of the volcanic complex did not support large-scale, high-temperature fluid flow in these rocks. Comparisons with a much smaller, barren volcanic complex in nearby Ben Nevis township reveal important differences in the alteration mineralogy between volcanoes of different size, with implications for area selection during regional-scale mineral exploration. The Ben Nevis Complex consists of a 3- to 4-km-thick succession of mafic, intermediate and felsic volcanic rocks centered on a small subvolcanic intrusion. Alteration of the volcanic rocks comprises mainly low-temperature assemblages of prehnite, pumpellyite, magnesium-rich chlorite (Fe/Fe+Mg 10 wt% Fe2O3) and calcite. Actinolite ± magnetite alteration occurs proximal to the intrusive core of the complex, but the limited extent of this alteration indicates only local high-temperature fluid circulation adjacent to the intrusion. A distal zone of carbonate alteration is located 4–6 km from the center of the volcano. Although iron-bearing carbonates are present locally within this zone, the absence of siderite argues against a high-temperature origin for this alteration. These observations do not offer positive encouragement for the existence of a fossil geothermal system of sufficient size or intensity to have produced a large massive sulfide deposit.

Spherulitic rhyolites of the Archean Blake River Group, Canada; implications for stratigraphic correlation and volcanogenic massive sulfide exploration

Based on the distinctive textural and chemical character of highly spherulitic rhyolites in the Late Archean, volcanogenic massive sulfide-rich, Blake River Group of Quebec, it is proposed that they represent key beds which may be used to infer the age relationships between the three subgroups (Rouyn-Pelletier, Misema, and Noranda) into which the Blake River Group is divided in this area. These rhyolites occur as flow and fragmental (volcaniclastic, probably pyroclastic) facies, both of which contain crystallization spherulites. They occur as three units, a flow facies exhibiting parallel-agglomerated lath-shaped plagioclase crystals (the Fish-roe rhyolite), a fragmental spherulitic rhyolite which is aphyric, and a second, very local, flow which is quartz-plagioclase porphyritic. Chemically, however, the three units are identical and distinct within the Blake River Group stratigraphy.The Ti, Y, Yb, and Zr contents are higher in the spherulitic rhyolites than in the other rhyolites in the Blake River Group, whereas the La content is similar between the two rhyolite populations. These values translate into higher Ti/Zr, and lower Zr/Y and La/Yb, ratios for the spherulitic rhyolites and result in their being important stratigraphic marker units in the group in Quebec. Statistical analyses of the geochemical data from the rhyolites in the Blake River Group show that there is a 95 percent probability that the spherulitic rhyolite analyses represent a single genetically related population and may, in fact, be a single stratigraphic unit.The occurrence of the fragmental highly spherulitic rhyolite in the andesitic Misema Subgroup and in the bimodal andesitic-rhyolitic Noranda Subgroup implies that these two domains formed, at least in part, concurrently. The same conclusion can be drawn for the basaltic Rouyn-Pelletier Subgroup and the Noranda Subgroup, as the Fish-roe rhyolite occurs in both these domains. This in turn suggests that a complex interplay of contemporaneous magmatic systems, rather than a single vertically stacked evolutionary system, was responsible for the formation of the Blake River Group and its volcanogenic massive sulfide deposits.

Volcanic lithogeochemistry and alteration at the Delbridge massive sulfide deposit, Noranda, Quebec

The Delbridge orebody occurs within a thick sequence (> 1 km) of porphyritic to aphyric massive rhyolite and rhyolite breccia of the Archean Blake River Group. The orebody produced ≈ 370,000 tonnes grading 0.61% Cu, 9.6% Zn, 110 g/t Ag and 2.1 g/t Au (1969–1971). The footwall consists of massive quartz porphyritic rhyolite mantled by proximal rhyolite breccias. An irregular chloritic alteration pipe with mineralization is subvertical to the ore lens. The orebody occurs at a thick cherty horizon within rhyolite breccia, and is overlain by a succession of mafic debris flows, porphyritic to aphyric massive rhyolite flows, and finally andesite. The main alteration assemblage in the rhyolite units is quartz-albite-sericite-chlorite-carbonate. Immobile element plots and rare-earth element data indicate that the footwall rhyolite flows and proximal breccias are tholeiitic to transitional ( Zr/ Y = 3.5–5.5; La N/Yb N = 1.7–2.6), whereas hangingwall rhyolite flows are mildly calc-alkaline ( Zr/ Y = 6.5–7.5; La N/Yb N = 2.8–3.8). These two rhyolite types also have separate alteration lines in Ti-Zr space and in various immobile element plots. The identification of chemically different rhyolites above and below the orebody provides markers that can be identified and traced even where strongly altered. An intrusive rhyolite mass in the footwall is chemically identical to the hangingwall aphyric rhyolite flows, and is interpreted as the feeder to these flows. Calculated mass changes in the footwall rhyolite commonly are large, and result from major silica change (±30%), significant loss of Na 2 O + CaO, and important additions of K 2O and FeO + MgO. The margins of the pipe show net mass gain, whereas the interior of the pipe shows net mass loss. Hangingwall rhyolite shows mass changes that generally are much smaller than in the footwall. Felsic rocks in the silica-sericite alteration zone up to ≈ 200 m from the orebody have high δ 18O values of 10–12‰, reflecting low-temperature alteration. The orebody occurs near the contact between a mainly tholeiitic rhyolite footwall and an overlying sequence of mildly calc-alkaline rhyolite then andesite.

Massive sulfide deposits of the Noranda area, Quebec. IV. The Mobrun mine

The Mobrun polymetallic deposit near Rouyn–Noranda comprises two complexes of massive sulfide lenses within mainly felsic volcanic rocks of the Archean Blake River Group. The Main lens contained 3.37 Mt of massive sulfides, with 1989 reserves of 0.95 Mt at 0.81% Cu, 2.44% Zn, 30.3 g/t Ag, and 2.2 g/t Au. The 1100 complex, located ~250 m to the southeast of the Main complex, contains estimated 1989 reserves of 10.4 Mt at 0.76% Cu, 5.43% Zn, 37.4 g/t Ag, and 1.35 g/t Au.Host volcanic rocks of the Main complex are mostly massive, brecciated, and tuffaceous rhyolites. The rhyolites are commonly strongly sheared parallel to lithological contacts, which are locally displaced by high-angle faults. Immobile-element plots such as Y–Zr and Nb–Zr show a separation of rhyolite data into two distinct alteration trends that generally correspond to massive and in situ brecciated rhyolite of the footwall, and tuffaceous rhyolite of the hanging wall. The hanging wall has tholeiitic Zr/Y ratios (3–5), whereas the footwall has mildly calc-alkaline Zr/Y ratios (7–9). Several immobile-element trends indicate that there was a subtle but clear change in rhyolite composition near the time of ore deposition. Identification of chemically distinct footwall and hanging wall rhyolites allows these units to be recognized and traced along strike, even where alteration is strong. Sericitization and silicification extend at least 100 m from the orebody, with local chloritic zones in the upper footwall. Calculated mass changes indicate that the footwall generally has lost silica mass relative to the hanging wall. Alteration zones associated with mineralization have mass gains in FeO + MgO and K2O gains, but mass loss in silica.The 1100 complex, located stratigraphically below the Main complex, is hosted by rhyolite, with one main andesite interval in the footwall. The footwall contains three chemically distinct rhyolite types, all tholeiitic. Hanging-wall rhyolites are, however, mildly calc-alkaline, and thus are chemically comparable to, and correlated with, the footwall of the Main complex. Rhyolites within ~100 m stratigraphically of the Main and 1100 complexes commonly have positively shifted δ18O whole-rock values of 11–13‰. These high values are interpreted as the result of an initial, widespread phase of low-temperature hydrothermal alteration that increased δ18O values by 3–5‰ relative to unaltered rhyolites. Some footwall rhyolites, however, are relatively depleted in 18O, strongly depleted in Ca–Na and depleted in Eu2+. Rhyolites with these chemical features have been overprinted by higher temperature alteration, presumably in localized feeder zones. All four rhyolite types near the 1100 complex are chemically recognizable despite contrasting alteration.The orebodies are interpreted as synvolcanic, based on their occurrence along distinctive volcanic contacts, and the presence of primary sulfide textures where deformation is minor. The chemostratigraphic framework defined for the host rhyolite sequence can be used to trace critical volcanic contacts through lithologically monotonous, strongly altered, and faulted stratigraphy.

Tholeiitic volcanic rocks of the late Archean Blake River Group, southern Abitibi greenstone belt: origin and geodynamic implications

The late Archean Blake River Group volcanic sequence forms the uppermost part of the southern Abitibi greenstone belt in Quebec. The group is mainly composed of mid-ocean-ridge basalt (MORB)-like tholeiites that show a progressive change of several incompatible trace element ratios (e.g., Nb/Th, Nb/Ta, La/Yb, and Zr/Y) during differentiation. The compositional variations are inferred to be the result of fractional crystallization coupled with mixing–contamination of tholeiites by calc-alkaline magma which produced the mafic–intermediate lavas intercalated with the tholeiites in the uppermost part of the sequence. The MORB-like tholeiites were probably emplaced in a back-arc setting.

Massive sulfide deposits of the Noranda area, Quebec. II. The Aldermac mine

The original Aldermac mine near Noranda contained several Cu–Zn massive sulfide lenses hosted by felsic to mafic volcanic rocks of the late Archean Blake River Group. The original Nos. 3–6 orebodies, which consisted of massive pyrite, with lesser magnetite, pyrrhotite, chalcopyrite, and sphalerite, contained 1.87 Mt of Cu–Zn ore that averaged 1.47% Cu (Zn was not recovered). The orebodies occurred within felsic breccias and tuffs up to 100 m thick that are stratigraphically overlain by an extensive dome of mainly massive rhyolite and rhyodacite (up to 250 m thick and at least 550 m across). Most of the volcanic rocks that laterally flank and overlie the felsic dome are dacitic to andesitic flows, breccia, and tuff, with minor rhyolites, and associated subvolcanic sills of quartz-feldspar porphyry and gabbro.The new massive sulfide deposit, discovered in 1988, lies 150–200 m east of the mined-out orebodies, at a similar stratigraphic level within altered felsic breccia and tuff. The sulfides are mainly in the No. 8 lens, which contains 1.0 Mt at an average grade of 1.54% Cu, 4.12% Zn, 31.2 g/t Ag, and 0.48 g/t Au. Pyrite forms porphyroblastic megacrysts in a groundmass of pyrrhotite, sphalerite, magnetite, and chalcopyrite. A funnel-shaped, chloritized stockwork zone underlies the No. 8 lens and contains Cu-stringer mineralization. The No. 8 lens appears to be zoned, with overall decreasing Cu:Zn ratios from the core to the fringes of the lens. Massive sulfides in this lens have high Ag, Cd, and Hg contents relative to other massive sulfide deposits near Noranda.Ti versus Zr trends for least-altered Aldermac volcanic rocks indicate a more or less continuous magmatic fractionation trend ranging from high-Ti andesite to andesite, dacite, rhyodacite, and two distinct rhyolites (A and B). Most volcanic rocks were derived from a common parental magma that was transitional between tholeiitic and calc-alkaline compositions, as indicated by Ti–Y–Zr–Nb data and rare-earth-element distributions.Ti versus Zr trends in altered volcanic rocks indicate that silicification (mass gain) has affected some of the andesitic to rhyodacitic rocks, whereas chloritization (mass loss) has affected many of the rhyolitic rocks. Intermediate to mafic volcanic rocks above and lateral to the felsic dome are commonly silicified, possibly the result of hydrothermally remobilized silica derived from underlying felsic volcanic rocks.The orebodies appear to have formed at an eruptive hiatus between mafic → felsic and felsic → mafic cycles, during explosive activity and accumulation of felsic breccia and tuff. Ore was deposited mainly within a felsic fragmental sequence (rhyolite A), but before emplacement of the dome of rhyolite B. In compositionally diverse volcanic terrains, the contact between successive mafic–felsic and felsic–mafic cycles may be a good exploration target, in particular specific geochemical contacts within the felsic stratigraphy.

Geochemistry of hydrothermally altered rocks at the Horne Mine, Noranda, Quebec

Low K rhyolites of the Archean Blake River Group that host the Horne volcanogenic massive sulfide deposit are altered to quartz-sericite-chlorite-albite (quartz-sericite zone) and chlorite + or - quartz (chlorite zone) assemblages. The quartz-sericite zone accounts for approximately 90 percent of the 5 km 3 of the altered hanging-wall and footwall rocks. Chlorite zone alteration is limited to portions of the footwall and flanks of the Cu-rich H orebodies. Clusters of gold-bearing chlorite-quartz veinlets crosscut large volumes of the quartz-sericite zone rocks.Alteration has added an average of 13 wt percent SiO 2 , 2.3 wt percent K 2 O, 430 ppm Ba, and 46 ppm Rb to the system and leached 3.6 wt percent Na 2 O, 2.2 wt percent CaO, and 41 ppm Sr. Middle and light REE were increasingly mobile from Yb to La; they were leached from the chlorite zone rocks and added to quartz-sericite zone rocks. Chlorite was more Fe rich (Fe/Fe + Mg = 0.65-0.85) in the chlorite zone rocks than in other altered lithologies (Fe/Fe + Mg = 0.2-0.5).Sulfur isotope values for massive ores are in the range delta 34 S = -2.1 to 1.9 per mil. Mineral separates from altered rocks yielded delta 18 O values of 10.2 to 12.1 per mil for quartz, 7.2 to 9.5 per mil for calcite, 2.2 to 3.0 per mil for chlorite, and 0.1 to 1.0 per mil for magnetite. Oxygen and sulfur isotope fractionation between coexisting minerals suggests formation from fluids having delta 18 O = 3 to 4 per mil at temperatures between 275 degrees to 380 degrees C. Oxygen isotope compositions of host rocks are delta 18 O = 8 + or - 1 per mil for freshest rhyolite, delta 18 O = 6.6 to 11.6 per mil for quartz-sericite zone rocks, and delta 18 O = 3.9 to 4.4 per mil for chlorite zone rocks. These measured values compare closely to estimated whole-rock delta 18 O values based on mass balance considerations using normative mineralogies.Alteration of wall rocks encompassing the massive sulfide orebodies is characterized by intense sericitization, a high degree of silicification, and a marked positive delta 18 O anomaly. Calculations based on SiO 2 solubilities and probable K contents of the hydrothermal fluids suggest a water-rock mass ratio of 50 to 300 during the mineralization event.

Progressive alteration associated with auriferous massive sulfide bodies at the Dumagami Mine, Abitibi greenstone belt, Quebec

Pyritite and massive sphalerite-galena bodies are surrounded by two distinct concentric alteration zones within the metafelsites of the late Archean Blake River Group at the Dumagami Au-Ag-Cu deposit, southern Abitibi belt, northwestern Quebec. The inner alteration zone is characterized by andalusite-bearing rocks and is interpreted as a metamorphosed advanced argillic alteration zone; the outer zone is represented by white mica phyllonite and phyllonitic breccia and is interpreted as a metamorphosed sericitic alteration zone. Surrounding least altered rocks, represented by massive quartziferous augen rocks and ribboned micaceous augen schists, are composed of blue quartz and albite phenocrysts, quartz, biotite, ilmenite, and chlorite. The sericite zone is characterized by the metamorphic mineral assemblage quartz-sericite-rutile-pyrite + or - andesine and by the absence of albite phenocrysts, biotite, and ilmenite. The andalusite zone is characterized by the metamorphic mineral assemblage quartz-andalusite-sericite-rutile-pyrite + or - kyanite and by the absence of albite phenocrysts, andesine porphyro-blasts, biotite, and ilmenite.Petrographic and geochemical features of least altered, sericitic, and andalusite-bearing rocks indicate that felsic effusive rocks were the principal protolith. Strong leaching of alkali and alkaline earths (especially Mg) within the inner alteration zone has resulted in a notable absence of chlorite, a feature atypical of the alteration associated with Archean massive sulfide deposits. Chemical and mineralogical changes from least altered to altered rocks suggest that highly acidic hydrothermal fluids were responsible for the premetamorphic formation of alteration assemblages at the Dumagami deposit. However, gold transport in these fluids would be minimal, and gold emplacement is linked instead to a late, structural event.

Evidence for contrasting compositional spectra in comagmatic intrusive and extrusive rocks of the late Archean Blake River Group, Abitibi, Quebec

The Flavrian pluton is a sill-shaped intrusion in the Blake River Group (BRG) volcanic rocks in the Noranda region of the Abitibi greenstone belt. The pluton is dominated by trondhjemites and tonalites, with minor peripheral quartz gabbro and hybrid phases. The BRG volcanic rocks consist of a bimodal suite of basalt–andesite and rhyolite. The Flavrian trondhjemites are geochemically identical to the rhyolitic lavas of the BRG (SiO2 ≥ 72%, La/Sm = 3.4, La/Yb = 3.6, Zr/Y = 3.9, Y/Nb = 3.1), and the Flavrian gabbroic and dioritic rocks are identical to the BRG basalts and andesites (SiO2 < 58%, La/Sm = 3.0, La/Yb = 5.5, Zr/Y = 4.2, Y/Nb = 3.3). However, the tonalitic rocks of the Flavrian pluton have no extrusive equivalents in the BRG. The different compositional spectra of the extrusive and intrusive rocks are interpreted as being a result of a transition in magma-chamber evolution from a zoned open system that was active during the evolution of the volcanic rocks to closed-system plutonic crystallization. The latter destroyed the compositional bimodality of the magma chamber and resulted in the evolution of intermediate compositions (tonalites) generated by both fractional crystallization and magma mixing.

Origin of a complex alteration assemblage, Four Corners Cu-Zn prospect, Quebec, Canada

Three types of hydrothermal alteration spatially associated with Cu-Zn mineralization within the Archean Blake River Group of greenschist facies metavolcanic rocks, Noranda, Quebec, were studied. Alteration was mapped around a partly fragmental rhyolite dome, enclosed in mafic flows. Sodic regional alteration and calcic-silicic background alterations were overprinted by potassic and magnesian pipe alteration. Pipe alteration was mapped only in conical breccia pipes within the rhyolite dome, which terminate at a pyroclastic rhyolite-rhyolite breccia contact. Cu-Zn sulfide mineralization was found within the pipes and in stratiform exhalative tuff on overlying volcanic rock contacts. Mass balance studies suggest a general trend from base-fixing regional and background alteration to base-leaching pipe alteration.Textural and geochemical evidence and similarities with presently active geothermal systems indicate alteration and mineralization were products of an Archean convective system, closely related in time and space to volcanism. Albite-rich rocks (regional alteration) are mineralogically similar but chemically distinct from greenschist facies metavolcanic rocks in the rest of the Noranda area. They are chemically similar to spilitic rocks from Thingmuli volcano, Iceland, and are the earliest evidence of seawater-rock interaction. This interaction, in conjunction with further reactions during subsequent background alteration, converted neutral to alkaline seawater into a hot, metal-enriched acid brine. Final reactions between this brine and the rocks through which it passed immediately beneath the sea floor resulted in pipe alteration and in Cu-Zn mineralization where the brine was expelled onto the sea floor. Stratiform pillowed andesite flows showing calcic background alteration were altered during their deposition by continuing exhalative activity. These distinctive altered flows represent excellent field indicators of base metal mineralization, best identified and traced through detailed mapping.