Volcanic red-bed copper mineralisation related to submarine basalt alteration, Mont Alexandre, Quebec Appalachians, Canada (original) (raw)
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Geochemistry and petrogenesis of basaltic rocks from Coppermine River area, Northwest Territories
Proterozoic basaltic flows (>2000 m thick) and associated dykes and sills from the Coppermine River area, Northwest Territories have chemical compositions typical of continental tholeiites. The low Mg/Fe ratio and abundances of Ni and Cr indicate that the lavas were extensively fractionated prior to extrusion. The variations of incompatible elements such as K, Rb, REE, Y, Zr, Nb, and Th suggest that the rocks were affected by interaction with continental crust. The samples least affected by contamination have trace-element compositions very similar to those of P-type mid-ocean ridge basalts. It is suggested that continental tholeiites have been generated from the same source as P-type oceanic tholeiites, and geochemical features, such as the enrichment of some lithophile elements in many of these rocks, may be related to crustal contamination. The variations within the volcanic pile of the Coppennine River area are related to those of an exposed part of the Muskox layered intrusion.
Volcanic rocks of the Jurassic Iskut River Formation (IRF) in northwestern British Columbia (Canada) host several volcanogenic massive sulfide (VMS) deposits, including the exceptionally high-grade Eskay Creek Ag–Au–Cu–Pb–Zn deposit. The IRF comprises voluminous pillow basalt (>90%), minor rhyolite, and sedimentary rock of late Early to early Middle Jurassic age, filling a series of sub-basins along a 300 by 50 km north-trending belt. Two geochemically distinct types of tholeiitic basalts interfinger; both resemble back-arc basin basalts formed from the melting of asthenospheric and sub-arc mantle sources. Group 2 basalts are more enriched in light rare-earth elements, Ba, K, Sr, Th, and U, and have lower positive Nd values than group 1 basalts (+3.2 to +6.3 versus +6.9 to +8.4, respectively). The compositional differences between group 1 and group 2 basalts are interpreted to result from crustal contamination in group 2. Group 1 basalts are most common in the southern part of the IRF belt where they are closely associated with the Eskay Creek, Bonanza, and Hidden Creek (Anyox) VMS deposits. Group 2 basalts are most abundant in the northern half of the belt and are not associated with exploited mineral deposits. The lack of crustal contamination in group 1 basalts indicates that they formed from rapidly ascending magma in an advanced rift setting and were associated with high heat flow that drove hydrothermal circulation. Group 1 and group 2 basalts are reliably discriminated by Ta/Th <2.5 in the former and >2.5 in the latter. This geochemical criteria can therefore be used as an exploration tool to identify VMS permissive sub-basins and (or) stratigraphy in the IRF.
Geochemistry of Volcanic Rocks Associated with Cu-Zn and Ni-Cu Deposits in the Abitibi Subprovince
Volcanogenic massive sulfide (VHMS) deposits in the Abitibi subprovince are preferentially associated with volcanic successions containing >150 m thicknesses of felsic volcanic rocks (• 50% by area of volcanic terranes) and are found within volcanic sequences of at least three distinct affinities. Group I, which is host to greater than half of the volcanogenic massive sulfide deposits by tonnage and which comprises only • 10 percent by area of volcanic terranes, is composed of bimodal, tholeiitic basalt-basaltic andesitc, and high silica rhyolite. The basaltic andesites and high silica rhyolites are characterized by high high field strength element and heavy rare earth element (REE) contents, low light to heavy REE ratios (most with LaN/YbN-0.8-3), and strong negative Eu anomalies. The Kamiskotia, Matagami, and Chibougamau (Lower cycle) volcanogenic massive sulfide areas, all of which are also underlain by large, synvolcanic gabbroic complexes, are associated with group I volcanic sequences. The Kidd Creek, Potter, Normetal, and Horne deposits are also included in this category. Group II, which is host to one-third of the volcanogenic massive sulfide deposits by tonnage and which is also • 10 percent by area of volcanic terranes, is composed of bimodal, transitional tholeiitic to calc-alkalic andesitc and rhyolite, characterized by intermediate high field strength element contents and slightly higher REE ratios (LaN/Yb • = 1-4: Noranda camp (excluding the Horne deposit) and Val d'Or camp). Group III is host to only one known deposit, the Selbaie mine, which is unusual in that much of its mineralization cuts strati-graphy. The Selbaie mine sequence contains calc-alkalic andesite-rhyolite with relatively low high field strength element and REE contents, and higher REE ratios (La • /Yb • = 3-9). The vast majority of volcanogenic massive sulfide-bearing mafic and felsic volcanic rocks in the Abitibi subprovince have Las/YbN < 5. Barren volcanic sequences are group IV-calc-alkalic basaltic andesitc to rhyodacite, with low high field strength element and relatively high REE ratios (Las/Ybs = 8-20), represented by the upper Skead Group, the Quebec Hunter Mine Group, and the Upper cycle Chibougamau rocks; and group V, mafic to felsic alkalic volcanic rocks, with high REE ratios (Las/Ybs = 12-62), represented by the Timiskaming, Opemisca, and Ridout series rocks. Group I is most similar to thickened oceanic rift suites (e.g., Galapagos spreading center, Iceland East riff), group II is similar to suites in rifled island arcs (e.g., Hokuroku district, Japan), and group III is comparable to continental arc suites (e.g., Southern Volcanic zone, central Chile). Groups IV and V are comparable to arc-related suites derived from metasoma-tized mantle, with variable amounts of crustal contamination (e.g., Setouchi area, Japan; Roman province, Italy). Known Ni-Cu deposits in volcanic sequences are hosted exclusively in komatiitic flows and hypabyssal sills, represented by chill compositions with high MgO contents (20-35 wt %, anhydrous), very low incompatible element contents, and depleted signatures (LaN/Sm • = 0.5-0.8; Zr/Y < 2.5; Alexo, Marbridge, and Shaw Dome deposits), in comparison to most barren komatiites in the Abitibi subprovince. Their depleted trace element signatures indicate minimal crustal assimilation, which would appear contrary to the widely held view that crustal contamination (e.g., addition of silica, sulfur, oxygen) of a mantle-derived melt is necessary for sulfur saturation and magmatic sulfide segregation. This problem may be reconciled by considering the dynamics of komatiite flows (Lesher and Arndt, 1990) where contam
GEOCHEMISTRY OF THE NORTH MOUNTAIN BASALTS (NOVA SCOTIA, CANADA)
The Early Mesozoic North Mountain basalts of Nova Scotia, Canada, are continental tholeiites similar in composition to numerous Triassic diabase dykes of the northern Appalachians. All these rocks probably originated during the initial opening of the Atlantic Ocean. The lavas have been affected by zeolite facies metamorphism which caused mobilization of some elements such as Li and Cu. The North Mountain basalts underwent extensive fractional crystallization dominated by pyroxene and plagioclase separation. Compared to oceanic tholeiites, these basalts have similar abundances of major, rare-earth and some high-field-strength elements, and corresponding element ratios, but they have higher contents of Th and K and associated elements. Their normalized trace-element patterns are closely similar to those of some other continental tholeiites and to the average continental crust, and display negative Nb anomalies. It is suggested that continental tholeiites were derived from an upper-mantle source similar to that for oceanic tholeiites but were affected by crustal contamination.
Initial Reports of the Deep Sea Drilling Project, 82, 1985
Sr and Nd isotopic composition of 23 basalts from Sites 556-559 and 561-564. are reported. The 87 Sr/ 86 Sr ratios in fresh glasses and leached whole rocks range from 0.7025 to 0.7034 and are negatively correlated with the initial 143 Nd/ 144 Nd compositions, which range from 0.51315 to 0.51289. The Sr and Nd isotopic compositions (in glasses or leached samples) lie within the fields of mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) from the Azores on the Nd-Sr mantle array/fan plot. In general, there is a correlation between the trace element characteristics and the 143 Nd/ 144 Nd composition (i.e., samples with Hf/Ta > 7 and (Ce/Sm) N < 1 [normal-MORB] have initial 143 Nd/ 144 Nd > 0.51307, whereas samples with Hf/Ta < 7 and (Ce/Sm) N > 1 (enriched-MORB) have initial 143 Nd/ 144 Nd compositions < 0.51300). A significant deviation from this general rule is found in Hole 558, where the N-MORB can have, within experimental limits, identical isotopic compositions to those found in associated E-MORB. The plume-depleted asthenosphere mixing hypothesis of Schilling (1975), White and Schilling (1978) and Schilling et al. (1977) provides a framework within which the present data can be evaluated. Given the distribution and possible origins of the chemical and isotopic heterogeneity observed in Leg 82 basalts, and some other basalts in the area, it would appear that the Schilling et al. model is not entirely satisfactory. In particular, it can be shown that trace element data may incorrectly estimate the plume component and more localized mantle heterogeneity (both chemical and isotopic) may be important.
Atlantic Geology, 2008
The Copper Lake area of mainland Nova Scotia is one of several vein-controlled mineralized (Cu-Au-Co) systems associated with widespread carbonate and iron-oxide alteration proximal to the east-trending Cobequid-Chedabucto Fault System. Although this mineralization has been known for decades, its metallogenic affinity remains poorly defined, and in recent years an IOCG (iron oxide-copper-gold) model has been suggested. In order to determine the age of mineralization and provide an important time constraint for developing a metallogenic model, direct dating of the mineralization and associated alteration was undertaken. At Copper Lake, mineralization occurs in a set of sulphide-carbonate fissure veins hosted by fine-grained metasedimentary rocks of the Middle Devonian Guysborough Group. Dating of the sulphide-alteration (pyrite) and phyllic-alteration (muscovite) stages of the ore system utilized the Re-Os and 40 Ar/ 39 Ar methods, respectively. The two different chronometers yield ages of about 320 Ma and provide an absolute age for the mineralization. As part of this study additional geochronological data were obtained for detrital zircon (U-Pb age of 1634 ± 11.2 Ma) from the host sedimentary rocks, as well as timing of thermal events at ca. 370-380 Ma, 350 Ma and < 300 Ma based on whole rock 40 Ar/ 39 Ar and chemical Th-Pb dating of host rocks and monazite. The Th-Pb dating of monazite indicates that rare-earth element mobility accompanied mineralization. Collectively, the data indicate that the area experienced multiple thermal events, but hydrothermal activity related to mineralization is constrained to about 320 Ma and is tentatively interpreted to relate to structural focusing of fluids that may have been driven by a mid-crustal level mafic heat source. The mineralizing event coincides with regional Alleghanian deformation in this part of the Appalachian orogen and thus reflects larger-scale tectonothermal processes. RÉSUMÉ Le secteur du lac Copper, dans la partie continentale de la Nouvelle-Écosse, constitue l'un de plusieurs systèmes minéralisés (Cu-Au-Co) régis par des filons associés à une altération étendue en oxyde de fer et en carbonates proximale du système de failles orienté vers l'est de Cobequid-Chedabucto. Même si l'on connaît cette minéralisation depuis des décennies, son affinité métallogénique demeure mal définie et des chercheurs ont avancé ces dernières années un modèle OFCO (oxyde de fer-cuivre-or). On a réalisé une datation directe de la minéralisation et de l'altération connexe pour déterminer l'âge de la minéralisation et établir une délimitation chronologique importante pour l'établissement d'un modèle métallogénique. Dans le secteur du lac Copper, la minéralisation est présente dans un ensemble de filons de fissures remplies de sulfures-carbonates à l'intérieur de roches métasédimentaires à grains fins du groupe du Dévonien moyen de Guysborough. Les méthodes Re-Os et 40 Ar/ 39 Ar, respectivement, ont permis la datation des stades de la sulfuration (pyrite) et de l'altération phylliteuse (muscovite). Les deux différentes méthodes chronométriques ont fourni des âges d'environ 320 Ma et confèrent un âge absolu à la minéralisation. On a obtenu, dans le cadre de cette étude, des données géochronologiques supplémentaires de zircon détritique (datation U-Pb de 1634 ± 11,2 Ma) des roches sédimentaires hôtes ainsi que le moment des événements thermiques, situés à environ 370-380 Ma, 350 Ma et < 300 Ma d'après une datation de la roche totale 40 Ar/ 39 Ar et une datation chimique Th-Pb des roches hôtes et de la monazite. La datation Th-Pb de la monazite révèle qu'une mobilité des métaux des terres rares a accompagné la minéralisation. Les données signalent collectivement que le secteur a connu plusieurs événements thermiques, mais que l'activité hydrothermale apparentée à la minéralisation est limitée à environ 320 Ma; on l'interprète provisoirement comme une activité liée à une concentration structurale des fluides qui pourrait avoir été provoquée par une source de chaleur mafique mi-crustale. L'épisode de minéralisation coïncide avec la déformation alléghanienne régionale dans cette partie de l'orogène des Appalaches et correspond en conséquence à des processus tectonothermaux à plus grande échelle.
1995
Differentiated rocks in thick flows ofthe Jurassic North Mountain Basalt, Nova Scotia, display evidence for fractionation of noble metals (Au, Pd, ft, Rh, Ru, and Ir). Meter-thick layers of mafic pegmatile and vesicular basalt high in the flbws are enriched in Au and Pd but depleted in Pt, Rh, Ru and Ir relative to undifferentiated basalt. Mineral precipitation (e.9., chromite
Generation of porphyry copper deposits by gas–brine reaction in volcanic arcs
Nature Geoscience, 2015
Porphyry copper deposits, that is, copper ore associated with hydrothermal fluids rising from a magma chamber, supply 75% of the world's copper. They are typically associated with intrusions of magma in the crust above subduction zones, indicating a primary role for magmatism in driving mineralization. However, it is not clear that a single, copper-rich magmatic fluid could trigger both copper enrichment and the subsequent precipitation of sulphide ore minerals within a zone of hydrothermally altered rock. Here we draw on observations of modern subduction zone volcanism to propose an alternative process for porphyry copper formation. We suggest that copper enrichment initially involves metalliferous, magmatic hyper-saline liquids, or brines, that exsolve from large, magmatic intrusions assembled in the shallow crust over tens to hundreds of thousands of years. In a subsequent step, sulphide ore precipitation is triggered by the interaction of the accumulated brines with sulphurrich gases, liberated in short-lived bursts from the underlying mafic magmas. We use high-temperature and high-pressure laboratory experiments to simulate such gas-brine interactions. The experiments yield copper-iron sulphide minerals and hydrogen chloride gas at magmatic temperatures of 700-800 • C, with textural and chemical characteristics that resemble those in porphyry copper deposits. We therefore conclude that porphyry copper ore forms in a two-stage process of brine enrichment followed by gas-induced precipitation.
9. Geochemistry of Hydrothermally Altered Basalts: Deep Sea Drilling
2006
This chapter documents the chemical changes produced by hydrothermal alteration of basalts drilled on Leg 83, in Hole 504B. It interprets these chemical changes in terms of mineralogical changes and alteration processes and discusses implications for geochemical cycling. Alteration of Leg 83 basalts is characterized by nonequilibrium and is heterogeneous on a scale of centimeters to tens or hundreds of meters. The basalts exhibit trends toward losses of SiO2, CaO, TiO2; decreases in density; gains of MnO, Na2O, CO2, H2O + , S; slight gains of MgO; increased oxidation of Fe; and variable changes in A12O3. Some mobility of rare earth elements (REE) also occurred, especially the light REE and Eu. The basalts have lost Ca in excess of Mg + Na gains. Variations in chemical trends are due to differing water/rock ratios, substrate control of secondary mineralogy, and superimposition of greenschist and zeolite facies mineralogies. Zeolitization resulted in uptake of Ca and H2O and losses of...
2014): Geochemical constraints on the tectonic setting of basaltic host rocks to the Windy Craggy Cu-Co-Au massive sulphide deposit, northwestern British Columbia, International Geology Review, Windy Craggy is an approximately 300 Mt Cu-Co-Au volcanogenic massive sulphide (VMS) deposit in northwestern British Columbia, Canada. The Windy Craggy deposit is hosted by the Middle Tats Volcanics (MTV), a Late Triassic volcano-sedimentary sequence of intercalated mafic pillowed to massive volcanic flows and sills and calcareous argillite that are part of the Alexander terrane. The host footwall and hangingwall flows and sills are predominantly alkalic basalts (Nb/ Y > 0.70). MTV alkali basalts at Windy Craggy are enriched in light rare earth elements (LREEs) >100X chondrite compared to chondrite, have steep REE patterns [(La/Yb) cn = 7.1-25.4], and generally lack the Ta and Nb depletions relative to primitive mantle (e.g. [Nb/Th] pm = 0.68-1.94) characteristic of arc environments, although most have [Nb/La] pm < 1. By contrast, volcanic rocks away from the deposit (and regionally; Lower Tats Volcanics, LTV) as well as late dikes that cross-cut all lithologies including metamorphic and deformational fabrics are sub-alkalic tholeiitic to calc-alkaline basalts and basaltic andesites that are less enriched in the LREEs (10-100X chondrite), have less steep REE patterns [(La/ Yb) cn = 0.41-10.6], and show well-developed Ta and Nb depletions (arc signatures; [Nb/Th] pm = 0.20-0.79), consistent