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Fluid inclusions in diagenetic cements in Upper Jurassic--Lower Cretaceous sandstones offshore Nova Scotia provide constraints on the fluid migration history in gas reservoirs of the Scotian basin. Diagenetic minerals from six wells in the Venture field were analysed by optical petrography, scanning electron microscopy (SEM), and electron microprobe. A total of 122 primary and secondary fluid inclusions were analysed from different cements. Primary aqueous inclusions in quartz overgrowths have homogenization temperatures([T.sub.h)] of 111.8 ± 7.1 °C (1σ) and in later carbonate cements 126.5 ± 2.1 °C; inclusions in both cements are highly saline (16-26.1 wt.% NaCl equivalent). Secondary aqueous and hydrocarbonbearing inclusion trails crosscutting silica cement and detrital quartz have [T.sub.h] of 121.6 ± 13.6 °C and low salinities (8.7 ± 6.0 wt.%). Secondary carbonic inclusions have C[O.sub.2] melting temperatures (-56.6 ± 0.1 °C) and [T.sub.h](-9.3 ± 0.8 °C) indicating a high-density carbonic phase. Late carbonate cements in the same sandstone units vary in chemical composition in different wells, and connected reservoirs show similar late carbonate assemblages, suggesting that the late carbonate cementation may be partly controlled by the reservoir fill and spill sequence. Silica and late carbonate cementation involved highly saline fluid flow, likely at about ~135 Ma. Hydrocarbon migration postdated silica cementation and was associated with secondary fracturing, suggesting that it corresponded to the onset of overpressure. Les inclusions de fluides dans les ciments diagenetiques de gres datant du Jurassique superieur--Cretace inferieur au large de la Nouvelle-Ecosse fournissent un cadre pour l'historique de la migration des fluides dans des reservoirs de gaz du bassin neo-ecossais. Des mineraux diagenetiques de six puits dans le champ Venture ont ete analyses par petrographie optique, microscopie electronique a balayage (SEM) et microsonde electronique. En tout, 122 inclusions primaires et secondaires de fluides provenant de differents ciments ont ete analysees. Les inclusions aqueuses primaires dans des excroissances de quartz ont des temperatures d' homogeneisation ([T.sub.h])de 111,8 ± 7,1 °C (1σ) et, dans des ciments carbonates plus tardifs, elles ont des [T.sub.h] de 126,5 ± 2,1 °C; les inclusions dans les deux ciments sont hautement salines (16 a 26,1 % poids en equivalent NaCl). Des trainees d'inclusions porteuses d'hydrocarbures et aqueuses secondaires recoupant le ciment de silice et le quartz detritique ont des [T.sub.h] de 121,6 ± 13,6 °C et de faibles salinites (8,7 ± 6,0 % poids). Les temperatures de fusion du C[O.sub.2](-56,6 ± 0,1 °C) et de [T.sub.h](-9,3 ±0,8 °C) des inclusions carboniques secondaires indiquent une phase carbonique a densite elevee. Les ciments carbonates tardifs dans les memes unites de gres ont des compositions chimiques qui varient selon les differents puits et des reservoirs relies montrent des assemblages carbonates tardifs similaires, suggerant que la cimentation carbonatee tardive puisse etre en partie controlee par la sequence de remplissage et de deversement du reservoir. La silice et la cimentation tardive par des carbonates impliquaient un ecoulement de fluides hautement salins, il y a probablement ~135 Ma. La migration des hydrocarbures a eu lieu apres la cimentation par de la silice et elle est associee a une fracturation secondaire, suggerant qu' elle corresponde avec le debut de la surpression. [Traduit par la Redaction], Introduction Fluid inclusions can record the migration of fluids, including hydrocarbons, through a basin during cement precipitation and microfracture annealing. These can be examined using microthermometry, ultraviolet microscopy, and other [...]

The major and trace element geochemistry of silicate melt inclusions was investigated within late Paleozoic felsic rhyolites from the Piskahegan and Harvey Formations of southern New Brunswick, Canada, in order to provide further insight into the genetic history of the volcanic- and caldera-related U mineralization that occurs in the region. Glassy melt inclusions analyzed by laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) and electron microprobe show enrichment in most incompatible trace elements, but a marked depletion in Ba, Sr, and Eu compared to whole rock. At Harvey, melt trapped in early quartz phenocrysts ("preeruptive" inclusions) and in late quartz aggregates ("syneruptive" inclusions) within the groundmass of the rhyolites was significantly more fractionated than melt trapped in quartz phenocrysts at Piskahegan. Fractionation was associated with the crystallization of feldspar and resulted in progressive enrichment of the melt in U, Th, B, LILE, LREE, and other metals, as well as an increase in the U/Th ratio of the melt. A higher degree of melt fractionation, combined with postmagmatic leaching, may have been prerequisites for mineralization at Harvey. Because felsic volcanic rocks are highly susceptible to alteration, melt inclusion analysis may be the only method capable of providing constraints on melt chemistry and evolution in such ancient volcanic terrains. This may enable the evaluation of the economic potential of such terrains if the initial U and Th concentration, as well as the U/Th ratio of the volcanic products, affect the ultimate mineralizing potential of the system. [ABSTRACT FROM AUTHOR]

Mafic pegmatites in the platinum-group element (PGE)-mineralized Roby zone, Lac des Iles Complex, northwest Ontario, Canada, comprise dikes, veins, and irregular pods of coarse-grained magnesiohornblende, pyroxene, and labradorite-andesine with minor biotite, apatite, Fe-Ti oxides and intercumulus quartz that grades into massive quartz or graphic quartz-base metal sulfide-vysotskite [Pd, Ni(S)] intergrowth at their cores. Quartz, apatite, and magnesiohornblende host primary and secondary assemblages of one- or two-phase carbonic fluid (CO2 ± up to ~10% CH4 ± minor H2O0, N2) inclusions that contain ore metals (Ni, Cu, Pd, Bi, Te, Fe). Rare trails of late-stage, high-salinity, aqueous fluid inclusions are secondary in origin and, therefore, unrelated to the crystallization of the pegmatites. Assemblages of primary carbonic fluid inclusions show considerable variation in mode and temperature of homogenization, reflecting large fluctuations in confining pressure at the time of quartz crystallization of as much as ~1 kbar (in single quartz crystals) and ~2.8 kbars (all data). Independent thermobarometric methods constrain conditions for the following two stages of pegmatite formation (and carbonic fluid entrapment): (1) the crystallization of magnesiohornblende-plagioclase inter-growth at T ~650° to 850°C, and P ~1 to 3 kbars; and (2) the crystallization of quartz at T ~535° to 650°C, and P ~0.4 to 3.2 kbars, setting the maximum depth of emplacement of the Lac des Iles Complex North Roby zone magma at 10 to 12 km. The results indicate that aqueous-dominant volatile phases were absent during the crystallization of pegmatitic gabbroic rocks at the Lac des Iles Complex and that water-poor, carbonic fluid entrapment persisted to well below solidus conditions. A role played by carbonic fluid as a potential transport medium for ligands involved in the precipitation and remobilization of the PGE and base metals is strongly suggested and warrants further investigation. [ABSTRACT FROM AUTHOR]

Alteration veins containing Cl-poor actinolite and actinolitic hornblende are a common feature throughout the Archean-aged gneisses and granitoids along the northern margin of the Sudbury Igneous Complex, Ontario, Canada. Crosscutting relations and structural measurements show that the veins postdate the Sudbury impact event but predate the emplacement of footwall style Cu-Ni-PGE (platinum-group element) sulfide ores. The veins are four to five times more abundant in brecciated, mineralized outcrop than in unbrecciated, barren outcrop. Analyses of vein-hosted amphibole by electron microprobe (EMP) and laser ablation ICP-MS (LA-ICP-MS) show elevated concentrations of Ni (up to ~100 times background; increasing from ~100 ppm to ~1.0 wt %), Cu (up to ~4 times background) and Sn (up to ~2.5 times background) in proximity to sulfide veins. Copper and Sn contents in amphibole are only anomalous within 10 to 20 m of mineralization and require quantification by LA-ICP-MS. However, anomalous Ni concentrations in vein-hosted amphibole may be detected with a high level of confidence solely by EMP up to ~700 m from mineralization in rocks that contain no visible sulfides in hand sample or thin section and show no bulk rock Ni or Cu anomalies. The relationship between the Ni content in amphibole (in ppm) and mineralization proximity (distance "D" in meters) is given by the following equation: [Ni]amp = 5100 (D) -0.45. The data indicate wide-scale Ni mobility in hydrothermal solutions in the Sudbury footwall. The formation of Ni-rich amphibole in the fracture networks, by either the growth of primary Ni-rich amphibole or metasomatic Ni enrichment of preexisting (Ni-poor) amphibole, was associated with circulation of a sodium- and halogen-rich fluid between ~390° and 550°C. Nickel may have been released into the footwall environment by extensive hydrothermal alteration of Ni-rich contact-style sulfide ores that lie in the hanging wall of the Cu-rich footwall-style ores. In the footwall rocks, pre existing fracture networks (now amphibole-bearing veins) acted as common structural pathways, first for the circulation of early Ni-rich fluids associated with leaching in the contact region and the onset of footwall mineralization, and second for the emplacement of Cu-rich footwall-style sulfide veins. Other metal concentrations in amphibole (Pb, Zn, Co) show no relationship to mineralization proximity. These metals may have been derived locally from country rocks during alteration reactions that promoted amphibole growth in the fractures. Identification of fracture-hosted amphibole with anomalous Ni contents in thin section, or Ni-Cu-Sn anomalous amphibole grains liberated from surface-weathered rocks hosting sulfide veins may serve as useful proximity indicators for hidden footwall-style sulfide deposits at Sudbury. [ABSTRACT FROM AUTHOR]

At the Dundonald Beach South Ni-Cu occurrence in the western Abitibi subprovince, Ontario, Canada, komatiite-hosted magmatic sulfide mineralization (pentlandite-dominated; 0.5-7 ppm Pt + Pd) contains millimeter- to centimeter-size blebs of polycrystalline gersdorffite and nickeline. The gersdorffite-nickeline blebs are concentrated (∼6 vol %) in a 10- to 20-cm-thick by 10-m-long lens that occurs at the base of a massive sulfide horizon. The lens is significantly more enriched in Pt and Pd (15-50 ppm Pt + Pd) than typical sulfide horizons in which sulfarsenide and arsenide phases occur only as trace to minor (up to 0.15 vol %) constituents. In the platinum group element (PGE)-rich lens, ∼29 and ∼74 percent of the bulk rock Pd and Pt, respectively, are dissolved in the sulfarsenide and arsenide minerals. Almost all the remaining Pd and Pt in the lens occur in inclusions of sperrylite, michenerite (PdBiTe), and an unidentified Pd-Bi-Te phase coexisting with inclusions of tellurobismuthite (Bi2Te3), altaite (PbTe), and electrum that fill former open spaces (vugs) in gersdorffite and nickeline. In the surrounding sulfide mineralization, PGE mineral inclusions are 4 to 10 times less abundant, and the bulk PGE grade is an order of magnitude lower. Overall, PGE grade on a deposit scale may be controlled by the abundance of gersdorffite and nickeline. Even in minor abundances (e.g., ∼ 1 wt %), sulfarsenide and arsenide minerals may produce economically favorable bulk PGE grades if dispersed throughout barren (PGE-poor) sulfide horizons. The PGE-rich lens may represent the accumulation of either crystallized droplets of an immiscible sulfarsenide melt phase or sulfarsenide phenocrysts that separated from a silicate or sulfide liquid. Arsenic-rich melt or mineral phases extracted (partitioned) the trace metals Pt, Pd, Au, Bi, Te, Sb, Pb, Zn, Ag, and Sn from non-As-bearing phases (silicate liquid, Fe-Ni-Cu sulfide liquid, or monosulfide solid solution [mss]) with inferred values of DAs-phase/sulfide on the order of ∼10 to 100. Textural, mineralogical, and geochemical observations highlight the importance of As-rich liquids or mineral phases as scavengers of the PGE and other chalcophile metals in komatiite-associated magmatic sulfide environments as well as the potential for high-grade PGE mineralization to occur where komatiites were contaminated by As-rich sediments or metasedimentary rocks. [ABSTRACT FROM AUTHOR]

Granitic rocks within the giant intrusion-related Au-Bi-Te deposit at Fort Knox, Alaska, contain a well-preserved inclusion record of magmatic-hydrothermal transition and processes that were potentially critical for the development of the sheeted ore system. Early magmatic titanite grains contain (in their cores) inclusions of an iron sulfide melt that were trapped coevally with silicate melt inclusions at a minimum temperature of 780 °C (based on O2 geobarometry). Textural and mineralogical evidence indicates that this sulfide melt phase was stable until a relatively late stage in the crystallization history of the granites, but was destabilized due to a progressive increase in oxygen fugacity. Laser ablation ICP-MS analyses of the sulfide melt inclusions show that they contain between 200 and 1800 ppm Cu, and concentrations of Ag, As, Bi, Sb, Te, W, Mo and Ni in the 10-100 ppm range. Notably, Au was detected in the sulfide melts at concentrations in the 1-3 ppm range. Analyses of the coeval silicate melt inclusions allow the calculation of sulfide-silicate melt partition coefficients. Values of D(sulfide/ silicate) are between 10 and 100 indicating that the sulfide droplets would have contained the majority of the ore metals present in the system while they where stable. Remarkably, selected metal ratios in the sulfide melt droplets (e.g., Bi/Au) are identical to those in the sheeted ore veins at Fort Knox, suggesting that the sulfide melts. This would suggest that ore-forming processes that occurred after the destabilization of the sulfide droplets (i.e., silicate melt-fluid partitioning, and metal coprecipitation) did not fractionate the ore metals from one another. Textural evidence shows that during titanite growth, apatite saturation occurred. Apatite grains trapped coexisting primary inclusions containing silicate melt and a low salinity (~ 4-6 wt% eq. NaCl based on clathrate melting temperatures), aqueous-carbonic fluid (CO2 ~ 21-39 vol%). These coeval melt and fluid inclusions in apatite provide unambiguous evidence for the saturation of the granitic magma in fluid phase. The fluid inclusions decrepitate at ~ 400 °C; however, apatite-biotite halogen exchange thermometry indicates that the inclusions were trapped at a minimum T of 560-680 °C. The results show that ore-forming granitic magmas were saturated in a Au-Bi-Te-rich sulfide melt phase. Resorption of the sulfide melt appears to have coincided with saturation of the crystallizing magma in a low salinity aqueous-carbonic fluid. Sulfide melt that persists to a relatively late stage of magma crystallization may act as an important storage phase for ore metals, preventing the loss of ore metals to (i) fluids that exsolve early on when the magma is insufficiently brittle to auto-generate mineralized sheeted veins, and (ii) crystallizing minerals in which metals such as Au are compatible (e.g., magnetite, biotite, titanite). Systematic identification and analysis of sulfide melt inclusions in granitic ore-forming settings significantly "inboard" of convergent plate margins may find application in locating highly mineralized granites and predicting the metal associations and metal ratios to be expected in the ores. [ABSTRACT FROM AUTHOR]