Fred J. Longstaffe

Alteration and metamorphism of Amitsoq gneisses from the Isukasia area, West Greenland: Recommendations for isotope studies of the early crust

Early Archaean gneisses (3400–3700 Ma) in the Isukasia area of West Greenland have been subjected to a series of local and regional metamorphic processes. Metasomatic alteration accompanied the intrusion of the early Archaean white gneisses (~3600 Ma) into the grey gneisses (~3700 Ma), and resulted in local formation of altered rocks. Pb-Pb isotope results on whole rock and feldspar from the gneisses reveal a major rearrangement of Pb isotopes at about 2600 Ma together with some local third stage Pb changes at a later time. This 2600 Ma event is also shown by Rb-Sr data for phengites in the ~3400 Ma Pegmatitic gneiss sheets and can be correlated with metamorphism accompanying the intrusion of a late Archaean pegmatite (~2600 Ma) swarm. Local hydrothermal alteration by fluids emanating from Proterozoic faults and fractures is petrographically recognised in and near major faults and lithological boundaries. This fault-controlled alteration, though only locally significant, may correlate with low grade thermal metamorphism reflected in a 1600 Ma Rb-Sr mineral isochron on biotites from both altered and unaltered gneisses. Oxygen isotope results show that this alteration probably was associated with influx of a low-δ18O fluid, perhaps meteoric water. Selection of the least altered Amitsoq gneisses can be made on the basis of petrographic and field criteria, and furnishes the best material to study original geochemical and isotopic systematics in these early Archaean rocks.

Oxygen Isotope Studies of Diagenesis and Pore-Water Evolution in the Western Canada Sedimentary Basin: Evidence From the Upper Cretaceous Basal Belly River Sandstone, Alberta

ABSTRACT The oxygen isotope compositions and paragenetic sequence of diagenetic minerals from the Upper Cretaceous basal Belly River sandstone, southwestern Alberta, have been used to identify changes in pore-water composition during diagenesis. These changes can be related to major geologic events within the western Canada sedimentary basin. Low-18O meteoric water has played an important role in the diagenesis of these Upper Cretaceous rocks on a basinwide scale during deposition and early diagenesis and following uplift of the basin in early Eocene time. Diagenetic phases include early chlorite (18O = +6.1 to +8.3) and calcite (18OSMOW = +21.0; 13CPDB = -1.6), followed by quartz overgrowths (18O = + 13. 1 to + 18.0), calcite (18OSMOW = + 10.4 to + 14.4; 13CPDB = - 10.6 to - 1.2), and then kaolinite (18O = +7.8 to + 11.9), illite/smectite (18O = +10.9 to +13.1) and smectite (18O = +12.6 to +13.7). The interpretation of these results is that shallow diagenesis, early in the burial history (chlorite, calcite) occurred in the presence of brackish or fresh water. With burial, the 18O value of the pore water increased due to water/rock interaction. Burial diagenesis was terminated in the early Eocene by uplift (Laramide Orogeny). Recharge of the basin by low-18O meteoric water occurred at this time. The meteoric water then became involved in the formation of diagenetic quartz and calcite at or near maximum burial temperatures, and in the crystallization at lower temperatures of kaolinite, illite/smectite, and smectite as post-Eocene cooling and erosion progressed.

Oxygen Isotope Studies of Diagenesis in the Basal Belly River Sandstone, Pembina I-Pool, Alberta

ABSTRACT The oxygen-isotope compositions of authigenic clay and carbonate minerals from the Pembina I-Pool (basal Belly River sandstone) provide useful constraints on the diagenesis of this unit. Early diagenetic chlorite has 18O values (+ 4.2 to + 6.3, SMOW) compatible with low-temperature crystallization from brackish water. The 18O values of diagenetic calcite (+ 11.8 to + 14.7, SMOW) and kaolinite (+ 10.4 to + 12.8, SMOW) are best explained by crystallization at temperatures of at least 45° to 70°C from pore fluids similar in 18O to the existing formation water (-9, SMOW). The implicatio of such temperatures is that the sandstone was much more deeply buried in the past than it is at present. At least 1,300-1,400 m of overlying material may have been eroded since maximum burial.

Separation and characterization of clay from Athabasca asphaltene

Abstract Athabasca bitumen separated from the associated mineral matter by Soxhlet extraction contains fine clay particles and inherent ash. Empirical relations have been developed to estimate the percentage of clay and inherent ash present in the asphaltene fraction which concentrates in large measure the mineral constituents present in the bitumen. The ash level, Y , of the asphaltenes is related to the weight per cent of clay, C t , by an expression of the form Y = 0.872 C t + 0.582. The ash level of the asphaltene fraction is also correlated with the infrared absorbance. A , at 1032 cm −1 which gives an approximate empirical relation of the form A = 0.0648 Y + 0.294. Greater accuracy at low ash levels can be achieved by measuring A at 1040 cm −1 above the base line drawn from 960 to 1140 cm −1 . This results in the equation A = 0.0709 Y + 0.0124 when a standard KBr pellet thickness of 0.833 mm and concentration of 2 mg asphaltene per 300 mg KBr is used. X-ray diffraction used to characterize the clay minerals shows decreasing crystallinity as the particle size diminishes. The infrared absorbance of this mineral matter indicates decreasing intensity of the band at 2930 cm −1 , associated with adsorbed and occluded organic matter, relative to the two characteristic clay bands at 3697 and 3620 cm −1 as the particle size decreases. Trace element analysis of the asphaltene inherent ash, by inductively coupled argon plasma, shows the major metallic constituents to be vanadium, nickel and iron with minor amounts of calcium, potassium, aluminium and sodium.

The Archean Lac du Bonnet batholith, Manitoba: Igneous history, metamorphic effects, and fluid overprinting

The 2.6 Ga Lac du Bonnet batholith of ~ 1000 km2 surface exposure is emplaced along a longlived regional fault. The batholith was generated by successive intrusions that have undergone different degrees of modification by internal processes or by tectonic effects: (i) minor early porphyritic hornblendebiotite granodiorite; (ii) extensive silicic leucogranite (with interior barren and exterior Be, Nb > Ta, Y, REE, F, Zr, Th, U, Ti-bearing pegmatites); (iii) dominant biotite granite with xenoliths of (basement?) tonalite, containing extensive K-feldspar porphyroblastesis; (iv) minor undeformed but porphyroblastic late biotite granodiorite. The chemistry of the two granodiorites is closely related to that of the biotite granite but the geochemical features of the leucogranite are markedly different. Isotopic constraints (δ18Oof +8.1, RSriof 0.7003) and K2O content suggest LIL-depleted tonalite with <25% greenstone-belt metasediments as the probable source of biotite granite. Very modest negative Eu anomalies and low HREE of the granite are compatible with a tonalitic protolith. Melting of short-lived felsic volcanics would also satisfy all constraints but encounters a potential mass balance problem. Extensive K-feldspar porphyroblastesis had no significant effect on bulk compositions, alkali-feldspar-bound elements or Rb-Sr isotope systematics but it disturbed magmatic oxygen to +6.7–+11.0 and remarkably depleted P, Zr, Hf, Th and all REE except Eu. The highly evolved leucogranite could have been affected by liquid fractionation, was depleted in volatile components and rare lithophile elements extracted into its pegmatite aureole, and its disturbed Rb-Sr isotope systematics could have resulted from pervasive shearing and recrystallization. The overall geochemical features of the leucogranite (δ180 + 7.9, RSri ~ 0.7000, high FeMg, LREE > HREE with very prominent negative Eu anomaly, and Nb > Ta, Y, F signature of derived pegmatites) rank it with A-type granites.

Oxygen isotopic compositions of Central Andean plutonic and volcanic rocks, latitudes 26°–29° south

Abstract Oxygen isotope data are reported for 27 igneous rocks of Mesozoic to Quaternary age from the Central Andes. 26–29°S. The plutonic rocks, and most of the volcanics, have δ 18 O values between 6.2 and 8.3‰. The whole-rock δ 18 O values show a weak correlation with initial 87 Sr/ 86 Sr data. This O-Sr array differs from documented trends for calc-alkaline plutonic suites from California, Scotland and northern Italy, but overlaps with data for volcanic and plutonic rocks from Ecuador, northern Chile and southern Peru. The oxygen isotope results indicate that the magmas evolved without significant contamination from supracrustal rocks (e.g., rocks that experienced 18 O enrichment during surficial weathering). The available O, Sr and Pb isotopic data for these rocks are best explained by magma generation in the upper mantle or lower crust. From the Late Mesozoic on, the 87 Sr/ 86 Sr values were modified at depth by isotopic exchange between the magma and a continually thickening crust of plutonic rocks of Late Precambrian to early Mesozoic age.

Rare-earth element modelling of Archean meta-igneous and igneous rocks, Lake Despair area, northwestern Ontario

Abstract Archean felsic to intermediate meta-igneous rocks from the Lake Despair area, northwestern Ontario, have highly-fractionated REE patterns (La N /Lu N 10–100). They are rich in LREEs (ca. 40–100 × chondrites) and poor in HREEs (ca. 1–7 × chondrites). Simple models for the REEs suggest eclogite—quartz eclogite parents for these rocks. The Footprint Gneiss, the major rock type of the Rainy Lake Batholith, was formed by limited melting (ca. 10%) of a quartz eclogite under hydrous conditions. The putative parent may have been transformed basalt derived from primitive, LREE-rich Archean mantle. The mafic metavolcanic rocks have a REE chemistry similar to modern island-arc or mid-ocean ridge tholeiites. Felsic metavolcanic rocks, and granodiorite from the Northwest Bay complex, have REE abundances compatible with an origin by partial melting (ca. 10%), under hydrous conditions, of quartz eclogite of tholeiitic REE chemistry. The Jackfish Lake plutonic complex, which is composed mostly of diorite, has REE abundances that are best described by 10% melting of eclogite (with tholeiitic-REE chemistry) under hydrous conditions. A small portion of the diorite magma was subsequently fractionate, largely by the early crystallization of amphibole, and formed more leucocratic rock types (e.g. leucodiorite and granodiorite).

Controls on Hydrocarbon Accumulation in Glauconitic Sandstone, Suffield Heavy Oil Sands, Southern Alberta

Hydrocarbon distribution in the Lower Cretaceous Glauconitic sandstone in the Suffield area of southeastern Alberta is controlled by three factors: sedimentology, structure, and mineralogy. The Glauconitic sandstone consists of six lithological facies interpreted to represent the lower-middle shoreface, middle shoreface, upper shoreface-foreshore, backshore, marsh, and lagoonal zones of a progradational, barrier-island system. Sediment deposited in the foreshore zone (laminated sandstone facies) has the best reservoir qualities: good porosity, low clay content, and good lateral continuity. The bioturbated, argillaceous sandstone, deposited in the backshore zone, has poor reservoir qualities: low porosity and high clay content with only isolated porous zones. Tidal inlet a d/or later stage fluvial channel deposits cutting through the sandstone trend form discontinuities in the reservoir. The hydrocarbon trapping mechanism is stratigraphic but with some structural influence. Deep faults, active during the deposition of upper Mannville sediments, caused differential subsidence and local thickening of sediment. This activity resulted in the apparent lateral juxtaposition of different facies. Parts of the Glauconitic sandstone form an exceptionally thick beach-shoreface sequence (up to 45 m or 148 ft thick). Faulting of sub-Cretaceous units may have controlled the rate of subsidence and the amount of sediment accumulation during deposition of the Glauconitic sandstone. The abundance of clay, mostly kaolinite, largely controls reservoir quality. Argillaceous backshore sandstones, which contain abundant detrital kaolinite, are poor reservoirs; clean foreshore deposits are good reservoirs. Porosity and permeability are only slightly reduced in the clean sandstone by formation of diagenetic phases such as kaolinite and quartz. During the wet forward-combustion recovery process, migration of kaolinite and dissolution-reprecipitation of silica could cause formation damage.

Oxygen-isotope geochemistry of metamorphosed, massive sulfide deposits of the Flin Flon — Snow Lake belt, Manitoba

Oxygen-isotope compositions have been measured for whole-rock and mineral samples of host and hydrothermally altered rocks from three massive sulfide deposits, Centennial (CL), Spruce Point (SP), and Anderson Lake (AL), in the Flin Flon — Snow Lake belt, Manitoba. Wholerock δ18O values of felsic metavolcanic, host rocks (+8.5 to +16.1‰) are higher than those of altered rocks from the three deposits. The δ18O values of altered rocks are lower in the chlorite zone and muscovite zone-I (CL=+ 5.3‰; SP=+5.4 to +8.3‰; AL= +3.7 to +5.9‰) than in the gradational zone (CL= +9.9 to +11.7‰; SP= +8.4 to +9.8‰; AL= + 6.6 to +7.7‰). Muscovite schist (Muscovite Zone-II) enveloping the Anderson Lake ore body has δ18O values of +7.2 to +8.3‰. Quartz, biotite, muscovite, and chlorite separated from the altered rocks have lower δ18O values compared to the same minerals separated from the host rocks. However, isotopic fractionation between mineral-pairs is generally similar in both host and altered rocks.It is interpreted that differences in the oxygen-isotope compositions of the altered and host rocks were produced prior to metamorphism, during hydrothermal alteration related to ore-deposition. Isotopic homogenization during metamorphism occurred on a grain-to-grain scale, over no more than a few meters. The whole-rock δ18O values did not change significantly during metamorphism. The generally lower δ18O values of altered rocks, the Cu-rich nature of the ore and the occurrence of the muscovite zone-II at Anderson Lake are consistent with the presence of higher temperature hydrothermal fluids at Anderson Lake than at the Centennial and Spruce Point deposits.

Oxygen-isotope geochemistry of Archean granitoid gneisses and related rocks in the English River Subprovince, northwestern Ontario

Whole-rock δ 18O values for Archean granitoid gneisses from the southern domain of the English River Subprovince, northwestern Ontario, increase from an average of +6.5 for 3000 Ma old tonalites in the eastern Lac Seul area to an average of +7.9 for 2750–2800 Ma old tonalites, trondhjemites and granodiorites that occur in the Kenora and eastern Lac Seul areas. Still younger granitoid plutons and dykes (2660-2560 Ma) that intrude the older gneisses also have average δ 18O = +7.9. These values lie towards the low end of the range known for normal granitoid rocks. Oxygen-isotope data for minerals from the granitoid rocks indicate minor isotopic disequilibrium, but no evidence exists for large-scale depletion in 18O subsequent to granitoid formation. Rocks rich in 18O are abundant only in the northern domain of the English River Subprovince, which is dominated by paragneiss and metasedimentary migmatite (δ 18O +9−12). Within the Kenora area of the southern domain, however, a remnant supracrustal sequence characterized by a thin metachert/iron formation unit (δ 18O = +8−10) has been recognized. The low whole-rock oxygen-isotope values indicate that pre-existing 18O-rich supracrustal rocks were not involved in the genesis of the 3000 Ma old gneisses and have made only limited, if any, contribution to the magmas that formed the younger units. This conclusion is supported by the low initial Sr-isotope ratios of these rocks (0.7009-0.7027). The oxygen- and strontium-isotope data require that most of the granitoid rocks in the southern domain of the English River Subprovince comprise juvenile or near-juvenile additions to the continental crust.