John F. Dehls MSc thesis abstract

Thesis Title: 
The Magnetic Fabrics and Strain History of the Archean Seine Group Metasedimentary Rocks Near Mine Centre, Northwestern Ontario
John F.

The Seine Group metasedimentary rocks are situated between two converging major dextral transcurrrent faults, the Quetico Fault to the north and the Seine River Fault to the south.  To the west lies the Bad Vermilion Intrusive Complex (B.V.I.C).  The Seine Group shows one major period of deformation resulting in major F1 folds.  Regional metamorphism of chlorite to biotite zone greenschist facies was synkinematic with the deformation.  Later minor deformation include localized crenulation cleavage and faults and shear zones.

Strain analysis of deformed conglomerates indicates 50-67% shortening in a north-south direction.  The B.V.I.C. has behaved rigidly during the deformation and produced a strain shadow in which only 18-42% shortening has occurred, deflected to a WNW-ESE direction.

The schistosity has an average trend of 252.3/85.7°NW, and the average trend of the stretching lineations is 063.4/43.4°.  However in the strain shadow of the B.V.I.C. the strikes are deflected to the NNE.

The AMS fabric in the Seine Group is controlled by paramagnetic chlorite and biotite.  The AMS foliation has an average orientation of 251.6/81.4°NW.  The AMS lineation has a peak trend of 059.7/41.7°.  The AMS ellipsoids are mostly oblate, with the least anisotropic fabric located in the strain shadow.  The AMS fabrics are deflected in the strain shadow of the B.V.I.C. in the same sense as the schistosity.

The anisotropy of isothermal remanent magnetization (ASIRM) of each sample was determined, using a saturating field of 500 mT, and then repeated at 800 mT.  The ASIRM fabric in the Seine Group is controlled by the ferromagnetic minerals magnetite and pyrrhotite.  The ASIRM lineation has a peak trend of 060.3/33.3°.  These fabrics are deflected in the same sense as the schistosity in the strain shadow of the B.V.I.C.

Although the three steep planar fabrics measured are similar in orientation, they are offset in plan view.  The angle between the schistosity and the AMS foliation is 4.4°.  The angle between the AMS and ASIRM foliation is 4.2°.  The schistosity, defined by deformed quartz and feldspar, is the oldest fabric.  The AMS fabric is younger, as the chlorite and biotite formed during metamorphism.  The magnetite and pyrrhotite which define the ASIRM formed later still, and thus represent the youngest fabric.

The successive offsets between the three fabrics is consistent with a dextral transpressional strain history operating throughout the fabric development.  The strain history had components of simple shear and non-coaxial shortening.  The initial shortening probably took place in a SE-NW direction.

A copy of the thesis can be downloaded here

Tomasz Werner MSc thesis abstract

Thesis Title: 
Paleomagnetism, structure and magnetic fabrics in a traverse of the Quetico Subprovince between Atikokan and Kashabowie, NW Ontario

The metasedimentary rocks along the traverse reveal low to high grade metamorphism from chlorite schist near Atikokan, through biotite schist to migmatites in the west within the centre of the Quetico Belt.  Continuing toward the ESE, near Huronian Lake, the metamorphic grade decreases symmetrically but with somewhat less steep gradients to Kahabowie.  The metamorphism was syn- to late-tectonic.

Only one pervasive tectonic fabric was observed in the interior of the belt, with NE-SW striking S1 foliations (mean direction:  256/85°NW, n=121) and extension lineations L1 plunging shallowly to the NE (mean direction:  70-20°, n=52).  In migmatite and pegmatite zones foliation was often less steep.  It was probably deflected due to the intrusion of pegmatite or granitoid bodies.

Minerals contributing to ferromagnetic properties are mostly monoclinic ferromagnetic pyrrhotite within the belt with some magnetite in medium grade amphibolite - rich outcrops in metavolcanics of Shebandowan Belt.

Anistropy of magnetic susceptibility fabric (AMS) is mainly controlled by paramagnetic biotite or chlorite and subordinate ferromagnetic pyrrhotite in metasediments of the Quetico Belt.  In some outcrops of mafic metavolcanics near Kashabowie, the high magnetite content controls bulk susceptibility more than its anisotropy.  Variations in AMS fabric are largely due to variations in relative composition of magnetite, pyrrhotite and paramagnetic sheet silicates, not due to changes in strain along the traverse.

AMS foliations and lineations within the belt and on its south-east margin are generally coaxial with tectonic fabric, with AMS foliation less steep (dip of 70°) in the interior of the belt.  However a slight (2-5°) anticlockwise offset of the mean AMS foliation from S1 can be postulated in the centre of the belt and at the southern margin.  When fabric data is studied separately for each sample, such offsets are not significant.  In the Atikokan area, AMS fabric is partly of sedimentary origin.  The offset of mean AMS lineations with respect to L1 was confirmed for the Kashabowie - Huronian Lake area (also reported by Borradaile and Spark, 1990) but it is not prominent in the centre of the belt.

Anisotropy of remanence based on acquisition of anhysteretic remanence (AARM) is controlled by preferred crystallographic orientation of pyrrhotite in metasediments and partly by preferred dimensional orientation of magnetite in metavolcanics.  It is substantially higher than AMS fabric (mean P' of order 3 to 5).  AARM foliation is usually oriented closer to the tectonic fabric than AMS foliation; it is vertical at the margins and less steep in the centre of the Belt.  AARM lineations usually plunge more gently than tectonic and AMS lineations in the centre of the Belt and at the southern margin in the Kashabowie - Huronian Lake area.  At the northern margin, AARM fabric is highly oblate, so that fabric lineations are difficult to define. 

At the belt margins, a slight anticlockwise offset of AARM foliations with respect to the tectonic fabric exists, when fabrics for individual samples are examined.  However, the relative orientation of tectonic and magnetic fabric (AMS and AARM) are not a region - wide consistent kinematic indicator of dextral transpression.

Correlation studies between magnetic susceptibility and ARM intensity indicate that a monotonic correlation between ARM and MS exists only in the Calm Lake - Perch Lake area, in which pyrrhotite controls ARM.  In other areas a higher content of magnetite increases magnetic susceptibility in several samples, but not ARM.

Both magnetic fabric (AMS and ARM) are usually oblate in low grade metasedimentary rock (at the Belt margins), but in metavolcanic rocks magnetite can produce prolate ARM, but in metavolcanic rocks magnetite can produce prolate ARM, whereas ARS is oblate.  In the interior of the Belt, 50% of samples have prolate AARM, but most of them are pyrrhotite-bearing.

The AARM fabric is believed to have arisen by the growth of ferromagnetic grains (at least in case of pyrrhotite) in the later stages of transpressive penetrative deformation with a dominant dextral shear component of deformation along an ENE - SWS, vertical plane.  The preferred orientation of pyrrhotite grains was probably controlled by the older biotite - chlorite matrix fabric.  This would explain the lack of a more significant deflection of the youngest ARM fabric from the oldest tectonic fabric.  However, the accuracy of determinations of ARM axes (at least 5-10°) limits the validity of these conclusions.

Characteristics directions of magnetic remanence obtained during thermal or AF step-wise demagnetization form a weakly developed girdle along the basal plane of ARM anisotropy.  Therefore natural remanent magnetization is probably entirely of chemical origin and controlled mostly by high anisotropy of pyrrhotite and its preferred crystallographic orientation.  The orientation of NRM toward basal plane of ARM is believed to be mostly an effect of deflection from the true Earth's magnetic field direction due to anisotropy of pyrrhotite (P' of values of 3-5, Fuller, 1963), and not the effect of grain rotation.  Stress control of post - metamorphic remagnetization of  primary ChRM is also a possibility.  Therefore, it is suggested that NRM was acquired in the same late stage of deformation as AARM, when sizes of individual ferromagnetic grains were suitable to carry a stable remanence.  Any primary magnetization acquired at the time of rock formation was not preserved.

The model of different ages of development of paramagnetic and ferromagnetic fabric is consistent with the dextral transpression model of the deformation for both margins and the fabric of the Quetico Belt.  The latest ferromagnetic sub-fabric formed along the shear plane when the paramagnetic matrix was already oriented subparallel to this plane and mimetically controlled the orientation of crystallization of ferromagnetic phase.  The ferromagnetic fabric is not therefore useful as a kinematic indicator of shear component in those areas.

A copy of the thesis can be downloaded here

Barbara S. Kowalski MSc thesis abstract

Thesis Title: 
Petrographic and Fluid Inclusion Studies on the Metalore - Golden Highway Deposit, Thunder Bay District, Ontario
Barbara S.

The gold-bearing Metalore shear zone and Golden Highway quartz-carbonate vein in the Beardmore-Geraldton Archean Greenstone Belt occur along the Paint Lake splay faults at the contact between metavolcanic rocks and metaconglomerates intruded by pre-ore diorite.  The Metalore and Golden Highway deposits were emplaced during a late tectonic event.  They consist primarily of quartz, clinochlore, ankerite, potassium feldspars, sericite, pyrite, argentite and chalcopyrite with native gold.  The minerals have been deformed and are cut by at least three healed fractures by fluid inclusions.  Gold typically occurs in recrystallized quartz.  Microthermometric and Raman spectroscopy techniques were used to study fluid inclusions in quartz, calcite, ankerite, chlorites and potassium feldspars.

Six types of fluid inclusions were found to occur in three separate generations of hydrothermal fluid activity in the Metalore and Golden Highway.  The three generations of hydrothermal fluids represented by fluid inclusions are:  (1) pre-ore, low-salinity (<2wt.%) NaCl-CaCl2 aqueous inclusions with small amounts of daughter mineral and H2O-CO2 inclusions with 10 and 40 mole percent CO2 occurring in the Golden Highway and Metalore respectively; (2) syn-ore, MgCl2 - H2O-CO2 aqueous and vapour rich inclusions with small amounts of daughter minerals with 50 and 80 mole percent in the Golden Highway and Metalore, respectively;  (3) post-ore, CO2-H2O liquid-vapour and CO2 -rich liquid-vapour inclusion with variable CO2 contents ranging from 10 to 50 mole percent CO2.  Homogenization temperatures of pre-ore H2O-CO2 inclusions during are 220°-230°C and 356°C; syn-ore 266°C; and post-ore 21°C to 66°C.

The aqueous and CO2-rich inclusions are interpreted to have been trapped as two immiscible phases in three separate generations.  Precipitation of gold may have been induced by pressure, temperature fluctuations and chemical changes from CO2 effervescence.  Metamorphic fluids are the likely source for the onset of precious-metal deposition from reduced sulphur complexes along the Metalore splay fault and precious- and base-metal deposition from both reduced sulphur and chloride complexes along the Golden Highway splay fault.

A copy of the thesis can be downloaded here

Jian Xiong MSc thesis abstract

Thesis Title: 
Cathodoluminescence Studies of Feldspars and Apatites from the Coldwell Alkaline Complex

The Coldwell alkaline Complex is located on the north shore of Lake Superior, and consists of three Centers representing three magmatic episodes.  Center I consists of gabbro, and layered and unlayered ferroaugite syenite.  Center II consists of alkalic biotite gabbro, miaskitic nepheline syenite, amphibole nepheline syenite, perthitic nepheline syenites and recrystallized nepheline syenites.  Center III consists of magnesio-hornblende syenite, ferro-edenite syenite, contaminated ferroedenite syenite, and quartz syenite.

The textures and compositions of the feldspars and apatite from these three Centers were examined using cathodoluminescence (CL) and the scanning electron microscope (SEM).  All of the colours described in the text refer to cathodoluminescence colours.  The feldspars in Center I consist of light blue to light violet blue, optically homogeneous alkali feldspar; braid microperthite and incipient perthite; and irregular vein or patch perthite.  The irregular vein patch perthite contain light violet blue cryptoperthitic patches, light blue exsolved albite and dull blue exsolved K-feldspar.  During late-stage fluid-induced alteration, these feldspars were replaced by violet secondary albite and purple secondary K-feldspar; and were coarsened by a deuteric antiperthitic rim.  The Fe-rich antiperthitic rim consists of deep red secondary albite and brown secondary K-feldspar.  The later dominates in the upper series of the layered and unlayered syenites.  The homogeneous alkali feldspar crystallized at high temperatures ≤700°C, exsolved into microperthite at 650-550°C and formed irregular vein and patch perthite at 520-420°C.  Late-stage fluid-induced replacement and deuteric coarsening occurs at relatively low temperatures 500-300°C.

The feldspars in Center II consist of light blue or light bluish grey oligoclase; light violet blue homogeneous alkali feldspar; and irregular vein or patch perthite.  A few oligoclase crystals are mantled by a light blue alkali feldspar rim.  Most of the irregular vein and patch perthite contain light blue exsolved albite and dull blue K-feldspar host; and some of them contain light violet blue to light violet unexsolved alkali feldspar patches in the core, and violet blue to dull blue cross hatched microcline along the margin.  During late-stage fluid-induced deuteric alteration, the alkali feldspar was replaced by light violet blue to light violet secondary albite or dull blue to dark brown secondary K-feldspar along the margins; and the alkali feldspar was coarsened by a red to deep red secondary albite rim.  The oligoclase crystallized at high temperatures 800-750°C, and the homogeneous alkali feldspar crystallized at ³710-620°C.  The perthite texture and the unexsolved alkali feldspar patches may have formed at 600-450°C.  The K-feldspar host may have transformed to the cross-hatched microcline at ≈300°C.  The deuteric coarsened and replaced secondary feldspar formed at low temperatures <350°C.

In the recrystallized nepheline syenites of Center II, the feldspar crystals are usually mantled with a relict core and a light violet blue recrystallized alkali feldspar rim.  The relict cores usually are a light blue albite or light violet homogeneous alkali feldspar.

The feldspars in Center III consist of zoned plagioclase crystals with light greenish blue andesine-oligclase cores and light violet blue to light blue oligoclase-albite rims; light blue to blue homogeneous alkali feldspar, K-rich feldspar, braid microperthite and oscillatory zoned alkali feldspar; irregular vein perthite consisting of light blue to light violet exsolved albite and dull blue K-feldspar host; and regular to irregular vein antipperthite consisting of dull blue to dark blue exsolved K-feldspar and dull red to deep red albite host.  Most irregular vein antiperthite grains reveal alternating dull red, violet and deep red oscillatory zones.  During late-stage fluid-induced alteration, most light blue to blue original alkali feldspar were coarsened by a deuteric antiperthite rim which consists of red secondary albite and dark brown secondary K-feldspar.  The plagioclase crystallized at high temperatures about 900-750°C, the homogeneous alkali feldspar crystallized at ³725-520, and the K-rich feldspar crystallized at 580-450°C.  The microperthitic texture may have formed at 650-420°C, and the perthite and antiperthite textures may have formed at non-equilibrium temperatures about 520-350°C.  The deuteric coarsened antiperthitic rim formed at temperatures <350°C.

The apatite crystals in the Coldwell complex exhibit a variety of CL textures which include: (1) uniform light pink to pink grains;  (2) growth zones with a small brownish pink core and light pink to pink rims;  (3) light pink to pink alternating oscillatory zones;  (4) mantled grains with light pink to pink cores and brownish pink reaction rims.  Some of the reaction rims are overgrown by light pink and yellow secondary apatite.

The light pink and pink CL colours in the apatite crystals are dominantly caused by Eu2+ activation, the brownish pink CL colour is dominantly caused by Dy3+, Sm3+ and Pr3+, and the yellow luminescent secondary apatite is characteristic of Dy3+ and Tb3+ activation.  The brownish pink apatites have higher contents of total REE and Si than the light pink apatites.  The yellow or light pink secondary apatites have relatively low contents of total REE and Si.  The apatite crystals in the Coldwell complex are characterized by LREE-enrichment which is caused by the coupled substitution of 2Ca2++P5+X(REE3++REE2+)+Si4+.  The Si content in the apatite crystals increase from Center I through Center II to Center III.

A copy of the thesis can be downloaded here

Robert Purdon MSc thesis abstract

Thesis Title: 
Lithostratigraphy and Provenance of the Neoarchean McKellar Habour Sequence, Superior Province, Ontario, Canada

The McKellar Habour Sequence is located on the shore of Lake Superior approximately 40 km west of the town of Marathon, Ontario.  The Sequence was examine in detail through stratigraphic and geochemical investigations in order to determine the depositional environment and provenance of these rocks.

Four subsequences were identified in the McKellar Harbour Sequence, consistent with the facies associated with a distal submarine ramp environment.  The Sequence shows thickening and coarsening upward trends, indicative of progradation of the ramp onto the basin floor.  This indicates that the rocks of the McKellar Harbour Sequence are the distal equivalents of the proximal submarine ramp facies identified in the Beardmore-Geraldton and Quetico terranes to the north of the study area.  Sedimentary strata from other potential source regions did not exhibit the characteristic features of a submarine ramp, and were likely deposited through different processes.  Insufficient data were collected to determine the depositional environment for these units.

Geochemical analyses indicate that the rocks of the McKellar Harbour Sequence have immobile element chemistry that is very similar to that of the Beardmore-Geraldton and Quetico terranes, and that a continuum of deposition from the north to south is present.  Sediments generated in the Beardmore-Geraldton terrane were transported to small basins associated with the Schreiber-Hemlo volcanic island system by a submarine ramp. 

A copy of the thesis can be downloaded here

Ken R. Kukkee MSc thesis abstract

Thesis Title: 
Rock magnetic and structural investigation of the Moss Lake stock and local area: western Shebandowan belt

The thesis area is located 120 km west of the City of Thunder Bay, Ontario, straddling the contact of the Quetico and Wawa subprovinces of the Archean Superior Province.

Metasedimentary rocks of the Quetico Subprovince with basic and granitic intrusions occupy the northwestern portion of the study area and are in contact to the southeast with metamorphosed mafic and felsic metavolcanic rocks of the Wawa Subprovince.

Tectonic compression has shortened the metasedimentary rock layers a minimum of 80%.  Bedding dips steeply to the northwest and a later cleavage, developed by transpression, is sub-parallel to bedding.  Graded beds young predominantly to the northwest.  Some graded beds young to the southeast and may represent pre-cleavage folding.  No large-scale folds are present.  Metamorphic grade increases northwest from greenschist to amphibolite facies over a distance of about 10 km.

Magnetite is the predominant magnetic component of the Moss Lake stock; hematite is present in trace amounts.  Magnetic susceptibility of the stock is high (12,000 X10-6 SI), making the intrusion amenable to anisotropy of magnetic susceptibility (AMS) work.

AMS study of the Moss Lake Stock shows that individual directions of maximum susceptibility have been reoriented, in most cases sub-parallel to the local planar fabric strike of the Quetico Subprovince.  Magnetic fabric parameters show that the rock magnetic fabric of the intrusion is deformed.  Vestiges of original magmatic fabric are evidenced by prolate (constricted) magnetic fabric associated with the central long axis of the stock by magnetic fabric parameters confirm that the intrusion margin is more deformed than the interior.  The predominant oblate (flattened) magnetic fabric of the Moss Lake stock is the product of northwest-southeast tectonic compression.

Alternating field and thermal demagnetization of oriented rock specimens confirm that the Moss Lake stock is deformed by tectonic compression.  Separation of magnetite and hematite magnetic contributions, by blocking temperature, reveals that primary natural remanent magnetization (NRM) orientations from hematite are parallel to the local planar fabric strike of the Quetico Subprovince.  Minor preservation of primary magmatic fabric is indicated by the mean principal component analysis (PCA) orientation for magnetite which corresponds closely in trend to the mean paleomagnetic orientation for the region, obtained by previous investigators.  Original Moss Lake stock magnetic fabric is overprinted by compression and shearing.

Comparison of magnetic studies (AMS and NRM) of the Moss Lake stock to structural data of country rocks argues in favour of a common tectonic control.

A copy of the thesis can be downloaded here

Christa Koebernick MSc thesis abstract

Thesis Title: 
Neoarchean Coastal Sedimentation in the Shebandowan Group, Northwestern Ontario

The study interpreted depositional environments from sedimentological data present in metasedimentary rocks of the Neoarchean Shebandowan Group of the Wawa Subprovince .  Outcrops in the study area contained sedimentary structures and bed sequences consistent with shallow water, coastal sedimentation, and represents an important record of Archean depositional processes.  Three depositional environments are represented in the rock record; tidal strandline, the shoreface, and the offshore.  The tidal strandline was further divided into the tidal flat channel sub-environments.  The presence of these three environments provides unequivocal evidence for the existence of shallow-water shelves in the Archean; a period during which sedimentation was dominated by deposition in alluvial fan, fluvial environments and deep water settings.

The three environments and associated sub-environments record processes reflective of differing current activity which controlled and influenced deposition.  The tidal environment was dominated by bidirectional tidal currents.  Deposition in the shoreface was predominated by unidirectional wave-produced currents which overprinted prevailing tidal current activity.  In the distal portions of the shoreface environment through deposition was once again controlled by tidal currents.  In the offshore, deposition was controlled by storm currents which generated distinctive beds of hummocky cross-stratification.

The tidal environment is composed of many sedimentary structures similar to those present in Phanerozoic and present-day tidal sequences.  In the tidal flat sub-environment, vertical sequences of flaser, lenticular, wavy and coarsely interlayered bedding reflect current velocity fluctuations intimately tied to spring - neap tidal cycles.  The tidal channel sub-environment lacks many of the features characteristic of tidal channels described in the literature; such an extensive point bar development.  In stead the tidal channels of the study area appear to represent sequences deposited in relatively straight channels.

Migration of sandwaves and dune fields deposited the cross-stratified lithofacies of the shoreface environment.  Similar to a high-energy non-barred coastline, the proximal portion of the shoreface lacks any evidence of beach development.  Instead, the shoreface records a rapid and discontinuous transition from the tidal strandline environment.

Hummocky cross-stratification (HCS), parallel laminated and massive sandstone beds as well as siltstone and mudstone beds typify the offshore environment.  The HCS differs greatly in thickness and internal structure from HCS described in the literature.  The HCS in the study area reflects restricted and/or variable sediment supply and flow conditions.

A paleotidal range was determined from the sediments of the tidal environment.  The range indicated a mesotidal environment and is comparable to Precambrian tidal ranges reported in literature.  Tidal rhythmites, present on the tidal flats, suggest a length of 26 days for the NeoArchean lunar month.  Currents which deposited the tidal rhythmites produced both sedmidiurnal and diurnal sediment sequences.  

A copy of the thesis can be downloaded here

Shannon Farrell MSc thesis abstract

Thesis Title: 
Crystallographic studies of selected perovskite-group compounds

This study investigates the tausonite-loparite solid solution series.  Members of this series are important in some alkaline complexes, thus making studies of their crystallography essential.  Rietveld refinement of the crystal structures of the tausonite-loparite solution series, using X-ray diffraction powder patterns, indicates that there is a reduction in symmetry from cubic (Pm3m) to orthorhombic (Pnma), by way of an intermediate tetragonal (P4/mbm) modification.  The symmetry changes appear to occur at about ~66.6 and ~33.3 wt% tausonite, which are consistent with formulae of approximately Sr2(NaLa)Ti3O9 and Sr(NaLa)2Ti3O9, respectively.  The pseudo-cubic cell parameter apdecreases with increasing loparite content, while the [111] tilt angle F (F=0 in Pm3m) on inception at 50 wt% loparite achieves a maximum and decreases thereafter with increasing loparite content.  Rietveld refinements indicate that no ordering at the A-site exists throughout the solid solution series. 

This study also investigates a titanium perovskite (Na2/3Th1/3TiO3), which is unusual in that it contains a tetravalent cation at the A-site.  This thorium titanium perovskite was synthesized in an attempt to determine its structure.  Although power diffractometry suggests an Fm3m space-group, attempts at Rietveld refinement of the structure show the actual space-group must be of reduced symmetry.

This study also provides data on the pseudo-binary system between hollandite (K2Cr2Ti6O16) and the n=3 member of the homologous series K2La2Ti3+nO10-2n, i.e., K2La2Ti6O16.  This series is important in understanding the location and environment of the rare-earth cations in natural hollandite specimens and the capability of hollandite (i.e., SynRock) to immobilize large elements of varying charge and size.  This pseudo-binary system is characterized by the presence of the following phases:  hollandite [K1.54(Cr1.43Ti6.52)7.95O16];  perovskite-2 (LaCrO3); and perovskite-3 (La2Ti2O7).  Complete solid-solution between the end-members of this system does not occur.  The hollandites (space-group 14/m) have an A-site occupancy of approximately 75-82%, and exhibit no significant substitution of La3+ at any of the cation sites.  Perovskite-1 is considered to be a non-stoichiometric A-site deficient perovskite.  Potassium hexatitanate is the only main phase that is stoichiometric and contains no substitution of Cr3+ in any of the cation sites.  All the Cr3+ excluded from the potassium hexatitanate structure is incorporated into perovskite-2.

A copy of the thesis can be downloaded here

Michael Julien Michaud MSc thesis abstract

Thesis Title: 
The Geology, Petrology, Geochemistry and Platinum-Group Element-Gold-Copper-Nickel Ore Assemblage of the Roby Zone, Lac des Iles Mafic-Ultramafic complex, Northwestern Ontario
Michael Julien

The Archean Lac des Iles Complex is a mafic to ultramafic intrusion emplaced into gneissic tonalite.  The Lac des Iles Complex is the largest of several mafic to ultramafic intrusions that form a circular outcrop pattern approximately 30 kilometers in diameter.  The Lac des Iles Complex is composed of two ultramafic intrusions exposed at Lac des Iles and a gabbroic intrusion located south of the lake.  The gabbroic rocks contain the economically significant Roby Zone PGE-Au-Cu-Ni deposit.

The Roby Zone deposit is composed of two texturally- and compositionally-distinct portions.  The northern portion of the deposit is composed of a relatively unaltered layered gabbroic sequence consisting of leucogabbro, gabbroonorite, gabbro and clinopyroxenite.  Field data, including the orientation and type of geologic contacts, indicate that the layers represent an intrusion of magma into a largely crystallized mush.  In-situ fractionation was identified within individual layers.  The southern portion of the Roby Zone consists of a lithologically and texturally complex unit containing numerous rounded and angular fragments varying in composition from leucogabbroic to pyroxenitic and grain size ranging from medium-grained to pegmatitic.  These rocks have experienced pervasive deuteric alteration that modified the original magmatic textures and compositions.  Numerous pegmatitic dikes and patches occur throughout the heterolithic gabbro.

PGE-Au-Cu-Ni mineralization within the northern layered sequence often forms net-textured sulphides and represents primary magmatic mineralization.  Within the heterolithic gabbro, PGEs occur as primarily sulphides and tellurides.  These PGE minerals occur as blebs within pegmatitic pods and as fine-grained inclusions and streaks within secondary silicates suggesting that deuteric fluids have concentrated and deposited the metals within the heterolithic gabbro.  Within the southern portion of the Roby Zone, higher PGE concentrations are associated with altered areas.

The model for the development of the Roby Zone and its attendant PGE-Au-Cu-Ni mineralization consists of  1) fractionation of tholeiitic magma in lower magma chamber and exsolution of immiscible sulphide liquid with associated PGE-Au-Cu-Ni,  2) intrusion of fractionated magma into Roby Zone and subsequent in-situ fractionation,  3) prior to complete solidification of the layers, a volatile-rich gabbroic magma injected the Roby Zone resulting in brecciation of the layered sequence and formation of the heterolithic gabbro composed of rounded and angular fragments within a gabbroic matrix,  4) partially solidified rounded fragments and partial melting of some of the remaining fragments by the gabbroic magma triggered liquid immiscibility,  5) deteuric fluids percolated through the fragmented gabbroic rocks modifying the original magmatic textures and compositions and concentrated and redoposited metals within the heterolithic gabbro.  Subsequent regional deformation tilted the Roby Zone to the east and shearing occurred within a portion of the clinopyroxenite.  Late-stage local faulting and hydrothermal fluids further modified the original magmatic textures, compositions and PGE-Au-Cu-Ni mineralization.

A copy of the thesis abstract can be downloaded here

David King MSc thesis abstract

Thesis Title: 
Depositional Environments of the 3.0 Ga Finlayson and Lumby Lake Greenstone Belts, Superior Province, Ontario, Canada

The Finlayson and Lumby Lake Greenstone Belts are located approximately 200km west of Thunder Bay, Ontario, north of Atikokan, Ontario.

Within both the Finlayson Lake and Lumby Lake Greenstone belts two distinct sequences of sedimentary rocks are present.  Each of the belts contains an upper and a lower sedimentary rock sequence which differ in age and chemical composition.

The lower sequence of the Finlayson Lake Greenstone Belt is represented by the Little Falls Lake metasedimentary rocks and the laterally equivalent lower Finlayson Lake metasedimentary rocks.  These rocks consist of coarse-grained sandstones, conglomerates, and lesser interbedded mafic detritus-rich metasedimentary rocks and are laterally continuous with felsic volcanic rocks to the south.  Deposition of these sedimentary rocks was by high-density turbidity current processes.  Their chemical composition is distinct from that of the upper Finlayson Lake metasedimentary rocks and suggest a single felsic volcanic source with composition similar to that of the Steep Rock Upper Felsic unit and the Old Tonalite unit.  U-Pb geochronology again supports a single source rock, with an age of 2996 ± 0.8 Ma.

The upper band of Finlayson Lake sedimentary rocks are distinct from the sedimentary rocks present in the Little Falls Lake area and lower Finlayson Lake areas.  Their chemical composition suggests that the upper Finlayson Lake sedimentary rocks are similar and were continuous with, although fault offset from, the upper Lumby Lake sedimentary rocks (Fenwick, 1976; Stone and Pufahl, 1995).  U-Pb data from conglomerate in the southern Finlayson Lake area yield zircon ages ranging from 2997 ± 2.5 to 3002 ± 0.9 Ma.  Sm-Nd data suggest that the basin received detritus derived from tonalitic intrusions as well as a slightly older mafic volcanic component.  These data agree with geochemical data which suggest that the composition of the upper Finlayson Lake sedimentary rocks lies on a mixing line between the Old Tonalite and the Steep Rock Upper Mafic unit or the Finlayson Lake mafic volcanic rocks.

A well developed coarsening upward sequence is preserved within the upper Finlayson Lake sedimentary rocks.  The sequence consists of iron formation and chemical sedimentary rocks at the base, overlain by DE turbidites, which coarsen to pebbly sandstones and conglomerates near the top of the sequence.  There is also some lateral facies variation with coarsest-grained metasedimentary rocks exposed in the southern part of Finlayson Lake.  These rocks are consistent with deposition from both high- and low-density turbidity currents and were likely deposited by a prograding delta system that was centered south of the area.

As in the Finlayson Lake Belt, the Lumby Lake Belt also contains two stratigraphically distinct sedimentary units.  The lower sedimentary unit is represented by the sedimentary rocks present within the Hock Lake area, whereas the upper sedimentary unit is represented by the sedimentary rocks near Norway Lake and west to the Keewatin-Hematite Lakes area.

The lower Lumby Lake sediments are laterally continuous with 2999 Ma old felsic volcanic rocks to the east (Jackson, 1985) and are the resedimented equivalent of them.  Their chemical composition is similar to that of the Little Falls Lake sedimentary rocks, and a chemically similar source is suggested.

The upper Lumby Lake sedimentary rock sequence is similar to the upper Finlayson Lake sequence and is the fault offset equivalent (Fenwick, 1976; Stone and Pufhal, 1995).  The chemical composition of the sedimentary rocks suggests that the upper Lumby Lake sedimentary rocks had source rocks of the same composition as the source of the upper Finlayson Lake sediments.  The upper Lumby Lake sequence is dominated by iron formations and chemical precipitates, with lesser fine-grained clastic sedimentary rocks.  There are lateral facies variations from a clastic dominance in the west to chemical precipitate dominance in the east.  The predominance of chemical precipitates is evidence of widespread hydrothermal activity throughout the area.  It is possible that the upper Lumby Lake clastic sedimentary rocks represent the distal equivalent of the turbidite system developed in the Finlayson Lake area.  Alternatively, the upper Lumby Lake portion of the basin may have been fed by a localized source centred in the Norway Lake area.  Evidence of this includes a dominance of clastic sedimentary rocks and the presence of debris flow conglomerates in this area.

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