The Sibley Peninsula is an approximately 300km² land mass that projects southwest into Lake Superior, approximately 30km east of Thunder Bay, Ontario, Canada.  The geology of the peninsula consists of Southern Province assemblages, including intrusive rocks of the ~ 1.1Ga Midcontinent Rift.  Due to their location within Sleeping Giant Provincial Park, the Sleeping Giant Sill (SGS) and numerous dikes on the Sibley Peninsula have not been geochemically classified.  With permission from Ontario Parks, this study was undertaken to 1) classify the SGS and dikes of the Sibley Peninsula based on their geochemistry and 2) determine relative ages of the SGS and dikes of various orientations.

Thirty-four dike samples and five SGS samples were collected and analyzed for whole rock geochemistry at the Geoscience Laboratories.  Dikes with three different orientations were sampled: 1) ~70˚ striking dikes, which represent the majority of the dikes on the peninsula, 2) two 110˚ striking dikes and 3) two 29˚ striking dikes. The ~70˚ striking dikes were classified as Pigeon River Dikes based on Mg# versus TiO₂and Gd/Yb_n versus La/Sm_n discrimination diagrams. The 110˚ striking dikes plotted near or within fields defined by Hollings et al. (2007a) for ultramafic intrusions of the MCR.  Despite geochemical similarities, the 110˚ strike directions of these dikes make them spatially unsuitable as feeders to any of the ultramafic assemblages found in the Nipigon Embayment, northeast of the study area.  The 29˚ striking dikes plotted in the field defined for mafic / ultramafic sills and intrusions on the Mg# versus TiO₂ plot and in the field defined for Nipigon Sills on the Gd/Yb_n versus La/Sm_n diagram. Although these dikes were geochemically similar to Osler volcanic rocks of the neighbouring Black Bay Peninsula, they were just 18 and 20 cm wide and tapered eastward, making them unlikely candidates as feeders to Osler volcanic rocks.  Using Mg# versus TiO₂ and Gd/Yb_n versus La/Sm_n discrimination diagrams, the SGS was determined to be of Logan affinity and now represents the easternmost recognized Logan Sill.  The thickness of the SGS was found to be 194 metres, thicker than any other previously recognized Logan Sill.

 Cross-cutting relationships indicate that the ~70˚ striking dikes are younger than both the SGS and the 29˚ striking dikes.  The SGS and the 29˚ striking dikes had lower silica and higher magnesium weight percentages than the ~70˚ striking dikes, which may suggest that more primitive magmas were locally emplaced earlier. The ~70˚ striking dikes displayed the largest negative niobium anomalies which could indicate an increase in contamination of magma sources over time, since these dikes were the youngest intrusions observed in this study.

The SGS was exclusively observed on the Sibley Peninsula's southern tip where it has a lowermost contact with Rove Formation shale.  The SGS is unexpectedly absent in drill core taken from the peninsula's northeastern shoreline from which the thickness of the Rove Formation was determined to be ~ 610 metres.  This can be explained by the timing of the Silver Islet Fault and the glacial erosion that removed the SGS from the northern portion of the peninsula and shaped the Sleeping Giant landform on the peninsula's southern terminus.

The Georgia Lake pegmatite field is located in the Quetico Gneiss Belt of the Superior Province.  Spodumene-bearing and subordinate beryl-bearing pegmatites of the Georgia Lake area are flanked to the south and east by an extensive granitoid terrain, which previously has not been subdivided.  Granitoids of the immediate Georgia Lake area were investigated in conjunction with rare-element pegmatites to determine the character of the granitoids as parental intrusions to rare-element pegmatites.  The granitoids include two-mica leucogranites occurring as a large plutonic mass south of the pegmatite field and as smaller satellitic intrusions, the Kilgour Lake Group granitoids centered on a small gabbroic-metagabbroic unit near Kilgour Lake tonalitic sills dispersed throughout the pegmatite field.  The distinction of the three types of granitoids was made on the basis of field observations, petrography and analytical geochemistry.  Two-mica leucogranites and tonalitic sills were derived as partial melts of pelitic metasediments and metagreywacke, respectively.  The Kilgour Lake Group granitoids were presumed to be the products of fractional crystallization of a mafic melt generated in the upper mantle or lower crust.

Mineralogical studies were carried out on perthitic microcline, tantalite-columbite and Sn oxide minerals from rare-element pegmatites.  Results indicate that perthitic microcline in all pegmatites is of the maximum microcline structural state, tantalite-columbite minerals occur in a partly to completely disordered structural state and the dominant Sn oxide mineral is staringite.

Division of spodumene-bearing rare-element pegmatites into Southern, Central and Northern Groups was made on the basis of internal textural variations, mineralogy and differences in geochemistry of perthitic microcline and muscovite.  The Southern Group consists of one pegmatite which is unique to the Georgia Lake pegmatite field with respect to development of mineralogical zones and strong internal fractionation of Rb and Cs.  Central Group pegmatites are linked by a fractionation trend, with respect to Rb and Cs, across the group.  A similar fractionation trend is not observed across the Northern Group pegmatites.  The pegmatite groupings reflect different modes of source fluid derivation, although all pegmatites of the Georgia Lake area originated as the result of a common anatectic event responsible for the intrusion of two-mica leucogranites.  Central and Southern Group pegmatites were derived from low viscosity fluids differentiated from granitic melts, while Northern Group pegmatites are presumed to be the products of fluids generated by direct anatexis of metasediments.

An occurrence of corona structures, resulting from a reaction between olivine and plagioclase to form a distinct, mineralogically zoned reaction rim between those two phases, was observed near Killala Lake in Northwestern Ontario.  These corona structures were seen to occur in a large ultramafic body, composed primarily of olivine, located in the midst of a terrain comprising granite gneiss and migmatite.  This body was veined by quartzo-feldspathic material that provided the plagioclase necessary for the reaction to occur.

A petrographic study was undertaken to determine the mineralogy of the coronite.  As a result of this, four distinct zones were defined - a core zone, comprising the original ultramafic material; an inner reaction zone, consisting mainly of orthopyroxene with a bronzitic composition; an outer reaction zone, consisting of biotite, which fills the role of an aluminous phase, and the amphiboles hornblende and cummingstonite; and a zoine of plagioclase, primarily, representing the original quartzo-feldspathic material.

The coronite was analyzed by zone by x-ray diffraction methods to confirm the above mineralogy, and by atomic absorption to determine elemental distribution.  In this manner, Fe, Mg, Ca, Al were found to be the major elements present; Na and Mn are minor elements and Ni, As and Ba trace elements.  Since the Fe and Mg are assumed to have originated in the olivine and Ca and Al in the plagioclase, the distribution of these elements amongst the four zones suggests that some form of diffusion took place.

With reference to earlier work, it is therefore postulated that the olivine reacted with water under conditions of middle to upper amphibolite facies metamorphism, that also caused the gneissification and migmatization of the surrounding rocks, to form the orthopyroxene of the inner zone and release Fe and Mg.  These elements diffused across the original olivine-plagioclase interface to react with plagioclase and water to form the amphiboles; biotite required an introduction of K into the system.  Incoming water brought this K into the system, along with As, which formed brandtite (Ca_2, Mn (AsO₄).2H₂0) in the core zone.  Water also acted as a catalyst for the reactions involved, by suppressing pressure requirements in an isothermal system, and as means of increasing P₀₂So that magnetite, a ubiquitous phase in the coronite, could be formed.  Extensive subsequent alteration by serpentinization is noted in the coronite.

Late Archean lamprophyre dikes are common throughout the Superior Province of the Canadian Shield, and are associated with the late stages of Archean deformation in the Canadian Shield.  Accurate age constraints on the timing of this late metamorphism would benefit both the economic and the academic fields of study in this area.  Limited geochronology suggests an approximate age of 2696"2Ma (Corfu and Stott, 1986), for the shoshonitic volcanism in the Superior Province.

Oriented samples taken from dikes outcropping between Thunder Bay and Atikokan were used to make 60 oriented core samples.  These samples underwent anisotropy of magnetic susceptibility measurements (AMS) to determine the origin of the internal fabric and define the cryptic metamorphic fabric.

Subsequently, the samples underwent low temperature demagnetization (LTD), to minimize domain-wall effects on the remnant magnetism contained within each sample.  The samples were then treated to ten steps of thermal demagnetization (100°C - 550°C) with the remnant magnetization measured between each step.  The data acquired from the thermal demagnetization was examined and A-and B-components of magnetization were isolated by Principal Component Analysis (PCA).  The principle components of magnetization were then used to determine the paleopole positions on a geographical map and compared with the best known apparent polar wander paths (APWP) for the Archean and the Proterozoic.  From this, the ages of magnetization may be estimated as 2500 Ma for the A-component and 2400-2300 Ma for the B-component.

An Archean sequence of rocks is exposed along Highway 527, approximately 90 kilometers north of Thunder Bay.  This sequence consists of metasedimentary and metavolcanic rocks which have been affected by lower greenschist facies metamorphism.

Structural elements in the area are S-surface and folds.  S-surface are designated S₀, S₁, S₂, and S₃, while folds are designated F₁, F₂, and F₃.

Cleavage-bedding relationships delineate the hinge lines of F₂ folds.  Younging information projected onto the axial planar cleavage (S₂) of F₂ folds, results in reversals in the structural facing directions.  These reversals indicate that the cleavage developed in previously deformed rocks.

The overprinting of structural elements indicates that three deformational events (D₁, D₂, D₃) can be defined.

D₁ produced nearly vertical and, in part, inverted F₁ folds.  D₂ produced a set of F₂ folds which resulted in refolding of the F₁ structures.  D₃ produced kink bands and crenulations that resulted in folding of the F₁ and F₂ structures.

The quartz-hematite laminations of Mink Mountain, Northwestern Ontario have become the subject of some considerable controversy with respect to the origin of these laminations.  To date, two schools of thought exist regarding the origin of these laminations.  The first concept is that these laminations are the result of hot spring activity producing geyserites or sinter (Walter, 1972).  The second view is that the laminations constitute biogenic results of the growth and preservation of stromatolites.  This thesis presents considerable evidence that indicates the quartz-hematite laminations of Mink Mountain are more likely the result of stromatolite activity and subsequent preservation by cementation and burial.

The quartz-hematite laminations occupy the Upper Algal Chert member of the Gunflint Formation.  A seventy-eight metre long section of the Upper Algal Chert member of Mink Mountain was extensively sampled and these samples were studied in detail by tracings of orthogonal cuts of individual samples and documentation of data into Hofmann's (1969b) stromatolite classification.  In addition, evidence was found to indicate biogenic activity had occurred and that these laminations were formed on a littoral margin.

Through Hofmann's (1969b) classification, it was found that Forms A, C, D, E and G stromatolites exist at Mink Mountain (Hofmann, 1969b).  Microfossils are fairly abundant as filamentous rods similar in appearance to Gunflintia, indicating a biogenic component of the laminations.  The characteristics and configuration of the bioherms suggests these stromatolites thrived in and around tidal pools of a tidal flat on a grainstone substrate.  It is possible that the stromatolite organisms originally secreted carbonate but during the early of late stages of diagenesis, almost total silicification of the bioherms by jaspilitic chert took place.

A section of the Quetico Gneiss Belt north of Thunder Bay, referred to as The Keelor Lake Area, was investigated to ascertain whether there was a relationship between metamorphism and the magnetic expression by the rocks of the area.

Three rock units were distinguished:  1) pelitic schists,  2) basic amphibole bearing schists and  3) quartzofeldspathic rocks.  Petrographic studies revealed the presence of upper amphibolite to granulite facies rocks in the central portion of the Quetico belt bounded both to the North and South by amphibolite facies rocks.  Chemical analysis of the pelitic rocks revealed that their chemical composition was very homogeneous thus providing evidence that the appearance of certain minerals was not due to a change in chemical composition.  Cordierite, garnet and sillimanite assemblages predominate near the centre of the Quetico Gneiss Belt.  Since the chemical composition consistently is the same, the appearance of higher grade mineral assemblages is suggested to be a result of their deeper crustal origin.  The highest grade rocks of the central portion of the Keelor Lake Area are in close proximity to the area which exhibits highest magnetic values.

The Winston Lake mine is located 145 km northeast of Thunder Bay, Ontario, in the Archean Superior Province.  Although the orebody is classified as a typical volcanogenic massive sulfide deposit, it differs in the fact that it was intruded by a large gabbro sill prior to regional metamorphism.  Presumably, the orebody experienced high T, low P contact metamorphism.  Based on the alteration assemblage of anthophyllite-cordierite-sillimanite-staurolite, the regional metamorphism that followed reached the sillimanite zone of the amphibolite facies.  The silicate assemblages suggest a temperature range for metamorphism of approximately 610-640°C and pressures of 2.2 to 5 kbars, assuming that P(H₂O) = P(Total) and that cordierite contains magnesium.

Sphalerite in 38 samples from seven drill-holes through the Winston Lake orebody and the adjacent Pick Lake zone was analyzed on a scanning electron microscope.  The samples are separated into three occurrences:  sphalerite-pyrrhotite-pyrite equilibrium assemblages, sphalerite-pyrrhotite contacts, and sphalerite encapsulated in pyrite.  The compositions of sphalerite from the triple junctions and sphalerite-pyrrhotite contacts are similar, yielding 11.04 ± 0.67 (P = 8.82) and 11.54 ± 0.87 mole % FeS.  Sphalerite enclosed in pyrite fell into three compositional ranges:  5 - 6 mole % FeS indicating sphalerite-pyrite equilibrium, approximately 11 mole % FeS indicating sphalerite-pyrrhotite-pyrite equilibrium, and a single sample yielding 15.2 mole % FeS (P = 4.36 kb).

Sphalerite compositions indicate that any effects of contact metamorphism has been obscured by regional metamorphism.  The relative lack of monoclinic pyrrhotite formation indicates that retrograde alteration is not responsible for the high calculated pressures obtained from the majority of the sphalerite compositions.  It is suggested that equilibrium of sulfides at peak pressure of metamorphism may have been attained before peak temperatures as represented by the silicate assemblages and thus account for the discrepancy in pressure between the sphalerite geobarometer and the silicates.

Bol©o is a 525 Mt sediment-hosted, stratiform Cu-Co-Zn district located along the eastern side of the Baja California Peninsula.  Mineralization consists of low-grade (0.71% Cu, 0.06% Co, and 0.71% Zn) sulfides and oxides disseminated through a series of laterally extensive conformable horizons of laminated and brecciated claystone.  Formation of such extensive mineralization implies a source(s) that is either highly enriched in metals or is volumetrically significant, or both.  Possible metal sources for the Bol©o district and mechanisms for their extraction include: i) low to high temperature leaching of arc volcanic and basement plutonic rocks; and, ii) leaching of red-bed conglomerate by low temperature fluids.

Mass balance calculations were carried out in order to evaluate leaching of arc to early rift volcanic rocks, plutonic basement rocks and intraformational conglomerate.  The calculated maximum metal yield from the conglomerate relative to the total metal budget of the entire Bol©o district is 14.3% for Cu, 81.3% for Co and 32.6% for Zn.  Mass balance determinations indicate that up to 8.6% of the Cu, 4.1% of the Co and 16.6% of the Zn in the total metal budget could be produced from leaching arc andesite.  Lead isotope compositions indicate leaching of the basement plutonic complex, but this source would have yielded only 23.2% of the Cu, 9.3% of the Co and 20.8% of the Zn in the Bol©o district metal budget.  Additional metal enrichment of the hydrothermal fluid is postulated to occur by the admixture of magmatic volatiles.  A magmatic component is inferred from: i) the inability of leaching to account for the total metal budget; ii) the spatial and temporal proximity of rift-related high-K andesite magmatism that is approximately coeval with the timing of sedimentation of ore mineralization; and, have the appropriately high 'O2 conditions of the high-K andesite melts required to transport large quantities of metals.
 
For more details about this thesis contact Dr. Andrew Conly

The Mattabi Mine, a volcanogenic massive sulfide deposit located 60 km north of Ignace, Ontario, in the Archean Superior Province, produced 13.5 mt of ore with an average grade 7.5% Zn, 0.88% Cu, 0.77% Pb and 3.10 oz/t Ag in the period 1970-1983.  Preliminary studies indicated the presence of an especially varied mineral assemblage, which was studied in detail.  As well as chalcopyrite and sphalerite, the deposit contains abundant galena accounting for its unusual lead production.  Argentian Tetrahedrite, or frelbergite, is also abundant, and in the absence of native silver and acanthite, is clearly a major carrier of silver.  Other sulfide minerals include pyrite, pyrrhotite, arsenopyrite, bornite, mackinawite, pyragyrite, boulangerite, freleslebenite, stephanite and veenite.  Oxide minerals include magnetite, hematite, llmenite, rutile, jacobsite, cassiterite and gahnite.

The mineral zoning in the orebody is pronounced.  Sphalerite, tetrahedrite, galena and the sulfosalts all occur in the upper portion of the orebody, whereas pyrrhotite and chalcopyrite are concentrated in the lower portion.  The oxide minerals, except for jacobsite, occur in the ore zone as well as in the footwall alteration zone.

A Brillouin zone model for tetrahedrite in which 204 to 208 electrons are accommodated in the 51^st Brillouin zone is verified in analytical data from 60 grains.  However, when three or more Ag atoms per unit formula are present, the number of valenic electrons increases from 204 to 208 as Ag atoms increase from three to six.  The presence of (Fe+Zn) in excess of two atoms per unit formula appears to stabilize the 52^nd Brillouin Zone with >208 valence electrons per unit cell.

Analyses of sphalerite yielded a mean mole % FeS = 12.11 " 0.70, indicating a pressure of metamorphism of 7.53 kbar, using the sphalerite geobarometer.  This pressure is approximately 2 to 3.77 kbar too high in comparison with pressures suggested by silicate systems.  A possible explanation is that sphalerite-pyrite-pyrrhotite equilibrated at peak pressure, whereas silicate assemblages indicate conditions of peak temperature in the P-T-t history of metamorphism of the deposit.  Retrograde alteration variably affected sphalerite composition, producing some scatter in data, as well, as evidenced by variable occurrence of late-stage monoclinic pyrrhotite.

The low gold content, high lead content, diversity of sulfosalt species and abundance of silver in tetrahedrite are features more similar to those of kuroko-type deposits of the Phanerozoic than to Noranda or primitive-type massive sulfide deposits characteristic of the Archean.