Katharine Masun MSc thesis abstract

Thesis Title: 
The Petrography and Mineralogy of the Lac de Gras Kimberlite Field, Slave Province, Northwest Territories: A Comparative Study
Katharine
Masun
MSc
1999

The kimberlites studied from the Lac de Gras area are composed of two broad textural types:  hypabyssal kimberlite and volcaniclastic kimberlite.  Hypabyssal kimberlite is present within small isolated dyke segments and in direct contact with volcaniclastic kimberlite within vent infill.  The latter likely represents small, sub-horizontal sills emplaced subsequent to vent excavation and infill.  Volcaniclastic kimberlite occurs as well-to-poorly sorted, often poorly consolidated vent infill.  Abundant xenoliths of wall and rock xenocrysts of mica, feldspar, and quartz characterize the volcaniclastic rock, suggesting that most of the volcaniclastic kimberlite has been subjected to resedimentation processes.  Juvenile lapilli (both vesiculated and non-vesiculated) are very common, but are often poorly-developed.  A conspicuous feature of these kimberlite vents is the presence of wood fragments, discrete xenoliths of non-kimberlitic sediment and well-laminated beds of non-kimberlitic mudstone and siltstone.  Pyroclastic kimberlite appears to be present within the kimberlites at Lac de Gras, but only accounts for a minor part of the vent infill.  The pyroclastic kimberlite contains abundant juvenile lapilli and may show textures indicative of welding and molding.  The Lac de Gras kimberlites are mainly small, steep-sided vents infilled with resedimented volcaniclastic kimberlite ±small hypabyssal sills.  No tuffisitic breccias, the hallmark of diatreme-facies kimberlite, were recognized within the kimberlite vents and appear to be absent from the Lac de Gras kimberlites. 

All features observed within the Lac de Gras kimberlites are characteristic of, but not exclusive to, kimberlites.  The composition of spinel, phlogopite and monticellite are typical of archetypal kimberlites, although spinels are conspicuously aluminous and phlogopites Ba-rich.  The nature of the primary texture, mineralogy and mantle-derived xenocrysts indicates that these rocks are archetypal kimberlites.

Comparing the character of kimberlite vents from the Fort à la Corne area (maar-like phreatomagmatic model) and the Orapa A/K1 kimberlite (southern African "classic" diatreme model) shows that there are two contrasting end-member emplacement mechanisms which are repeated in time and space.  The former is driven by meteoric water in phreatomagmatic processes, while much debate exists whether the later is driven by juvenile gases (fluidization model) or hydrovolcanic processes.  Near-surface geological setting at the time of emplacement appears to have played a critical role in determining the emplacement process of the kimberlite magma.  Kimberlites discovered in the Lac de Gras area do not conform to either end-member emplacement models and serve to highlight that a third, intermediate model will need to be developed to account for the features observed.

Clearly, two distinct processes were responsible for the emplacement of the Lac de Gras kimberlites: vent excavation and vent infill.  Phreatomagmatic processes likely contributed significantly to the excavation of the kimberlite vents and a flaring explosion crater or maar was excavated into the soft, overlying sediments.  The resultant ejecta was deposited as extra-crater material as a tuff ring or cone.  This material was subsequently reworked and redeposited within the evacuated vent by debris flow and mass wasting processes incorporating a large amount of xenolithic material.  The thin pyroclastic kimberlite beds within the vents are the result of either minor pyroclastic activity that continued through a central conduit during vent infill, or primary pyroclastic kimberlite initially deposited within the tuff cone/ring, which subsequently fell back into the vent with little reworking, as a coherent mass preserving the primary textures of the tuff.  In the later scenario the "pyroclastic kimberlite" units are in fact resedimented "pseudopyroclastic" volcaniclastic kimberlite.

Clearly, more detailed studies need to be undertaken on these enigmatic kimberlites to constrain further the nature of their infill and emplacement.  This can only occur with the creation of better exposures during mining and advanced exploration activities.

A copy of the thesis can be downloaded here

France Lagroix MSc thesis abstract

Thesis Title: 
Neo-Tectonic and Rock Magnetic Study of the Circum Troodas Sedimentary Succession, Cyprus
France
Lagroix
MSc
1999

The circum Troodos Sedimentary Succession (Late Cretaceous to Recent) overlies the Troodos ophiolite of Cyprus, located in the Eastern Mediterranean.  The pattern of neo-tectonic deformation was investigated through magnetic analysis of the tectonic petrofabrics.  Anisotropy of magnetic susceptibility (AMS), anisotropy of anhysteretic remanence magnetization (AARM) and hysteresis loop parameters were determined in order to define the magnetic fabric and the magnetic mineralogy.

The study area extends over approximately 1000 km2 mainly to the south of the exposed Troodos ophiolite.  The sample suite includes 432 oriented hand samples, predominantly of the Lefkara and Pakhna Formations.

Field measurements indicate bedding predominantly dips less than 10° to the south, while stylolitic cleavage dips steeper than bedding in various directions.  The bedding-cleavage relationship yields variable vergence directions suggesting gravity sliding of the sediments towards local sedimentary basins.  Southwest of the Limassol Forest Block axial planar cleavage consistently defines a SW vergence with respect to bedding due to a locally different, Early to Middle Miocene compression along NNE to NE - SSW to SW azimuth. Hysteresis loop analysis show that pseudo-single domain magnetitel is the ferromagnetic contributor.  Furthermore, in 71% of the samples, the ferromagnetic contribution provided more than 50% of the total susceptibility.  Thus, the petrofabrics of traces of magnetite largely control the magnetic fabrics.

The AMS fabric, in part tectonic, is controlled by the preferred crystallographic orientation of diamagnetic calcite and paramagnetic clay minerals as well as magnetite.  Orientation directions of the principal axes relative to bedding and cleavage indicate incomplete overprinting of the primary sedimentary fabric in many cases.  AMS foliation preferentially dips shallowly to the east and west.  AMS lineation varies regionally, from west to east across the study area, from a NNE to NNW trends, respectively.  The tectonic AMS fabric registers either a late Miocene supra-subduction extension regime due to southward migration of the reactivation Cyprean Arc or due to Pleistocene gravity sliding due to uplift of the Troodos Ophiolite Complex. 

The AARM fabric is controlled exclusively by the preferred dimensional orientation of pseudo single domain magnetite.  Exluding the area in proximity to the Limassol Forest Block, the AARM  fabric orientations are regionally consistent.  AARM foliation planes dip ~45° to the NW and AARM lineation is directed NE and SW, almost orthogonal to AMS lineation.  In some cases, in proximity to the Limassol Forest, the AARM lineation results from the combined magnetic fabrics, parallel to cleavage and bedding and is parallel to the bedding-cleavage intersection lineation.  The actual extension direction is to the WNW, represented by the AARMint principal axes.  Southwest of the Limassol Forest Block of the AARM fabric registers the Early to Middle Miocene SW - NE compression, and its tectonic expression is conventional, with AARMmax oriented NW and SE, perpendicular to the maximum compression direction.

A copy of the thesis can be downloaded here

Kirk Ross MSc thesis abstract

Thesis Title: 
X-Ray Diffraction and Rietveld Structural Refinement of Selected Fluoroperovskites
Kirk
Ross
MSc
2000

This study presents an X-ray diffraction analysis and Rietveld structural refinement of selected synthetic fluoroperovskite-type compounds including the Na1-xKxMgF3 solid solutions series in addition to synthetic analogues of cryolite (Na2NaAlF6) and simmonsite (Na2LiAlF6).

The Na1-xKxMgF3 solid solution series is comprised of three structurally distinct perovskite phases. In order of increasing potassium they are: orthorhombic (Pbnm, a=5.3609(1), b=5.4862 (1), c=7.6661(1), Z=4) in the x=0-0.35 compositional range, tetragonal (P4/mbm, a=5.444(3), c=3.9217(3), Z=2) in the x=0.40~0.46 compositional range and cubic (Pm3m, a=3.9903, Z=1) in the x=0.50-1.0 compositional range. The orthorhombic and tetragonal members are derived from the cubic aristotype by octahedral rotation, a-a-c+ and a0a0c+ respectively. Introduction of potassium into the Na 4c crystallographic site results in a decrease of octahedral rotation and an overall reduction of structural distortion from f = 4.42 for the neighborite end member to  f = 5.0 for cubic members. This is accompanied by an increase in the pseudocubic cell dimension in addition to a reduction in A-site cationic displacement.  Angular and bond length distortion of the MgF6 octahedron are at a maximum in the x=0.20 intermediate member of the series and decrease linearly with additional potassium.  These effects are a direct result of cations approaching special positions in the unit cell as potassium is introduced into the neighborite structure.

Synthetic cryolite (Na2NaAlF6) and synthetic simmonsite (Na2LiAlF6) investigated exhibited similar structural characteristics such as 1:1 B-site cationic ordering and an a+b-b- octahedral rotation scheme. Synthetic cryolite exhibited the greatest structural and octahedral distortion (f = 4.07, ΔB' = 0.016, δB' = 11.75) and octahedral rotation (ΦB' = 18.6) as compared to simmonsite (f = 4.52, ΔB' = 0.06, δB' = 2.74 and ΦB' = 14.96). These ordered monoclinic perovskites (P2(1/n), a≈b≈√2ap, c≈2ap) are derived from the cubic aristotype by octahedral rotation and B-site cationic ordering.
 
A copy of the thesis can be downloaded here

Ekaterina Reguir MSc thesis abstract

Thesis Title: 
Aspects of the mineralogy of the Murun alkaline complex, Yakutia, Russia
Ekaterina
Reguir
MSc
2001

The  Murun complex contains a number of unusual and mineralogically unique rocks.  Many of them are problematic in terms of their genesis and petrographic interpretation.  These enigmatic rocks include charoite assemblages, as well as unique Ba-Sr-rich carbonatites and alkaline ultramafic dykes referred to as lamproites or lamprophyres.

Charoitites occur in about 25 localities along the southern margin of the Little Murun intrusion.  Obtained compositional data for charoite suggests, in general, the empirical formula:  (K,Na)3(Ca,Sr,Ba,Mn)5Si12O30(OH,F).  3H2O. X-ray diffraction patterns of mosaic-fibrous and schistose charoite are indexed using a primitive monoclinic cell with the following parameters: a =19.86(1) D,  b =32.13(2) D,  c =7.952(9) D,  $=97.24E (mosaic-fibrous) and a =32.13(2) D,  b =19.64(2) D, 
c =8.509(6) D,  $=95.23E (schistose).  Some of the patterns of mosaic-fibrous charoite can also be refined on a triclinic cell (a =19.96(1) D,  b =32.17(2) D,  c =7.258(4) D,  "=93.51E, $=99.45E, (=89.50E).  Comparison of the compositional and structural data of charoite and "tube"-chain silicates of similar composition (canasite and miserite) suggests that charoite has a significantly higher relative proportion of SiO4tetrahedra to octahedrally-coordinated cations than two other minerals.  Possible similarities between the structural motifs of charoite and phyllosilicates are demonstrated.
  
The carbonatite bodies are confined to the aegirine-microcline fenite aureole in the southern contact zone of the Little Murun massif with the Precambrian crystalline basement.  Three mineralogical types of the studied carbonatites are distinguished:  calcite carbonatite (i), Ba-Sr-Ca  carbonatite  (ii), and phlogopite-calcite carbonatite (iii).  In addition, the quartz-feldspar-carbonate rock is classified as a distinct lithological type.  Carbonatites of types (i) and (ii) noticeably differ in terms of the composition of rock-forming silicates (potassium feldspar and clinopyroxene) and primary carbonates (Sr-rich calcite or barytocalcite, respectively).  In both cases, the primary carbonates underwent complex exsolution processes.  Typical exsolution textures are represented by primary carbonates in a core, and the subsolvus mineral assemblage confined to marginal parts of the crystal.  The composition of the primary carbonates becomes progressively depleted in Ba + Sr (calcite) or Sr (barytocalcite) towards the margin, suggesting that interstitial fluid played an important role in the onset of exsolution processes promoting an outward diffusion of components in the peripheral zones of carbonate crystals.  In contrast to the calcite-carbonatite, the phlogopite-calcite carbonatite [type (iii)] is devoid of clinopyroxene and Ba-Sr-Ca carbonates, and rarely exhibits exsolution textures.  Potassium feldspar and phlogopite from the carbonatite of type (iii) are Ba-rich, and shoe core-to-rim zonation pattern of decreasing Ba content.  The mineralogical differences between the carbonatites of types (i) and (iii) clearly indicate that the latter crystallized from a volatile-rich magma depleted in Na, under more reducing conditions, and upon crystallization, underwent a rapid loss of residual volatiles.  Textural and mineralogical features of the quartz-fledspar-carbonate rock suggest that the rock is transitional from carbonatites to quartz-calcite rocks locally known as "torgolites".
 
The youngest igneous suite at Murun is represented mostly by dykes and sills.  Studied hypabyssal and volcanic rocks are divided into eight petrographic groups: lamprophyre, pseudolueucite syenite, pseudoleucite italite, kalsilite melasyenite, eudialyte-bearing syenite, "potassium-batisite" syenite, lamprophyllite- and barytolamprophyllite-bearing syenite, and aegirinite.  The compositional trends of mafic minerals from different groups clearly indicate that their parental magmas could not be derived from the same source, and that the magma mixing cannot account for the diversity of the hypabyssal rocks found at Murun.  The compositional data of mafic minerals are in agreement with the previous findings, supporting the consanguineous nature of lamprophyres, phlogopite clinopyroxenites and skonkinites (including melasyenites).  The syenitic rocks containing major proportions of such "agpaitic" minerals as eudialyte, "potassium batisite", and lamprophyllite-group minerals clearly represent the most evolved magmas.  Compositions of their characteristic accessory minerals crystallized early in the evolutionary history suggest enrichment of parental magmas in Sr, Ba and Zr.  The compositions of mafic minerals from pseudoleucite syenites fall off major evolutionary trends, therefore the source of these rocks is uncertain.
  
Clearly, further studies of the geochemistry and mineralogy of the alkaline ultramafic rocks, unique Ba-Sr-rich carbonatites and charoitites of the Murun complex are required. 
 
A copy of the thesis can be downloaded here
 

Lionel Charpentier MSc thesis abstract

Thesis Title: 
Magnetic fabrics of granitic plutons and gneisses, northwestern Ontario
Lionel
Charpentier
MSc
2003

Specimens of plutonic granitic gneisses have been collected from different locations of the Superior Province.  The study of their Koenigsberger ratio (related to natural remanent magnetization) and the theoretical Koenigsberger ratio (related to maximum artificial remanent magnetization) shows that the theorietical Koenigsberger ratio of the specimens is less dispersed than the Koenigsberger ratio.  There is a power law relation between the remanent magnetization and induced magnetization for the Koenigsberger ratio and the theoretical Koenigsberger ratio and this power law relation is due to variation of amount of ferromagnetic minerals in the specimens.

A structural study of the McKenzie granite (NE of Thunder Bay), the Rice Bay dome and the Sawbill dome (NE of Fort Frances) have also been performed.  The McKenzie granite magnetic fabrics cannot be used as kinematic indicators because of the non-coaxiality of the direction of the magnetic ellipsoid axes and also because a primary fabric has been overprinted by a secondary one.  The Rice Bay dome fabrics are non-coaxial but are clearly related to the regional strain and mineral lineation: they may be related to the diapiric emplacement of the dome.  Sawbill dome fabrics are not related to the diapiric emplacement of the dome but to a later event related to the southern border of the dome adjacent to the Quetico fault.

A copy of the thesis can be downloaded here

Geoff Heggie MSc thesis abstract

Thesis Title: 
Whole rock geochemistry, mineral chemistry, petrology and Pt, Pd mineralization of the Seagull Intrusion, Northwestern Ontario.
Geoff
Heggie
MSc
2005

The Seagull Intrusion emplaced during formation of the 1.1 Ga Mid-continental Rift centred around Lake Superior was studied in several drill core and surface samples and consists of a lower ultramafic section (~650 m) and an upper mafic section of unknown thickness (<100m). The lower ultramafic section is texturally dominated by cumulate olivine and poikilitic clinopyroxene with lithologies consisting of dunites, peridotites and pyroxenites. The upper mafic section is variable in lithology, with increasing upwards in abundance of cumulate pyroxene and poikilitic feldspar, dominated by olivine gabbro and gabbros.  

Olivine, pyroxene and oxide mineral chemistry from the lower ultramafic section, indicates limited fractionation throughout the section. Forsterite compositions range from Fo75.8 to Fo86.3. Pyroxene also exhibits a limited range of variability with clinopyroxene restricted to the compositions of En42Wo48Fs10 to En54Wo39Fs7. Oxides occur as inclusions in olivine and in pyroxene with compositions reflecting subsolidus re-equilibrium with the host mineral. Olivine compositions from the mafic section exhibit a more differentiated nature and range in composition from Fo57.4 to Fo75.8.

Whole rock geochemistry is consistent with the homogeneous nature of the ultramafic section and more differentiated character of the mafic section. Major elements only exhibit minor variations in abundance through the ultramafics, and exhibit enrichment in Fe and Si only in the mafic section. Trace elements and primitive mantle normalised patterns identified five geochemical units, characterised by total trace element abundances and Eu, Nb and Th anomalies. Sm-Nd and Rb-Sr isotopes indicate the intrusion was variably contaminated by older continental crust. The most extensive contamination is found in the basal section of the intrusion and decreases away from the contact.

Platinum, palladium, nickel and copper mineralization is found at the base of the ultramafic section of the intrusion, formed during initial emplacement and sulphur saturation at that point in time. A second mineralized horizon (RGB Horizon) occurs ~100 to 150m above the basal contact. The RGB Horizon is continuous over 700 m and contains three distinct horizons separated by at least 2 m of unmineralized ultramafic rock. These layers are interpreted to have formed as individual events related to the injection of primitive magma refreshing the chamber.

Geoff is currently working on his PhD at the University of Western Australia in Perth

A copy of this thesis can be downloaded here

Clarence Surette MSc thesis abstract

Thesis Title: 
The Potential of Microfossil use in Paleodiet and Paleoenvironmental Analysis in Northwestern Ontario
Clarence
Surette
MSc
2008

Historically wild rice was important economically and spiritually across much of central and eastern Canada, but the antiquity of its use by Native communities is unclear.  Unlike plant macrofossils, which have traditionally been used to identify this plant in prehistoric contexts, silicophytoliths preserve well in archaeological sites and in carbonized food residue encrusted on clay pots.  This proxy, therefore, promises to yield considerable new insight into the antiquity and intensity of wild rice harvesting in this region.

Various phytolith types from various grasses, sedges and aquatic plants were examined.  However, the focus was kept on rondel phytoliths, a form only produced in grasses.  Thirty-eight grass species were examined, including two species of wild rice (Zizania aquatica and Zizania palustris).  A minimum of three hundred rondels from each grass species were counted from various parts of the plant including the inflorescence, the leaf and the stem.  Based on extensive morphological comparisons of phytoliths produced by wild rice (Zizania spp.), and other native Boreal and Prairie grasses and maize (Zea mays), several phytolith morphotypes were identified that are produced only in wild rice (Zizania spp.).  In general, rondels with four spikes, with one and three indentations, are characteristic of Zizania (spp.).  However, differences between the two wild rice species were not established.

Lake sediments from Lulu Lake in the Lake of the Woods area where modern wild rice grows were analysed to determine if the types identified as being diagnostic of wild rice would be present.  As a preliminary analysis, the presence of wild rice (Zizania spp.) can be identified in small quantities in lake sediments.  Therefore, wild rice (Zizania spp.) phytoliths can be a powerful tool in the identification of the plant in Holocene sediments.

Potsherds with encrusted carbonized residues from the Lake of the Woods and surrounding area were also examined for the presence of diagnostic wild rice (Zizania spp.) and maize (Zea mays) phytoliths.  These archaeological samples are attributed to the Laurel (Middle Woodland), Selkirk (Late Woodland), Blackduck (Late Woodland), and Sandy Lake (Late Woodland) cultures.  Based on the use of diagnostic phytolith types for both wild rice (Zizania spp.) and maize (Zea mays), the presence of both these cultigens was identified in the residues of all four cultures mentioned above.

This is the first time maize (Zea mays) and wild rice (Zizania spp.) have been positively identified in prehistoric carbonized food residues from the Boreal Forest.  Based on pottery types, wild rice (Zizania spp.) and maize (Zea mays) were consumed as early as the Middle Woodland period (Laurel phase).  Based on the samples examined, the evidence of maize (Zea mays) phytoliths in the residue is greater than those of wild rice (Zizania spp.).  However, this might reflect sample bias or most likely biases due to processing of the plants before consumption.  Therefore, the absence of a wild rice (Zizania spp.) phytolith signature might not represent that the plant was not consumed, rather that the parts of the plant with the diagnostic phytolith types were removed before consumption.  

Maize (Zea mays) horticulture during the Late Woodland period in the Lake of the Woods and surrounding area does not seem likely because there is no evidence of gardening, or heavy consumption of this plant.  However, the latter might not be necessary for local horticulture.  In contrast, wild rice (Zizania spp.) stands are common in the Lake of the Woods and surrounding area, and therefore local harvesting of this plant is inferred.

Clarence is currently working in  the Anthropology Department at Lakehead University

A copy of the thesis can be downloaded here

Amy Shute's MSc Thesis Abstract

Thesis Title: 
Geology and Alteration Associated with the Hamlin Lake VMS System, Shebandowan Greenstone Belt, Northwestern Ontario, Canada
Amy
Shute
MSc
2008

The Hamlin Lake area is located approximately 120 km southwest of the City of Thunder Bay, Ontario within the Shebandowan greenstone belt of the Wawa Subprovince, Superior Province. The purpose of this study was to understand the relationship between an extensive felsic volcanic rock package and the hydrothermal alteration.

Mapping of the area exposed five major lithologies consisting of mafic metavolcanic rocks, intermediate metavolcanic rocks, felsic metavolcanic rocks, felsic intrusive rocks and iron formation. The felsic metavolcanic rocks vary from rhyolites and banded ash to lapilli tuffs and debris flows, whereas the intermediate metavolcanic rocks are made up of dacites and andesites. The only mafic metavolcanic rocks are the debris flows. The felsic intrusive rocks consist of a quartz-eye porphyry and pink breccia. Primary textures, such as fiamme, can still be observed and which are indicative of a subaqueous environment.

Major and trace element geochemical analyses were undertaken on 200 samples of andesitic to rhyolitic volcanic and volcaniclastic rocks. The major elements have become mobile, whereas the trace elements have remained relatively immobile. Hydrothermal alteration affected the primary textures of the Hamlin Lake area so it was necessary to use trace elements to correctly classify the volcanic rocks. Primitive mantle-normalized (PM) and Zr versus Ti plots were used to separate major suites of rocks and also to determine the tectonic setting. The PM plots showed negative anomalies of the Nb and Ti elements, an indication of a supra-subduction zone setting.

A plot of Zr versus Y was used to establish that the rhyolites and andesites are both of a mildly calc-alkaline affinity and the dacites are strongly calc-alkaline. The VMS classifications of Lesher et al. (1986) and Hart et al. (2004) were used to classify the rhyolites and andesites as FII felsic volcanic rocks, whereas the dacites were classified as FI felsic volcanic rocks. FI felsic volcanic rocks are thought to have originated from a deeper source than the FII felsic volcanic rocks suggesting that the Hamlin Lake volcanic rocks were produced in a mature arc-setting.

Mapping and geochemistry was further supported by analysis of Sm-Nd and oxygen isotopes. Nine samples were analyzed for Sm-Nd isotopes and yielded εNd2700 values from -6.59 to +2.62 suggesting some samples had undergone contamination by older material. Several of the samples were close to the value for 2.7 Ga depleted mantle, but one particular sample with a εNd value of -6.59 showed that it had been affected by contamination that could only be explained by a contaminant that was much older than the surrounding 2.7 Ga rocks. The source of the contamination is likely the result of continentally derived sediments being subducted into the mantle wedge, because oceanic arcs are not underlain by older continental crust.

Thirty-seven samples were analyzed for O isotopes to recognize alteration patterns in the Hamlin Lake area. The area mapped at Hamlin Lake was not large enough to clearly show an alteration halo, but it does show that the samples at Hamlin Lake have elevated δ18O values, the result of interaction with low temperature hydrothermal fluids.

Amy is currently working as an exploration geologist

A copy of the thesis can be downloaded here

Patrick Moran MSc thesis abstract

Thesis Title: 
Lithogeochemistry of the sedimentary stratigraphy and metosomatic alteration in the Musselwhite gold deposit, North Caribour Lake greenstone belt, Superior Province, Canada: Implications for deposition and mineralization
Patrick
Moran
MSc
2008

The Musselwhite gold deposit, 100% owned and operated by Goldcorp Inc., has cumulatively produced in excess of 2 million ounces since opening in 1997 and has a projected mine life through 2013. It is situated in the North Caribou Lake metavolcanic/metasedimentary belt in the central northwestern portion of Superior Province, approximately 430 km northwest of Thunder Bay, Ontario, Canada. The belt occurs along the contact between the North Caribou Terrane and the Island Lake Domain, with a large, crustal-scale deformation zone forming its eastern margin. The Musselwhite gold deposit is hosted by amphibolite facies metamorphic rocks dominated by banded iron formation (BIF). This study primarily focuses on the Northern Iron Formation (NIF) metasedimentary assemblage, host to the majority of gold mineralization at Musselwhite.

Stratigraphic and geochemical analyses suggest that the lithologies of the NIF assemblage were deposited on Mesoarcheaen mafic to ultramafic volcanic rocks forming the ocean floor. The NIF assemblage and another iron formation lower in the stratigraphy, the Southern Iron Formation (SIF), record hydrothermal regimes associated with, and interrupted by, eruptive volcanic activity. The stratigraphically lowest lithologies in the NIF assemblage, meta-argillite, quartz-grunerite BIF, and magnetite-dominant BIF, were deposited in deep, calm water, in association with venting hydrothermal fluids. These ancient chemical sediments are analogous in geochemistry to modern day deposits in places such as the Red Sea and the East Pacific Rise. Differing Eu contents between chert and magnetite layers indicate that rhythmically changing temperature variations drove the hydrothermal system, imparting the banded nature. The chemically pure chert and magnetite layers of the lower portion of the NIF assemblage contrast with silicate-dominant banded iron formation; the silicate-dominant BIF increases in frequency up stratigraphically. It represents a decreasing hydrothermal system and/or an increase in the rate of clastic sedimentation. Hornblende-garnet and biotite-garnet schists were formed by metamorphism of mudstones composed of eroded material. The sediment that formed the hornblende-garnet schist is the same sediment that comprises the siliciclastic component of the silicate-dominant NIF. Similarly the biotite-garnet schist represents a mudstone, but unlike the hornblende-garnet schist, it is primarily derived from intermediate to felsic volcanic rocks. Lastly the garnet-bearing quartzite represents metamorphosed sandstone eroded from the same intermediate to felsic igneous source rocks as the biotite-garnet schist. Just as there is an overall increase in clastic content upwards through the approximately 30 m thick sedimentary succession, there is also a change from more mafic-sourced debris to a more intermediate/felsic source.

The majority of samples collected from Musselwhite did not experience significant remobilization of typically immobile elements. This is indicated by the relatively linear geochemical ratios between the immobile elements (Al2O3, TiO2, Zr, U, Th, etc.). Even elements that are commonly more mobile (K2O, Na2O, etc.) appear to have remained relatively immobile at Musselwhite. The only samples that show significant geochemical change are from shear zones. The gold mineralization is primarily associated with shear zones within the siliciclastic-rich, upper NIF assemblage, where pyrrhotite (possibly originally pyrite) replaced iron oxides and iron silicates. This indicates that the control on areas of gold mineralization was a combination of: 1) the presence of structural zones allowing gold-bearing fluids to move through the NIF, which could act as a geochemical trap for gold; and 2) structural conditions in the siliciclastic-rich NIF that favoured hydrothermal fluid involvement with this unit.

A copy of the thesis can be downloaded here

Carissa Isaac MSc thesis abstract

Thesis Title: 
Stable isotope (N, O, H) geochemistry, petrology and compositions of biotite of the Musselwhite Mine, Ontario: implications for mineralisation
Carissa
Isaac
MSc
2008

Musselwhite Mine is located on the south shore of Opapimiskan Lake, approximately 480 km north of Thunder Bay. Hosted in the  ~2.8Ga North Caribou Lake greenstone belt of the North Caribou Terrane, Superior Province, Musselwhite is currently classified as a shear hosted orogenic gold deposit. The deposit is hosted in an iron formation within a volcanic pile of intermediate to felsic metavolcanic rocks, metabasalts and komatiitic metabasalts and has been metamorphosed to amphibolite grade.

Twenty biotite and 30 quartz samples have been analyzed for δ15N, δ18O and δD from Musselwhite Mine as well as 12 biotite samples from the granitoid rocks surrounding the North Caribou Lake Greenstone Belt. Nitrogen isotopes in biotite from Musselwhite Mine are characterized by a δ15N range from -1.3 to 11.1 per mil. Oxygen and hydrogen isotopes of biotite samples from the mine range from +7.1 to +10.1 per mil for δ18O and -55 to -100 per mil for δD.

Oxygen isotope signatures from quartz samples from Musselwhite range from +12.4 to +17.1 per mil. Values for the silicate facies iron formation and mineralized zones are consistent with previous work by Otto (2002) and indicate fluid compositions that fall within both the magmatic and metamorphic range.

Biotite samples from granites and metasedimentary rocks adjacent to the deposit have a δ15N range of -6.9 to +6.1 per mil. Oxygen and hydrogen isotopic ranges for the granitoid plutonic rocks are +2.0 to +4.0 per mil and -59 to -80 per mil respectively; values are typical of felsic plutonic rocks.

The δ15N, δ18O and δD stable isotopic data generated for Musselwhite Mine suggest that magmatic fluids played a role in the formation of the deposit.  

Carissa is working on her PhD at the University of Western Australia in Perth

A copy of the thesis can be downloaded here

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