Placer accumulations are formed by the preferential sedimentation of heavy minerals from the general population of detritus being transported by a fluid.  The depth, velocity, and grain size conditions under which placers form on beaches, sand-dominated meandering, and braided fluvial systems is at present only partially understood.  Our knowledge of the controls on alluvial placer formation in gravel-dominated longitudinal bars of braided rivers is even more poorly developed despite their obvious economic importance.

The accumulation of heavy minerals in coarse-grained longitudinal gravel bars was studied by examining and sampling surficial and matrix sediments from modern, naturally occurring bars, and by simulating these bars under a variety of controlled flow conditions in a sediment-water recirculating flume.

Two processes dominated the deposition of sediments in both the natural and artificial systems studied:  1) suspension rain out; and  2)  avalanche face progradation.  Sediments which were deposited as a result of avalanche face progradation were found to contain significantly higher concentrations of heavy minerals in both the naturally occurring and experimental longitudinal gravel bars.  Data also indicate that the difference in heavy mineral content amongst sediments deposited as a result of these two processes will increase substantially with increasing density of the detrital minerals present.  This suggests that denser heavy minerals are more likely to be deposited amongst less dense surficial sediments whereas less dense heavy minerals are more likely to be vertically distributed throughout the bar sequence.

In the natural systems studied, heavy mineral content was found to be much higher in poorly sorted, coarse-grained sediments deposited amongst pebble sized clasts.  Flume tank experimentation similarly revealed that detrital lead content was highest amongst pebble sized clasts during the fastest velocity runs.  In addition, an increase in clast size resulted in a decrease in the amount of heavy minerals accumulating in surficial sediments.

This study has also highlighted two processes which result in the formation of alluvial depositional placer accumulations in coarse-grained braided river systems.  The first process occurs as a result of heavy minerals in channel bottom sediments becoming progressively enriched through the winnowing of less dense sediments, resulting in the formation of an erosional placer deposit.  Flume experimentation revealed that when high concentrations of heavy minerals armouring the stream-bed were reached, this often resulted in the initiation of their movement downstream.  This process can also be triggered by catastrophic events such as large floods or regional tectonic uplift.  A sudden increase in energy typically associated with such events results in the flushing of erosional placers and their eventual deposition in areas of higher preservation potential.  Therefore, a catastrophic adjustment helps to flush out erosional placer deposits in to the basin to form a depositional placer accumulation.  The second process of depositional placer formation results from heavy minerals traveling in bed load transport, while less dense sediments are kept mostly in suspension.  With a decrease in velocity, heavy minerals are sedimented with hydraulically equivalent sized, less dense sediments in open framework gravels.  

The Sibley Group is an unmetamorphosed Mesoproterozoic red bed sequence, commonly flat-lying, that formed in an intracratonic basin between 1450 Ma and 1500 Ma.  It covers an area of 15,000 km², and reaches a total thickness of 950 m.  Recent drilling projects in the Nipigon Plate have provided a unique opportunity to study the basin using data obtained from a combination of 25 drill holes and surface outcrops.  This has allowed a re-examination of the lithostratigraphy, as well as providing insight into the basin architecture.  Previously, the Sibley Group consisted of only three formations: the Pass Lake Formation, the Rossport Formation, and the Kama Hill Formation (Cheadle, 1986a).  Two additional formations have been introduced here: the Outan Island Formation and the Nipigon Bay Formation.  The Pass Lake Formation, interpreted as a fluvial-lacustrine system, is divided into the Loon Lake Member and the Fork Bay Member (Cheadle, 1986a).  The Rossport Formation is separated into the Channel Island Member, the Middlebrun Bay Member, and the Fire Hill Member (Cheadle, 1986a).  These consist of cyclic dolomite-siltstone layers, stromatolites and red mudstone, which represent a playa lake, sabkha, and mudflat environments.  The Kama Hill Formation is not subdivided, and is composed of purple shales and siltstones interpreted as subaerial mudflat deposits.  The Outan Island Formation has been divided into the deltaic Lyon Member and the fluvial Hele Member.  The Nipigon Bay Formation consists of cross-stratified sandstone beds, and is thought to denote an aeolian environment.

Geochemistry has been used to examine formation-scale trends in weathering chracteristics and provenance.  Samples were collected from drill holes spanning the entire thickness of the Sibley Group, and the concentrations of various elements were plotted against depth to appraise variations between Formations.

A paleomagnetic study was also conducted on the Sibley Group.  The first part involved comparing paleopoles from samples of the Pass Lake, Kama Hill, and Nipigon Bay Formations.  This revealed a probable depositional paleopole age between 1450 Ma and 1500 Ma, with remagnetization events at approximately 1350 Ma and 1100 Ma.  The second part involved a study of paleo-secular variation in the Rossport Formation, which resulted in the documentation of one of the oldest known examples of paleo-secular variation yet discovered.

This study concentrates on a section through a lateral correlative of the cyclic facies contained in the Rossport Formation present in the Noranda drill core NI-92-5.  The location of this drill hole is north of other cored sections and outcrops of the cyclic facies and represents a more basin center environment.  The cyclic facies to the south consists of alternating layers of dolomite and red shale with individual layers, in the approximately 40 m thick assemblage, varying from mm- to dm-scale.  In drill hole NI-92-5 the layering is at a similar scale, but is composed of alternations between what was first thought to be dolomite-rich and gypsum-rich intervals, reflecting wet and dry seasonality in the central playa environment.

S.E.M. and X.R.D. analysis indicate that the sequence was metamorphosed.  The following minerals represent the progression of metamorphic facies, from lower to higher T towards the diabase sill:  talc, tremolite, and pargasite.  Clinochlore and calcite are found throughout the metamorphic series.  The metamorphism masks the primary layering in places.  However, S.E.M. analysis clearly shows the mineralogical layering, reflecting variations in primary geochemical constituents between individual laminae.  ICP-AES analysis also highlights the varying compositions of the sequence.  The pargasite tends to exist in layers with differing K and Na proportions.  The K⁺ and Na⁺ ions may reflect the incorporation of KCl and NaCl.  Both of the minerals are found in S.E.M. sections taken near the middle of the sequence.  Nb and Ba show unusual concentrations and fluctuations, and need to be studied further.

Clastic input is variable between individual layers, but shows an increase at the top of the section.  This increase is related to the appearance of sand sheets, which typically mark the end of the cyclic facies in the Sibley Group sediments.  Clastic material becomes slightly enriched in elements denoting a mafic rock source up-section.

New exposure of the Paleoproterozoic Animikie Group at the Terry Fox Monument northeast of Thunder Bay reveals several thrust faults that cut through the Gunflint Formation, Sudbury Impact layer and the overlying Rove Formation.  This discover has implications for the timing of thrust-fault deformation in the area north of Lake Superior.  Previously, thrust-fault deformation in the Gunflint Formation was interpreted to be associated with the Penokean orogeny.  New evidence collected at the Terry Fox Monument suggests thrust faulting may be related to late Penokean deformation or possibly a younger orogeny.  Thrust faulting at this location is observed cutting the impact layer and overlying Rove Formation meaning that this deformation may be too young to be attributed to the Penokean orogeny.

The thrust faults at the Terry Fox Monument rock cut are mainly expressed as small discrete bedding plane faults with few kinematic indicators or piercing points to quantify the displacement.  The faults are most easily identified where they cut through stratigraphy, once identified faults can be traced laterally along the outcrop.  Similar to other units present, thrust faults within the impact layer are identified by slickenlines or slickenfibres on fault plane surfaces.  The faults can be traced along strike until they are either covered by overburden or cut by a prominent normal fault which displaces all units in the hangingwall down to the south several meters.

The carbonate fraction of recent of recent sediments from the northern Lake Superior basin was analysed as to mineralogy and trace element distributions.

Three carbonate phases; low-magnesium calcite, protodolomite and dolomite are present.  Detrital minerals include quartz, alkali and plagioclase feldspars, illite, vermiculite and chlorite.

The trace elements analysed are strontium, manganese and barium.  The distribution of these elements appears to be controlled by the carbonate mineralogy.  The strontium is concentrated in the calcite while the barium is found preferentially in the dolomite.

The study area includes Hazelwood Lake and vicinity and is situated north of Thunder Bay, Ontario.  Metasedimentary rocks dominate and form part of the Shebandowan-Wawa volcanic-plutonic belt.  They form sequences of interbedded greywackes and mudstones and exhibit many of the features characteristic of turbidites.  The framework minerals, in order of abundance, are quartz, feldspar and rock fragments.  The rock fragments are mainly felsic volcanics with lesser amounts of plutonic rock fragments.

The Bouma sequence of internal sedimentary structures present in the ideal turbidite bed occurs, in whole or part, throughout the area.  It appears, that in the area, the rocks are folded in a series of gently plunging folds with steeply dipping axial surfaces.  To some extent, the evidence suggests multiple episodes of folding to have occurred.

The majority of the turbidites are characteristic of distal deposition.  The presence of proximal beds intermixed with distal beds is probably the result of contribution of detritus from multiple sources.  The rock fragments indicate a mixed felsic-volcanic and granitic provenance.

The Neys Diatreme is a sharply defined structure consisting of large, subround fragments set in a finely comminuted matrix of the same material.  It is located on the west side of the Coldwell Peninsula where it is seen to cut rocks of the Coldwell Alkaline Complex.  The diatreme itself is cut by later dykes.

Detailed mapping and sampling of the diatreme and the immediate host rock have been undertaken to determine the shape, extent and lithologic relationships of the structure.  Studies of the petrography, mineralogy and geochemistry of the clasts and dyke rocks have also been made.

The Neys Diatreme contains clasts of alkali feldspar syenite, layered olivine monzo-gabbro and nepheline syenites, all of which belong to the Coldwell alkaline complex.  Minor exotic clasts of undeformed porphyritic felsite and basic volcanics are also present.  In addition basic hornfels clasts are found which are believed to originate from the contact aureole of the Coldwell intrusion.

The shape of the diatreme suggests structural control of its emplacement.  Turbulent flow, the result of high velocities achieved by a gas-solid phase of a volatile-rich magma, is postulated as generating the diatreme breccia.  Block faulting is interpreted to have occurred in the Coldwell Complex.  This may be assumed to have initiated the intrusion of this type of structure.

The thesis area is located in the Keewatin District of the Northwest Territories on the east shore of Smiling Ghost Lake.  An approximate 1.2 km² area of metamorphosed Archean volcanic and sedimentary rocks of the Henik Group was studied in detail.  The rocks vary in metamorphic grade from greenschist to epidote-amphibolite facies.  Pillowed and massive basic flows are the major lithology in the thesis area and form the upper and lower volcanic units.  Medium grained porphyritic rocks of andesitic composition occur as irregular shaped bodies throughout the sequence. 

Volcaniclastic rocks vary from tuffaceous to breccia textures and are both oligomict and polymict.  These rocks interlayer the basic flows in the central portion of the thesis area.  Macroscopic and microscopic examination of the volcaniclastic breccias combined with geochemical whole rock analyses indicates derivation from a multicompositional source area.  Therefore it is postulated that the Smiling Ghost Lake area is a section of the epiclastic rocks represent reworked units.  Gravity sliding and slumping in a subaqueous environment are suggested as possible methods of deposition, and were likely similar to a laharic type deposition present in the cone-complex facies.

Strike-slip faults caused by compressive forces are present throughout the thesis area and have developed well defined shear zones, jointing and minor folds.  Shearing and metamorphism cause variable amounts of deformation and foliation in the rocks, which have been subjected to several periods of deformation.

Sulphide mineralization is present in a fracture filled shear zone found in basic volcanic rocks.  Chalcopyrite, sphalerite, pyrite and iron oxides are found with deformed and foliated quartz-sericitic rocks.  Cataclastic textures in both the ore minerals and the fracture filling material indicate that deformation along the shear zone was intense.  Likely the fracture filling itself was sheared and brecciated by later movements along the fault.

Early Precambrian metavolcanic and metasedimentary rocks of the Schreiber-White River greenstone terrain have been intruded by rocks of the Coldwell alkaline complex.  A thermal metamorphic aureole produced by the complex attains pyroxene hornfels facies in adjacent country rocks.

The earliest magmatism, center 1, is exposed at the western margin of the complex and consists of gabbro and ferroaugite syenite.  Younger, center 2, silica-undersaturated syenites intrude both the gabbro and ferroaugite syenite.

The gabbro is characterized by igneous layering and evidence of deformation, in the form of cataclasis and recrystallization of the feldspar.  Ferroaugite syenite intrudes the gabbro and contains inclusions of an ilmeno-magnetite bearing cumulate rock and country rock xenoliths.

The principal rock units are crosscut by a dyke suite exhibiting a complex intrusive history.  The most common dyke type is lamprophyre.

In a north-south traverse of the Quetico Subprovince (of the Superior Province, Canadian Shield) a metasedimentary sequence and a migmatitic sequence are exposed.  The metasedimentary sequence consists of slates and turbiditic greywackes.  Interbedded with these are calc-silicate rich horizons and nodules.  The metasediments are folded into a major F₁ antiform and synform which are probably downward facing on the schistosity.  In the northern part of the study area a schistose migmatitic sequence is exposed separated from the metasediments by intrusive rocks.

Grade of the syn and post-tectonic metamorphism increases from south to north.  Low grade (chlorite, biotite), middle grade (staurolite, cordierite) and high grade (sillimanite) zones are defined.  A somewhat higher pressure assemblage, including staurolite, is probably post dated by a lower pressure assemblage, including cordierite.

A zone of largely ductile dislocation metamorphism in a zone known as the Quetico Fault, post dates the regional metamorphism.