Mainland Belt silver deposits of the Thunder Bay District are hosted in steeply dipping normal faults that cut Aphebian Rove Shale and Neohelikian Logan diabase sills.  The Mainland Belt is located in the southern province of the Canadian Shield.

This study examines the Creswel silver deposits which occur in and around a group of diabase sills 1.7 km west of Rabbit Mountain.  The term, Creswel silver deposits includes the following mines and prospects:  the Badger Mine, Porcupine Mine, Porcupine Jr. Mine, Keystone Veins, Eschweiller Veins, Animike Mines Vein 22 and the West Beaver Mining Property (Little Pig Vein and Vein No. 2).

The veins usually have a common mineralogy consisting of quartz, calcite, fluorite, and variable amounts of sphalerite, galena, pyrite, chalcopyrite, argentite and silver.  Silver bearing veins show evidence of secondary fracture, which crosscuts pre-existing vein development.  Silver bearing vein material is emplaced after secondary fracture and is richer in sulphides than pre-existing vein material.  Furthermore, calcite after secondary fracturing is coarser than calcite of the primary vein development.  Secondary pulses of vein material may also be more voluminous and calcite rich than the pre-existing vein.

Four controls may have been responsible for the development of silver bearing veins, these are:

1. A geochemical control, in that silver veins are commonly associated with lower argillite member of Rove Shale.
2. A structural control apparently exists such that secondary fracturing and a subsequent secondary pulse of vein material is necessary for the development of silver bearing veins.
3. Silver may be deposited at the intersection of vein systems.  This theory is somewhat tenuous and more work must be done to prove or refute this supposition.
4. A second sill is known to occur beneath some of the Creswel silver deposits.  The importance of the sill is, as yet, uncertain but it may have acted as a permeability barrier effectively trapping fluids between the two sills.  Furthermore, the added heat and concentrated circulation may have enhanced the "scavenging " ability of the fluids thus producing silver bearing veins.

At any rate, the common theory of genesis states that, heat from the intrusive diabase sills stimulated water circulation in the metal rich lower argillite unit of the Rove Shale.  Soon after diabase had crystallized a faulting event occurred and structural dilatent zones opened and vein material was deposited.

Adrian Simunovic - April 1985

The study area is located approximately 90 km north of Thunder Bay on Highway 527 which leads to Armstrong, Ontario.

Four outcrops of a polymict conglomerate were examined in detail.  The four exposures are all part of the same stratigraphic unit which forms a part of the Archean bedrock in this area.

R_f / ∅ analysis was used to determine an approximation of strain.  Granitic and volcanic clasts were treated separately.

Results show that due to competency contrasts the volcanic clasts reflect greater strain than the granite clasts. 

The harmonic mean of final pebble shape (R_f) was used as an approximation of R_s (strain ellipse shape) after Lisle (1977).

The averaged strain ellipsoid for the granite clasts and the volcanic clasts both, approximated a uniaxial pancake type ellipsoid (K = 0).

The Haughton impact structure is a 23 Ma old unaltered meteorite crater, located in the Paleozoic sediments of Devon Island in the Canadian Arctic Archepelago.  Fluid inclusions found within shock metamorphosed sandstone fragments were microthermometrically investigated.  The Onaping Formation of the 1850 Ma old Sudbury Structure is breccia zone that is of controversial origin.  The fluid inclusions found within these rocks were also analyzed and compared to the Haughton study and other previous studies.

Fluid inclusions found in shocked Haughton sandstones exhibit a wide range of eutectic and homogenization temperatures.  Eutectic temperatures are found to be extremely variable, suggesting the presence of complex, low- to high-salinity, aqueous solutions.  CO₂ was positively identified to be present in only one relict inclusion.

Fluid inclusions found within the rocks of the Onaping Formation also have variable eutectic and homogenization temperatures.  Many of the inclusions from the Onaping Formation decrepitated prior to homogenization.  The inclusions found within the matrix of the Onaping Formation are generally of low salinity and are found to contain some metastable ice phases.  Fluid inclusions found within the xenoliths are of highly variable compositions and often contain CO₂ phases and saturated salt solutions.

The presence of low salinity inclusions and metastable ice from the matrix of the Onaping Formation, is alalogous to the fluid inclusions studied in the melted rocks of the Haughton Structure.  The wide range of homogenization and eutectic temperatures, recorded from inclusions in shocked xenoliths of the Sudbury Structure, is also consistent with the measurements on shocked, unmelted rocks from Haughton and other meteorite impact sites.  This study supports an impact origin of the Onaping Formation rocks and thus the Sudbury Structure.

A paleoclimatic study of the laminated dolomite and shale sequence in the Rossport Formation of the Sibley Group in Northwestern Ontario was executed to determine the environment of deposition and the controlling climatic conditions.  The seventeen metre thick sequence was logged using a petrographic microscope and the colours of the layers were recorded on a scale of one to seven based on the fact that the higher the dolomite content, the lighter the sediment will appear.  Colour index numbers seven to two represents the laminae with the least and most dolomite respectively according to carbon-hydrogen-nitrogen (CHN) analyses.  The laminae do not alternate from pure shale to pure carbonate, but rather though a whole spectrum of compositions with shale and carbonate as the end members.  It is assumed that one cycle represents on year of deposition.  One cycle is defined in this study as one period on a sinusoidal-like line plot of the colour index data, not as two adjacent laminae.

This sequence was deposited in a playa and lacustrine complex.  The carbonate minerals formed as a chemical precipitate within the alkaline and saline ephemeral lake.  Periodic rain events washed the clastic sediment off of the subaerially exposed playa into the lake.  The fresh water would transport the coarse-grained quartz and feldspar fragments, lower the lake water salinity and supply calcium ions, and renew the nutrient supply in the water column producing an algal bloom.  Thus, carbonate mineral deposition could be the result of a combination of eutrophication-triggered precipitation and the mixing waters with different compositions that formed the cycles seen in the rock record today.

The climatic cycles represented by the sediment are arranged in at least a four-order hierarchical structure.  Annual deposition is transposed onto El Niño events.   El Niño events are identified within the eleven-year sunspot hemicycle.  The twenty-two year sunspot cycle is host to variations caused by annual, El Niño and the eleven-year sunspot hemicycle.  The first three cyclic events are also transposed onto a thirty-year cycle of unknown origin.  Additionally equinox cycles are documented.  Alternations between long hot summers and short cold winters to short warm summers and long mild winters are represented in the plotted data as areas with extreme variation in colour index (or dolomite content) and areas with slight variations in carbonate content respectively.

The Ash Bay Gneiss Dome is located in the most northern part of Rainy Lake.  The Quetico Fault and the Seine River-Rainy Lake Faults are located to the south of the dome.  Rainy Lake is wedged between the Wabigoon Subprovince to the north and the Quetico Subprovince to the south, all being apart of the Superior Province.  The mineralogy indicates that the rocks of the dome underwent amphibolite facies metamorphism at approximately 500°C and was emplaced during solid-state metamorphic flow by a diapiric inflation.  The indicating minerals found in samples RL-01 - Rl-29 are mostly quartz and oligoclase, but sillimanite, epidote, and hornblende are also found within the samples in smaller amounts.  There were no kinematic indicators located in the field, so a closer examination of the microstructures was necessary to understand the extent of the deformation of the crystals and their deformation mechanisms.  In all, 29 oriented samples were studied that were taken from the eastern side of the Ash Bay dome by G. Borradaile and D. Gauthier.  These samples were thin sectioned and quartz c-axis petrofabric studies were performed on them by the use of the universal stage.  The petrofabric reveals a northeast-southwest orientation of the quartz crystals c-axes maximum compressive direction of the stress on the rocks, which is interpreted to be in a northwest-southeast direction.  Anisotropy of magnetic susceptibility was also performed on the 29 oriented samples and was later compared with the petrofabrics.  It was found that the quartz c-axis petrofabrics were strongly clustered and dipping at higher angles than the magnetic fabrics.  This showed that the quartz fabrics were aligned before the magnetic fabrics and therefore record a longer period of shear.  The magnetic fabrics could not have formed until the later stages of the domes inflation to result in nearly horizontal magnetic fabrics.

Amy is currently working as an exploration geologist
 
For more details about this thesis contact Dr. Graham Borradaile

The Big Duck Lake metavolcanic belt is an east - west trending, steeply north-dipping sequence of mafic and felsic intrusive and extrusive rocks.  Field mapping has revealed two distinct porphyritic felsic rock types.  They are pyroclastic porphyritic tuffaceous rocks and quartz feldspar porphyritic intrusive rocks.  The intrusive felsic rocks in the Big Duck Lake area occur as sill-like bodies and are generally massive except for foliated contact zones.  The extrusive units are comprised of laterally extensive fragment and crystal-rich beds, which have been selectively altered by hydrothermal solutions.  Field relationships, petrography and chemistry were used to compare the two felsic porphyries.  These studies indicate that the extrusive and intrusive phases are distinct.  On the microscopic scale the intrusive rocks exhibit primary crystallization textures displayed by gradational quartz and feldspar phenocryst reduction into the fine-grained groundmass.  The extrusive rocks display distinctive bimodal phenocryst and groundmass phases and are dominated by secondary metamorphic textures.  Whole rock and trace element analysis indicate that the intrusive rocks have more Sr and Al and less Zr than the extrusive rocks.  Zinc is preferentially concentrated in the tuffaceous rocks while higher values of Cu and Au are associated with the main intrusive sill.

However, similar concentrations of Pb, Ni, Co, Cr, Ti and P imply that both units are derived from the same parent magma.  A model based on intracontinental rifting is presented to show a possible origin for the bimodal volcanic activity.  This model also proposes that the synvolcanic intrusives are feeders for the extrusive felsic crystal tuffs.

A three to four meter thick limestone assemblage (Ardness Formation) contained within a thick redbed succession (Lismore Formation) was deposited in a semiarid climatic setting during an expansion of the Windsorian Sea (Late Vis©an).  Schenk's (1969) generalized facies model for the Windsor Group described eight depositional environments.  However, alluvial fans associated with an uplifted horst, mudcracks and authigenic sulphate minerals found in the salt flat, and algal mats of the intertidal zone are all absent in the Ardness and proximal Lismore Formations.  Shallow marine depositional systems controlled sedimentation in the offshore but a fluvial depositional system dominated the nearshore and terrestrial environments.  From the offshore landward the depositional environments in the Ardness Formation are:  1) oolitic shoal;  2) lagoon (subtidal);  3) nearshore; and  4) fluvial floodplains. 

A network of small meandering channels, represented by channel, point bar, and floodplain lithofacies, transported by siliclastics through the fluvial floodplain environment.  These high-sinuosity channels emptied into a restricted brackish lagoonal system dominated by siliciclastics on the landward side and carbonates on the seaward side.  During storm events ooids and shell debris were washed into the lagoon from the offshore.  An ooid shoal complex marked maximum transgression into the basin.  Decreasing water depth led to the stabilization of the oolitic shoals and to the development of minor amounts of evaporate minerals and algal stromatolites.  Fluvial channels cut into the top of the carbonate assemblage and mark progradation of the fluvial depositional system into the area.

Numerous gold bearing quartz vein systems are found in oxide facies banded iron formations of the Beardmore-Geraldton area.  They exhibit a spatial relationship with zones of dilated cleavage and fracture networks that transect the limbs and hinges of asymmetric folds.  Many of the gold deposits in the region are located in close proximity to east-west trending fault zones that may have acted as plumbing systems for the auriferous hydrothermal fluids.

In this thesis the geological setting of the gold mineralization is discussed with particular emphasis on mineralogy, alteration and structural control of the vein systems.  Several gold bearing veins were studied from a select number of mine sites and gold occurrences to obtain information on the mineralization processes.

The mineralogy of the quartz veins and spatially associated sulphidation and carbonatization alteration zones were examined in detail.  The results suggest the precipitation of gold from hydrothermal solutions was concomitant with the replacement of the iron formations by pyrite and arsenopyrite.

Gold may have been transported as reduced sulphur complexes in CO₂ rich fluids of low salinity derived by the metamorphic devolitization of a greenstone terrain at the transition between greenschist and amphibolite facies.  The emplacement of the veins is predominantly a late stage event although evidence suggests some veining may have occurred concomitant with a regional episode of deformation.

The iron formations in the study area were important as both chemical and structural traps for gold precipitation.  The competency of the units allowed for ductile and brittle deformation providing low pressure zones for the hydrothermal fluids.  They also supplied abundant Fe for sulphidation processes initiating the reduction of gold in solution to elemental gold following the reaction:

  2Fe₃O₄    +    6Au(HS)₂- +  6H +         6FeS_{2 +} 8H₂O  +  6Au
(magnetite)     (bisulphide gold complex)      (pyrite)           (gold)

Ivanhoe Lake is located 220km east of Wawa, Ontario, Canada. The Ivanhoe Lake structure strikes northeast to southwest and is the boundary between the high grade metamorphism of the Kapuskasing structural zone and the low grade metamorphism of the Abitibi subprovince. This difference in grade of metamorphism is what makes the Ivanhoe Lake structure so interesting to study. Through analysis of the microstructures seen in thin section a long deformation history was revealed. Ductile deformation, characteristic of deep burial, as well as deformation at or near the brittle-ductile transition zone were evident. This affects a wide area that extends to either side of the structure. This was followed by brittle fracturing at a shallower depth of burial. The brittle deformation is confined to a narrow zone found along Ivanhoe Lake. Examples of ductile deformation within the thin sections include: undulatory extinction in quartz, subgrains in quartz and feldspars, deformation twins in plagioclase and microcline, as well as irregular grain boundaries for all grains. The brittle-ductile transition zone is documented by the existence of pseudotachylites. These glassy rocks are characteristic of large faults. The brittle history is documented by quartz or calcite filled fractures found at both the outcrop and microscopic scale. These brittle structures always crosscut the ductile features. This crosscutting relationship suggests that the structure has had a long history which began with ductile deformation and progressed into brittle deformation.

The Zenith zinc deposit, near Schreiber, Ontario, is a small massive sulphide deposit yielding sphalerite, chalcopyrite, and pyrrhotite.  It is hosted by a metadiorite sill.  The country rocks are metasediments and metapyroclastic rocks in the foot wall in the western part of the map area, while a mafic metavolcanic unit forms the hanging wall in the northeast part of the area.  The entire series is bounded on the west and south by granite.  Metamorphic grades vary from high-grade (Winkler, 1974), indicated by migmatites at the contacts with the granite, to amphibolite and greenschist facies (medium and low-grade) in the country rocks.

The main sulphide ore, massive sphalerite, occurs in fracture fillings which form the matrix of a zone of fault breccia.  The mechanism of hydraulic fracture is proposed to explain the localization of the sulphide ore.  These sulphides are postulated to have been remobilized from sulphide rich xenoliths contained in the sill which originated as metasedimentary-metapyroclastic foot wall rocks.