Immediately north of Thunder Bay, Ontairo, Archean rocks of the Western Superior structural province are exposed.  Near the small community of Lappe, in the present study area, well stratified metasedimentary rocks are in contact with metavolcanic rocks.

The metasedimentary rocks consist of greywackes, siltstones, mudstones and turbidites.  The metavolcanic rocks are predominantly massive flows of basic to intermediate composition.  Both major rock types have been affected by a low grade metamorphism.  Graded bedding and flame structures in the metasedimentary rock exposures are generally well preserved, but the directions of younging and bedding orientations of the metavolcanic sequences could not be determined.

A variably developed cleavage is present in exposures of both major rock types.  The cleavage is shown to be axial planar to dominant, sideways-closing folds in the region.  Reversals in structural facing directions indicate that the cleavage developed in previously deformed rocks.

Structural facing also indicates that the stratigraphy, as a whole, becomes younger to the southwest or northeast.  In parts of the study area, the metavolcanic rocks appar to stratigraphically underlie metasedimentary rocks, while in other parts of the area, the metavolcanic and metasedimentary rocks appear to be interlayered.  Elsewhere, the stratigraphic relationship is not clear.  In any case, it is quite possible that deformation events affected both metavolcanic and metasedimentary rocks.  Evidence of polyphase deformation indicates that the contact between the two major rock types is complex.

The Hele Sill is an elliptically shaped ultramafic intrusion emplaced within the sedimentary rocks of the Kama Hill Formation. It is recognized as being part of the earliest phase of magmatism associated with the onset of the 1.1 Ga Midcontinent rift. These early melts are picritic in composition and have rare earth element compositions that show enrichment in the light rare earth elements and fractionation through the heavy rare earth elements interpreted to indicate a primitive, mantle source.

The stratigraphy of the Hele Sill can be divided, using petrographic and geochemical characteristics, into five zones. The Upper Marginal Zone comprises vari-textured leucogabbro that contains fragments of partially digested hanging wall rocks, and potassic pegmatitic pods. The Upper Gabbroic Zone is a layered sequence of subophitic gabbro to cumulate melagabbronorite, and displays a trend of Ca and Mg enrichment and Na and Si depletion towards its base. A rapid transition to lherzolites and wehrlites of the Ultramafic Zone is due to the crystallization and accumulation of cumulus olivine and pyroxenes, which show a strong cumulate texture. Megacrystic clinopyroxene, orthopyroxene and plagioclase poikilitically enclose cumulate minerals indicating residual liquids diffused efficiently. The Lower Gabbroic Zone and Lower Marginal Zone form a 10 - 20m marginal reversal, with rock compositions becoming more felsic towards the lower chill margin. Rock types change from melagabbronorite to gabbro, and show the strongest negative Nb anomalies found in the intrusion.

The rocks of the Hele Sill are interpreted to have formed from an ultramafic magma that underwent fractionation and accumulated mafic mineral phases. The effects of contamination from the Kama Hill Formation are strongest in the Lower Marginal Zone, which show increases in alkalis and incompatible elements. The bulk composition of the Hele Sill is not significantly affected by this local contamination. However, an earlier, more pervasive contamination may have occurred as the parent magma staged within the basement rocks of the Quetico Subprovince as indicated by the overall negative Nb anomaly not expected from a primitive mantle source rock. This trend is observed in the middle section of the nearby Osler Group volcanic rocks. It is suggested that the Hele Sill is coeval with this stage of volcanism and represents the intrusive equivalent of the Middle Osler Group Volcanics.

Steve is currently working as a geologist for HTX Minerals, in Sudbury

For more information about this thesis contact Dr Pete Hollings

The "Agglomerate Unit" at the Pamour One Mine in Timmins, Ontario has been the source of some confusion since its discover.  The unit appears to be composed entirely of volcanic fragments, but its mode of deposition has never been adequately identified.  When the unit was first identified, it was designated the "Agglomerate Unit", as it closely resembled an eruptive, agglomeratic unit under macroscopic examination.  Several examinations of the unit over the last three decades have suggested that the unit was probably a sedimentary rock formed from fragmented volcanic materials.  However, the unit is relatively economically unimportant at the Pamour One Mine, and little work was devoted to the resolution of this problem.  The unit is not regionally extensive, only having been identified at the Pamour One Mine and the Hallnor Mine which lies to the west, adjacent to the Pamour One Mine.  At the Pamour One Mine the unit occupies paleotopographic lows in the surface of unconformity above the Keewatin volcanic rocks.  The unit is conformably overlain by a greywacke turbidite sequence.  Mineralogical and textural examinations of the unit suggest that the unit is derived from the Keewatin volcanics.  The lack of welding within the matrix material indicates that this was not a hot deposit.  The unit displays sedimentary bedding, though this is often hard to discern, and this bedding is conformable with the bedding of the greywacke turbidites.  Macroscopic and microscopic examinations of the clasts and matrix materials indicate that the unit is in fact an immature sediment, which has undergone little transport.  The presence of isolated beds of greywacke within the unit indicate that this unit was deposited in a transitional sedimentary environment and that the unit was deposited continuously over time and not in one rapid pulse.  There is no evidence of localized unit thickening or increases in the density of larger clasts to suggest the presence of a central vent for an agglomeratic eruption.  Indeed the unit is relatively homogeneous under large scale observation.  Consideration of this evidence indicates that the unit is not an agglomerate but that it is an immature basal conglomerate belonging to the Timiskaming sediments.  The unit was not formed as part of a submarine fan sequence as were the sedimentary rocks that lie directly above it.  For these reasons, the unit should be elevated to formation status and renamed the basal volcanic conglomerate formation.

Metallogeny and Soil Geochemistry of the Lakehead Gold Mines Property, Thunder Bay, Ontario

Timothy J. Twomey - April 1983

Exploration for gold deposits utilizing applied geochemistry is becoming more popular in glaciated Archean terranes at the present time.  This has brought about the need for a greater awareness and understanding of the dispersal of gold in the environment in relation to its source rock.  In light of this, the Lakehead Gold Mines property is studied, which is located just north of Thunder Bay, Ontario.  The gold response in the altered wallrock, soil and humus of the study area is related to the mineralized quartz vein.  The accumulation of gold in the soil horizons, due to the mineralization beneath them, is controlled by the tenor and dimensions of the deposit, by subsequent glaciation, and subsequent to that by processes of drainage, soil formation and contamination from trenching.  Thus planning, implementing and interpreting an exploration survey is based on many parameters.  All of these parameters must be investigated in an orientation study in order for the survey to be effective.  The soil geochemistry technique used in the study area is considered to be effective in locating auriferous quartz veins beneath a meter of till.  Finally, the thesis briefly discusses the present theories on the origin of gold deposits as related to the study area.  This has importance on how exploration geochemistry surveys can be used and suggests a specific model for further exploration in the area.

The Archean ultramafic flows found in the Henik Group, Griffin Lake Area, Northwest Territories, are texturally and chemically similar to peridotitic komatiites found in Munro Township, Ontario, and in the Barberton Mountain region, South Africa.  The Henik Group flows, however, show no development of cumulate zones at the bases of the spinifex textured flows.

A small area (10.000 m²) was mapped in detail, which permitted the accurate positioning of sample locations, as well as maintaining control of the textural zone of a flow from which the sample was taken.  Each flow displays a textural zonation fro a polygonal jointed quenched flow top, grading into increasingly coarser bladed spinifex in the middle of the flow, then rapidly decreasing in grain size, terminating in a fine-grained brecciated base.

A correlation exists between the texture and the geochemistry, as demonstrated in a single flow which was studied in detail.  The preservation of the spinifex textures and the relatively low degree of serpentinization in this flow suggest that chemical zonation is the result of a compositional difference in the original crystalline material.  The enrichment of nickel and magnesium at the top and the base of the flow is considered due to the partitioning of these elements into the earliest crystallizing phases.  This partitioning results in the enrichment of silica, alumina, titanium, and iron in the residual phases in the middle portions of the flow.

This report will describe the geology and briefly comment on the economic potential of a feldspar pegmatite located in the north central Goldie Township, approximately 33 miles by air northwest of Thunder Bay, Ontario.  Suggestions will be made as to how to evaluate the full potential of the occurrence.  The pegmatite forms the crest of a northerly trending ridge located between the Trans-Canada Pipeline and the Canadian Pacific Railway tracks.  This ridge is located approximately one mile northwest of Buda, a siding on the CPR.  The dike intrudes quartz-biotite schists, hornblendite, and a hornblendite-granite agmatite.  The pegmatite attains a maximum apparent width of 85 feet where it crosses the hornblendite/quartz-biotite schist contact.  The dike is exposed for a strike length of over 1200 feet.  Where seen in section, the dike exhibits a dip of 45° to 50° south.  Microcline feldspar of economic grade and undermined tonnage exist in the Buda pegmatite.  From the chemical analysis, the dike has some potential for rare metals, namely rubidium, yttrium and cesium.  Mineralogically the dike consists of 80-90 percent feldspar, 5-8 percent quartz, 1 percent muscovite, and garnet, apatite, pyrite and all other accessory minerals make up the balance.  Commercial development of this dike will depend on the viability of the larger pegmatite found by Steep Rock Iron Mines approximately 0.75 miles to the west of the Buda dike.

The Quetico Fault is a significant regional structure forming part of the Archean terrane in Northwestern Ontario.  The fault transects medium to high grade gneisses of variable origin for much of its strike length.  The absence of easily identifiable stratigraphic marker horizons makes it difficult to readily determine the net slip or the sense of motion or the fault.  Microscopic examination of samples taken from the fault zone show a number of  structures and fabrics which permit the determination of movement sense locally.  Based on the geometry of porphyroclast systems and shear banding a dextral sense of motion is indicated for the fault zone at this locality.

The Little Pig Vein, which is thought to be associated with the West Beaver Mine due to its proximity, occurs within the northeast trending Mainland Belt of silver mines.  These deposits are associated with a zone of steeply dipping normal faults within the Southern Province.  These veined faults cut Archean basement rocks, Aphebian metasedimentary and Keweenawan instrusive rocks.

Hand samples and structural data were obtained from the Little Pig Vein and surrounding outcrop.  The sampling was tied into a cut grid.  Investigative methods included hand sample study and petrographic microscopy.

The vein is located within a fault-breccia zone in the Rove argillite.  The vein has a strike of 75 degrees and a dip of 70 degrees to the south.  The argillite is flat-lying with a gentle southerly dip of three to five degrees.  The vein does not enter the diabase.

Within the Little Pig Vein, there appear to have been two pulses of mineralization.  In these open-space filled veins, variable amounts of gangue and sulphide minerals are seen to be present.  The sulphides which are noted in the Little Pig Vein are sphalerite, galena, pyrite and chalcopyrite, in their respective order of abundance.  The gangue minerals which constitute a major portion of the vein are calcite, quartz and fluorite.  No silver was viewed in the samples collected for study.

Migration of metals in the Rove argillite was initiated by cooling diabase dikes and sills.  Faulting formed structural traps for the mineral-bearing solution moving along bedding planes to areas of lower pressures.  Precipitation in the vein proceeded according to sequential deposition.

No silver mineralization was found in the Little Pig Vein.  The silver mineralization may be lacking due to the absence of the mineralization pulse containing the silver metals.  The structural data note the lack of the phase of faulting during which silver mineralization has been noted to occur.  No opening occurred, thus no mineralization was found.  This later stage pulse of silver mineralization has not occurred in the Little Pig Vein.

The Finmark metasedimentary belt is situated west of Thunder Bay, Ontario.  Metasedimentary rocks predominate and form part of the Shebandowan-Wawa greenstone belt where they overlie a Keewatin succession of metabasalts.  The metasediments form interlayered sandstone-siltstone-mudstone sequences which alternate with thick deposits of cross-stratified sandstones.  The interlayered sequences contain many of the primary sedimentary structures characteristic of tidal flat deposits.

The rocks in the area have undergone very low grade greenschist metamorphism and are structurally complicated.  Reversals of top directions and the presence of both easterly and westerly plunging minor folds, suggest that one or more episodes of folding have occurred.  The attitude of cleavage within the area does not vary considerably, indicating that it may have developed after one or more episodes of folding.  Numerous left lateral displacements and the intrusion of mafic dikes have contributed to the structural deformation in the area.  One continuous section of outcrops have remained stratigraphically upright and were used to determine the depositional environment of the metasediments.

The sedimentary rocks are composed of feldspar, rock fragments, quartz, and some mafic minerals.  Modal analysis has revealed that most of the sandstones in the area are arkosic arenites.  The rock fragments are felsic to intermediate and predominantly volcanic, with lesser amounts of igneous and sedimentary rock fragments.  The volcanic clasts have been derived from the Lake Shebandowan metavolcanics which are part of a calc-alkaline suite of rocks.  Therefore, the metasediments probably represent immature detritus from proximal volcanic centers.

This study examined metavolcanic and metasedimentary strata from the about 3.0 Ga Balmer assemblage of the Birch-Uchi greenstone belt, Western Superior province.  ICP-AES wholerock geochemical analyses coupled with field and petrographic observations were used to better define the tectonic setting in which these rocks developed.  The geochemistry of 21 of 34 samples was taken to be representative of geochemical variation amongst the volcanic rocks of the study area and showed them to be comprised of two distinct suites.  The first suite is characterized by mafic volcanic rocks of basaltic to andesitic composition and was found to be tholeiitic in character.  The second suite is characterized by intermediate to felsic volcanic rocks of andesitic to rhyolitic and rhyodacitic composition and is calc-alkaline in character.  The geochemistry of these samples seems consistent with an arc-like setting.  Sedimentary rocks crop out poorly in the study area but may represent a submarine volcaniclastic apron or submarine fan/ramp feeding into an arc-related basin. The geochemistry of 16 samples of sedimentary rocks were analyzed in order to study their provenance.  Results from these analyses suggest that the Balmer assemblage sedimentary rocks are derived from the Balmer assemblage volcanic rocks and primarily the intermediate to felsic calc-alkaline suite.  This is consistent with the Balmer assemblage sedimentary rocks being deposited in an arc-related basin.  Suggestions made here should be tested through geochronological study of detrital zircons in the sedimentary rocks and high precision geochemical techniques with a more complete suite of trace elements.