The Lac de Gras Kimberlite field is located approximately 295 kilometers north of the town of Yellowknife, within the Slave structural province.  Since the discovery of the first kimberlite pipe in 1991, nearly two hundred kimberlite pipes have been identified in the Lac de Gras region, more than fifty of which are diamond-bearing.  The kimberlites studied thus far have been of two broad textural types, hypabyssal and volcaniclastic (pyroclastic).  Three kimberlites of each textural type were selected for study, and the spinel grains within each kimberlite studied.  Roughly eighty spinel grains were analyzed, producing approximately four hundred analyses.

From these analyses, it was found that the hypabyssal kimberlite spinels followed essentially two evolutionary trends.  The majority of the spinel grains analyzed followed the magnesian ulvöspinel magmatic trend, the characteristic magmatic trend observed in archetypal kimberlites.  This trend consisted of euhedral titanian magnesium aluminous chromite (TIMAC) cores mantled by magnesian ulvöspinel - ulvöspinel magnetite series (MUM) compositions.  When plotted on a reduced spinel prism, the trend begins near the MgCr₂O₄-FeCr₂O₄ join and heads towards the rear rectangular face and up towards the Mg₂TiO₄-Fe₂TiO₄ apex.  The secondary evolutionary trend observed was the pleonaste reaction trend.  This consisted of either: euhedral pleonaste cores mantled by TIMAC or MUM compositions, or more commonly, TIMAC cores, mantled by pleonaste compositions followed by an iron-rich rim.  In terms of morphology, the hypabyssal kimberlites were dominated by atoll grains rimmed by a thin magnetite layer.  In addition, the atoll grains appear to have preferentially nucleated about phlogopite phenocrysts.  In the pyroclastic kimberlites, the spinel grains displayed little or no spinel evolution in terms of core and mantle textures.  Rather, the magnesian ulvöspinel trend was preserved in two distinct grain populations, unlike the hypabyssal units, which exhibited core and mantle textures.  In terms of morphology, these units were dominated by corroded atoll grains of TIMAC compositions, with subeuhedral to euhedral groundmass spinel grains of MUM compositions forming the rest of the population.  

The Hadley Prospect is a massive Zn-Pb mineral occurrence situated in the Savant Lake area of northwestern Ontario.  This occurrence displays many of the characteristics typical of volcanogenic massive sulphide deposits found in the Canadian Shield.  The thesis area is found in a felsic volcanic pile of bedded tuffs intercalated with fine to medium grained flows and lapilli/tuff breccia units with localized metasedimentary and quartz porphoritic intrusive units.

Geochemical sampling was implemented in an attempt to locate an alteration zone in the footwall rocks.  A total of 16 samples were taken for analysis by atomic absorption techniques for the elements Na, Ca, Mg, K, total Fe, Co, Cr, Cu, Zn, Mn, and Ni.  The survey failed to intersect an alteration zone and showed the hanging wall and footwall units to be essentially the same.

Textural analysis of samples taken from the thesis area revealed a distinct systematic sequence of metamorphic and cataclastic events.  Recrystallization, cataclastic, and replacement textures indicate a major metamorphic event accompanied by a shearing event with a possible later minor thermal event have acted on the occurrence since deposition.  The shearing event is believed to be responsible for the tectonic transport of the alteration zone to the western extension of the occurrence thus removing it from the vicinity of the geochemical sampling.  The alteration zone is contained in a pyroxene-amphibole-olivine breccia unit at the western extremity of the occurrence.

The major metamorphic period is a skarn-forming type metamorphism which has produced a contact areole over the regional metamorphism produced during the Kenoran Orogeny.

A comparison of the occurrence to the model of Sangster (1972) for fumaroltic volcanogenic massive sulphides revealed an excellent correlation and indicated a similar origin for the Hadley Prospect.

Forty eight rock powder samples from Freemans Cove volcanic suite were analysed for cesium using neutron activation analysis.  The samples analysed vary in composition and may fit into the following three series:

    i)  A sodic nephelinite-basanite (phonolite) series

    ii) A mildly potassic nepheline-basanite (phonolite) series

   iii) A mildy alkakine (sodic transitional) basalt suite.

The cesium was radio-chemically separated using an inorganic cation exchanger, Ammonium 12 moybdophosphate.  The gamma ray spectrometry was carried out using a Norland 5400 Ino Tech Multichannel Analyser.

The cesium concentration ranged from a low of 0.03 ppm, in a basalt to a high of 5.87 ppm, in a basanite or nephelinite.  The K/Cs ratio ranged from 3900 for a basalt to 702500 for a Ne bearing trachytoid of phonolitoid.  The majority of the samples have K/Cs ratios between 14,500 and 40,000.  The Rb/Cs ratio ranges between 8.6 for a basalt to 3747 for a Ne bearing trachytoid or phonolitoid.  The majority of points lye between Rb/Cs ratios of 25 to 100.

Graphs of Rb vs Cs and K vs Cs show trends that are very similar.  These trends show a loss of cesium without the loss of either rubidium or potassium.  These trends may indicate a degassing of cesium due to it's volatile nature, but these trends are found only in two rock groups, basalts and Ne bearing trachytoids and phonolitoids.

The graph of K vs Rb shows a linear trend indicating not only comagmatism, but also a coherence of potassium and rubidium through magma evolution.

AMS (anisotropy of magnetic susceptibility) is a useful tool for identifying petrofabrics, especially when the petrofabrics may be cryptic as in the case of the limestones of the present study.  Although a conventional penetrative tectonic fabric did not overprint the sedimentary fabric in the Polis limestones, the sedimentary fabrics were modified by the stress fields of fault block rotations.  The principal extension directions are recognized from the imposed alignments of certain paramagnetic minerals (micas and clay) and by ferromagnetic magnetite.  By comparing AMS data with structural evidence and seismic data (in the form of Centroid-Moment Tensors) it is possible to compare paleotectonics with neotectonic to prognosticate surface expression of the current tectonic regime.  The paleotectonic regime was expressed as ENE-WSW extension as indicated by AMS and fault orientations and movements; but the current tectonics exhibit ENE-WSW compression as inferred from fault-plane solutions.  This neotectonic regime has expressed itself surficially as a dextral strike-slip motion along one of the major extension faults of the former regime.  The data is also useful to compare with models of development of the tectonic regime of western Cyprus, which displays non-coaxial progressive strain history as a result of the Eratosthenes Seamount interfering with subduction rates of the Cyprus Trench.

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.